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Self-assembly of single electron transistors and related devices Daniel L. Feldheima and Christine D. Keatingb a Department of Chemistry North Carolina State University Raleigh NC 27695 USA b Department of Chemistry Pennsylvania State University 152 Davey Laboratory University Park PA 16802 USA For the past 40 years since the invention of the integrated circuit the number of transistors on a computer chip has doubled roughly every 18 months. As the limits of photolithography are rapidly approached however it is becoming clear that continued increases in circuit density will require fairly dramatic changes in the way transistors are designed and operated. This review summarizes current strategies for fabricating transistors which operate based on the flow of single electrons through nanometre-sized metal and semiconductor particles; i.e.single electron transistors (SETs). Because the room temperature operation of SETs requires nanoparticles < 10 nm in diameter we focus mainly on devices which have the potential for being assembled from the solution phase (non-lithographic systems). Several applications of SETs are discussed in addition to the major hurdles which must be overcome for their implementation in electronic device technology. 1 Introduction 1.1 History and impact of the transistor The fabrication of the first transistor by Shockley Brattain and Bardeen nearly 50 years ago is arguably the most important technological development of the 20th century.1 Indeed it is difficult to think of an area of our lives on which the transistor has not had a significant impact.Transistors are major components in such comforts as compact disc players highperformance automobiles portable telephones and televisions and countless electronic devices. Perhaps more important to basic human health transistors are found in portable sensors for rapid medical and environmental screening may soon provide more freedom to diabetics via electronic wristwatch insulindelivery systems and one day may aid in returning sight in certain cases of blindness through ‘vision chips’ implanted in the back of the eye.2 Of course the greatest triumph of the Dan Feldheim was born in 1967 in Ventura CA. After studying under the direction of Professor Kevin Ashley at San Jose State University he joined Professor C.Michael Elliott’s research group at Colorado State University. He then conducted postdoctoral studies in Professor Thomas E. Mallouk’s laboratory at Pennsylvania State University as a National Science Foundation post-doctoral fellow before moving to North Carolina State University as an assistant professor. His research interests range from the development of single molecule detectors for ‘on-chip’ chemical sensing to the synthesis of highly ordered nanoparticle superstructures. Christine Dolan Keating Dan Feldheim transistor is the personal computer which now possesses more memory in the space of a small briefcase than computers which once filled large rooms. The above examples were made possible because the transistor has shrunk incredibly in dimensions over the past 40 years.Fig. 1 shows a timeline of the transistor minimum feature size vs. year. The size of the transistor has decreased by a factor of 2 every 18 months a trend first pointed out by Gordon Moore in the 1960s (Moore’s Law) and one that continues today.3 Today electronic devices employed in state-of-the-art integrated circuitry have dimensions of the order of 0.35 mm (350 nm). Thus well over 1 million transistors can be integrated in the space taken up by the first transistor. Fig. 1 “Moore’s Law” plot of transistor size vs. year. The trend line illustrates the fact that the transistor size has decreased by a factor of 2 every 18 months since 1950.This review addresses the question of how current trends may be continued in transistor miniaturization ideally down to the molecular level (tens of nanometres or less). It is tempting to Christine Dolan Keating was born in 1969. As an undergraduate she did summer research in Professor Janos Fendler’s laboratories at Syracuse University. Chris received her BS degree in Chemistry and Biology from St Francis College after which she joined Professor Michael J. Natan’s group at Pennsylvania State University. There she has conducted research aimed at controlling the assembly of colloidal metal particles. These assemblies have been used in studies involving surface-enhanced Raman scattering of adsorbed biological molecules. 1 Chemical Society Reviews 1998 volume 27 suggest that as the resolution of surface patterning techniques such as electron beam lithography improves conventional transistors could simply be made even smaller.Unfortunately the electronic properties of solids and solid–solid interfaces are inherently different on the nanometre level. Thus it is evident that if electronic integrated circuitry is to reach its ultimate potential (molecular-scale computing) radical changes in the way transistors are fabricated and operated will be necessary. But what will these changes entail? What materials will futuristic transistors be made from? How will they be assembled? How will they operate? The answers to these questions promise a revolution in the electronics industry.Consider that if the transistor could be scaled down to 5 nm in size over 10 000 of these ‘nanotransistors’ would fit in the same area as one of today’s transistors. Many schemes for building nanometre-scale computer components have been proposed.4 These include logic based on single molecules molecular shuttles resonant tunnelling diodes and atomic relays. Of all the designs proposed for use in futuristic integrated circuitry the one receiving the most attention lately is perhaps the single electron transistor (SET).5 (Coincidentally this year marks the 100th anniversary of the discovery of the electron). The SET is similar in principle to the conventional field effect transistor (vide infra). Logic operations in the SET are based however on the tunnelling of single electrons through nanometre-sized metal or semiconductor quantum dots.In the discussions which follow we describe the operating principles of the SET and related single electron devices. Several possible applications are highlighted some of which have already been reduced in practice. The major obstacles to implementing single electron devices in computer technology are discussed in addition to some strategies which are being pursued to overcome these problems. While lithographic techniques are mentioned briefly throughout the review because of size and cost constraints imposed on SETs we focus mainly on devices which have the potential for assembling themselves from solution via chemical interactions. Our objectives are to present a chemist’s view of the basics physics behind the SET (Section 2.1) and to summarize current efforts toward the chemical synthesis of SETs (Sections 3 and 4).Because space does not permit a general review of selfassembling systems we have respectfully omitted many important works in this area including those of Whitesides,2,4 Stoddart,4 Petty Ferguson and others4. However many of these Fig. 2 Schematic illustration of (a) an NMOSFET and (b) an n-channel MOSFET Chemical Society Reviews 1998 volume 27 2 papers are referenced within the literature cited at the conclusion of this manuscript. 1.2 The metal oxide semiconductor field effect transistor (MOSFET) Before describing the SET it will prove useful to briefly review the design and operating principles of conventional MOSFET transistors.The MOSFET is the most common type of transistor found in modern digital circuitry. A schematic diagram of an ‘NMOSFET’ is shown in Fig. 2(a).6 The NMOSFET consists of highly conductive n-type Si source and drain regions separated by an insulating p-type Si channel and body. The letters n and p refer to atomic impurities or ‘dopants’ which add excess free negative or positive charges respectively (the N in NMOSFET signifies n-type). Typical dopants for Si are boron or arsenic. Source and drain are terms which describe where current flows from and to respectively. A metal electrode commonly known as a gate separated by a thin oxide layer is attached to the Si channel. In the absence of an applied voltage bias between the gate and body in an NMOSFET current cannot flow between the source and drain because the p-type channel is insulating (the ‘off’ state).Upon application of a positive potential to the gate electrons migrate into the channel essentially creating an n-doped conductive pathway between the source and drain (the ‘on’ state). Since the NMOSFET is off in the absence of an applied bias it is sometimes called a ‘normally off’ transistor. This behaviour may be contrasted with the n-channel MOSFET in which a thin n-type channel has been inserted under the oxide layer between the source and drain regions [Fig. 2(b)]. In the absence of an applied gate bias a conductive path exists between the source and drain. When a negative potential is applied to the gate electrons are forced out of the channel.This renders the channel p-type (insulating) and eliminates the current path between the source and drain. Since the n-channel MOSFET is on in the absence of an applied bias it is called a ‘normally on’ transistor. Similar devices are made with p-type channels (PMOSFETS). When these two-state (on/off 1/0) transistors are integrated together on one chip they are called complimentary MOSFETs (CMOSFETS). Various combinations of CMOS transistors provide the NOT OR AND etc. logic functions upon which computer operations are based. A second important function of the MOSFET is signal amplification. Amplification in a transistor is due to the acceleration of electrons as they move through the strong electric fields in the channel region.This allows signals to propagate through the computer without losing their strength. 1.3 Scaling problems MOSFET devices have dominated computer technologies for several reasons including their low operating voltages (0.1 V) low power consumption (low heat) high speed and the ease with which they have been scaled down in dimension. Indeed in the past MOSFETS could be scaled down simply by shrinking each component part by a constant factor (i.e. the channel source gate leads etc.) and operating the device as usual. Unfortunately it is not at all certain that the operating principles of the MOSFET will scale as the size decreases even below 100 nm. As the n–p–n regions in the transistor shrink their ability to control the flow of electrons is overcome by the quantum mechanical probability that the electrons simply tunnel through the n–p interface.Furthermore as the transistor density increases the probability that an electron can tunnel between neighbouring transistors increases. These tunnelling processes cause errors in data manipulation and storage. There is also concern that as the size of a MOSFET decreases the ability to make any two transistors with the same electronic properties will be lost (i.e. achieving a specific dopant density in any two devices will be difficult).4 The rather obtrusive laws of quantum physics have left researchers with an ‘if you can’t beat ’em join ’em’ attitude thus initiating a search for a way to capitalize on quantum effects rather than circumventing them.2 Single electron nanoelectronics 2.1 Single electron tunnelling basic theory The discreteness of charge does not show up at the macroscopic level. Consider for example the charging of a large-area capacitor by a battery. The capacitor is charged by displacing electrons from their fixed positively-charged ions on one plate and transferring them to a second plate. The work required by the battery to perform this operation is given by eqn. (1) where (1) W = q2/2C q is the total charge stored (ne) C the capacitance and e the electron charge.7 A typical computer capacitor has picofarad (pF) capacitance. If one wanted to charge this capacitor with a single electron it would be necessary to apply a potential of Vext = e/2C Å 1028 V.Furthermore in order to avoid thermal effects the capacitor would have to be cooled to a temperature such that 1028 V > kT (corresponding to a temperature of 0.0005 K!). Conversely if more conventional potential increments are applied say 100 mV then not one but q = CV Å 106 electrons are stored in the capacitor. Importantly if the capacitor junction was thin enough and a single electron was able to tunnel from one plate to the other there would be no observable effect on the charging potential Vext. Thus although electrons are constrained to integer values once the capacitor is charged the ‘granularity’ of electrons is not apparent in macroscale devices. If the junction capacitance is small ( < ~ 10218 F) and the resistance is high however the charging energy and tunnelling of single electrons in the circuit can affect the current–voltage (I–V) characteristics of the capacitor.5 Consider the device shown in Fig.3(a) consisting of a bulk metal–insulator– nanocluster–insulator–bulk metal double tunnel junction (MINIM). (We use the terms cluster and particle interchangeably throughout the review to describe both semiconductor and metal particles less than ~ 50 nm in diameter). When the MINIM is biased by an external voltage source an extremely unusual current response is observed as the nanocluster capacitor is charged. Current steps are observed separated by voltage plateaus which may span hundreds of mV (‘the Coulomb staircase’ Fig.4). Each current step corresponds to the addition of a single electron to the cluster. Below the models and equations which describe this I–V behavior are examined and their consequences for future electronic device technologies are highlighted. In the semiclassical approach the MINIM device is treated as two capacitors with capacitances and resistances C1 R1 and C2 R2 placed in series and driven by an ideal voltage source Vext [Fig. 3(b)]. (The term ideal is used to describe a battery with zero internal resistance which can deliver charge instantly). The state of the system is described by the voltage drop across each junction (V1 V2) and Q0 the number of electrons on the cluster all classical variables. The dynamics of the system are then determined by the probabilities that an electron will tunnel across junction 1 and/or junction 2 thus altering Q0 (i.e.a stochastic approach). These tunnelling events are dependent on the change in energy of each electron as it tunnels from the bulk metal through junction 1 and onto the cluster. To quantify this dependence consider what happens to a MINIM device upon contacting the two metal electrodes but before an external bias is applied. The Fermi levels of the two bulk metal electrodes and the nanocluster will try to align by tunnelling electrons from the electrodes to the cluster. In general the Fermi levels will not be able to align exactly but will be offset in energy by one electron or more because of the discrete nature of charge and any impurities present in the junction region.We will ignore these details for now and consider the case of perfect alignment. One further initial assumption is that the quantum mechanical energy levels are closer in energy than the electrostatic energy levels. Now that the system is in electrostatic equilibrium a potential is applied by the voltage source and n electrons tunnel through the thin insulating barrier and onto the cluster. Our goal is to find n as a function of the applied potential (or applied energy). To describe this process energetically we focus on junction 1 alone (the local view) seeking the quantity DE1 = Ef 2 Ei where DE1 is the difference in the energy of junction 1 before (Ei) and after (Ef) the electron tunnels. This quantity represents the energy that must be supplied by the external voltage source to place an electron on the cluster.The initial state is the energy of junction 1 charged by n electrons [Fig. 3(c)]. This energy is given by eqn. (2) where C Ei = (ne)2/2CT T = C1 + C2 is the total cluster capacitance. Note (2) that CT is not the circuit capacitance [1/CT = 1/C1 + 1/C2 = (C1 + C2)/(C1C2)] but is the capacitance an electron ‘sees’ when tunnelling across the first junction. Global views in which the entire circuit capacitance is considered result in identical energy equations.5 The final state energy Ef is the energy of the system with an electron on the cluster. Placing an electron on the cluster lowers the potential across V1 which causes a polarization charge to flow through the circuit.Consequently the battery does work eV1 to bring an electron from metal electrode 2 to electrode 1. Combined with the energy associated with changing the cluster charge by one electron one obtains eqn. (3). (3) Ef = eV1 + [(Q02e)2/2CT] Upon expanding term 2 in eqn. (3) and subtracting eqn. (2) we obtain eqn. (4). (4) Ef2Ei = eV12(Qoe/CT) + (e2/2CT) Note that the energy of the system is fully described by the change in the cluster charge and the work done by the voltage source. To calculate the external voltage that must be applied by the battery or potentiostat a relation between V1 and Vext is needed. This is obtained using Kirchoff’s loop laws and charge conservation. First note from charge conservation that eqn.(5) holds. (5) C1V1 = C2V2 From Kirchoff’s laws we obtain eqn. (6). (6) Vext = V1 + V2 Combining eqns. (5) and (6) yields eqn. (7) 3 Chemical Society Reviews 1998 volume 27 Fig. 3 (a) Schematic diagram of a metal–insulator–quantum dot–insulator–metal (MINIM) device; (b) an equivalent circuit diagram for the MINIM; (c) the equations and equivalent circuit representations which describe the single electron charging and tunnelling events in a MINIM. Fig. 4 Schematic depiction of the I–V behaviour of an ideal MINIM. V (7) 1 = C2Vext/CT and finally eqn. (8). (8) DE1 = (eC2Vext/CT)2(eQo/CT) + (e2/2CT) Close examination of eqn. (8) reveals that the first term is the work performed by the voltage source to maintain V1 after an electron has tunnelled to the cluster.Terms 2 and 3 represent the single electron charging effects. Term 2 is the additional work required to tunnel an electron to the cluster if electron(s) are already present on the cluster. This term provides the voltage Chemical Society Reviews 1998 volume 27 4 feedback necessary to prevent the tunnelling of more than n electrons to the cluster per voltage increment where n is the step number (e.g. 1e2 2e2 etc. in Fig. 4). In contrast to the macroscale capacitor where the tunnelling of a single electron would not be noticed the transfer of a single electron through a nanoscale capacitor causes a substantial energy change in the circuit. This prevents more than the allowed number of electrons (n) from residing on the cluster simultaneously.The current staircase shown in Fig. 4 can now be rationalized by considering the allowed voltage change of the junction DV > 0. If this were not the case the electron would immediately tunnel back to where it came from. Thus eqn. (9) holds. (9) Vext > Q0/C22e/2C2 In the case of an initially neutral nanoparticle (Q0 = 0) an external voltage of e/2C2 is required before current may flow through the circuit (the Coulomb gap or blockade). When this voltage is reached a single electron tunnels to the cluster. The electron does not remain on the cluster indefinitely but quickly tunnels off through the next junction (ca. 100 ps depending on the ratio R2C2/R1C1). It does remain long enough however to provide the voltage feedback required to prevent additional electrons from tunnelling simultaneously to the cluster.Thus a continuous 1 electron current of I = e/2R2CT flows through the circuit (notice that e/RC contains units of charge per time). Each additional electron placed on the cluster requires a full e/C2 in voltage. This leads to the overall 1/2 3/2 5/2 etc. voltage increments in the current staircase in Fig. 4 (with each current step after the first of magnitude e/R2CT). A number of important assumptions regarding eqns. (1)–(8) must be emphasized at this point. (i) The only electron transfer events considered were from the electrodes to the nanocluster. Other tunnelling pathways such as those from electrode 1 to electrode 2 between particles were not considered.(ii) The voltage source was assumed to deliver charge as fast as the electron tunnels but the time between tunnelling events was long. (iii) Misalignments in the Fermi level due to charge offsets or impurities were not considered. These can be accounted for simply by adding a voltage offset term to eqn. (8). (iv) The quantum mechanical energy level spacing was assumed to be smaller than the electrostatic energy spacing. This assumption is valid for metal particles > ca. 5 nm in diameter. Semiconductor particles display quantum effects at sizes much greater than this however. These effects have been treated successfully within the context of the above models. (v) Tunnelling from one metal electrode onto the nanocluster was considered exclusively.The opposite case tunnelling from the cluster to the metal electrode occurs by reversing the applied bias. This results in the identical staircase structure with current steps of opposite sign. Finally (vi) the resistances of the junctions are so large (R > h/e2) that the electrons are localized on one side of the junction or the other. 2 and R2 9 C1 and R1 [Fig. 5(a) and (b)].8 As In addition to the assumptions stated above a number of subtleties exist which make the experimental observation of the Coulomb staircase challenging. One challenge is in designing a system with optimum junction capacitances and resistances. Simulated MINIM I–V curves show that the sharpest steps are observed for C C2/C1 and R2/R1 approach 1 the zero-current plateau at 0 V remains but the current steps disappear.This represents a departure from assumption (ii) above. If R and C for the two junctions are equal the electron will tunnel through both junctions with identical rates. The voltage feedback required to 2/C1 and R2/R1 = 1. Both plots assume e2/CT9kT. Fig. 5 Calculated I–V curves for a MINIM with (a) C2/C1 and R2/R1 = 100 and (b) C see current steps is thus lost. Unfortunately since C decreases but R increases as the junction thickness increases these ratios can only be optimized by constructing the two junctions from materials with different dielectric properties. The biggest obstacle to designing a MINIM device was mentioned briefly above—that is thermal effects. To avoid thermally-activated tunnelling processes e/2C2 9 kT.As T increases the single electron current steps are gradually washed out and an ohmic response (linear I–V curve) is observed. The room temperature operation of single electron devices is therefore limited to clusters < ca. 12 nm in diameter. Finally we must point out that many experimental configurations involve measuring the I–V properties of parallel arrays of clusters [e.g. metal–insulator–(nanocluster)N–insulator– metal devices see Section 3.2]. The arguments above hold for these systems with the exception that the current steps are of magnitude Ne/RC. In other words each cluster acts as a single MINIM with their currents additive. An additional challenge to the observation of Coulomb charging effects in these systems is that the dispersity in the diameter of the clusters must be low.Otherwise the varying capacitances (and charging energies) of the nanoclusters will cause the steps to blend together and the I–V curve will again appear ohmic. An interesting analogy to the behaviour of MINIM devices has been drawn by Kastner.9 He has used the term ‘artificial atom’ to describe the controlled addition of single electrons to nanoparticles. In the analogy adding electrons to a particle is similar to adding electrons to an atomic nucleus in moving across a row of the Periodic table. If a gate electrode is included in the structure the analogy can be stretched further. A positive gate bias pulls charge away from the cluster allowing excess electrons to tunnel from the source to the cluster.The gate electrode can thus be used to control the number of extra electrons on the dot. This is equivalent to adding protons to an atom. Of course adding positive charge to a nucleus changes the number of electrons that must reside on a neutral atom. In fact the analogy does have a mathematical foundation. The potential energy of a two-particle system (the hydrogen atom) is 21/4pe0(e2/r) and the capacitance of an isolated sphere is 4pe0r (r is the sphere radius). Combining these terms yields the energy of a hydrogen atom in terms of its capacitance. Conversely the energy of a hydrogen-like nanoparticle ‘atom’ is obtained. This is eqn. (2) above. 2.2 Single electronics a brief history Predictions of single electron charging effects date back to the 1950s.10 Soon after the existence of the Coulomb blockade of electrons was demonstrated for electron hopping in granular metal films.11 It was not until 1987 however that broad current steps were observed in the low-temperature (4 K) I–V curves of Cu–Al2O3–sputtered Ag island–Al2O3–Ag sandwich structures.12 The ‘smearing’ out of the steps was attributed to polydispersity in the size of the Ag islands. Sharper current steps were revealed by Ammen and coworkers when the tip of a scanning tunnelling microscope (STM) was placed over a single Au island.13 During this time experiments were also being performed on devices fabricated lithographically. In pioneering experiments Fulton and Dolan fabricated spatially well-defined MINIM double tunnel junctions in which a gate electrode was placed near the central island.14 These workers showed that charging effects could be modulated by applying a gate bias.This threeterminal device by analogy to the MOSFET described above was named a single electron transistor (SET). The flow of single electrons from source to drain in the SET was controlled by injecting (or removing) single electrons from the metal dot through the gate lead. Once again however because of the large size of the device these experiments were performed at extremely low temperatures (1 K). The fundamental experiments described above provided important evidence that single electron tunnelling effects 5 Chemical Society Reviews 1998 volume 27 existed and gave hope that SETs might one day form the basis of advanced computing devices.At this point however an impasse was reached with regard to the fabrication of Coulomb blockade devices. On one hand photolithographic techniques were capable of fabricating complex SET structures easily and cheaply but with minimum size features of only ca. 100 nm. This limited the operation of SETs to sub-Kelvin temperatures. (Likewise electron beam lithography while capable of producing features of the order of 5 nm is expensive slow and still not readily available.) On the other hand relatively simple metal evaporation methods provided metal islands with features down to 10 nm but the precise placement and dispersity of the islands was difficult to control.In contrast to lithography and metal evaporation wet-chemical synthesis can provide clusters of almost arbitrary size. This has prompted research aimed toward single electron devices which assemble themselves from solution. 3 Self-assembled single electron tunnelling devices 3.1 Synthesis and self-assembly of colloidal particles on solid surfaces For the purposes of this review self-assembly is defined as the solution phase chemically directed organization of materials into pre-designed composite structures. The composite structure of interest here is the MINIM containing a metal or semiconductor nanoparticle < 20 nm in diameter. Chemically-synthesized nanoparticles offer several advantages as SET components most important of which is their small size.Metal and semiconductor nanoparticles can be prepared in solution with average diameters tens of Ångstroms and larger. Adsorbed or covalently attached ligands can act as stabilizers against agglomeration and can be used to impart chemical functionality to nanoparticles. Importantly nanoparticles can be immobilized between insulating thin films through electrostatic or covalent attachment chemistries. Countless methods for synthesis of metal and semiconductor particles have been published; these have been reviewed elsewhere.15,16 For example II–VI semiconductor nanoparticles (CdS ZnS) have been prepared by introducing H2S or Na2S into a solution containing the appropriate cation (ZnCl2 CdCl2) or by pyrolysis of organometallic precursors (alkylcadmium silylchalcogenides) in hot coordinating solvents (trin-octylphosphine).17 Colloidal metals are typically made by addition of a reducing agent (citrate NaBH4) to a solution of the metal salt (HAuCl4 H2PtCl4); the smaller metal clusters (73 nm) are often prepared by gas phase or liquid two-phase systems containing ‘capping ligands’ (RSH).Capping ligands or surfactants can be used to stabilize the nanoparticles and prevent the formation of larger particles and agglomerates. The capping ligand metal ratio is used to control the final cluster size. While methods for preparing colloidal particles are numerous the goal of producing monodisperse clusters of a target diameter has been attained only in a few cases most notably Au.Au nanoparticles can be prepared with mean diameters from 0.84 to more than 200 nm.15 In fact many sizes are commercially available (Nanoprobes Goldmark Biologicals). The stability and reactivity of colloidal particles is determined largely by the ligand shell adsorbed or covalently bound to the surface of the particle. Nanoparticles tend to aggregate and precipitate; this can be prevented by the presence of a ligand shell. Water-soluble sulfonated phosphine ligands [P(m- C6H4SO3Na)3] have been used to stabilize CdS and Au nanoparticles. The phosphine-stabilized particles can be isolated and resuspended without agglomeration. Unfortunately these ligands degrade slowly in the presence of H2O or O2 limiting their long-term stability.16 Recently Brust and coworkers prepared ligand-stabilized Au clusters from a two-phase solvent system containing C12H25SH.These clusters exhibit solubility in organic solvents can be evaporated to dryness and Chemical Society Reviews 1998 volume 27 6 resuspended and are air stable.18 Since this work was first published several groups have shown that bifunctional organothiol ligands (RCnH2nSH where R = Br CH2NCH ferrocene etc.) can be used to control the surface chemistry and reactivity of Au nanoparticles.19,20 The capping ligand may then be employed in coupling reactions to produce more complex assemblies. An elegant example of this was provided recently by Alivisatos and coworkers who reactively coupled SH– terminated single stranded DNA oligonucleotides to maleimido-funtionalized 1.4 nm Au clusters.21 Upon addition of complementary oligonucleotides these particles self-assembled to form dimers and trimers.In similar work Mirkin and coworkers prepared 3D superstructures of 13 nm Au colloids capped with SH-terminated DNA nucleotides.22 Such directed assembly using chemically-functionalized ligand shells holds great potential for control and direction of nanoparticle placement in device fabrication. Construction of electronic devices such as SETs requires the assembly of nanoparticles onto solid supports. Solution-based approaches to surface assembly of metal and semiconductor nanoparticles typically involve electrostatic or covalent binding of the particle to a surface-bound molecular or polymeric thin film.For example surfactant structures (monolayers bilayers etc.) have been used to direct assembly of metallic semiconducting and magnetic particles.23 This can be accomplished by adsorbing particles electrostatically to charged surfactant headgroups or by in situ generation of particles beneath monolayers at the air–water interface. Surfactant monolayers with or without attached nanoparticles can be transferred to solid supports using standard Langmuir–Blodgett techniques. Nanoparticles can also be assembled on solid supports using polyelectrolytes. Schmitt et al. and Mallouk et al. have prepared multilayered insulator–Au particle–insulator structures with alternating anionic and cationic polyelectrolyes as the insulating layers.24,25 The thickness of the polyelectrolyte layers between particles was varied by increasing the number of cation and anion deposition cycles (see Section 3.2).Covalent attachment strategies often take advantage of the reactivity of the outer shell atoms in the cluster. Many metallic and semiconducting clusters (Au Ag CdS CdSe) have a high affinity for amine and/or thiol moities. For example Alivisatos and coworkers have covalently attached CdS particles to bulk Au and Al substrates using bifunctional crosslinkers (dithiols thioglycolate).20 Natan and coworkers have assembled Au and Ag colloidal particles on NH2- and SH-terminated organosilane polymers on SiOx and SnO2 substrates. The kinetics of this surface-assembly reaction have been investigated in some detail affording control over the number of particles on the surface.26 Alternatively close-packed monolayers of alkanethiol stabilized clusters have been formed by solvent evaporation.18,27 In this case the length of the organic ligand defines the distance between particles. This distance has a pronounced effect on the electronic properties of the resulting 2D array (see Section 3.2). Recently lines and grids of Au particles have been fabricated with features less than 1 mm by combining Au selfassembly with microcontact printing28 or conventional lithography techniques.29 Using wet-chemical approaches to nanoparticle organization such as those described here one can envision assembly strategies for nanoparticles of nearly any material on almost any substrate.The versatility of these immobilization methods makes it possible to design a number of self-assembled electronic devices including the insulator–cluster–insulator tunnel junction of the SET. Despite these recent advances numerous challenges remain in the area of nanoparticle synthesis and assembly. Control over size monodispersity or ligation is not currently available for most metal and semiconductor materials in the size range of interest for SETs ( < 12 nm). Increasing the monodispersity of nanoparticles would improve the operation of single electron devices. In addition methods for arranging nanoparticles into more complex 2D and 3D assemblies (other than submonolayers and simple closestpacked geometries) are completely lacking.A self-assembled 2D square lattice of clusters for example would be an interesting analogue of large 2D arrays which have been fabricated lithographically. 3.2 Current–voltage characteristics of self-assembled single electron devices Andres and coworkers have investigated the I–V properties of self-assembled Au nanocluster films extensively. In one type of experiment 1.9 ± 0.6 nm diameter Au clusters were bound to a bulk Au substrate via a SAM of the dithiol p-xylene-a,aA-dithiol (XYL dithiol).27 An STM tip was placed over a single nanocluster to complete the Au–dithiol–Au nanocluster–air gap–STM tip double tunnel junction. I–V measurements revealed a Coulomb gap and one clear current step at positive bias even at room temperature.A number of important electrical parameters were also ascertained from their data. For example the resistance of a single XYL molecule was estimated to be 18 ± 12 MW and the dithiol junction capacitance was estimated to be 1.7 3 10219 F. These values agreed well with theoretical predictions. In a second type of experimental arrangement Andres and coworkers assembled a 2D array of decanethiol-coated Au nanoclusters between two Au electrodes separated by ~ 450 nm.27 Coulomb blockade effects were again marked by the appearance of a high resistance gap around 0 V in the I–V curves. Interestingly when the nanoclusters were exposed to a conjugated aryl diisonitrile molecule the I–V response became ohmic. Presumably this was due to a combination of increased electronic overlap between particles resulting from the p system of the conjugated molecules and the distance dependence of electron hopping between Au centres (the Au–Au distance increased by 0.4 nm as the diisonitrile bound).Experiments similar to those described above were conducted by Murray et al.19 In their work Au clusters stabilized by alkane thiol SAMs of varying alkyl chain lengths (C8 C12 C16) were assembled across the gaps of interdigitated array electrodes and investigated electronically. Non-linear I–V curves were reported for these systems which depended on the length of the alkane chain. Conductivities of the 2D arrays calculated from the I–V curves revealed a two order of magnitude decrease for every four carbons in the alkane chain.Murray pointed out that the I–V properties observed in Au cluster monolayers in the high potential limit fit well to models usually employed in interpreting electron transfer in redox polymer systems. These models provide additional insight into the Au cluster–cluster electron transfer mechanism rate coupling coefficient and charging energy. Mallouk and coworkers have used a combination of layer-bylayer inorganic polyelectrolyte and Au nanocluster selfassembly methods to fabricate MINIM devices.25 This scheme is depicted in Fig. 6. First a clean bulk substrate was immersed in a solution of mercaptoethylamine hydrochloride to immobilize cationic sites on the surface. The substrate was then alternately soaked in aqueous solutions containing single anionic sheets of lamellar inorganic solids [KTiNbO5 a-Zr(HPO4)2·H2O (ZrP)] and organic polyelectrolyte cations [polyallylamine hydrochloride (PAH)].A ‘monolayer’ of the desired polyelectrolyte ion exchanges onto the oppositelycharged material deposited on the substrate during the previous immersion step (i.e. anionic ZrP to cationic PAH). Multilayers of the same material cannot form on the surface during a single immersion step because of electrostatic repulsion. The thickness of the resulting film was thus defined by the number of immersion cycles the substrate was subjected to. Once the desired junction thickness was assembled Au nanoparticles were introduced into the film by soaking the substrate in a solution containing citrate-stabilized Au nanoclusters.Au colloids bind readily to the amine functionalities contained in PAH. Following Au cluster deposition a second insulating junction was formed by simply reversing the adsorption sequence used to form the first junction. Note that the two junctions may be designed to vary in thickness and/or composition (i.e. a different inorganic may be chosen for junction 2). A thin layer of the organic conducting polymer poly(pyrrole) was polymerized on top to complete the MINIM device. Fig. 6 Illustration of the polyelectrolyte sequential adsorption route to MINIM devices developed by Mallouk and coworkers. The plot at the bottom shows ellipsometry data of layer thickness vs. layer number for a typical device (see ref.25 for details). I–V curves of MINIM devices fabricated with 2.5 ± 1.5 nm diameter Au nanoclusters displayed Coulomb gap potentials at room temperature which agreed well with predictions based on eqn. (9). The magnitude of the gap potential was somewhat tunable via the junction thickness; decreasing the junction thicknesses from 80–30 Å (by decreasing the number of polyelectrolyte pairs) decreased the Coulomb gap potential from 400 to 275 mV. In addition changing the Au nanocluster size also affected the I–V properties of these devices. Fig. 7 shows a series of I–V curves recorded at various temperatures for a device fabricated with 12 nm diameter Au clusters. At temperatures close to 25 °C an ohmic response was observed because the capacitance of the particles was such that kT > e/2C.Upon cooling slightly however the I–V curves became increasingly non-linear as the single electron charging energy began to dominate the tunneling process. Although single electron current steps were not observed in these devices (probably because C2/C1 = 1) the inorganic polyelectrolyte self-assembly approach appears to be a promising route to MINIM devices since (i) lamellar inorganic solids with a wide range of dielectric properties are amenable to the assembly methods outlined above (providing a means of optimizing C1 and C2) and (ii) defects do not seem to be a concern. Note that single electron charging effects were observed in devices consisting of a parallel array of ~ 1011 clusters covering large areas (1–2 cm2).This indicates that the 7 Chemical Society Reviews 1998 volume 27 Fig. 7 I–V curves at three temperatures for a MINIM device consisting of an Au substrate–60 Å ZrP/PAH–12 nm Au nanoparticle–70 Å ZrP–PAH– poly(pyrrole). devices do not short-circuit themselves through defects. Scaling the device down in size by assembling the films on prepatterned surfaces should therefore be straightforward and will only diminish the defect density. A self-assembled MINIM structure was recently scaled down to the level of a single particle by Alivisatos McEuen and coworkers.30 To fabricate the structure a combination of optical lithography and angle evaporation techniques were first used to define a narrow gap (a few nm) between two Au leads on a Si substrate [Fig.8(a)]. The substrate was then placed in an isopropyl alcohol solution containing hexane-1,6-dithiol. The dithiol binds to Au surfaces linearly with one end attached to the surface and the other end facing the solution. The free end was used to assemble 5.8 nm Au or CdSe clusters in the region between the leads [Fig. 8(b)]. A Au–dithiol–nanocluster– dithiol–Au device resulted from these procedures. An I–V curve for a device with a 5.8 nm Au cluster displaying slight current steps is shown in Fig. 9. Fitting the curve to the Coulomb blockade models presented above gave C1 = 2.1 aF C2 = 1.5 aF R1 = 32 MW and R2 = 2 GW. While it is unclear why the same dithiol linker would result in two junctions with such different capacitances and resistances Chemical Society Reviews 1998 volume 27 8 ( a) 50 nm Au Nanocrystals ( b) 5 nm Hexane-1,6-dithiol Fig.8 (a) Field emission scanning electron micrograph of a lead structure prior to the assembly of nanocrystals; (b) schematic cross section of nanocrystals bound to the leads. (Taken from ref. 30). this approach to single electron devices is exciting in part because the gate electrode is built-in. The underlying Si substrate was used recently as a gate to externally control the flow of single electrons from source to drain to make a true SET. Recent work by Moskovits and coworkers while probably not self-assembly in the strictest sense deserves mentioning.31 Moskovits used porous Al2O3 membranes as a template for the synthesis of metallic and semiconductor wires.Membranes with pore diameters ranging from 4 to 250 nm have been synthesized by oxidizing an Al substrate in acidic media. The underlying Al can then be used as the working electrode for the electrochemical deposition of a number of metallic magnetic and semiconductor materials. For example Ni wires were electro-deposited in the pores of a 10 nm diameter porous membrane and the Ni was oxidized at the tips. Sputterdepositing Ag on top resulted in a Ag–NiO–10 nm Ni wire– Al2O3–Al MINIM device [Fig. 10(a)]. I–V curves for the template-synthesized MINIMs show remarkably well-defined current steps [Fig. 10(b)]. Interestingly each voltage plateau is of the same magnitude ( ~ 1 V).Recall from the discussion above that the first current step should require half the voltage of each successive step. This observation and the large background current associated with each step is likely a consequence of electronic coupling between the closely spaced wires. (Each wire was separated by approximately 10–20 nm.) The electronic coupling between wires effectively adds an additional charging term to eqn. (9) and can shift the entire I–V curve up and to the right (Fig. 11). Similar effects were briefly noted for a gate bias or impurity Fig. 9 I–V characteristic of a 5.8 nm diameter Au nanocrystal measured at 77 K. (Taken from ref. 30). Fig. 10 (a) Schematic diagram of a 10 nm diameter MINIM fabricated in the pores of an anodically-etched Al2O3 film; (b) I–V characteristic of the device depicted in (a) showing several current steps as a function of potential.(Taken from ref. 31 copyright 1996 IEEE). charge in section 2.1. These experiments illustrate the importance of the local environment for the electronic properties of single electron devices. Fig. 11 Calculated I–V curve for a MINIM with an initially uncharged particle (solid line) in the presence of an external charge source (dashed line). Both plots assume e2/CT9kT; C2/C1 and R2/R1 = 100. 4 Applications of single electron devices Numerous applications for single electron transistors have been suggested. These include ultra-high density information storage supersensitive electrometry near-infrared radiation receivers and dc current standards.5 Several applications have already been demonstrated at low temperatures in devices fabricated lithographically.Below we highlight the operating principles of SETs as they pertain to two of the more advanced applications—computing and electrometry. 4.1 Single electron memory Perhaps the ultimate application of the SET is as a memory cell in which information is stored as the presence or absence of a single electron on the cluster. Two routes have been suggested for implementing SETs into digital circuitry. The first is to mimic conventional MOS technology. In this scheme a single electron injected onto the cluster from the gate electrode modulates the source–drain current. As with the MOSFET current flow (on or off) would represent ‘1s’ and ‘0s’.Single electron memory of this type was demonstrated independently by Chou and Chan recently.32,33 Their SETs consisted of a Si nanoparticle (or several particles in Chan’s device) embedded in a thin SiO2 insulator. Conductive Si source drain and gate electrodes surrounded the particles. Chan’s devices displayed read/write times of ca. 20 ns lifetimes in excess of 109 cycles and retention times of days to weeks (meaning the charge does not leak out of the dot during this time). Although these are not exceptional quantities read/write times of ~ 30 ps are possible in CMOS transistors they are certainly acceptable when one considers that it may be possible to integrate 4–5 orders of magnitude more SETs cm22 than is viable with the current state-of-the-art transistor.The incorporation of Si cluster devices into existing Si technologies is also appealing. A second proposed method for utilizing SET-based memory is to make 1 bit = 1 electron rather than using the source–drain dc current flow. As proposed arrays of typically 4 to 7 SETs are connected in series and the positions of single electrons in the array are used to designate different memory states. This design has been difficult to realize in practice. If successful however this type of memory may have advantages over the MOSFETtype memory described above. One advantage could be gained when the time comes for the large scale integration of SETs to form logic gates. Integrated SETs operating on conventional principles may have problems due to their inherently low voltage gain.Coding memory by single electrons rather than voltage signals avoids this problem. 4.2 Supersensitive electrometry While single electron computing continues to be the ultimate goal of SETs the most advanced practical application currently for SETs is probably as an electrometer (a device used to measure charge). The SET electrometer is operated by capacitively coupling the external charge source of interest to the gate. Changes in the SET source–drain current are then measured as the unknown charge quantity is placed on the gate. Esteve has reported a charge sensitivity of 600 pA per e2 for an SET electrometer fabricated lithographically.5 Fulton and coworkers recently built a scanning SET electrometer on the end of a sharp glass tip.34 Sub-single electron charges placed near the tip caused measurable changes in the SET source–drain current.For example when placed in close proximity to an illuminated GaAs–AlGaAs heterostructure individual photo-ionized charge sites in the semiconductor could be mapped across the surface with a resolution of 100 nm. Extremely sensitive capacitance measurements have also been performed using a similar configuration. The SET electrometer is loosely considered to be the charge analogue of the SQUID device used for magnetic flux measurements (although not quite as sensitive). The SET electrometer is in principle not limited to the detection of charge sites on a surface but should be applicable to a wide range of sensitive chemical signal transduction events as well.For example if the nanoparticle of an SET is capped with alkane thiols containing an analyte receptor moiety the I–V properties should be extremely dependent on any binding or redox events that occur at the particle surface (Fig. 12). There are two possible mechanisms which would alter the I–V curve upon analyte binding a change in particle capacitance or charge (in analogy to the scanning SET when it approaches a surface charged surface). It is difficult to predict a priori the magnitudes of these changes; however Murray and coworkers have recently employed rotated-disk voltammetry to measure the average capacitance change per particle during the oxidation of ferrocene-terminated alkane thiols attached to Au clusters 9 Chemical Society Reviews 1998 volume 27 (these experiments were conducted on 0.1 mm solutions of Au clusters).19 The average capacitance increased by a factor of 8 upon oxidation an extraordinary change considering that only a few molecules on the particle surface were oxidized.Our calculations suggest that in the configuration shown in Fig. 12(a) a capacitance change of this magnitude would result in large shifts in the I–V curve [Fig. 12 (b)]. These calculations demonstrate the potential to detect a redox event occurring on even a single molecule attached to a metal nanoparticle thus enabling fundamental studies of the kinetics and thermodynamics of single-molecule electron transfer events.Fig. 12 (a) Schematic illustration of a Au nanoparticle assembled between two metal electrodes. The nanoparticle is capped with alkane thiols and a single generic redox active a,w-substituted alkane thiol which can undergo the redox reaction shown; (b) calculated I–V curves for the structure shown in (a) assuming an eight-fold capacitance change (see ref. 19) in going from Red (solid curve) to Ox + 1e2 (diamonds). Fig. 13 Transmission electron micrograph showing the Au nanoparticle dimers and trimers which formed as a result of the reaction shown at top. The Au particles were initially 1.4 nm in diameter but were ‘enhanced’ by selective Ag reduction (Nanoprobes) for better viewing. Chemical Society Reviews 1998 volume 27 10 4.3 Lage-scale integration of SETs Looking ahead to the all-SET computer one might envisage a number of problems.For example although 1 SET has demonstrated useful memory capabilities how will 10x SETs (with x being very large) be integrated? How will the integrated SET systems be connected to the outside world? Chemical selfassembly is in principle an ideal way of solving these problems. Recently the first steps toward the integration of nanoclusters were taken by Alivisatos. His group synthesized CdSe clusters capped with N-methyl-4-sulfanylbenzamide (MBAA). Reaction of the MBAA with bis(acyl hydrazide) crosslinked the particles and CdSe dimers were isolated from the mixture by centrifugation.35a The DNA-crosslinked Au dimers and trimers described in Section 3.1 constitute a second important milestone in the integration of nanoclusters.We have taken a similar approach to integrated systems by attaching 1.4 nm Au clusters to tetrakis(p-aminophenyl)porphyrins. Au–porphyrin dimers (20%) and trimers (5%) were observed in the product mixture (Fig. 13). Au–porphyrin tetramers were not produced presumably because of steric hindrance. Porphyrins were chosen as the ‘scaffolding’ from which to build cluster arrays because of their rigidity and well-developed coupling chemistry. In fact very large porphyrin arrays have been synthesized which could be modified to accommodate metal clusters in the preciselydefined arrangements required for single electronics. Johnson and coworkers have recently developed an approach to integrating nanoparticle structures whereby Au clusters are linked up directly on a planar surface or between source and drain electrodes.35b This was accomplished by first adsorbing a single layer of well-spaced 10 nm diameter Au clusters to the surface treating the particles with hexane-1,6-dithiol and finally treatment with a second layer of Au clusters.The second layer of Au clusters attached to the first layer via the thiol linker in many cases forming Au cluster trimers tetramers etc. When applied to source and drain electrodes this method produced Au cluster trimers which spanned the gap between leads enabling electronic characterization. Single electron tunnelling was observed for these systems the capacitances in accord with those observed by Alivisatos.The question remains as to how the SET arrays will be wired to the outside world. (It is not an unreasonable task to make electrical connections to a single nanotransistor. Contacting 1012 transistors is quite a different demand.) One way to accomplish this may be to employ a hybrid approach where SETs and related devices are integrated together with existing MOSFETs. This approach is appealing because it could increase the integrated circuit density while building on 50 years of existing technology. Notice from the Moore’s law plot (Fig. 1) that the era of nanoelectronics would be ushered in more quickly using this strategy. A second approach proposed separately by Lent and Korotkov is to forgo the wires altogether.4 This scheme appropriately named quantum cellular automata (QCA) is based on the electrostatic interactions present between cells of connecting clusters.In Korotkov’s design the basic cell is a line of nanoclusters connected by insulating material (Fig. 14 top). An electric field applied in either direction polarizes the string to give a ‘1’ or ‘0’ state. Lent’s QCA is similar in principle but square cells of nanoclusters carry the polarization states (Fig. 14 bottom). Again two states are possible depending upon the direction of the applied field. In either design the cells are connected in various configurations to make more complex logic circuits. Fig. 15 illustrates how Lent’s cells are connected to form a logic gate. The dark and open circles correspond to one-electron rich and one-electron deficient clusters respectively.Note that the configuration of the overall circuit provides a vehicle for controlling the polarization state of individual cells (i.e. the entire circuit relaxes to its low-energy configuration). Alternating 1s and 0s result from the design shown in Fig. 15. Fig. 14 Nanoparticle–insulator structures proposed in the wireless computing schemes of Korotkov (top) and Lent (bottom).4 The circles represent quantum dots the lines are insulating spacers. The advantage that QCA offers over conventional circuit technology is that signals are rapidly transferred between interconnecting cells via electrostatic interactions only. These signals travel at the speed of light so the time required for one cell to influence another is negligible.Furthermore electrostatic signals can be transmitted over long distances making communication between large arrays of cells possible without extensive wiring. This advantage along with the small size of each cell (as low as ~ 2.5 nm2) makes the prospects of ultrahigh density data storage excellent. A four-dot QCA logic cell was recently demonstrated by Lent and coworkers.4b Their device fabricated lithographically consisted of four Al islands situated at the corners of a square with Al2O3 serving as tunnel barriers between islands. Gate electrodes were used to switch the single-electron polarization states trapping electrons on specific islands for periods of Fig.15 A simple logic circuit consisting of an array of individual quantum dot cells as described by Lent and coworkers.4 The dark and open circles represent quantum dots with an excess and deficiency of charge respectively. minutes. Although the device was much larger (8 mm) than current MOSFETS and operated at much lower temperatures (1 K) this work demonstrated QCA logic for the first time experimentally. Two important factors must be considered in designing more complicated QCA structures. First one must consider that when relying on electrostatics to set up a memory state the location and size of each dot must be controlled precisely. Deviations from a particular structural design will lead to undesirable tunnelling events and hence data manipulation errors.Second as with the SET if QCA is to function at room temperature the islands must be < 10 nm in diameter. In this respect Au– porphyrin or Au–DNA building blocks may prove useful for the solution-assembly of QCA circuits that operate at much higher temperatures than devices fabricated lithographically. 5 Summary and future challenges If Moore’s first law states that integrated circuitry roughly doubles in density every 18 months his second law might be that the cost associated with the first law quadruples every 18 months. If integrated circuit density is to continue to increase into the next century it is clear that fairly dramatic changes in the way transistors are fabricated and operated need to be made. This review has outlined strategies for fabricating transistors which operate by controlling the flow of single electrons.Research to date has shown that a single SET can function as an extremely sensitive electrometer and memory cell. If nanoscale electronics is to come to full fruition however three challenges must be met. First if these devices are to operate near room temperature large quantities of monodisperse nanoparticles less than 10 nm in diameter must be synthesized. Significant progress has been made toward this in the past few years. Second methods must be developed for connecting the individual structures into patterns which function as logic circuits. Third these circuits must in turn be arranged into larger 2D patterns. From both physical and economic perspectives photo and electron beam lithographies are currently not suited to meet any of these challenges.Chemical self-assembly methods however are becoming quite adept at arranging large numbers of small structures into well-ordered macroscopic architectures. It is likely that these methods will have much to offer the chemist interested in designing more complex nanoparticle structures for use in advanced electronics. 6 Acknowledgements The authors wish to thank Drs L. Andrew Lyon and Susan M. Hendrickson and Professors Michael J. Natan and Thomas E. 11 Chemical Society Reviews 1998 volume 27 Mallouk for helpful discussions. Professor A. P. Alivisatos is gratefully acknowledged for discussing as yet unpublished results on the single cluster SET.Portions of this work were financially supported by the National Science Foundation (DLF #CHE-9504672) North Carolina State University start-up funds (D. L. F.) and the Henkel Corporation/ACS Division of Colloid and Surface Chemistry (C. D. K.). References 1 J. Bardeen and W. H. Brattain Phys. Rev. 1948 74 230. 2 A special issue on Key Technologies for the 21st Century appeared in the September 1995 issue of Scientific American. The impacts of transistor-based devices on science and medicine are discussed throughout. 3 B. E. Deal Interface 1997 6 18. 4 (a) M. S. Montemerlo J. C. Love G. J. Opiteck D. Goldhaber-Gordon and J. C. Ellenbogen Technologies and Designs for Electronic Nanocomputers Mitre Corporation McLean VA 1996; (b) A.O. Orlov I. Amlani G. H. Berstein C. S. Lent and G. L. Snider Science 1997 277 928. 5 Single Charge Tunneling Coulomb Blockade Phenomena in Nanostructures ed. H. Graber and M. H. Devoret New York Plenum 1992 NATO ASI Series B 294. 6 R. E. Hummel Electronic Properties of Materials 2nd edn. New York Springer-Verlag 1993. 7 D. Halliday R. Resnick and J. Walker Fundamentals of Physics 5th edn. New York 1997 pp. 603–643. 8 SET simulation programs are available on the internet courtesy of A. Korotkov. See http://qt.tn.tudelft.nl/SET/Korotkov/index. html 9 M. A. Kastner Physics Today January 1993 24. 10 C. J. Gorter Physica 1951 17 777. 11 C. A. Neugebauer and M. B. Webb J. Appl. Phys. 1962 33 74. 12 J. B. Barner and S. T. Ruggiero Phys. Rev.Lett. 1987 59 807. 13 M. Amman R. Wilkins E. Ben-Jacob P. D. Maker and R. C. Jaklevic Phys. Rev. B 1991 43 1146. 14 T. A. Fulton and G. J. Dolan Phys. Rev. Lett. 1987 59 109. 15 Clusters and Colloids ed. G. Schmid New York VCH 1994. 16 G. Schmid Chem. Rev. 1992 92 1709. 17 C. B. Murray D. J. Norris and M. G. Bawendi J. Am. Chem. Soc. 1993 115 8706. Chemical Society Reviews 1998 volume 27 12 18 M. Brust M. Walker D. Bethell D. J. Schiffrin and R. J. Whyman J. Chem. Soc. Chem. Commun. 1994 801. 19 S. J. Green J. J. Stokes M. J. Hostetler J. Pietron and R. W. Murray J. Phys. Chem. B 1997 101 2663 and references therein. 20 V. L. Colvin A. N. Goldstein and A. P. Alivisatos J. Am. Chem. Soc. 1992 114 5221. 21 A. P. Alivisatos K. P. Johnsson X. Peng T. E. Wilson C. J. Loweth M. P. Bruchez Jr. and P. G. Schultz Nature 1996 382 609. 22 C. A. Mirkin R. L. Letsinger R. C. Mucic and J. J. Storhoff Nature 1996 382 607. 23 J. H. Fendler and F. C. Meldrum Adv. Mater. 1995 7 607. 24 J. Schmitt G. Decher W. J. Dressick S. L. Brandow R. E. Geer R. Shashidhar and J. M. Calvert Adv. Mater. 1997 9 61. 25 D. L. Feldheim K. C. Grabar M. J. Natan and T. E. Mallouk J. Am. Chem. Soc. 1996 118 7640. 26 K. C. Grabar P. C. Smith M. D. Musick J. A. Davis D. G. Walter M. A. Jackson A. P. Guthrie and M. J. Natan J. Am. Chem. Soc. 1996 118 1148. 27 R. P. Andres T. Bein M. Dorogi S. Feng J. I. Henderson C. P. Kubiak W. Mahoney R. G. Osifchin and R. Reifenberger Science 1996 272 1323. 28 L. A. Lyon D. L. Feldheim M. J. Natan and T. E. Mallouk unpublished results. 29 T. Sato D. G. Hasko and H. Ahmed J. Vac. Sci. Technol. B 1997 15 45. 30 D. L. Klein P. L. McEuen J. E. Bowen Katari R. Roth and A. P. Alivisatos Appl. Phys. Lett. 1996 68 2574. 31 D. Routkevitch A. A. Tager J. Haruyama D. Almawlawi M. Moskovits and J. M. Xu IEEE Transactions on Electronic Devices 1996 43 1646. 32 G. Lingjie E. Leobandung and S. Y. Chou Science 1997 275 649. 33 S. Tiwari F. Rana H. Hanafi A. Hartstein E. F. Crabbe and K. Chan Appl. Phys. Lett. 1996 68 1377. 34 M. J. Yoo T. A. Fulton H. F. Hess R. L. Willett L. N. Dunkleberger R. J. Chichester L. N. Pfeiffer and K. W. West Science 1997 276 579. 35 (a) X. Peng T. E. Wilson A. P. Alivisatos and P. G. Schultz Angew. Chem. Int. Ed. Engl. 1997 36 145; (b) T. Sato H. Ahmed D. Brown and B. F. G. Johnson J. Appl. Phys. 1997 82(2) 696. Received 11th June 1997 Accepted 16th September 1997

ISSN:0306-0012    

DOI:10.1039/a827001z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Asymmetric synthesis of amino acids using sulfinimines (thiooxime S-oxides) Franklin A. Davis,* Ping Zhou and Bang-Chi Chen Department of Chemistry Temple University Philadelphia PA 19122 USA The occurrence of a- and b-amino acids in biological systems and their exceptional utility as chiral building blocks underlies the importance of new and improved methods for their synthesis in enantiomerically pure form. The intent of this review is to highlight the applications of a new class of chiral imine building block sulfinimines (thiooxime S-oxides) for the enantioselective synthesis of amino acids and their derivertives. 1 Introduction The N-sulfur bonding imines 1 are versatile intermediates in organic synthesis particularly for the preparation of amine derivatives (Scheme 1).1–3 Among them sulfinimines (thiooxime S-oxide N-alkylidenesulfinamides 1b) display unique reactivity and stereoselectivity due to the existence of the chiral electron withdrawing sulfinyl group.Like sulfoxides sulfinimines undergo thermo-elimination to give sulfenic acids.4–6 As expected sulfinimines are strong Michael acceptors and undergo addition reactions with alcohols,7 thiols,8 amines,9 hydrazines9 and hydrides.10–12 Sulfinimines also react with carbon nucleophiles.12,13 More importantly in many of these Franklin A. Davis was born in Des Moines Iowa. He received his BS degree in 1962 from the University of Wisconsin and was awarded a PhD in organic chemistry from Syracuse University in 1966 where he worked with Donald C.Dittmer. After two years with Michael J. S. Dewar as a Welch Postdoctoral Fellow at the University of Texas he joined the faculty at Drexel University in 1968. He was the George. S. Sasin Professor of Chemistry until 1995 when he joined the Chemistry Department at Temple University. In 1980 he received the Philadelphia ACS Section Award and was a Fellow of the Japan Society for the Promotion of Sciences in 1992. Dr Davis is a member of the executive committees of the Fluorine and Organic Divisions of the American Chemical Society and served as Program Chair (1988–91) and Chair (1994) of the Organic Division. Ping Zhou Franklin A. Davis O H CN N S R p-MeC6H4 H ( Ss S)-21 70–86% 6M HCl Me- p H CO2H N R R H2N R1 (O) n S R N R2 1a n = 0 sulfenimine b n = 1 sulfinimine c n = 2 sulfonimine Scheme 1 Bang-Chi Chen reactions the chiral centre of the sulfur atom makes it possible to control these reactions in a highly diastereoselective manner.The purpose of this article is to review the asymmetric synthesis of amino acids and their derivatives from enantiomerically pure sulfinimines with particular attention to applications in the synthesis of biologically active molecules. 2 Preparation of enantiomerically pure sulfinimines Several methods have been developed for the preparation of enantiomerically pure sulfinimines and can be divided into three categories asymmetric oxidation of sulfenimines asymmetric iminolysis of sulfinates (e.g.the Andersen’s reagent) and asymmetric iminolysis of sulfinamides. Ping Zhou was born in Shengxing Zhejiang China. She received her BS degree in 1984 from Hangzhou University. After working for four years in Zhejiang Agricultural University as an instructor she joined Professor Davis at Drexel University in 1988 and received her PhD degree in organic chemistry in 1994. After one and a half years of postdoctoral work with Professor Edward C. Taylor at Princeton University she joined Wyeth Ayerst Research in 1996 as a Research Scientist in the Department of Medicinal Chemistry. Her research has resulted in over 20 publications and patents. Bang-Chi Chen was born in Ruian Zhejiang China. He received his BS degree in 1984 from Hangzhou University.He worked for three years in Hangzhou University as an instructor while conducting research with Professor Xian Huang. In 1987 he joined Professor Davis at Drexel University in 1987 and received his PhD degree in organic chemistry 1991. In the same year he joined Bristol- Myers Squibb Company in Syracuse New York as a Research Scientist and now is a Senior Research Investigator in the Department of Discovery Chemistry Pharmaceutical Research Institute Bristol-Myers Squibb in Princeton New Jersey. His research efforts have resulted in over 50 publications and patents. Chemical Society Reviews 1998 volume 27 13 2.1 Asymmetric oxidation of sulfenimines The chemoselective oxidation of sulfenimines 1a to racemic sulfinimines 1b without over-oxidation to sulfonimines 1c was first reported by us over two decades ago.7 The chemo- and stereo-selective oxidation of sulfenimines to enantiomerically enriched sulfinimines however was realized much more recently during our investigations of N-sulfonyloxaziridines (Scheme 2).14,15 In these studies it was found that (2)-N-(phenylsulfonyl)(3,3-dichlorocamphoryl)oxaziridine 3 oxidizes sulfenimines 2 to give sulfinimines (Rs)-4 in 87–90 ee and 89–96% yield.Simple crystallization upgrades 4 to enantiomeric purity.15,16 The antipodal sulfinimines (Ss)-4 can be readily prepared using enantiomeric oxaziridine (+)-3. Cl R O R CCl4 Cl + SO2Ph Ar N S Ar¢ Ar N S Ar¢ N 87-90% ee O (–)-3 2 ( Rs)-4 Ph Ph Ph H N H N H N S S S [O] O O + OR OR OR ( Ss)-6 5 Scheme 2 Diastereoselective oxidation of sulfenimines has also been reported for the preparation of non-racemic sulfinimines (Scheme 3).13 Oxidation of sulfenimines 5 with m-CPBA or MMPP afforded sulfinimines 6 in 83–99% yield.The diastereoselectivity however was highly dependent on the R group in the chiral auxiliary. For example when R = H in 5 the sulfinimine (Rs)-6 was obtained in diastereomerically pure form. ( Rs)-6 Scheme 3 2.2 Asymmetric iminolysis of sulfinates Another method for the preparation of sulfinimines is the iminolysis of sulfinates (Scheme 4). Enantiomerically pure sulfinimines 10 have been prepared from the Andersen’s reagent 7 and imino-metallo reagents 9 in moderate to low yields.10,12,17 This reaction is highly stereoselective taking place at the chiral sulfur atom in an SN2 fashion.The iminometallo reagents 9 are usually prepared in situ via the reaction of aromatic nitriles 8 with lithium or Grignard reagents. This means that R and Ar in 10 cannot be hydrogen and alkyl respectively. ArCN 8 RLi or RMgBr R O O R + N–M+ S S O Ar N p-MeC6H4 p-MeC6H4 Ar ( Ss)-10 ( Ss)-7 9 Scheme 4 Recently we devised an efficient ‘one pot’ procedure for the asymmetric synthesis of aromatic and aliphatic aldehyde derived sulfinimines 14 ( > 95% ee) making these versatile building blocks available for the first time.18,19 This procedure entails the reaction of N,N-bis(trimethylsilyl)-p-toluenesulfi- Chemical Society Reviews 1998 volume 27 14 namide 11 prepared in situ by treatment of the Andersen’s reagent 7 with lithium bis(trimethylsilyl)amide (LiHMDS) with aromatic or aliphatic aldehydes (Scheme 5).This method is highly effective for the preparation of arylidene and alkylidene sulfinamides 14 (R = aryl alkyl) which are usually obtained in 57–90% yield. The mechanism of this transformation involves the reaction of silyl sulfinamide anion 13 with the aldehyde in a Peterson type olefination reaction. Anion 13 is thought to be generated by reaction of 11 with the byproduct lithium menthoxide (12).19 O O S p-MeC6H4 ( Ss)-7 THF LiHMDS O TMS + N S p-MeC6H4 TMS TMS p-MeC6H4 Scheme 5 O S O OMe ( Rs)-15 Scheme 6 O 1.LiHMDS 2. RCHO/CsF O O O ( Ss)-11 O S N p-MeC6H4 13 Another enantiomerically pure sulfinate available for the preparation of sulfinimines is menthyl 2-methoxy-1-naphthalenesulfinate 15 (Scheme 6).20 In a manner similar to that outlined in Scheme 5 enantiomerically pure sulfinimines such as 16 were also obtained.21,22 1. LiHMDS –78 oC 2. RCHO/CsF S-Alkyl sulfinimines can also be prepared using this method (Scheme 7). Thus reaction of sulfinate 17 with LiHMDS followed by addition of the aldehyde in the presence of CsF afforded S-tert-butyl sulfinimines 18 in enantiomerically pure form.23 O O S O But 17 Scheme 7 LiO 12 RCHO O H N S R ( Ss)-14 O H S N R OMe ( Rs)-16 H O S R But N 18 2.3 Asymmetric iminolysis of sulfinamides Analogous to the iminolysis of sulfinates Wills and co-workers reported that the reaction of sulfinamide 19 with the lithiated imines 9 gave sulfinimines 20 as a single isomer (Scheme 8).24,25 As noted in the other examples an SN2 inversion of the chiral centre at sulfur atom is observed and R and Ar in 20 cannot be H or alkyl respectively.H Me Ar THF Ar N O S N–Li+ + N R R S But Me O ButCONH O 19 Scheme 8 Et2AlCN H O H CN S R N R p-MeC6H4 H ( Ss)-14 6M HCl C6H4Me- p H O CO2H N R = aromatic aliphatic R S Al C R 9 36–42%de Et2AlCN/PriOH 82–96%de Et EtO p-MeC6H4 70–86% H2N N 20 3 Asymmetric synthesis of a-amino acids from sulfinimines As an extension of the Strecker synthesis first reported in 1850 addition of cyanide to sulfinimines is expected to give a-amino nitriles which on hydrolysis give a-amino acids.Our initial attempts to add common cyanide sources such as KCN TMSCN etc. to sulfinimines were unsuccessful.26 However reaction of sulfinimine (Ss)-14 with diethylaluminium cyanide afforded a mixture of diastereoisomers 21 in good yield but modest diastereoselectivity; e.g. 36–42% (Scheme 9).26 Formation of the major product (Ss,S)-21 is consistent with complexation of Et2AlCN with the sulfinyl oxygen activating the imine for intramolecular cyanide addition via chair-like transition state 22.Significantly it was observed that addition of ethyl- (alkoxy)aluminium cyanide [Et(RAO)AlCN] prepared by treatment of Et2AlCN with isopropyl alcohol (RAOH) to the sulfinimine results in a dramatic improvement in the diastereoselectivity (de) e.g. from 36–42% to 82–94%.27 The enhanced des are attributed to the reduced Lewis acidity of Et(RAO)AlCN vs. Et2AlCN which makes it more selective. Simple crystallization of the amino nitriles affords a diastereomerically pure product 21 ( > 96% de) in good yield. Acid catalysed hydrolysis of the diastereomerically pure 21 not only removes the sulfinyl auxiliary but hydrolyses the nitrile group affording the enantiomerically pure ( > 95% ee) a-amino acids 23.Importantly racemization of the sensitive arylglycines was not detected in this practical asymmetric Strecker synthesis. O N S ( Ss S)-21 ( S)-23 22 Scheme 9 A new method for the synthesis of a-amino acids from sulfinimines was reported by Hua and co-workers (Scheme 10).28 Reaction of sulfinimine 24 with 9-borabicyclo[3.3.1]nonane gave 25 exclusively in 95% yield.28 Hydrolysis of the ortho-ester on silica gel followed by removal of the N-sulfinyl group resulted in formation of alanine ethyl ester 27 in excellent yield. Similarly reaction of 24 with allylmagnesium bromide afforded 28 in 95% yield as a single isomer. The high stereoselectivity observed with the allyl Grignard reagent was attributed to a chair-like six-membered transition state.12,13,28 Compound 28 has been converted to (S)-2-amino- 2-methylbut- 4-enoic acid 29 in 91% yield.The sulfinimine 24 was prepared in 68% yield by treatment of the Andersen reagent (Rs)-7 with the imino-metallo reagent prepared from triethoxyacetonitrile and methyllithium. O CH3 S N + 9-BBN C(OEt)3 p-MeC6H4 Et2O/0 oC CH3 C(OEt)3 p-MeC6H4 2H H2N Scheme 10 R 24 CH2=CHCH2MgBr O N S H O S Ph p-MeC6H4 Et2O/0 oC 84–98% 28 95% 1. TFA/MeCN/H2O 2. LiOH CH3 CO 29 91% 4 Asymmetric synthesis of b-amino acids from sulfinimines N ( Ss)-30a R = Me b R = Bu b-Amino acids are important constituents of natural products and precursors of the b-lactam class of antibiotics.By taking advantage of the high diastereoselectivity obtained in the addition of allyl Grignard reagent to sulfinimines,12,13,28 Hua et al. developed a method for the synthesis of b-amino acids (Scheme 11).12 Diastereoselective addition of allylmagnesium bromide to sulfinimines (Ss)-30 gave sulfinamides 31 in 82–98% de and 92–96% yield.12 Following separation of the diastereoisomers sulfinamides 31 were converted to b-amino acids 32 in 50–52% yield via a sequence of reactions. 27 R Ph Scheme 11 A simpler route to b-amino acids involves the diastereoselective addition of enolates to enantiopure sulfinimines (Scheme 12).16,29–31 For example treatment of (Ss)-sulfinimine Chemical Society Reviews 1998 volume 27 31 1.TFA/MeOH 2. Ac2O Et3N 3. O3 –78 oC 4. AgNO3 KOH 5. HCl R 32 O THF/0 oC 3 S CH3 C(OEt) H N p-MeC6H4 silica gel overnight CH3 CO2Et H p-MeC6H4 26 100% TFA/EtOH CH3 CO2Et H H2N O MgBr S Ph p-MeC6H4 H 25 95% O N S H N H 50–52% CO2H H2N 15 30a with the lithium enolate of methyl acetate afforded b-amino ester 33 in > 97 de and 84% yield.16 Removal of the N-sulfinyl group with TFA afforded b-phenylalanine 34 in 85% yield.16 O Me O Me CH3CO2Me/LDA/THF S S CO2Me N Ph Ph N p-MeC6H4 p-MeC6H4 H ( Ss)-30a ( Ss R)-33 >97% de 84% TFA/MeOH Me NH2 CO2Me Ph ( R)-34 85% Scheme 12 Fujisawa and co-workers reported the addition of the enolate of tert-butyl acetate to sulfinimine 35.32 Interestingly the lithium enolate gave (Ss,S)-37 while the titanium enolate afforded (Ss,R)-37.A non-chelated transition state was used to explain the formation of (Ss,S)-37 while a six-membered chairlike transition state containing a four-membered metallocycle and/or a seven membered counterpart was attributed to the formation of the (Ss,R)-37. Treatment of (Ss,S)-37 with TFA gave b-amino acid 38 in 70% yield,32 Scheme 13. Mikolajczyk et al. reported that the addition of a-phosphonate carbanions to sulfinimines gives rise to b-amino phosphonic acids (Scheme 14).33 For example reaction of sulfinimine 39 with the lithium a-phosphonate carbanion afforded 40 in 82% de which can be isolated in 52% yield diastereomerically pure by flash column chromatography.Treatment of 40 with TFA–MeOH gave dimethyl b-aminophosphonate 41 in 66% yield. On the other hand b-amino phosphonic acid 42 was obtained in 78% yield by treating 40 with HCl–AcOH. A sevenmembered chelated transition state was proposed to explain the stereochemistry of the product. 5 Asymmetric synthesis of aziridine-2-carboxylate esters from sulfinimines Aziridine-2-carboxylate esters are a special class of amino acids. Enantiomerically pure aziridine-2-carboxylic acids are versatile intermediates for the asymmetric synthesis of many biologically active materials because they undergo highly regioand stereo-controlled ring opening reactions with nucleophiles to give b-substituted a-amino acids.34 In this regard we developed a highly diastereoselective Darzens’ type condensation involving addition of the lithium enolate of a-bromoacetate THF/HMPA M = Li O H OM S N + p-MeC6H4 OBut O O 36 ( Ss)-35 THF M = Ti(OPri)2 Scheme 13 Chemical Society Reviews 1998 volume 27 16 O O H H CH3PO(OMe)2 S S PO(OMe)2 N N Ph Ph LiHMDS/THF p-MeC6H4 p-MeC6H4 H ( Ss)-39 ( Ss R)-40 82% de 52% TFA/MeOH aq.HCl/AcOH NH2 NH2 PO(OH)2 PO(OMe)2 Ph Ph ( R)-41 ( R)-42 78% 66% Scheme 14 O H OLi R O H THF S + N S OMe N R p-MeC6H4 CO2Me Br H ( Ss)-14 ( Ss S S)-44 R (L) n OMe Li N H Br O S O CO2But O H S CO2H 43 Scheme 15 6 Applications in the synthesis of biologically important molecules Enantiomerically pure sulfinimines have found a new role in the asymmetric synthesis of biologically important nitrogen containing molecules.This section highlights some of these applications. For example sulfinimine 39 has been used in the synthesis of the Taxol C-13 side chain 5016 and its fluoro analogue 51 as outlined in Scheme 16.30 Novel aspects of these syntheses are the highly diastereoselective syn hydroxylation of TFA N p-MeC6H4 O O O H to sulfinimines (Ss)-14 for the preparation of cis-aziridine- 2-carboxylates (Scheme 15).35 The corresponding N-sulfinylaziridine-2-carboxylic esters (Ss,S,S)-44 were obtained in 94–98 de and 60–74% yield. A chair-like transition state 45 was suggested as being responsible for the high selectivity and stereochemistry.a-Substituted aziridine-2-carboxylates can be prepared in a similar manner.36 p-MeC6H4 C6H4Me- p 45 NH2 O 38 70% ( Ss S)-37 96% de 68% CO2But O H S N p-MeC6H4 O O H ( Ss R)-37 92% de 89% the enolate of 46 with (+)-(camphorylsulfonyl)oxaziridine 47 and the fluorination of 46 with the electrophilic fluorinating reagent N-fluoro-o-benzenedisulfonimide 48. O H 1. CH3CO2Me/NaHMDS/Et2O 2. TFA/MeOH S 3. Et3N/PhCOCl N Ph p-MeC6H4 ( Ss)-39 1. LDA SO2 F N 2. SO O 2 Ph O NH OMe Ph F 49 yield.21 O H S N OMe N ( RS)-51 H O N H HN O CO HN N 54 48 Scheme 16 CH3CO2Et/LiHMDS 85% N Scheme 17 (S)-Ethyl b-amino-3-pyridinepropanoate 53 is a key component of 54 a peptidomimetic for the Arg-Gly-Asp-Phe sequence of fibrinogen and may be useful in the treatment of heart disease (Scheme 17).This compound is conveniently prepared from sulfinimine 51 in > 97% ee and 68% overall (R)-(2)-Dysidazirine 57 is a cytotoxic antitumour antibiotic isolated from a marine sponge,37 belonging to the smallest class of nitrogen-unsaturated heterocycles 2H-azirines (Scheme 18). Its first enantioselective synthesis was recently reported by us by treating enantiomerically pure N-sulfinylaziridine 56 prepared from sulfinimine 55 with lithium diisopropylamide (LDA).38 d-erythro-Sphingosine 58 the major constituent of the lipid backbone of the sphingolipids which play important roles in cell recognition events was synthesized from the same aziridine.39 This was accomplished using a new trifluoroacetic anhydride (TFAA) induced Pummerer-type rearrangement of 50 H NH OMe ( RS S)-52 TFA EtOH 90% NH 53 56 discovered in our laboratory.39 The threo isomer of 58 is available by treatment of 56 with aqueous trifluoroacetic acid.O Ph NH p-MeC6H4 2Me Ph N O O 46 1. LDA/LiCl 2. NH Ph Ph 57 O S N 7 Conclusions CO S O2 OMe 47 O OH CO2Et 2 CO2Et 2Et N 8 Acknowledgements (+)-Thiamphenicol 62a and its fluorinated analogue (2)-florfenicol 62b are broad spectrum synthetic antibacterial agents used in the animal health industry (Scheme 19).threo- (1R,2R)-(2)-1-[(4-Methylthio)phenyl]propane-1,3-diol 61 is a common precursor to both these compounds the manufacture of which involves a multi-step sequence ending with a classical resolution of racemic 61. This compound is conveniently prepared from the enantiomerically pure sulfinimine 59 via aziridine 60.40 Conversion of 61 to thiamphenicol is straightforward involving treatment with dichloroacetyl chloride and oxidation with m-chloroperbenzoic acid (m-CPBA). a-Alkyl-a-amino acids are important in the study of enzyme mechanism and in altering the conformational properties of peptides. Once incorporated into peptides these amino acids result in increased rigidity enhancing stability and altering secondary structures.These amino acids can be prepared from sulfinimine derived N-sulfinylaziridines such as 63 because they undergo highly regio- and stereo-selective hydrolysis to give for example a-methyl-b-phenylserine 64 (Scheme 20).36 The work outlined in this brief review illustrates the applications of sulfinimines (thiooxime S-oxides) 1b as chiral imine building blocks for the asymmetric synthesis of a- and b-amino acids aziridine-2-carboxylate esters and other biologically relevant molecules. The usual limitations of imines in these reactions low reactivity enolization and poor stereocontrol are avoided with sulfinimines because the chiral sulfinyl group activates the C–N bond for addition and is a powerful stereodirecting group.Furthermore the product sulfinamides [ArS(O)NH-CHRRA] represent readily separable diastereoisomers that on hydrolysis afford the primary amine derivative without racemization. An added advantage of the sulfinyl group is that it can be used for further elaboration of the product; e.g. Pummerer rearrangements and oxidation to sulfonamides a useful amine activating and protecting group. It is a pleasure to acknowledge the important efforts of our coworkers whose names appear in the references. Our own 56 3. K 1. TFAA/CH2Cl2 2. LiBH4/MeOH 2CO3/EtOH 58 31% Chemical Society Reviews 1998 volume 27 R O H H O R LDA/THF BrCH2CO2Me N S S 67% N p-MeC6H4 ( Rs)-55 CO2Me H R = n-C12H25 1. LDA/–78 oC 2. MeI 3.H2O OH N H OH CO2Me NH2 42% Scheme 18 17 O H O S BrCH2CO2Me LDA/THF N p-MeC6H4 N S p-MeC6H4 ( Ss)-59 ( Ss S S)-60 Scheme 19 HO HO O H H SMe 50% TFA/MeCN : Ph Ph S N p-MeC6H4 CO CO2Me 2Me 2 NH NH2 Me Me Me Me MeO MeO2C ( R R)-(+)- )-(+)-64 64 75% Scheme 20 ( Ss R S)- )-63 63 contributions to this review were supported by the National Science Foundation and the National Institutes of Health. 9 References 1 P. K. Claus in The Chemistry of Sulfenic Acids and Their Derivatives ed. S. Patai Wiley 1990 pp. 723–741. 2 J. G. Tillett in The Chemistry of Sulfinic Acids Esters and Their Derivatives ed. S. Patai Wiley 1990 pp. 603–622. 3 L. Craine and M. Raban Chem. Rev. 1989 89 689.4 F. A. Davis A. J. Friedman and U. K. Nadir J. Am. Chem. Soc. 1978 100 2844. 5 F. A. Davis and A. J. Friedman J. Org. Chem. 1976 41 897. 6 F. A. Davis S. Q. A. Rizvi R. Ardecky D. J. Gosciniak A. J. Friedman and S. G. Yocklovich J. Org. Chem. 1980 45 1650. 7 F. A. Davis A. J. Friedman and E. W. Kluger J. Am. Chem. Soc. 1974 96 5000. 8 K. Burger J. Albanbauer F. Kafig and S. Penninger Liebigs Ann. Chem. 1977 624. 9 T. Yoshida S. Naruto H. Uno and H. Nishimura Chem. Pharm. Bull. 1982 30 2820. 10 R. Annunziata M. Cinquini and F. Cozzi J. Chem. Soc. Perkin Trans. 1 1982 339. 11 M. Cinquini and F. Cozzi J. Chem. Soc. Chem. Commun. 1977 723. 12 D. H. Hua S. W. Miao J. S. Chen and S. Iguchi J. Org. Chem. 1991 56 4. 13 T.-K. Yang R.-Y.Chen D.-S. Lee W.-S. Peng Y.-Z. Jiang A.- Q. Mi and T.-T. Jong J. Org. Chem. 1994 59 914. 14 F. A. Davis and B.-C. Chen Chem. Rev. 1992 92 919. 15 F. A. Davis R. T. Reddy W. Han and R. E. Reddy Pure Appl. Chem. 1993 65 633. 16 F. A. Davis R. T. Reddy and R. E. Reddy J. Org. Chem. 1992 57 6387. 17 M. Cinquini and F. Cozzi J. Chem. Soc. Chem. Commun. 1977 502. Chemical Society Reviews 1998 volume 27 18 MeS H NH2 SMe 3 eq. LiAlH4 OH 87% CO2Me H 61 OH 1. Cl2CHCOCl/Et3N 2. m-CPBA 83% O H MeSO2 N CHCl2 X OH 62a X = OH b X = F 18 F. A. Davis R. E. Reddy J. M. Szewczyk and P. S. Portonovo Tetrahedron Lett. 1993 34 6229. 19 F. A. Davis R. E. Reddy J. M. Szewczyk G. V. Reddy P. S. Portonovo H. Zhang D.Fanelli H. Zhang R. Thimma Reddy P. Zhou and P. J. Carroll J. Org. Chem. 1997 62 2555. 20 S. G. Pyne A. R. Hajipour and K. Prabakaran Tetrahedron Lett. 1994 35 645. 21 F. A. Davis P. Zhou C.-H. Liang and R. E. Reddy Tetrahedron Asymmetry 1995 6 1511. 22 F. A. Davis J. M. Szewczyk and R. E. Reddy J. Org. Chem. 1996 61 2222. 23 J. L. Garcia Ruano I. Fernandez M. D. Prado Catalina and A. A. Cruz Tetrahedron Asymmetry 1996 7 3407. 24 D. R. J. Hose T. Raynham and M. Wills Tetrahedron Asymmetry 1993 4 2159. 25 D. R. J. Hose M. F. Mahon K. C. Molloy T. Raynham and M. Wills J. Chem. Soc. Perkin Trans 1 1996 691. 26 F. A. Davis R. E. Reddy and P. S. Portonovo Tetrahedron Lett. 1994 35 9351. 27 F. A. Davis P. S. Portonovo R. E. Reddy and Y.-H. Chiu J. Org. Chem. 1996 61 440. 28 D. H. Hua N. Lagneau H. Wang and J. Chen Tetrahedron Asymmetry 1995 6 349. 29 J. Jiang K. K. Schumacher M. M. Joulli�e F. A. Davis and R. E. Reddy Tetrahedron Lett. 1994 35 2121. 30 F. A. Davis and R. E. Reddy Tetrahedron Asymmetry 1994 5 955. 31 F. A. Davis R. E. Reddy and J. M. Szewczyk J Org. Chem. 1995 60 7037. 32 T. Fujisawa Y. Kooriyama and M. Shimizu Tetrahedron Lett. 1996 37 3881. 33 M. Mikolajczyk P. Lyzwa J. Drabowicz M. W. Wieczorek and J. Blaszczyk Chem. Commun. 1996 1503. 34 D. Tanner Angew. Chem. Int. Ed. Engl. 1994 33 599. 35 F. A. Davis P.hou and G. V. Reddy J. Org. Chem. 1994 58 3243. 36 F. A. Davis H. Liu and G. V. Reddy Tetrahedron Lett. 1996 37 5473. 37 T. F. Molinski and C. M. Ireland J. Org. Chem. 1988 53 2103. 38 F. A. Davis G. V. Reddy and H. Liu J. Am. Chem. Soc. 1995 117 3651. 39 F. A. Davis and G. V. Reddy Tetrahedron Lett. 1996 37 4349. 40 F. A. Davis and P. Zhou Tetrahedron Lett. 1994 35 7525. Received 27th June 1997 Accepted 19th August 1997

ISSN:0306-0012    

DOI:10.1039/a827013z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Lanthanide(iii) chelates for NMR biomedical applications Silvio Aime Mauro Botta Mauro Fasano and Enzo Terreno Dipartimento di Chimica I.F.M. Universit`a di Torino Via P. Giuria 7 I-10125 Torino Italy The peculiar magnetic properties of lanthanide(III) ions may be exploited for the development of powerful NMR probes for biomedical applications. GdIII chelates are in current clinical use as contrast agents for magnetic resonance imaging. Other paramagnetic lanthanide(III) complexes endowed with shift reagent capabilities are used for the separation of NMR resonances of species present in the inner and outer cellular compartments and for the measurement of pH and temperature. 1 Introduction In the last decade there has been a renewed interest in paramagnetic lanthanide(iii) complexes because their peculiar magnetic properties have provided the route to tackle a number of problems in different fields of relevance to biomedicine by means of NMR techniques.1 The largest part of these studies has been devoted to GdIII chelates used as contrast agents (CA) in conjunction with magnetic resonance imaging (MRI).2,3 This is a powerful diagnostic technique which allows one to obtain images of Mauro Fasano was born in 1965 in Asti Italy.He received both Laurea (1989) and doctoral (1992) degrees in Chemistry from the University of Torino. In 1992 he was appointed to a position of assistant professor at the Faculty of Sciences at Torino. He is author of 40 publications in the fields of inorganic biochemistry and biological coordination chemistry.Silvio Aime was born in 1948 near Torino Italy. He received the Laurea degree from the University of Torino in 1971. Following a postdoctoral appointment at the University of East Anglia (with R. K. Harris) he returned in 1974 to Torino where he spent all his career. He is currently Professor of General and Inorganic Chemistry in the Faculty of Pharmacy. He is coauthor of ca. 250 papers and 4 patents in the field of organometallic chemistry in particular the application of NMR spectroscopy to investigate solution and solid state properties of metal carbonyl clusters and in bio-inorganic chemistry with projects in the fields of relaxation and shift reagents for NMR Silvio Aime Mauro Fasano Chemical Society Reviews 1998 volume 27 tissues and organs which are topological representations of NMR parameters.Among these longitudinal (R1) and transverse (R2) relaxation rates of water protons are the most important. The presence of paramagnetic GdIII complexes causes a dramatic enhancement of the water proton relaxation rates and then allows one to add physiological information to the impressive anatomical resolution commonly obtained in the uncontrasted images. Thus administration of Gd-based contrast agents has entered into the pool of diagnostic protocols and is particularly useful to assess organ perfusion and any abnormalities in the blood–brain barrier or in kidney clearance. Several other applications primarily in the field of angiography and tumor targetting are currently under intense scrutiny with the promise of soon being available in clinical practice.Besides GdIII complexes there is another important class of contrast agents for MRI which is based on polysaccharidecoated iron oxide particles. Their peculiarity stems from the fact that their blood half-life and distribution to different organs of the reticuloendothelial system (RES) depend upon the particle size. In general larger particles are quickly sequestered by RES cells of the liver and spleen whereas smaller particles remain in applications in biomedicine and on the role of metal ions in the etiology of Parkinson’s disease. Enzo Terreno was born in 1965 in Rome Italy. He graduated in Pharmaceutical Chemistry in 1990 in Torino.Currently he is research associate with Silvio Aime. He is author of 15 papers on the chemistry of lanthanide complexes of interest as contrast agents for Magnetic Resonance Imaging. Mauro Botta was born in 1958 near Cuneo Italy. He received the Laurea degree in chemistry from the University of Torino in 1985. After three years spent as research assistant in the Department of Chemistry he was appointed in 1990 as assistant professor at the faculty of Pharmacy of the University of Torino. He is co-author of 70 papers and 2 patents in the field of organometallic and coordination chemistry mainly for biomedical applications. Mauro Botta Enzo Terreno 19 the blood for a longer time and accumulate mainly in the lymph nodes.4 Nowadays about 35% of the MRI examinations make use of contrast agents but this percentage is predicted to increase further following the development of more effective and specific contrast media than those currently commercially available.Another major field of interest for the application in biomedicine of paramagnetic lanthanide chelates deals with their use as shift reagents to separate NMR signals of species present in the inner- and outer-cellular compartments.5 The prototype application is the relative quantification of Na+ and K+ ions inside and outside red blood cells by using DyIII or TmIII complexes endowed with a high residual negative charge which does not allow them to cross the cellular membrane. It follows that the paramagnetic perturbation is confined to the extracellular environment and results in an alteration of the local magnetic field strength which in turn causes a significant shift in the resonance frequency of Na+ or K+ ions present in this compartment.In principle this approach may be extended to other cationic anionic and zwitterionic species provided there is the presence in the extracellular compartment of a shift reagent endowed with suitable electric charge distribution and molecular recognition properties. Furthermore the resonances themselves of the paramagnetic shift reagents may be used as reporters of physico-chemical parameters like pH temperature etc. In general although no protocol for the in vivo use of shift reagents for humans has been approved yet it is likely that the increasing availability of MRI instruments at 1.5 T (or more) incorporating high resolution NMR capabilities will increase the attention towards procedures able to exploit fully the diagnostic potential of these agents.2 GdIII complexes as contrast agents for MRI The use of paramagnetic substances for increasing and controlling the magnetic relaxation of water protons has found wide application in the NMR techniques for medical imaging and diagnosis.6 The attention has been primarily focused on complexes of GdIII since this metal ion with a S ground state electronic structure couples a large magnetic moment with a long electron spin relaxation time ( ~ 1029 s at the magnetic field strengths of interest for MRI applications) two properties that ensure an optimum efficiency for nuclear spin relaxation of the interacting nuclei.Other general requirements of CA for MRI are low toxicity rapid excretion after administration good water solubility and low osmotic potential of the solutions clinically used. Moreover since the free metal ions are poorly tolerated they must be coordinated by a strongly binding ligand that occupies most of the available coordination sites. Eventually the preferred metal complexes in addition to showing high thermodynamic (and possibly kinetic) stability should present at least one water molecule in their inner coordination sphere in rapid exchange with the bulk solvent in order to affect strongly the relaxation of all solvent protons. The anionic complexes Gd(DTPA)22 (MAGNEVIST) and Gd(DOTA)2 (DOTAREM) were the first complexes entered into clinical practice and they represent the reference compounds for the development and the evaluation of new agents.Later two neutral complexes Gd(DTPA–BMA) (OMNISCAN) and GdHPDO3A (PROHANCE) have been introduced with the aim of providing systems with reduced osmotic potential for applications requiring higher doses of CA. Fig. 1 reports the schematic structure of the four ligands and the thermodynamic stability constants of their GdIII complexes measured at 25 °C and m = 0.1. It may be surprising to find out that a complex like Gd(DTPA–BMA) whose stability constant is more than five orders of magnitude lower than Gd(DTPA)22 is considered safe enough for its clinical use.However it has Chemical Society Reviews 1998 volume 27 20 been pointed out that the toxicity7 in vivo of GdIII complexes with polyaminocarboxylate ligands does not necessarily correlate to the overall thermodynamic stability. Rather the lack of displacement of GdIII by endogenous CaII and the selectivity towards LnIII ions introduced by two amide substituents are effective in setting off the net loss in the overall thermodynamic stability. COOH N HOOC COOH N N COOH HOOC DTPA log KGd = 22.5 COOH O O C N MeHN NHMe C N N COOH HOOC DTPA–BMA log KGd = 16.85 HOOC HOOC COOH COOH N N N N N N N N HOOC OH (1) (2) COOH = Ris 55.6 HOOC T M H 1 HP–DO3A log KGd = 23.8 Fig.1 Schematic representation of the four ligands whose GdIII chelates are currently used as CA for MRI. Thermodynamic stabilities of the complexes at 25 °C and m = 0.1 m are reported. An important step in the design and characterization of more effective CA is represented by the investigation of the relationships between the chemical structure and the factors determining their ability to enhance the water protons relaxation rates. In the last few years we have investigated many of these complexes by combining relaxometric and high resolution NMR techniques (on related complexes with Ln ­ Gd) and here we summarize the most relevant results. 2.1 Structural and dynamic determinants of the relaxivity of GdIII complexes The observed water proton longitudinal relaxation rate in a solution containing a paramagnetic metal complex is given by the sum of three contributions:1,2,6 1p + R1 os p + R1 w where Rw is the water relaxation rate in the absence of the paramagnetic compound Ris 1p represents the contribution due to exchange of water molecules from the inner coordination sphere of the metal ion to the bulk water and Ros 1p is the contribution of solvent molecules diffusing in the outer coordination sphere of the paramagnetic center.The overall paramagnetic relaxation enhancement (Ris 1p + R1 os p) referred to by a 1 mm concentration of a given GdIII chelate is called its relaxivity. A schematic representation of these two relaxation mechanisms operating the relaxation enhancement of the solvent water protons in solution by GdIII complexes is shown in Fig.2. The inner sphere relaxation rate is described in terms of the following set of equations:1,2,8 1 M DOTA log KGd = 24.7 obs R1 1 R p is 1 c � q � = + t Fig. 2 Schematic view of the relaxation mechanisms (and the main relaxation parameters) operating in a aqueous solution containing a paramagnetic GdIII chelate h 2 2 2 2 S H 0 = 1 T 15 4 1M g g r6 GdH 1 ÆÙ 1 + ci 2 mp 11 ¡Ò ZFS = t t M ZFS 12 5 D2t D2t v = =12 10 1 ¡Ò t S S t 2 In eqns. 2¡V6 c is the molar concentration of the paramagnetic complex; q is the number of water molecules coordinated to the metal ion; tM is their mean residence lifetime; T1M is their longitudinal relaxation time; S is the electron spin quantum number; gS and gH are the electron and the proton nuclear magnetogyric ratios respectively rGd¡VH is the distance between the metal ion and the protons of the coordinated water molecules; wH and wS are the proton and electron Larmor frequencies respectively; tR is the reorientational correlation time; and tS1 and tS2 are the longitudinal and transverse electron spin relaxation times.These last two are frequency dependent according to eqns. 5 and 6 and characterized by the correlation time (tv) of the modulation of the transient zero-field splitting (expressed by the square of its trace value D2). The outer-sphere relaxivity which depends on the electronic relaxation time of the metal ion on the distance of closest approach of solute and solvent (a) and on the sum of solvent and solute diffusion coefficients (D) is usually treated on the base of the set of equations developed by Freed.9 For small-sized complexes with q = 1 [such as Gd(DOTA)2 and Gd(DTPA)22] it makes a contribution of roughly 40¡V50% to the observed relaxivity that at high-field ( > 10 MHz) is about the same for complexes of similar size and molecular ¡Ò 7 3 S(S t cl 2 2 1 1 w t + t c2 2 2 S c2 H cl 1 wi + 4 + 2 2 S v t R v 1 w t £»£»¢X(3) (4) (5) (6) 3 + + t = 1,2 1+ 4w t 2 1 + 4 +1) 1 + t Si 12 2 S v 5 + + 2 2 S v +1 w t ¡Ò 2 2 S w t v weight.The solvent proton relaxation rate has a magnetic field dependence through eqns. 3¡V6 (and through the outer sphere equations as well) and thus the set of parameters involved in the paramagnetic relaxation theory can be best obtained through a magnetic field dependent study.Experimentally this is performed by measuring solvent longitudinal relaxation rates over a wide range of magnetic fields with a field-cycling spectrometer that rapidly switches magnetic field strength over a range corresponding to proton Larmor frequencies of 0.01¡V50 MHz.2 The data points represent the so-called nuclear magnetic relaxation dispersion (NMRD) profile that can be adequately fitted to yield the values of the relaxation parameters. On dealing with multi-parameter equations it is desirable in the fitting procedure to fix the values of the parameters that can be determined through independent experiments. For instance the q value may be obtained from luminescence studies of the corresponding EuIII and TbIII complexes tR may be estimated from 13C-T1 measurement of a suitable C¡VH fragment in the related diamagnetic YIII LaIII or LuIII chelates tM may be assesed (vide infra) from variable temperature measurements of the 17O NMR transverse relaxation rate.R S0 The inner sphere relaxivity in the high magnetic field region is mainly controlled by the reorientational correlation time t which mainly depends upon the molecular dimension of the complexes as shown by the good correlation between relaxivity and molecular weight for a number of structurally similar complexes. The low-field region of the NMRD profiles substantially differs among different complexes according to the zero-field value of their electronic relaxation time (tS0). The latter parameter [which may be easily calculated from D2 and tv values through tS0 = (12D2 tv)21] is highly sensitive to the symmetry of the complex and to the chemical nature of the coordinating groups.In Fig. 3 the NMRD profiles of Gd(DOTA)2 and Gd(DTPA)22 are reported. Since both complexes have one coordinated water molecule and very similar size and molecular weight their relaxivities at high fields which depend on qtR/r6Gd¡VH are also very similar. However the two profiles differ considerably in the low field region as a consequence of their different electronic relaxation times. The axially symmetric Gd(DOTA)2 complex has a t value higher than 700 ps while for Gd(DTPA)22 this parameter assumes the value of 80 ps. Fig. 3 1/T1 NMRD profiles of Gd(DTPA)22 and Gd(DOTA)2 at 25 ¢XC. The lower curves represent the outer sphere contribution to the profiles.Currently the search for new CA for MRI is mainly directed toward the synthesis of GdIII complexes of functionalized derivatives of DTPA and DOTA ligands without altering their chelating abilities. We studied a number of new complexes derived from the macrocyclic structure of DOTA by introducing one or more b-benzyloxy-a-propionic residues (Fig. 4). These GdIII chelates have been designed to interact through the aromatic groups with hydrophobic sites in biological molecules in order to improve their relaxivity (vide infra) and to increase their lifetime in the circulating blood. The NMRD profiles of the complexes are very sensitive to the chemical 21 Chemical Society Reviews 1998 volume 27R¢ R¢¢ –OOC COO– N N Gd3+ N N –OOC COO– R GdDOGdDOTA(BOM) R = R¢ = R¢¢ = H O cis-GdDOTA(BOM)2 R = R¢ = R¢¢ = H O trans-GdDOTA(BOM)2 R = R¢ = R¢¢ = H O R = R¢ = R¢¢ = R = R¢¢ = R¢ = H O GdDOTA(BOM)3 Fig.4 Gd(DOTA)2-like complexes bearing the b-benzyloxy-a-propionic residues (BOM) modification of the chelate basic structure as shown in Figs. 5 and 6. All four GdIII complexes have significantly higher relaxivities than Gd(DOTA)2 over the entire magnetic field range investigated. Fig. 5 Comparison between the 1/T1 NMRD profiles for Gd(DOTA)2 Gd(DOTA–BOM)2 and Gd(DOTA–BOM3)2 complexes at 25 °C. The lower curves represent the outer sphere contribution to the profiles. The differences in relaxivity among the chelates are due to their different values of tR and tS0. At high fields the relaxivities show an almost linear dependence on tR which in turn is strictly related to the molecular weight and the size of the complexes whereas at lower fields the relaxivity differences are well accounted for by the different values of tR and tS0.10 The effect of the latter parameter is particularly evident when the relaxivity profiles of the disubstituted isomeric complexes are compared (Fig.6). In this case the low field differences in the inner and outer sphere relaxivities are completely accounted for by the different electronic relaxation times of the two chelates. The value of tS0 seems to reflect the changes in symmetry introduced in the coordination sphere of the GdIII ion by the insertion of one two or three b-benzyloxy-a-propionic residues.In fact tS0 of the mono-substituted complex (417 ps) is lower than that of the highly symmetric DOTA-complex. Moreover the difference in tS0 between the GdIII complexes of cis (275 ps) and trans (443 ps) disubstituted ligands is particularly impressive and may result from the lower symmetry of the 1,4-disubstituted isomer. The value of tS0 depends not only on the change introduced in the molecular geometry but also on the nature of the substituent group. In fact the amidation of a carboxy group produces a dramatic decrease in tS0 which results in a lower water proton relaxivity at low fields. Moreover the data obtained on a series of monoamide Chemical Society Reviews 1998 volume 27 22 Fig. 6 1/T1 NMRD profiles for the two isomers of Gd(DOTA– BOM2)2 at 25 °C.The lower curves represent the outer sphere contribution to the profiles. derivatives of DOTA indicate that tS0 is almost independent of the nature of the amide substituent (120–140 ps).11 It is likely that the observed decrease in this parameter depends on the decreased donor ability of the amide oxygen with respect to the carboxylate oxygen. Therefore the tS0 parameter acts as a molecular amplifier of the minor differences in the coordination between the carboxylate and the carboxamide groups. M 2.2 The role of the exchange lifetime t Although this parameter may affect the observed relaxivity either through eqn. 2 or eqn. 4 only recently has it been realized12–14 that its value may be long enough to represent a limiting factor to the attainable relaxation enhancement promoted by GdIII complexes.Graphical simulations through eqn. 2–6 (Fig. 7) show that an optimum value for this parameter Fig. 7 3D representation of the tM and tS dependence of the longitudinal relaxivity for an immobilized (tR = 30 ns) GdIII chelate (q = 1 r = 3 Å) when the GdIII chelate is bound to a slowly tumbling substrate like a protein is in the range of few tenths of nanoseconds. An accurate determination of the exchange lifetime of the coordinated water molecule is pursued through the measurement of the transverse 17O NMR relaxation time at variable temperature. The observed relaxation rates are dominated by the M contact interaction and it is dependent either on tM or DwO (which is the 17O chemical shift difference between coordinated and bulk water) (eqns.7–10):13,14 2 2 R R t + O M M O 2M O -1 2M R P t = M M O 2 p 2 Dw O (7) - -1 M + R M + t DwM O 2 1)2 + ( 2 R S(S ) t + E1 O 2M (8) 1 2 2 2 s E2 21 (9) æ è ç 21 + t21 M = t M - (10) + 1 = tSi exp ë ê 1 æ Aö è ø 3 h tEi -1 298,15 T M 298.15 ö ø ÷ 1 ö T æè 298 15 . ø t Mj -1 ù û ú where (A/�h) is the Gd–17O scalar coupling constant (3.8 3 106 rad s21 for the polyaminocarboxylate GdIII chelate with only one metal bound water molecule) tEi represent the correlation times of the processes modulating the scalar interaction and M is the activation enthalpy for the exchange process.R2 O p values increase as the temperature increases until tM 2M thus causing a Fig. 8 Temperature dependence of the paramagnetic contribution to the q 8 t E + w t 1 tM/ns 1.2 303 244 77 350 2200 68 19000 éDH R charge 3+ 22 12 12 0 0 12 3+ = ( ) Ligand aquo-ion 3 DTPA DOTA DOTA(BOM) HP-DO3A BMA–DTPA DOTMA DTMAa 1 1 1 1 1 1 1 DH becomes short enough with respect to RO decrease of RO 2pwith a further increase of temperature. It is worth noting that RO 2M is significantly higher than R1 H M making this method much more sensitive to tM values with respect to the measurement of the longitudinal water proton relaxation rate.The resulting bell-shaped behaviour [Fig. 8 shows the profile water 17O NMR transverse relaxation rate (R2p) for Gd(DOTA)2 (0.05 m) at 2.1 T obtained for Gd(DOTA)2] may be fitted through the above eqns. 7–10 affording the activation energy for the exchange process and the actual tM value at any temperature. From the data now available (Table 1) it is evident that tM (at 298 K) in GdIII complexes may fall over an extended range of values from a few nanoseconds to few microseconds. A simple qualitative assessment of the occurrence of relatively long Table 1 Water exchange lifetime at 25 °C for several GdIII chelates as determined from the analysis of the temperature dependence of water 17O transverse relaxation rate a DTMA = 1,4,7,10,tetrakis-[(N-methylcarbamoyl)methyl]-1,4,7,10- tetraazacyclododecane.exchange lifetime ( > 0.5 ms) of the coordinated water molecule may also be drawn by measuring the proton relaxivity at temperatures lower than 25 °C; the flattening of the resulting profile of RH 1p versus T at low temperature is a clear indication of the ‘quenching’ effect of the exchange lifetime (eqn. 2).12 Inspection of the data reported in Table 1 indicates that for systems with q = 1 the exchange is primarily determined by the residual electric charge and the structural properties of the complex. Now since the exchange mechanism is dissociative the exchange rate is determined by the difference in energy (DE) between the nine-coordinated ground state and the eightcoordinated activated state.On DOTA-like structures the introduction of substituents on the square-antiprismatic coordination cage causes a decrease in the stability of the ground state (with respect to the parent DOTA complex) which results in a decrease of DE and in turn in a shortening of tM. Furthermore Powell et al. have shown that a direct insight into the exchange mechanism of the coordinated water may be gained through 17O NMR experiments at variable pressure.15 Interestingly the occurrence of a long exchange lifetime of the coordinated water molecule in some of these paramagnetic chelates allows us to determine the contribution to the overall relaxivity arising from the exchange of protons only.16,17 Let us consider the GdIII complex of the bisbenzylamide–DTPA ligand analogous to Gd(DTPA–BBA) and look at the pH dependence of the longitudinal water proton relaxation rate at room temperature and 20 MHz (Fig.9). The higher relaxivity observed at basic pH arises then from the contribution of the base catalyzed prototropic exchange to the relatively slow exchange of the whole water molecule. Thus the fast prototropic exchange atH 12 removes the ‘quenching’ effect of the long tM and the R1 H p value measured at this pH is then the expected value for this complex on the basis of its molecular size owing to the linear dependence of RH 1p versus the molecular weight. Although not straightforward one might envisage the possibility to shorten tM by increasing the prototropic exchange at physiological pH through the introduction of functionalities with suitable pKa values in the close proximity of the coordinated water molecule.Fig. 9 pH dependence of the water proton longitudinal relaxivity (RH 1P) for Gd(DTPA–BBA) at 20 MHz and 25 °C 2.3 Long tR values for enhanced relaxivities At the magnetic field strengths currently employed in MRI (0.5–1.5 T corresponding to proton Larmor frequencies of 20–60 MHz) the ability of GdIII chelates to enhance the longitudinal water proton relaxation rate is mainly determined by the value of their molecular reorientational time tR. Therefore the achievement of higher water proton relaxation rates may be pursued through an increase of this parameter since the increase of the number of the metal bound water molecules (q) which would lead to the same result is likely accompanied by a decrease of the stability of the complex.It has been shown that the effectiveness of GdIII complexes as CA may be significantly improved by using protein–chelate 23 Chemical Society Reviews 1998 volume 27 conjugates in which the metal complex is covalently attached to amino acid residues of the protein; this approach then couples the strong chelation of the metal ion with the slow molecular tumbling of the protein.18 However severe limitations may arise from a poorly controlled addition of the ligand molecules to the protein which may result in a decreased thermodynamic stability of the chelate and in an alteration of the hydrophobic– hydrophilic domains of the macromolecule.An alternative route for increasing tR may be pursued through the formation of hostguest non-covalent interactions between suitably functionalized complexes and slowly tumbling macromolecules.19 We have investigated the non-covalent interactions between a variety of substituted derivatives of Gd(DTPA)22 and Gd(DOTA)2 and put forward a general picture accounting for the main determinants of the relaxation enhancement observed when a paramagnetic GdIII complex is bound to human serum albumin (HSA).20 In addition to providing high relaxivities (which in turn allows one to reduce the administered doses of CA) the formation of adducts between GdIII complexes and HSA are of notable interest for the design of novel angiographic experiments for which an increased residence time and a better compartmentalisation in the circulating blood is required.As schematically shown in Fig. 10 we have shown that the observed relaxation enhancement in these systems receives a substantial contribution also from water molecules in the hydration shell of the macromolecule and protein exchangeable protons which lie close to the interaction site of the paramagnetic complex. Fig. 10 Schematic representation of the non-covalent interaction between a protein and a GdIII chelate bearing an hydrophobic residue According to the type of substituent the thermodynamic association constant of these complexes with HSA varies from 102 to 104 m21 whereas the number of binding sites has been found to vary from 1 to 3.Competition assays performed with suitable probes allow us to map the interaction sites. For instance we found that the GdIII complex of a DOTA-like ligand bearing three b-benzyloxy-a-propionic substituents displays two equivalent binding sites on HSA located in subdomains IIA and IIIA of the protein. The macromolecular adduct resulting from the interaction of this complex with HSA Chemical Society Reviews 1998 volume 27 24 has a relaxivity of about 56 mm21 s21 (at 39 °C and 20 MHz) which represents the highest value so far reported for a GdIII chelate.20 NMRD profiles are highly diagnostic of the formation of adducts between GdIII complexes and slowly moving substrates like proteins as they show a relaxivity peak centered at about 20–30 MHz (Fig.11). Indeed slowly rotating systems t R and t are characterised by a tR value which is too long to contribute to C at low fields where tS is short. Since the actual tS value increases with frequency (eqn. 5) it becomes longer than t C goes from tS to tR at a frequency corresponding to tS21. At higher frequency the conditions wHtR ! 1 occurs. Fig. 11 1/T1 NMRD profiles for Gd(DOTA–BOM3)2 with (-) and without (8) bovine serum albumin (25 °C pH 6.9) 2.4 Relaxation enhancement in systems with q = 0 As mentioned before when a GdIII complex does not possess any water molecule in its inner coordination sphere the enhancement of the solvent relaxivity is exclusively due to the electron–nucleus dipolar interaction between the metal ion and the water molecules diffusing in the outer coordination sphere of the complex.This interaction is modulated by the translational diffusional motion of solute and solvent and by the electronic relaxation time. This mechanism makes a contribution of roughly 40–50% to the overall relaxation rate for low molecular weight GdIII chelates with octadentate ligands and q = 11,2 and in principle it could be evaluated experimentally by considering systems with q = 0. However because of the preference of GdIII for a coordination number of 9 this situation is rather uncommon. Recently we have studied a highly rigid kinetically stable 8-coordinate GdIII complex of a macrocyclic benzylphosphinate ligand (BzDOTP Fig.12).21 In this case the O O Ph Ph H P CH2 2C P N N OH OH O O N N Ph Ph P CH2 H2C P OH OH Fig. 12 Scheme representing the BzDOTP ligand magnitude of the relaxivity and the nature and form of the NMRD profile are fully consistent with the behaviour of a complex which does not possess any contribution from a bound water molecule. The analysis of the profile suggests that the nearest water molecule is 4.25 Å distant from Gd. This complex represents then a good example of a pure ‘outer-sphere’ contrast agent. Furthermore Gd(BzDOTP)2 has been found to form a relatively strong complex with bovine serum albumin (BSA; KA = 3.6 3 103 m21 at 25 °C) leading to a marked (for a q = 0 GdIII complex!) relaxivity enhancement (Fig. 13). In fact a remarkably high efficacy of the complex in liver and bile has been observed in MRI examinations.A major contribution to Fig. 13 Proton relaxation enhancement for Gd(BzDOTP)2 (0.2 mm) at 20 MHz and 25 °C as a function of bovine serum albumin concentration the relaxivity enhancement is likely to be due to the exchange of the mobile protons of the protein which are dipolarly relaxed by the proximity to the paramagnetic center. Another possible contribution could arise from the high structural organisation and the consequent reduced mobility of the solvent molecules in the hydration sphere of the protein near the binding site of the complex which allows the observation of second-sphere interactions. This contribution to the overall relaxivity has been recently described in the case of complexes of 1,4,7,10-tetraazacyclododecane containing one carboxamide and three phosphinate substituents.22 It has been found that the observed relaxation enhancement of water protons (Fig.14) is determined in addition to the expected outer-sphere mechanism by a relatively distant water molecule in the second coordination sphere. This is explained by the participation of the amide carbonyl oxygen in hydrogen bonding to a local water molecule which results in a short enough metal–proton distance. Fig. 14 1/T1 NMRD profiles for Gd(BzDOTP)2 and for a macrocyclic GdIII chelate containing three methylphosphinate groups and one bisn-buthylcarboxoamide group In some cases the outer sphere relaxivity may be so high as to induce an erroneous estimation of the q value.This happened in the case of Gd(DOTP)52 [DOTP = 1,4,7,10-tetraazacyclododecane-N,NA,NB,NAAA-tetrakis(methylenephosphonic acid)] which displays a relaxivity of 4.7 s21 mm21 at 25 °C and 20 MHz a value quite reasonable for a system with q = 1 [e.g. Gd(DOTA)2 has the same relaxivity under the same experimental conditions]. Later 17O NMR R2p measurements unambigously showed that the complex has no water molecule directly coordinated to the paramagnetic center. Then the high relaxivity appears determined by the ability of the phosphonate groups to form a strong second hydration sphere around the complex. These water molecules are in fast exchange with the bulk solvent and their contribution to the overall relaxivity may be evaluated by the same set of eqns.2–6 above introduced for the assessment of the inner sphere term. Moreover this complex displays a strong interaction with hemoglobin at the binding site of the natural 2,3-diphosphoglycerate allosteric effector.23 Very interestingly this makes the observed relaxivity dependent upon the conformational state of the protein. 2.5 Improved contrast agents The search for a new generation of CA appears strongly orientated to provide them with high tissue and/or organ specificity. In this context there is much interest to develop CA able to detect malignant focal lesions and to differentiate them from non-malignant ones. In particular in order to target hepatocytes the route to exploit among various possibilities the known properties of the transport system of the baso-lateral membranes has been pursued.24 Accordingly a GdIII complex is expected to enter the hepatocytes if it bears on the surface of the ligand a synthon already known to be recognized by the transport system of the hepatocyte.Schematically these CA are formed by three components the recognition synthon a spacer and the GdIII containing moiety (Fig. 15). Synthon Gd Fig. 15 As recognition synthons either bile acid residues or iodinated species (namely 3-amino-2,4,6-triiodobenzoic acid and iopanoic acid) have been used. Both synthons are particularly able to enter the hepatocytes as well documented in studies of related hepatospecific X-ray CA. By this approach the hepatobiliary excretion of DTPA- and DOTA-like systems may reach values as high as 50–60%.It is worth mentioning that the highly hydrophilic parent Gd(DTPA)22 and Gd(DOTA)2 have an almost 100% renal excretion. Animal biodistribution of a given contrast agent is usually assessed in addition to the direct MRI evaluation by means of the investigation with radioactive 153Sm and 159Gd chelates.25 3 Applications of GdIII complexes to in vitro quantitative assays A feedback of the research efforts to afford new CA for MRI has provided a new route to in vitro quantitative determination of a number of species through the observation of the effects on the relaxation properties of solvent water protons caused by the interaction that suitable paramagnetic complexes are able to set up with the analytes to be determined.As a first test to probe this idea we chose the determination of glycated albumin which may be present in significant amount in blood serum in the presence of high glucose levels.26 The non-enzymatic glycation of proteins is a quite common process and it is known to involve mainly e-terminal amino groups of lysine residues. As a paramagnetic probe we dealt with a functionalized derivative of Gd(DTPA)22. To recognise the glycated protein we chose the boronic functionality whose ability to form a stable ester bond with syn-diols is well established and an affinity chromatography method based on boronic functionalised resin is currently used among others in clinical practice to determine glycated hemoglobin.The synthesis of the DTPA ligand functionalised with boronic acid was carried on by reacting DTPA anhydride with 3-aminophenylboronic acid. The obtained bisamide ligand was then reacted with an equimolar amount of Gd2(CO3)3 to afford the corresponding GdIII complex. In the presence of glycated proteins the boronate on the complex forms a stable ester bond with the syn-diol of the protein sugar (Fig. 16). As a consequence the reorientational correlation time of the complex will become longer and the water proton relaxation rate will increase significantly. In order to check the potential utility 25 Chemical Society Reviews 1998 volume 27 CH2OH HO OH O Protein OH – B O NH H2O Gd Fig. 16 Schematic representation of the covalent binding between the syndiol group of a glycated protein and the GdIII chelate functionalised with a boronic residue of this approach to quantify the correct amounts of glycated protein in an actual specimen we compared the proton relaxation rates of solutions of this GdIII bisamide complex containing variable amounts of HSA at different degrees of glycation to the results obtained from the fructosamine method used in the clinical chemistry practice.The good linearity found between the observed relaxation rate and the protein sugar concentration in the albumin solutions (Fig. 17) is very promising to support such proton relaxation enhancement approaches as a new method for the determination of glycated proteins.More generally we believe that these results introduce a novel area of development of paramagnetic GdIII complexes in addition to their well established role as contrast agents for MRI. The use of functionalised paramagnetic complexes may allow the easy and quick determination of a variety of macromolecular substrates through the detection of the increase of solvent water proton relaxation rate as the result of the formation of slowly tumbling macromolecule–complex adducts. Interestingly the proposed procedure represents the NMR counterpart of the EPR free radical assay technique based on the detection of changes in the linewidth of nitroxide resonance caused by the interaction of the labeled reagent with macromolecules.27 Fig. 17 Water proton relaxation rates of 0.56 mm solution of the GdIII chelate bearing the boronic functionality as a function of the concentration of glycidic groups on human serum albumin as determined by the fructosamine method Very recently another interesting strategy for constructing relaxation reagents whose relaxivity is dependent upon the biochemical environment has been proposed.The prototypical example is represented by a Gd–DOTA like complex containing the galactopyranosyl ring bound to the tetraazamacrocycle in a way that prevents the coordination of the water molecule to the GdIII ion. When this substituent is removed by the activity of the b-galactosidase enzyme the access of water becomes allowed with a consequent increase of the observed relaxivity.These agents then generate distinct ‘on’ and ‘off’ states by controlling Chemical Society Reviews 1998 volume 27 26 the access of water molecules to a chelated paramagnetic GdIII ion.28 4 LnIII chelates as shift reagents 4.1 Metal cations NMR active metal cations of clinical importance such as 7Li+ 23Na+ and 39K+ are normally found in biological systems in both the intra- and extra-cellular compartments and in routine high resolution NMR spectra are characterized by a single resonance. The application of metal NMR spectroscopy to biomedical studies implies obtaining distinct signals from the the two compartments that may then be simultaneously monitored and individually analyzed.5 This is made possible by the use of aqueous shift reagents (SR) water soluble paramagnetic metal chelates which only distribute in the extracellular space and thus are able to remove the signal degeneracy by selectively affecting the extracellular resonance.On the basis of the experience gained over the last fifteen years by different laboratories it has been established that in order to be an effective SR a metal chelate must satisfy several requirements and present certain well defined characteristics (a) The paramagnetic metal ion in a non-S ground state electronic configuration should present a high magnetic moment and a short value of the electronic relaxation time ( ~ 10213 s). This requirement normally restricts the choice among the lanthanide(iii) ions to DyIII TbIII and TmIII. (b) High negative charge.The efficacy of a SR (i.e. its ability to produce a large shift of the cation resonance in the extracellular compartment) is strictly related to the possibility of favouring multiple strong electrostatic interactions (ion-pairs) with the metal cations. Obviously the binding requires negatively charged groups on the surface of the coordination cage (carboxylic phosphonic etc.) either coordinated to the paramagnetic centre or pendant. Furthermore the overall negative charge of the SR does not allow the crossing of the phospholipidic membranes and thus favours their distribution in the extracellular space. Unlike T1-water relaxation reagents described above the cations to be monitored do not enter the first coordination sphere of the paramagnetic metal ion and therefore all the coordination sites may be occupied by the donor atoms of the ligand.(c) The extent of the shift effect also has a dependence from a term of the type (3cos2q 2 1)/r3 where r and q are the polar coordinates of the nucleus under study with respect to the LnIII ion and with the main magnetic axis. It follows that the effect will be maximum for axially symmetric complexes having the cation binding sites along the symmetry axis. Moreover due to the dependence of the shift on the 1/r3 term the complexes bearing the negative charge on unbound side chains are expected to be less effective SR. Up to now in laboratory practice four complexes have been studied in detail and applied to the answering of several questions of biomedical interest:5 the DyIII complexes of PPP52 (triphosphate) and TTHA62 (triethylenetetraminehexaacetate) and the DyIII and TmIII complex of DOTP82.The most effective SR so far reported is the chelate Dy(PPP)2 72 first introduced in 1982. The Dy(TTHA)32 complex is much less toxic as a consequence probably of the high stability constant of Dy3+ with this multidentate ligand. However the reduced value of the complex negative charge and the fact that it is mainly localized on unbound carboxylic groups away from the paramagnetic centre make this metal chelate less effective in removing the signal degeneracy. The DyIII and TmIII complexes of DOTP represent an important improvement in the search for safer and more effective SR for metal cations of biological relevance (Fig.18). In fact these metal chelates are extremely resistant to dissociation processes over a wide range of pH and present interaction sites for the cations very close to their four-fold axis of symmetry thus ensuring a maximum shifting efficency. Actually it has been Fig. 18 23Na NMR spectra of human blood before (a) and after (b) the addition of 5 mm of Dy(DOTP)52 (39 °C at 2.1 T) shown that TmDOTP42 produces resolved 23Na resonances in the in vivo rat liver with relatively little compromise in commonly measured physiological indices.29 Moreover its application to the in vivo rat kidney showed that this shift reagent produces three resolved resonances from intracellular Na+ combined interstitial and vascular Na+ and filtrate Na+.30 In conclusion it is worth commenting that although several studies have been possible by the use of the available SR much remains to be done in terms of synthetic strategy and ligand design in the search for safer and more effective compounds.4.2 Anions In principle analogous considerations may be used for the development of LnIII complexes acting as SR for anions like phosphate carbonate chloride lactate etc. In this context we have considered the cationic macrocyclic Eu(DTMA)3+ complex (DTMA = 1,4,7,10-tetrakis-[(N-methylcarbamoyl) methyl]-1,4,7,10-tetraazacyclododecane) as SR for the phosphate ion.31 As shown in Fig. 19a the 31P NMR spectrum of human blood shows a single resonance for the inorganic phosphate ion flanking one of the two diphosphoglycerate (DPG) resonances.Upon addition of Eu(DTMA)3+ a new signal is clearly detectable (Fig. 19b) which is assigned to the extracellular phosphate experiencing the additional field of the paramagnetic metal ion. Up to now this represents the first tentative step in a field that is likely to receive much more attention in the near future. 5 Water signal suppression in 1H NMR spectra of biological fluids promoted by DyIII complexes Solvent suppression in 1H NMR spectra has received considerable attention in recent years in biochemical applications of NMR spectroscopy. Spectral selection methods have been proposed over the years based on the differences between the longitudinal or transverse relaxation rates of water protons compared to those of other species present in solution.Among 2 them the WATR method (water attenuation by T2 relaxation) introduced by Rabenstein et al.32 is based on the increase of water transverse relaxation rates by chemical means resulting in a selectively attenuated solvent signal in the spin-echo spectra. Chemicals which cause such a decrease of water T relaxation times are species containing mobile protons which can exchange with the solvent water. Aqueous solution of DyIII complexes with DOTA-like ligands can act as paramagnetic Fig. 19 31P NMR spectra of human blood before (a) and after (b) the addition of Eu(DTMA)+ (39 °C at 2.1 T) (2,3-DPG = 2,3-diphosphoglycerate; Pi = inorganic phosphate) WATR agents at low concentration and over a wide pH range and were tested in water signal suppression experiments by using the CPMG pulse sequence as suggested in the WATR method.33 In principle their ability to suppress the water signal depends upon the paramagnetic shift the relaxation time and the exchange lifetime of the coordinated water protons.The pH of the solution (from 2 to 11) had no detectable effect on R2. An illustrative spectrum obtained by the addition of a DyIII DOTAlike complex (2.5 mm) to a normal human CSF (5% D2O) is shown in Fig. 20. Typical resonances from glucose lactate acetate citrate creatinine etc. are readily detected in agreement with previously reported 1H NMR spectra of CSF specimens. We have observed neither any detectable paramagnetic shift nor broadening of these resonances.This may be the result of the tight molecular geometry of the complex which largely limits the access of organic substrates into the inner coordination sphere of the metal ion.34 Furthermore the proton resonances of the paramagnetic complex fall in a wide absorption range (over 200 ppm) but their very short T2 values do not allow their detection in the spin-echo 1H NMR spectra. 6 Paramagnetic LnIII complexes as reporters of the physico-chemical environment 6.1 pH indicators As just mentioned a characteristic NMR property of paramagnetic LnIII complexes is the large chemical shift range usually shown by ligand resonances. Thus it is common that the 1H resonances of these complexes usually fall outside the diamagnetic region and then do not overlap with those arising from endogenous diamagnetic molecules.Moreover these paramagnetically shifted resonances are highly sensitive to slight structural and electronic variations. We found that these compounds may represent excellent NMR pH indicators provided that they contain acid/base functionalities whose pKa values fall in the pH range of interest. As a representative example of this class of complexes we report the results obtained with Yb(DOTP)52.35 As we have seen in a previous paragraph the corresponding Dy(DOTP)52 and Tm(DOTP)52 chelates are used as shift reagent to differentiate extra- and intra-cellular signals of NMR-active cations. The 1H NMR 27 Chemical Society Reviews 1998 volume 27 ( a) ( b) Fig.20 1H-NMR spectra of normal human CSF in 95% H2O–5% D2O solution containing a 2.5 mm concentration of DyIII-complex (25 °C at 9.4 T). (a) single pulse method. (b) Carr–Purcell–Meiboom–Gill spin-echo spectrum. spectrum of Yb(DOTP)52 (Fig. 21) is consistent with a stereochemically rigid system of D4-symmetry. At basic pH values there are five residual negative charges on the complex located on the eight uncoordinated oxygen atoms of the phosphonate groups which progressively decrease when the pH of the solution is lowered. The stepwise addition of H+ ions causes large changes in the chemical shift of all the resonances Fig. 21 1H NMR spectrum of a 25 mm solution of Yb(DOTP)52 at 39 °C 2.1 T and pH 7.1 (w = HOD resonance; r = tert-butyl alcohol resonance used as internal reference d = 0 ppm) 28 Chemical Society Reviews 1998 volume 27 whose values become then reporters of the pH of the solution.In order to avoid the use of an internal reference to quote the shift of the pH-dependent resonances it is advantageous simply to consider the chemical shift separation between a selected pair of resonances. For instance the dependence of dax1 2 dac1 upon pH appears to be linear in the pH range between 5.0 and 7.5 with a slope of 7.0 ± 0.1 ppm (pH unit)21. 6.2 Temperature sensitive probes It is well known that the chemical shift of proton resonances in paramagnetic complexes is strongly temperature dependent. In the absence of specific interactions between the complex and endogenous substrates one may safely assume that changes in chemical shift of the ligand resonances may act as a reporter of temperature changes in a given organ or tissue.For this application we chose Yb(DOTMA)2 whose 1H NMR spectrum approximately spans 170 ppm (Fig. 22).36 Its fourfold symmetry axis reduces the number of 1H resonances to six. Analogously to the approach described for the pH indicator it is more advantageous simply to compare the chemical shift difference between a pair of resonances falling respectively at the low and high frequency side of the normal (diamagnetic) proton spectrum. In Fig. 23 the temperature dependence of the change in chemical shift difference between dac and dax1 proton resonances (measured in human serum) between 35 °C and 45 °C is reported.A straight line fit of the experimental data points gave a slope of 20.41 ± 0.01 ppm °C21. Fig. 22 1H-NMR spectrum of Yb(DOTMA)2 at 27 °C 2.1 T and pH 7.1 (w = HOD resonance; t-Bu = tert-butyl alcohol resonance used as internal reference d = 0 ppm) Fig. 23 Temperature dependence of ax1-ac proton chemical shift difference as measured in human serum containing 25 mm of Yb(DOTMA)2 From this work we draw the suggestion that the interesting properties of the high resolution NMR spectra of paramagnetic LnIII complexes (when Ln ­ Gd) may find a novel application in MRI.36 In fact in principle it may be possible to map the spatial distribution of a paramagnetic complex provided a sufficiently intense signal with a chemical shift that is far enough from those of water and other tissue constituents is available.This has been shown for Yb(DOTMA)2 by selectively exciting the most intense methyl group resonance (12 protons 214.2 ppm at 27 °C) for complex concentration ranging from 0.003–0.1 m. A distinct advantage of dealing with paramagnetic complexes is that the protons in the complexes are expected to exhibit extremely short T1 relaxation times allowing very rapid acquisition times. 7 Concluding remarks The maturation of magnetic resonance approaches to tackle in vivo biochemical problems as well as to improve the specificity of clinical investigations will prompt the development of novel lanthanide complexes in order to enhance the physiological information from this technique.The increased availability in hospitals of MRI instruments capable of providing spectroscopic in addition to morphologic information introduces stronger links between chemists biochemists and physicians leading towards a molecular view of biological problems. This in turn will contribute to the design and use of suitable chemicals which allow a better understanding of complex systems. On the basis of the results herein reviewed we believe that much innovative work may come from exploiting the peculiar magnetic properties of paramagnetic LnIII complexes. 8 Acknowledgements We gratefully acknowledge the research team led by F. Uggeri (Bracco S.p.A. Milano Italy) for a long and fruitful collaboration. Their skillful contribution has been invaluable to the foundations of the present work.We thank S. H. Koenig for stimulating discussions and for providing us with the possibility for having the Field-Cycling Relaxometer facility in Torino. 9 References 1 J. A. Peters J. Huskens and D. J. Raber Prog. NMR Spectrosc. 1996 28 283. 2 S. H. Koenig and R. D. Brown III Prog. NMR Spectrosc. 1990 22 487. 3 K. Kumar and M. F. Tweedle Pure Appl. Chem. 1993 65 515. 4 J. Petersein S. Saini and R. Weisslader MRI Clin. N. Am. 1996 4 53. 5 A. D. Sherry and C. F. C. G. Geraldes Shift Reagents in NMR Spectroscopy in Lanthanide Probes in Life Chemical and Earth Sciences ed. J. G. Bunzli and G. R. Choppin 1989 Elsevier Amsterdam ch. 4 p. 93. 6 R. B. Lauffer Chem. Rev.1987 87 901. 7 C. Paul-Roth and K. N. Raymond Inorg. Chem. 1995 34 1408. 8 L. Banci I. Bertini and C. Luchinat Nuclear and Electron Relaxation 1991 VCH Weinheim. 9 J. H. Freed J. Chem. Phys. 1978 68 4034. 10 S. Aime M. Botta G. Ermondi F. Fedeli and F. Uggeri Inorg. Chem. 1992 31 1100. 11 A. D. Sherry R. D. Brown III C. F. C. G. Geraldes S. H. Koenig K.-T. Kuan and M. Spiller Inorg. Chem. 1989 28 620. 12 S. Aime M. Botta M. Fasano S. Paoletti P. L. Anelli F. Uggeri and M. Virtuani Inorg. Chem. 1994 33 4707. 13 G. Gonz`alez D. H. Powell V. Tissi`eres and A. E. Merbach J. Phys. Chem. 1994 98 53. 14 D. H. Powell O. M. Ni Dhubhghaill D. Pubanz L. Helm Y. S. Lebedev W. Schlaepfer and A. E. Merbach J. Am. Chem. Soc. 1996 118 9333. 15 L. Helm D.H. Powell A. E. Merbach K. Micskei and E. Br�ucher High Pressure Res. 1994 13 739. 16 S. Aime A. Barge M. Botta D. Parker and A. S. De Sousa J. Am. Chem. Soc. 1997 119 4767. 17 S. Aime M. Botta M. Fasano S. Paoletti and E. Terreno Chem. Eur. J. 1997 3 1499. 18 R. C. Brasch Magn. Reson. Med. 1991 22 282. 19 B. G. Jenkins E. Armstrong and R. B. Lauffer Magn. Reson. Med. 1991 17 164. 20 S. Aime M. Botta M. Fasano S. Geninatti Crich and E. Terreno JBIC 1996 1 312. 21 S. Aime A. S. Batsanov M. Botta J. A. K. Howard D. Parker K. Senanayake and G. Williams Inorg. Chem. 1994 33 4696. 22 S. Aime M. Botta D. Parker and G. J. A. Williams J. Chem. Soc. Dalton Trans. 1996 17. 23 S. Aime P. Ascenzi E. Comoglio M. Fasano and S. Paoletti J. Am. Chem. Soc. 1995 117 9365. 24 P. L. Anelli L. Calabi C. de Haen F. Fedeli P. Losi M. Murru and F. Uggeri Gazz. Chim. Ital. 1996 126 89. 25 C. F. G. C. Geraldes A. D. Sherry I. Lazar A. Miseta P. Bogner E. Berenyl B. Sumegi G. E. Kiefer K. McMillon F. Maton and 26 S. Aime M. Botta W. Dastr`u M. Fasano M. Panero and A. Arnelli 27 R. K. Leute E. F. Ullman A. Goldstein and A. Herzenberg Nature 28 R. A. Moats S. E. Fraser and T. J. Meade Angew. Chem. Int. Ed. Engl. 29 N. Bansal M. J. Germann V. Seshan G. T. Shires C. R. Malloy and 30 V. Seshan M. J. Germann P. Preisig C. R. Malloy A. D. Sherry and 31 S. Aime A. Barge M. Botta D. Parker and A. S. de Sousa 4th SMRM 32 D. L. Rabenstein S. Fan and T. T. Nakashima J. Magn. Res. 1985 64 33 S. Aime M. Botta L. Barbero F. Uggeri and F. Fedeli Magn. Res. 34 S. Aime M. Botta M. Fasano M. P. M. Marques C. F. C. G. Geraldes 35 S. Aime M. Botta L. Milone and E. Terreno Chem. Commun. 1996 36 S. Aime M. Botta M. Fasano E. Terreno P. Kinchesh L. Calabi and R. N. Muller Magn. Reson. Med. 1993 30 696. Inorg. Chem. 1993 32 2068. (London) New Biol. 1972 236 93. 1997 36 726. A. D. Sherry Biochemistry 1993 32 5638. N. Bansal Magn. Reson. Med. 1995 34 25. Meeting New York 1996 vol. 3 1869. 541. Chem. 1991 29 S85. D. Pubanz and A. E. Merbach Inorg. Chem. 1997 36 2059. 1265. L. Paleari Magn. Res. Med. 1996 35 648. Received 22nd July 1997 Accepted 22nd September 1997 29 Chemical Society Reviews 1998 volume

ISSN:0306-0012    

DOI:10.1039/a827019z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Nonplanar porphyrins and their significance in proteins John A. Shelnutt,*a Xing-Zhi Song,a Jian-Guo Ma,a Song-Ling Jia,a Walter Jentzena and Craig J. Medfortha,b a Catalysis and Chemical Technologies Department Sandia National Laboratories Albuquerque NM 87185-0710 USA† and Department of Chemistry University of New Mexico Albuquerque NM 87131 USA b Department of Chemistry University of California Davis CA 95616 USA Nonplanar distortions of tetrapyrroles are prevalent in the hemes of hemoproteins the pigments of photosynthetic proteins and cofactor F430 of methylreductase. The nonplanarity of these porphyrin cofactors is currently believed to influence factors in the biological activity of the proteins in part because the porphyrin deformations are often conserved within functional classes of proteins.The occurrence † Sandia is a multiprogram laboratory operated by Sandia Corporation a Lockheed Martin Company for the United States Department of Energy under Contract DE-AC04-94AL85000. John Shelnutt is a Distinguished Member of Technical Staff in the Catalysis and Chemical Technologies Department at Sandia National Laboratories and SNL/UNM Distinguished Professor in the Department of Chemistry at the University of New Mexico. He was a consultant at Bell Laboratories after receiving his BS MS and PhD from the School of Physics at Georgia Tech. Xingzhi Song is currently a postdoctoral associate at the Department of Molecular Physiology and Biophysics Baylor College of Medicine. He received his PhD in Chemistry from the University of New Mexico in 1997 MS in 1990 from East China John Shelnutt Walter Jentzen Jianguo Ma Xingzhi Song Songling Jia classification and study of nonplanar porphyrins in proteins and synthetic nonplanar porphyrin analogs are reviewed.1 Introduction Macrocyclic tetrapyrroles including porphyrins are found as cofactors in a bewildering array of proteins. Tetrapyrrole derivatives occur biologically in many enzymes as heme (iron porphyrin) in photosynthetic proteins as chlorophyll and pheophytin and in other proteins as corrin (vitamin B12) and Normal University and BS in 1983 from Beijing Normal University. Jianguo Ma is currently a graduate student in the Department of Chemistry at the University of New Mexico and Sandia National Laboratories studying for his PhD degree.He received his MS degree in 1990 and BS degree in 1987 from the Department of Chemistry at East China Normal University. Songling Jia is currently a graduate student in the Department of Chemistry at the University of New Mexico and Sandia National Laboratories studying for his PhD degree. He received his MS degree in 1991 and BS degree in 1985 from the Department of Chemistry at Nanjing University. Walter Jentzen after receiving his Diploma and PhD degrees in physics at the University of Bremen Germany moved in 1992 to Sandia National Laboratories as a postdoctoral fellow. He then returned to Germany in 1997 as a research scientist at the University of Essen where he is currently involved in tracer kinetic modeling.Craig Medforth is a Graduate of the Royal Society of Chemistry and received a PhD from the University of Liverpool in 1988. He was subsequently a Fulbright Postdoctoral Scholar under the direction of Professor Kevin Smith at the University of California at Davis. He is currently involved in the synthesis and spectroscopic characterization of nonplanar porphyrin systems. Craig Medforth 31 Chemical Society Reviews 1998 volume 27 corphin (methylreductase). Macrocyclic tetrapyrroles in biological systems are either metal free (pheophytins biosynthetic intermediates catabolites) or contain iron (hemes) magnesium (chlorophylls) cobalt (vitamin B12) nickel (cofactor F430) and copper (pigments). Some of these porphyrin derivatives are illustrated in Fig.1. Their biological functions range from O2 transport (hemoglobins) and storage (myoglobins) collection and transport of light energy (antennae complexes) conversion of solar energy to chemical energy (photosynthetic reaction centers) electron transfer (cytochromes) oxygen reduction (oxidases) and a large number of other enzymatic reactions (peroxidases catalases cytochromes P450 methylreductases methyltransferases etc.) Beginning over 70 years ago the heme proteins have been investigated intensely with the aim of determining the structural mechanisms controlling their varied biological functions. The major questions remaining concern the role of the protein in modulating the properties of the iron-porphyrin cofactor to yield its specific biological function.The immediate surroundings of the heme active site certainly have a dominant influence on function. In particular axial coordination to the central iron atom covalent attachment of the heme to the protein and the nature of amino acid side chains in the immediate vicinity of the active site are undoubtedly important. More subtle influences on the structure of the active site are also sometimes observed to modify the activity of the protein. For example the number and type of axial ligands of the iron atom the axial ligand binding geometry and the nature of the hydrogen bonds between the axial ligands and the protein appear to be of importance in governing the redox properties.1 For hemoglobins the O2 affinity of each of the four hemes depends on whether the other hemes have O2 bound as an Fe axial ligand.The differences in heme O2 affinity for hemoglobin have been ascribed to subtle structural changes in axial coordination of the iron atom that are transmitted from one heme to the others through the protein’s tertiary and quaternary structure.2 In a similar manner proteininduced conformational differences in the porphyrin itself might influence enzymatic reactivity. Early X-ray crystal structures of heme proteins led to the false impression that the porphyrin macrocycle in the proteins was planar or nearly planar partly because the heme was constrained to a planar conformation during the refinement procedure. Later crystal structures of heme proteins and photosynthetic proteins almost invariably show nonplanar heme conformations.For the photosynthetic proteins the nonplanar distortions observed in the crystal structures have been suggested to influence the photophysical and redox properties of chlorophyll pigments with consequent effects on electron-transfer rates in photosynthetic reaction centers and antennae complexes.3 Currently attempts are underway to determine whether these nonplanar porphyrin distortions in proteins have functional significance N N N N Fe Mg N N N N H H COOH HOOC H MeOOC Protoheme Chlorophyll-b Fig. 1 Structures of protoheme (iron protoporphyrin IX) chlorophyll b and cofactor F430 phytyl-OOC Chemical Society Reviews 1998 volume 27 32 and if so to determine the mechanism by which nonplanarity influences activity.Some typical nonplanar structures of porphyrin macrocycles are shown in Fig. 2. Only the atoms of the porphyrin macrocycle and the central metal are shown i.e. substituents that are present at the twelve atoms around the perimeter of the macrocycle have been omitted. The distortions illustrated are simple symmetric deformations; more complicated asymmetric distortions that combine these simple distortions are often observed especially in the proteins. Arriving at a detailed structural understanding of the function of tetrapyrrole-containing proteins requires a thorough knowledge of the various influences of structure on function. Toward this end researchers often look for structural features that are conserved across functionally related proteins from many species for example conserved residues in the amino acid sequence.These conserved structural features are most likely to influence enzymatic function and are where one normally looks for relationships between structure and function. Yet the structure of the macrocycle has often been ignored in proposed structural mechanisms. This in spite of the fact that crystallographers have recently noted some highly nonplanar conformations in X-ray structures of proteins4 and these distortions are often conserved (vide infra). The hemoproteins provide a representative example of the occurrence of nonplanar porphyrins in proteins. It has been recognized for about 10 years that the hemes in many hemoproteins are highly distorted from planarity and that these nonplanar distortions might play a role in their biological function.3 Further by using a new normal-coordinate structural decomposition (NSD) procedure5–7 for characterizing and quantifying heme distortions our group has recently found that these distortions are often of different types for hemoproteins with different functions.Moreover the types of distortion observed are conserved for proteins belonging to the same functional class.4,7 Since nonplanar distortion is energetically unfavorable for hemes,8 conservation of the heme conformation strongly suggests that the biological function of hemoproteins might be modulated by protein control over the conformation of the heme prosthetic group.The possible importance of the nonplanar distortions of the heme is also emphasized by recent studies of model nonplanar porphyrins showing first that hemes are expected to be nearly planar in the absence of interactions with the protein moiety8 and second that the nonplanar structure of the heme influences relevant chemical and photophysical properties (e.g. axial ligand affinity redox potentials transition dipoles and energies). 3,6,9–12 Moreover the advent of many model nonplanar porphyrins has contributed to an improved understanding of the origin of nonplanar distortions of porphyrins. A great variety of sterically constrained nonplanar porphyrins has been synthe- HOOC O CHO HN COOH H H2NOC N N Ni+ N N H COOH HOOC H O O COOH Cofactor F430 ruf ( B1 u) 88 cm–1 sad ( B2 u) 65 cm–1 wav ( x) [ Eg( x)] 176 cm–1 dom ( A2 u) 135 cm–1 wav ( y) [ Eg( y)] 176 cm–1 pro ( A1 u) 335 cm–1 Fig.2 Symmetric normal-coordinate deformations used to decompose the structures of hemes. (1 Å displacements are shown). sized and their photophysical and chemical properties have been determined. These model studies indicate how the functional role of the porphyrin might be altered when the protein induces a particular distortion of the macrocycle. Currently a few spectroscopic techniques for distinguishing the magnitude of nonplanar distortion have been found although distinguishing the different types of distortion (e.g.doming ruffling saddling illustrated in Fig. 2) proves to be more challenging. If the nonplanar conformers are not interconverting too rapidly NMR spectroscopy can sometimes distinguish the different distortion types. Improved spectroscopic probes of porphyrin structure are needed to characterize fully the different distortions for a wide range of environments and time scales. Molecular modeling is a recent development in the investigation of nonplanar porphyrins. Molecular mechanics calculations in particular provide insight into the possible mechanisms by which the surroundings of the porphyrin may induce various nonplanar distortions especially when coupled with spectroscopic data and NSD analysis. Molecular mechanics force fields have been developed for porphyrins and exhaustively validated experimentally for the prediction of porphyrin conformations.6,8,12–14 The molecular modeling studies provide information about the energetics of nonplanar distortions as well as additional structural information. The computational capabilities are essential because present spectroscopic and crystallographic methods alone are inadequate for fully distinguishing and measuring the types of distortions shown in Fig. 2 under all conditions. Molecular mechanics calculations are helpful for interpreting experimental results but more importantly they can predict the presence of conformers that may be energetically accessible though not populated (and thus not observed spectroscopically or in X-ray structures).These stable highenergy conformational states of the heme may also have functional significance. The NSD method is described here briefly first since it provides the framework appropriate for reviewing the nonplanar distortions of porphyrins that occur in the X-ray crystal structures of proteins and model porphyrins. Next the novel properties associated with porphyrin nonplanarity are described along with the spectroscopic methods for investigating these nonplanar structures. Subsequently the question of how the environment of the porphyrin influences its conformation is addressed. Finally investigations of the relationship between chemical and biological function and heme structure are reviewed. 1.1 Normal-coordinate structural decomposition of the out-of-plane distortions of porphyrins The NSD method is simple in concept.5,6 It relies on the fact that the distortions of the 24 macrocycle atoms from ideal squareplanar geometry can be given in terms of the 3N 2 6 = 66 (N is the number of atoms) normal coordinates of the macrocycle instead of simply giving the x y and z displacements of each atom in the porphyrin skeleton.The normal coordinates are special linear combinations of the x y and z displacements of each of the atoms from their equilibrium positions. Only in the normal coordinate system is the molecular vibrational energy expressed simply as a sum of energies for each of the 3N 2 6 coordinates. In addition vibrations along the normal coordinates are what one observes in vibrational spectroscopy i.e.Raman and infrared spectroscopy. The advantage of a description in terms of the normal coordinates is that the distortional energy of the macrocycle takes its simplest form in this representation. Further because the restoring forces are smallest for displacements along these coordinates the distortion of the porphyrin takes place primarily along only the lowestfrequency normal coordinates. In other words the largest deformations are usually observed for the normal coordinates of lowest-frequency because they are the softest modes of distortion. A mathematical procedure has been described5,7 which projects out the displacements from an ideal geometry [chosen to be a planar copper(ii) porphyrin macrocycle] along the normal coordinates.5 The coordinate eigenvectors of each symmetry type are obtained from a normal coordinate calculation.Fig. 2 illustrates a 1 Å distortion along the lowestfrequency vibrational mode of each out-of-plane symmetry type. One easily identifies each of these normal deformations ruffling (B1u) saddling (B2u) doming (A2u) waving (Eg) and pyrrole propellering (A1u) with various nonplanar macrocyclic conformations that commonly occur in X-ray crystal structures of symmetrically substituted porphyrins. A 1 Å distortion means that the square root of the sum of the squares of the z-displacements from the mean plane is equal to one. For a particular porphyrin structure the NSD computational procedure ascertains the contribution of each normal mode to the structure.Combining only the individual displacements along the lowest-frequency normal coordinates (ruf sad dom wav and pro)5,15 gives a simulated structure that typically closely matches the observed structure. In general an exact representation of the observed conformation usually requires that the deformations along all normal coordinates be added into the simulated structure. However in practice an essentially exact representation of the structure requires far less than the total number of out-of-plane normal coordinates (N 2 3 = 21). When applied to protein crystal structures (Fig. 3) the poorer resolution (compared to porphyrin crystals) usually precludes the determination of displacements for more than the lowestfrequency normal coordinates.That is the high-frequency normal coordinate displacements are smaller than the statistical positional uncertainties in the X-ray refinements. The dotted lines in Fig. 3 show the simulated structures based on just the displacements of the lowest-frequency normal coordinates for some hemes in protein X-ray structures. All of the hemes of the approximately 350 X-ray crystal structures of hemoproteins contained in the Protein Data Bank have been analyzed by NSD. The heme conformations have been examined with regard to structural motifs that are maintained within functional classes of proteins. In most of the proteins the nonplanar heme structures have been found to be characteristic of the specific protein type.4,5,9 Detailed examination of the NSD results for the X-ray structures of hemoproteins has delineated a variety of structural effects of natural amino acid sequence variation mutation axial ligation and other protein differences on the conformation of the heme thus tying the primary secondary and tertiary structure of the protein moiety to the conformation of the heme.In the case of the mitochondrial cytochromes c a structural mechanism by which the protein produces the strong ruffling of the heme skeleton has been suggested by the NSD results.4,5,7 NSD characterization of the hemes of other proteins will lead to other detailed mechanisms by which protein structure modulates heme conformation and function. 33 Chemical Society Reviews 1998 volume 27 Fig.3 Linear or ‘clothesline’ displays of the hemes in the a- and b-subunits of deoxyhemoglobin A and in isoenzyme-1 of cytochrome c isolated from baker’s yeast. The dotted line represents the simulated structure obtained from a linear combination of displacements along only the lowest-frequency normal coordinates of each of the out-of-plane symmetries (see text). 1.2 Occurrence and characterization of nonplanar heme conformations in proteins Nonplanar porphyrin macrocycles are observed in many heme proteins with the largest heme distortions observed so far in the c-type cytochromes and the peroxidases. Fig. 3 shows the conformations of typical hemes in X-ray crystal structures of human hemoglobin and yeast mitochondrial cytochrome c.7 In Fig.3 the z-displacements of the atoms relative to the mean plane of the macrocycle are shown in linear or ‘clothesline’ displays with the z-displacements expanded to show clearly the deviations from planarity. These clothesline displays also clearly illustrate the variety and complexity of the heme distortions occurring in proteins. 4h) The NSD method allows the nonplanar distortions to be quantified in terms of a few displacements along the most deformable normal coordinates of the square-planar (D symmetric porphyrin macrocycle.5,7 Thus the NSD procedure greatly simplifies the discription of these geometrically complex structures. A simple bar graph of the displacements (Fig. 4) clearly shows the similarities and differences in the structures of hemes from different proteins.The lengths of the bars represent the normal-coordinate displacements that best represent the X-ray structure. The great variety in the types and magnitudes of the deformations in the hemoproteins is clear from Fig. 4. (Also see Fig. 9 of reference 5). For example the heme of myoglobin (not shown) is predominantly domed with a significant wav(y) component.5 In contrast cytochrome c peroxidase (Fig. 4) is mainly saddled and much more distorted. What is surprising is that in many cases these distortions are conserved for proteins of the same type but from different species. Fig. 4 shows the NSD results for the hemes of three Chemical Society Reviews 1998 volume 27 34 Fig. 4 Normal-coordinate Structural Decomposition results for several types of hemoproteins (peroxidase cytochrome P450 cytochrome c cytochrome cA and cytochrome c2).Hemes from three different species are shown. The color scheme for the displacements is as shown in Fig. 7 [redsad green-ruf pink-dom cyan-wav(x) yellow-wav(y) blue-pro]. For full details see ref. 7. species for several different types of hemoproteins. Considering the approximately 0.1 Å uncertainties in the atomic positions inherent in the X-ray crystal data and thus in the displacements determined using NSD the heme conformations for each type of protein are remarkably similar. Individual displacements along the normal deformations as large as 1.0 Å are observed. Although only three proteins are shown in Fig. 4 the conformation is generally conserved for all known crystal structures of a particular protein.For example the hemes of all of the more than 25 peroxidases and their mutants are predominately saddled as Fig. 4 indicates. (Prostaglandin synthase which is also a peroxidase is an exception.) It is clear from the NSD results that the heme conformation is conserved in many instances and thus may be expected to play a role in the function of these enzymes. An alternative view is that the structure of the heme may have no functional significance but simply reflects the protein’s tertiary structure which is known to be remarkably similar for proteins within a class. From this point of view the NSD analysis of the hemes in proteins provides a useful probe of the protein’s tertiary structure at the active site.That is the heme serves as a reporter group for the structure of the protein. In either case NSD analysis is necessary in order to further develop spectroscopic methods for precisely determining the conformations of porphyrins in hemoproteins. 1.3 Occurrence and characterization of the nonplanar structures in synthetic porphyrins A large number of studies of model nonplanar porphyrins has given new insight into the conformational flexibility of porphyrins and the energetics of the interactions necessary to produce nonplanar distortions.6,8–13,16 Some of these nonplanar porphyrins simulate the structures of hemes in specific proteins. Over the last decade a variety of porphyrins with nonplanar structures has been synthesized.At least four methods have been used to induce nonplanarity in porphyrins. One method of inducing nonplanarity is by substituting sterically bulky groups at some or all of the peripheral positions of the macrocycle. Even for tetra-substituted porphyrins large distortions from planarity are observed when the substituents are sufficiently bulky. Fig. 5 (top) shows the ruffling and doming that occurs for Fig. 5 Crystal structures of the mono-pyridine complex of zinc(ii) tetra-tertbutylporphyrin and thallium(iii) tetraphenylporphyrin iodide. Structure taken from the Cambridge Structural Database. the Zn(py) complex of meso-tetra(tert-butyl)porphyrin.16 Octasubstituted porphyrins with bulky groups at the b-pyrrole positions are also nonplanar.Nonplanarity relieves the steric strain by increasing the volume available for the substituent groups to occupy. More highly substituted porphyrins generally exhibit large distortions even for relatively small substituents. For example dodeca-substituted porphyrins are highly sterically crowded at the periphery of the macrocycle and consequently they show large deviations from planarity. Good examples are dodecaphenylporphyrin,11,17 octaethyltetraphenylporphyrin, 12 and octabromotetraphenylporphyrin18 shown in Fig. 6. The structures of these porphyrins are generally ruffled or saddled or a combination of these deformations. A second method that has been used to create nonplanar porphyrins is to incorporate very small [e.g.Ni(ii)] or very large [e.g. Ag(ii)] metal ions into the porphyrin core. When the optimum metal–nitrogen(pyrrole) distance for a metal is at Fig. 6 X-ray crystal structures of nickel(ii) octaethyltetraphenylporphyrin (OETPP) free base dodecaphenylporphyrin (DPP) and nickel(ii) octabromotetramesitylporphyrin taken from the Cambridge Structural Database variance with optimum core size of the planar porphyrin macrocycle ( ~ 2.01 Å) then the macrocycle distorts to accommodate the metal ion. The deformations can be of the inplane [Sn(iv)] or the out-of-plane [Ni(ii) Ag(ii)] variety. While ruffling and saddling often are necessary to accommodate small metals doming is often observed for large metals. The doming for large metals is illustrated by the X-ray structure of Tl(iii)TPP iodide in Fig.5.16 Doming is usually small as seen in Fig. 5 because of the large energy required for deformation along the dom normal coordinate. A third strategy for inducing nonplanar distortion is to incorporate ‘straps’ or ‘basket-handles’ between peripheral substitution sites that are too short for a planar porphyrin. Chandrashekar Ravikanth and coworkers have most recently investigated and reviewed the properties of these nonplanar porphyrin models.19 Axial ligand–metalloporphyrin interactions are also known to induce nonplanar distortions of the porphyrin. This effect 35 Chemical Society Reviews 1998 volume 27 may also play a role in the doming of the thallium porphyrin shown in Fig. 5. Safo et al.have used X-ray crystallography and other techniques to investigate the electronic and steric forces influencing axial ligand orientation in porphyrins.20 These porphyrin systems serve as models for the observed eclipsed (parallel) and staggered (perpendicular) orientations of the axial histidine ligands of hemes in proteins. In these systems the steric crowding at the periphery is small thus the conformation of the macrocycle is directly influenced by the weak steric interactions of the axial ligands with the macrocycle. The results obtained strongly support the suggestion that axial ligand orientation may alter the spectroscopic and redox properties of heme proteins. Our group has investigated axial ligand orientation effects in highly substituted porphyrins for which the deformation of the macrocycle is determined by the substituents rather than by the axial ligands.21 For the bis-ligand complex of Co(iii) octaethyltetraphenylporphyrin which has the sad macrocyclic conformation the planes of the ligands are perpendicular to each other and aligned with the metal–nitrogen bonds i.e.along the saddle. In contrast for the bis-ligand complexes of Co(iii) tetra(tert-butyl)porphyrin which has the ruf conformation the planes of the ligands are perpendicular and aligned with the direction of opposite meso carbons and the groove formed by the ruf macrocycle.21 The observed orientations of the ligands are thus those that minimize steric interactions with the macrocycle. In some cases steric interactions between the ligand and porphyrin were sufficient to result in hindered rotation of the axial ligands.0 at 561 nm and Ni 0 at 648 nm. The red shifts are a result 2.1 Novel spectroscopic properties and consequences of nonplanar distortion of porphyrins 2.1.1. Photophysical properties UV–visible absorption spectra The most commonly observed spectroscopic consequence of porphyrin nonplanarity is a red shift in the p–p* absorption bands in the UV–visible spectrum. Shifts in the Soret or B band typically near 400 nm of as much as 50 nm have been observed as a result of nonplanar distortion. For example the Soret band of NiTPP a mixture of planar and nonplanar conformers is at 424 nm while highly ruffled Ni meso-tetraadamantylporphyrin has its Soret band at 478 nm a 54 nm red shift.The size of the red shift is proportional to the magnitude of the distortion albeit in a nonlinear fashion.6 For a series of ruffled Ni tetraalkylporphyrins with increasingly bulky alkyl substituents ruffling angles (angle between the planes of adjacent pyrroles) of up to 20° produce only small shifts; however small increases in the ruffling angle beyond 20° begin to give large red shifts. A similar nonlinear relationship holds when the normal coordinate displacements are used to quantify the deformation. This must be the case since there is a linear relationship between ruffling angle and the size of the ruf deformation.6 In addition other types of distortions besides ruffling have a similar but not quantitatively equivalent effect on the absorption spectrum.13,22 The Q band located in the red region of the absorption spectrum red shifts to an even larger extent than the B (Soret) band. For example NiTPP has Q adamantylporphyrin has Q of a decrease in the energy separating the filled a1u(p) and a2u(p) orbitals and the empty eg(p) orbitals of the macrocycle. 6,13 In addition to the spectral shifts a broadening of the absorption bands is usually observed as the porphyrin becomes more distorted. Iron porphyrin complexes have not been investigated sufficiently. Many questions remain as to whether the spectroscopic markers of nonplanar conformation primarily deduced for nickel porphyrin complexes can be reliably used for iron porphyrins.For example the oxidation and spin state of the iron atom likely influence the dependence of these absorption bands Chemical Society Reviews 1998 volume 27 36 on the magnitude of distortion. Systematic studies of a series of progressively more distorted iron porphyrins are needed. 2.1.2 Non-linear optical properties Nonlinear optical properties such as optical limiting and harmonic generation are closely connected with molecular optical spectral properties. Various molecular factors such as p-delocalization length donor-acceptor groups conformation and orientation influence the nonlinear optical properties of porphyrins. Porphyrins including some nonplanar Cu baskethandle porphyrins,23 have been investigated as nonlinear optical materials.Tailoring the nonplanarity of porphyrins offers a method for conformational control over optical properties such as lifetimes and intersystem crossing rates which influence nonlinear optical properties. In addition photo-conversion between stable nonplanar conformers provides another opportunity for controlling nonlinear optical properties. 2.1.3 Luminescence spectra Ravikanth Chandrashekar and coworkers19 and Holten et al.24 have investigated the fluorescence from nonplanar baskethandle porphyrins and sterically crowded porphyrins respectively. They demonstrated that the quantum yields for fluorescence are reduced as a consequence of the nonplanarity. The reduced yield is a result of the decreased lifetime of the excited singlet states which in turn is a result of increased intersystem crossing to the triplet manifold and increased nonradiative decay rates.2.1.4 Vibrational spectra resonance Raman and IR spectroscopy Resonance Raman spectroscopy has proved to be one of the best probes of the conformation of the porphyrin. In fact it was the first technique to detect,25 and the only method currently known that can quantify the conformational equilibrium between planar and nonplanar conformers of some metal porphyrins. 8,9,10,13,25 We now outline the major conclusions of recent resonance Raman studies of model nonplanar porphyrins. Our group’s contribution6,8,10–13,15,25 to the development of resonance Raman spectroscopy as a means of quantifying the type and magnitude of distortion began in 1988 when we investigated a new crystalline form of NiOEP.At that time two other crystalline forms were known—a ruffled form and a planar form. The new crystal morphology also exhibited a planar porphyrin ring. Having three crystals with known porphyrin conformations we used single-crystal resonance Raman spectroscopy to verify that certain structure-sensitive Raman lines could be used as indicators of the nonplanar conformation of the porphyrin.26 A year later we found that NiOEP in solution is a roughly equal mixture of planar and nonplanar conformers,25 identifying these solution forms with the planar and nonplanar crystalline forms investigated earlier. This finding was subsequently confirmed in a study by Czernuszewicz and coworkers.13 We later showed this to be a general property of b-pyrrole-substituted Ni porphyrins including biological porphyrins like protoporphyrin.9 We also showed that the environment of Ni porphyrin influences the equilibrium between these conformers. Specifically the formation of p–p dimers and insertion into the active site of hemoglobin and its a-subunits shift the equilibrium in favor of one conformer or the other. In particular binding NiProtoP to the hemoglobin active site forces the macrocycle to be nearly planar. This occurs for the hemoglobin binding sites that do not have the proximal histidine coordinated to the Ni atom. This was a particularly interesting finding in that the protein was shown to exercise direct control over the macrocycle structure through only nonbonding interactions.Subsequent to these findings we published a series of Raman studies6,11 of nonplanar synthetic porphyrins and the Nicorphinoid cofactor F430 of methylreductase.10 F430 has a highly 430 is reduced porphyrin ligand. In the enzyme F430 catalyzes methylcoenzyme M reduction. The reduced porphyrin ring of F thought to facilitate changes in the size and oxidation state of the Ni atom by increasing the out-of-plane flexibility of the macrocycle.27 The principal result of our studies with model nonplanar porphyrins has been the elucidation of the role of the substituents and their orientations in determining the macrocycle conformation. In addition these Raman studies have also clarified the existence of stable nonplanar conformational isomers at higher energy than the ground-state conformer.With regard to the latter these stable conformers (local minima) are sometimes thermally occupied at room temperature.7,21 The studies of the model compounds have also provided some useful correlations between the frequencies of structure-sensitive Raman lines and the degree of nonplanarity of the macrocycle. 6,11,12 Some of the different types of deformations in Fig. 2 were examined in more recent Raman investigations.6,13 For example a series of nickel meso-tetraalkyl-substituted porphyrins with alkyl groups of varying steric size exhibit nearly pure ruf deformations.6 The magnitude of the distortion increases with the bulkiness of the substituent so that the variations of the Raman-line frequencies and absorption-band positions were determined and correlated with molecular mechanics structural parameters and transition energies calculated using INDO/s semiempirical methods.A study of 5,15-dialkyl substituted porphyrins investigated the consequences of the addition of a dom deformation component to a ruf deformation.13 Differing dependencies of the Raman frequencies on the magnitude of distortion are found when the dom and ruf deformations are both present compared with when only the ruf deformation occurs. Both calculated and X-ray structures were available for direct comparison in this study. Because almost all substituted Ni porphyrins show ruffled conformers we became interested in whether Ni porphyrin (NiP) with the reduced steric requirements of its hydrogen substituents would also be nonplanar.16 In other words is at least some steric interaction of the peripheral substituents necessary to give nonplanar conformers? An X-ray crystal structure of NiP was obtained and showed a nearly planar macrocycle.The Raman spectra taken both in solution and in the crystal show that NiP also exists in solution as only the planar species.15 Other resonance Raman investigations of nonplanar porphyrins looked at the influence of the size of the central metal on nonplanar structure by using resonance Raman spectroscopy. 6,8,12 These studies also served to develop suitable forcefield parameters for the molecular mechanics calculations for additional metals including Co(ii) Cu(ii) Zn(ii) and Fe(iii).The results show that large metals reduce the magnitude of the distortion for sterically crowded porphyrins. These studies also explained why small metals reduce the slope of the well-known core-size correlations for the structure-sensitive Raman lines. For porphyrins that coexist in both planar and nonplanar conformers it was also found that increasing the metal size shifts the equilibrium in favor of the planar form.8 The latter work also provides an estimate of the steric repulsion energy necessary to induce nonplanar distortion. It demonstrated that biological Fe porphyrins like FeProtoP should exhibit only the planar conformer in the absence of a perturbing protein environment.8 2.1.5 NMR spectroscopy As the prevalence of nonplanar porphyrin conformers and their possible importance in biological systems has become evident studies of nonplanar porphyrins have become more diverse.They now include a wide range of nonplanar porphyrin models and other spectroscopic probes such as NMR. Proton NMR studies of model nonplanar porphyrins have revealed the presence of numerous dynamic processes including inversion of the porphyrin macrocycle hindered rotation of aryl or alkyl substituents at the meso or b-pyrrole positions and hindered rotation of axial ligands.21 Some of these processes are unique to nonplanar porphyrins. Proton NMR studies have also been used to determine the solution structures of cobalt(ii) complexes of nonplanar porphyrins and most recently to measure the effect of nonplanarity on the porphyrin ring current.Using a double-dipole model of ring current effects it was shown that nonplanarity caused little if any decrease in the ring current even for extremely nonplanar sad or ruf porphyrins compared with planar porphyrins. 2.1.6 Electron paramagnetic resonance EPR spectroscopy of copper octaethyltetraphenylporphyrin has added support to the proposal by Reed Scheidt and coworkers that nonplanarity controls the magnetic coupling between paramagnetic metals and the macrocycle radical cation giving antiferromagnetic coupling with the cation for highly nonplanar porphyrins.28 This was also verified by Ravikanth Chandrashekar and coworkers using other techniques.19 Time-resolved EPR measurements of the photoexcited triplet states of free base and zinc derivatives of OETPP reveal fast exchange between different conformers suggesting fluxional excursions from the saddled ground-state conformer observed in X-ray crystal structures.29 2.2 Novel functional properties of nonplanar synthetic porphyrins 2.2.1 Redox potentials The most studied influence of nonplanarity on porphyrin chemistry is its effect on redox potentials.Fajer et al.,3 Ravikanth and Chandrashekar,19 and Reddy have shown that nonplanar porphyrins are easier to oxidize and harder to reduce than planar porphyrins. This was initially shown for ZnOETPP,3 but the same trend has now been verified for many other nonplanar porphyrins.In a particularly striking example of this effect it was shown for a series of increasingly brominated tetraphenylporphyrins (Fe and H2 BrxTPP x = 0 to 8) that the porphyrin initially becomes harder to oxidize due to the electron withdrawing ability of the added bromine substituents but subsequently becomes easier to oxidize as the added bromines make the porphyrin more nonplanar.30 Electron withdrawing groups appear to effect oxidations and reductions equally. Kadish and others have also shown that nonplanarity affects the site (metal or ring) of oxidation30 and whether 2-electron versus 1-electron oxidation is observed. 2.2.2 Axial ligand affinity Our group has shown that the affinity of nickel porphyrins decreases as the ruffling of the macrocycle increases if the electron-withdrawing capabilities of the substituents are held constant.Based on resonance Raman spectra Desbois et al. have associated the increased macrocycle distortion in a series of strapped hemes with a decrease in O2 ligand off-rates. The decrease does not occur for CO off-rates in the models. Thus heme distortion provides a mechanism for differentiating CO and O2 binding in hemoglobins and myoglobin.31 2.2.3 Iron spin states Novel spin states have been suggested to result from distorted hemes. FeOETPPCl has been claimed to be in a quantum-mixed S = 5/2 3/2 intermediate spin state32 as have some cytochromes cA and peroxidases all of which have been noted to have saddled deformations. 2.2.4 Chirality Some porphyrins that are not chiral when planar become chiral when nonplanar.The chirality results from an out-of-plane location of the substituents. Furthermore inversion of the macrocycle geometry and substituent rotation result in racemization. A good example is the single-armed porphyrin derived 37 Chemical Society Reviews 1998 volume 27 by mono-meso-substitution of metalloetioporphyrin I with a single pivaloylamino group [(CH3)3CCONH].33 Mono-substituted etioporphyrin I though asymmetrically substituted with alternating methyl and ethyl groups at the b-pyrrole carbons is not chiral when planar since the mirror image can be superimposed by a C2 rotation. However steric interaction with the adjacent substituents moves the meso substituent out-ofplane making the porphyrin nonplanar and the porphyrin becomes chiral.The enantiomers can be separated by chiral chromatography and slow racemization occurs at room temperature by flipping of the bulky arm through the macrocycle mean plane and rotating about the meso position. Free base porphyrins apparently racemize more rapidly than metalloporphyrins and a metal dependence of the racemization rate is observed. A number of workers have also reported photoinduced atropisomerization of these chiral porphyrins i.e. the racemization rate increases on radiation by visible light. Aida and coworkers have very recently shown that a chiral nonplanar porphyrin can sense the chirality of asymmetric carboxylic acids and retain a memory of the chirality of the acid even after the asymmetric acid has been removed.2.2.5 Excited state lifetimes The lifetimes and other dynamic photophysical properties of the excited states of porphyrins are altered by nonplanarity.19,24 For example the formation of the excited (d,d) state via the (p,p*) state of NiDPP (see Fig. 6) exhibits complex spectral evolution involving both cooling and conformational changes. The results are interpreted in terms of photoinduced access to multiple lowenergy nonplanar conformers. 2.2.6 Basicity and metallation rate Nonplanarity alters the basicity of the porphyrin nitrogen atoms and the rate constants for metal insertion. For example proton dissociation constants for the mono- and di-cations of dodecaphenylporphyrin are at least 109 times less than those for TPP.17 Copper insertion in the nonplanar DPP system is accelerated by a factor of 6 3 105.17 Moreover increased metal insertion rates have been observed for nonplanar porphyrins with substituents that decrease the basicity of the nitrogen atoms e.g.metal insertion is faster by a factor of 102–103 in Br8TPP versus TPP. Nonplanarity also appears to be responsible for the unusual optical spectra of H2DPP in certain solvents where it has been suggested that the exposed porphyrin NH protons can hydrogen bond with solvent molecules.17 3.1 Non-protein environmental influences on porphyrin conformation 3.1.1 p– pAggregation and complexation The formation of p–p dimers usually results in a flattening of the porphyrin macrocycle.9 Presumably the stacking of porphyrins is favored when the macrocycles are nearly planar.The effect is most clearly seen for porphyrins that exist in solution as a mixture of planar and nonplanar species. Aggregation causes the equilibrium to shift in favor of the planar conformer. Similarly p–p complex formation has an influence on the equilibrium between planar and nonplanar conformers. For highly nonplanar porphyrins p–p aggregation and complex formation is apparently disfavored or brings about unusual aggregation behavior. Specifically cobalt(ii) complexes do not stack with 1,3,5-trinitrobenzene. In addition p–p dimerization does not occur for some dodeca-substituted porphyrins most likely because of the interference caused by the nonplanar structure and the bulky substituents surrounding the macrocycle.This leads to some unusual properties when nonplanar porphyrins are incorporated into surfactant micelles or when the porphyrin moiety of a lipoporphyrin is nonplanar. 13,34 The altered aggregation properties can lead to interesting polymers of porphyrins as in the case of zinc octaethyltetranitroporphyrin.16 Chemical Society Reviews 1998 volume 27 38 3.1.2 Surfactant interactions Incorporation of porphyrins into detergent micelles and films also influences the equilibrium between planar and nonplanar conformers.34 Often the micellar environment induces a shift toward the planar conformer; however in general the effect on porphyrin structure depends on the nature of the detergent molecules.For example incorporation into cholate micelles induces a shift toward nonplanarity. 3.2 Protein influences on porphyrin conformation and possible roles of nonplanar conformers in protein function Since the isolated heme group is nominally planar the distortions evident in Figs. 3 and 4 are a consequence of the protein environment of the heme. In some cases it is easy to see how the protein might strongly influence the macrocyclic structure. In particular hemes that are covalently linked to amino acid residues such as those of the cytochromes c and myeloperoxidase might easily be distorted through these strong interactions with the protein. Indeed the hemes in these proteins show some of the largest nonplanar distortions.4,5,7 On the other hand many proteins that lack covalent connections to the heme also show large deviations from planarity.A specific example of the latter case is the peroxidases whose deformations are characterized in Fig. 4. In fact the crystal structures of myeloperoxidase and all of the approximately 50 peroxidases exhibit a strong saddling of the heme. 3.2.1 Mitochondrial cytochrome c Examination of Fig. 3 shows that the most out-of-plane atom of the heme for yeast cytochrome c is the meso-carbon between pyrroles I and II. This is a characteristic feature of the nonplanar conformation of the hemes of most c-type cytochromes and has led to the suggestion that the covalent attachments to the protein at the 2- and 4-positions of pyrroles I and II cause the distortion from planarity.4 Specifically the short protein segment between the cysteine residues could contract the distance between the thioether linkages to the heme causing the porphyrin to buckle.In particular we proposed4 that the hydrogen-bonding network in this segment might act to contract this protein segment giving rise to the nonplanar distortion. Molecular mechanics calculations show that the heme-pentapeptide unit alone does account for the ruf wav(x) and wav(y) components of the observed heme distortion. 3.2.2 Cytochrome c3 Additional support for this structural hypothesis comes from the NSD results for other cytochromes. The NSD results for the four-heme cytochromes c3 reproduced in Fig. 7 are particularly convincing.5,7 Hemes 2 and 4 typically have four intervening residues between the cysteines whereas hemes 1 and 3 have only two intervening residues as for the mitochondrial cytochromes c.The relationship between the short segment and the heme conformation does not simply correlate with the number of residues but depends on the detailed assembly of the amino acids in the peptide unit. Fig. 7 shows that the heme conformations for these proteins are generally conserved for different strains even though there is very little amino acid sequence identity among these proteins. In fact excluding the eight cysteines and eight histidines bound directly to the hemes only a handful of residues (out of more than 110) are conserved for the strains listed.The maintenance of the conformation of the heme given so little sequence identity between the strains suggests that only a small portion of the protein is required to generate the major part of the distortion. The NSD results for heme 4 directly point to the short segment that includes the cysteines the intervening residues and the adjacent histidine ligand. Notice that hemes 4 of the baculatum strains (Fig. 7) are distinctly different from those of the other hemes 4. The structural origin of this difference is likely the result of the differences in the number and folding of the amino acids between the cysteines for heme 4; the Fig. 7 NSD results for X-ray crystal structures of cytochromes c3 from four different strains of Desulfovibrio desulfuricans (ATTC 27774) Desulfovibrio vulgaris (Miyazaki Hildenborough) Desulfomicrobium baculatum (Norway 4) and Desulfovibrio gigas baculatum strains have two residues not the four residues common to the other strains.3.2.3 Microperoxidase Desbois et al.31 have investigated Fe(iii) microperoxidase-8 using resonance Raman spectroscopy. Microperoxidase is a digestion product of cytochrome c that leaves only a covalently attached 8-amino acid segment of the protein. Resonance Raman spectroscopy shows that by itself the short protein segment induces a nonplanar distortion of the heme. 2 3.2.4 Cytochrome c The influence of conserved amino acids on the structure of the heme of the cytochrome c2 has been examined by Desbois et al.31 They find evidence for structural heterogeneity in the macrocycle distortion for the wild type protein in both oxidation states although higher deformability is found for the ferric state.Similar conformational flexibility was observed for microperoxidase. The distortion of the macrocycle is sensitive to mutation of some conserved residues particularly tryptophan 67 which is hydrogen bonded to one of the heme propionates. The distortion could have an obvious influence on the redox 39 Chemical Society Reviews 1998 volume 27 potential but in the case of R. capsulatus cytochrome c2 the distortion is not large (Fig. 4). 3.2.5 Cytochrome oxidase Hildebrandt et al. have noted species-specific differences in the distortion of the macrocycle in the hemes a of cytochrome oxidases.35 Specifically for the oxidized heme a of Paracoccus denitrificans oxidase Raman frequency differences are interpreted in terms of a nonplanar porphyrin structure for the Paracoccus enzyme but a planar conformation for the beef heart oxidase.Differences in the oxidases are also observed by EPR. The conformational changes influence the formyl substituent and its electronic coupling with the porphyrin ring. Since the hemes a are apparently planar in the fully reduced oxidases a more large-scale redox-linked conformational transition is indicated for the bacterial oxidase. Flattening of the heme is the main conformational change in the bacterial enzyme upon reduction and it occurs because of a reorganization of the protein surroundings of the heme.The protein reorganization also influences the interaction with the heme a formyl substituent. No significant redox-linked conformational changes can be inferred for heme a3. The heme a protein transition could have significance for the function of oxidase. 3.2.6 Nickel-reconstituted hemoglobin Resonance Raman studies of nickel-reconstituted hemoglobin and myoglobin further underline the influence of the protein on the conformation of the heme.9 Nickel protoporphyrin in solution exists as an equilibrium mixture of planar and nonplanar conformers. Upon binding to the active site of hemoglobin only the planar conformer is observed. This is true even for the NiProtoP molecules that are bound in the active site but not coordinated to the proximal histidine.NiProtoP is known to be in the active site because the histidine ligand is transiently acquired during 10 ns pulsed photo-excitation. These Raman results show that the heme-binding site strongly favors a planar rather than a ruffled macrocyclic conformation more so in hemoglobin than in the isolated a-subunits. Further since the ruffled form has lower axial ligand affinity its prevalence in the a-subunits of hemoglobin may account for their lower affinity for proximal histidine. This macrocyclic distortion effect on axial ligand affinity is similar to the noted influence of macrocyclic distortion on the relative CO and O2 affinities of strapped heme models. 3.2.7 Methyl-coenzyme M reductase Methyl reductase is the terminal enzyme in methanogenesis.The enzyme has an (abg)2 protein composition and contains two molecules of coenzyme F430 coenzyme M (mercaptoethane sulfonate) and coenzyme B (mercaptoheptanoyl threonine phosphate). It catalyzes the final step in the methanogenic pathway reducing methyl-coenzyme M to methane and forming the disulfide coenzyme M–S–S–coenzyme B. The nickel atom of cofactor F430 is apparently coordinated to an oxygen atom of a glutamine amino acid. Coenzymes M and B are located on the other side of the Ni-corphin plane at the active site. Cofactor F430 is the prosthetic group of the enzyme. Its nickel atom is reduced to Ni(i) in the catalytic cycle which entails a large change in metal size. The reduction of the porphinoid ring increases the out-of-plane flexibility for F430 permitting facile changes in the metal core size.Nonplanarity gives F430 the ability to accommodate large changes in core size and may also influence its axial ligand affinity.27 Ligand affinity differences are noted for the F430 and its 12,13-di-epimer and these differences may be attributable to differences in macrocycle conformation resulting from epimerization.36 Multiple coexisting nonplanar conformers have been observed by resonance Raman scattering for model nickel hydrocorphinates related to cofactor F430.36 The Ni corphinate models also show altered Chemical Society Reviews 1998 volume 27 40 photodynamics compared with Ni porphyrins and this too has been attributed to the corphin’s higher out-of-plane flexibility.3.2.8 Photosynthetic pigments Although this review has focused primarily on nonplanarity in heme proteins and model porphyrins crystallographic structures of antenna and reaction center proteins have revealed nonplanar conformations for many of the photosynthetic pigments. Fajer and coworkers in particular have pointed out that the protein may provide a microenvironment that defines a protein scaffolding which controls the out-of-plane conformation of the bacteriochlorophylls.3,28 Further many experimental and theoretical studies have demonstrated that modulating the conformation can vary the optical redox and electron-transfer properties of the photosynthetic chromophores. Temperaturedependent conformational changes in the bacteriopheophytins of Rhodobacter sphaeroides reaction centers that are detected by Raman spectroscopy have been related to changes in function.37 Undoubtedly the entire microenvironment including chromophore conformation will be involved in directing the flow of energy and electrons in photosynthetic proteins.4 Summary New tetrapyrrole-containing proteins are being found each year generating the continued interest in an understanding of the molecular basis of their function. Studies of synthetic nonplanar porphyrins are providing an improved understanding of the porphyrin’s role in the biological function of these proteins. Conserved heme structural motifs within the proteins indicate the importance and richness of the heme’s share in determining the function of these versatile prosthetic groups.In addition nonplanar porphyrins may have practical uses in biomimetic processes and other commercial applications. Continued experimental and theoretical investigations of nonplanar porphyrins may lead to important new processes materials and technological applications in electronics photonics catalysis and other chemical technologies. 5 References 1 H. Senn and K. Wuthrich Q. Rev. Biophys. 1985 18 111. 2 B. M. Hoffman The Porphyrins ed. D. Dolphin Academic New York 1979 vol. VII p. 403. 3 K. M. Barkigia L. Chantranupong K. M. Smith and J. Fajer J. Am. Chem. Soc. 1988 110 7566. 4 J. D. Hobbs and J. A. Shelnutt J. Protein Chem. 1995 14 19 and references therein. 5 W. Jentzen X.-Z.Song and J. A. Shelnutt J. Phys. Chem. B 1997 101 1684 and references therein. 6 W. Jentzen M. C. Simpson J. D. Hobbs X. Song T. Ema N. Y. Nelson C. J. Medforth K. M. Smith M. Veyrat M. Mazzanti R. Ramasseul J. C. Marchon T. Takeuchi W. A. Goddard III and J. A. Shelnutt J. Am. Chem. Soc. 1995 117 11085 and references therein. 7 W. Jentzen J.-G. Ma and J. A. Shelnutt Biophysical J. 1997 in press and references therein. 8 K. K. Anderson J. D. Hobbs L. A. Luo K. D. Stanley J. M. E. Quirke and J. A. Shelnutt J. Am. Chem. Soc. 1993 115 12346 and references therein. 9 R. G. Alden M. R. Ondrias and J. A. Shelnutt J. Am. Chem. Soc. 1990 112 691 and references therein. 10 J. A. Shelnutt J. Phys. Chem. 1989 93 6283. 11 J. A. Shelnutt C. J. Medforth M.D. Berber K. M. Barkigia and K. M. Smith J. Am. Chem. Soc. 1991 113 4077. 12 L. D. Sparks C. J. Medforth M. S. Park J. R. Chamberlain M. R. Ondrias M. O. Senge K. M. Smith and J. A. Shelnutt J. Am. Chem. Soc. 1993 115 581. 13 X.-Z. Song W. Jentzen S.-L. Jia L. Jaquinod D. J. Nurco C. J. Medforth K. M. Smith and J. A. Shelnutt J. Am. Chem. Soc. 1996 118 12975 and references therein. 14 O. Q. Munro H. M. Marques P. G. Debrunner K. Mohanrao and W. R. Scheidt J. Am. Chem. Soc. 1995 117 935. 15 W. Jentzen E. Unger X.-Z. Song S.-L. Jia I. Turowska-Tyrk R. Schweitzer-Stenner W. Dreybrodt W. R. Scheidt and J. A. Shelnutt J. Phys. Chem. A 1997 101 5789. 16 Cambridge Structural Database and references therein. 17 J. Takjeda and M. Sato Chem.Lett. 1995 11 971 and references therein. 18 P. Bhyrappa V. Krishnan and M. Nethaji J. Chem. Soc. Dalton Trans. 1993 1901. 19 M. Ravikanth and T. K. Chandrashekar Structure and Bonding 1995 82 105 and references therein. 20 M. K. Safo F. A. Walker A. M. Raitsimring W. P. Walters D. P. Dolata P. G. Debrunner and W. R. Scheidt J. Am. Chem. Soc. 1994 116 7760 and references therein. 21 C. J. Medforth C. M. Muzzi K. M. Shea K. M. Smith R. J. Abraham S.-L. Jia and J. A. Shelnutt J. Chem. Soc. Perkin Trans. 2 1997 833 and references therein. 22 M. W. Grinstaff M. G. Hill E. R. Birnbaum W. P. Schaefer J. A. Labinger and H. B. Gray Inorg. Chem. 1995 34 4896. 23 A. Sevian M. Ravikanth and G. R. Kumar Chem. Phys. Lett. 1996 263 241. 24 C. M. Drain C. Kirmaier C. J. Medforth D. J. Nurco K. M. Smith and D. Holten J. Phys. Chem. 1996 100 11984 and references therein. 25 R. G. Alden B. A. Crawford R. Doolen M. R. Ondrias and J. A. Shelnutt J. Am. Chem. Soc. 1989 111 2070 and references therein. 26 T. D. Brennan W. R. Scheidt and J. A. Shelnutt J. Am. Chem. Soc. 1988 110 3919. 27 A. Eschenmoser Ann. N. Y. Acad. Sci. 1986 471 108 and references therein. 28 M. W. Renner K. M. Barkigia Y. Zhang C. J. Medforth K. M. Smith and J. Fager J. Am. Chem. Soc. 1994 116 8582 and references therein. 29 A. Regev T. Galili C. J. Medforth K. M. Smith K. M. Barkigia J. Fajer and H. Levanon J. Phys. Chem. 1994 98 2520. 30 K. M. Kadish E. Van Caemelbecke F. Dsouza C. J. Medforth K. M. Smith A. Tabard and R. Guilard Inorg. Chem. 1995 34 2984 and references therein. 31 S. Othman J. Fitch M. A. Cusonovich and A. Desbois Biochemistry 1997 36 5499 and references therein. 32 R. J. Cheng P. Y. Chen P. R. Gau C. C. Chen and S. M. Peng J. Am. Chem. Soc. 1997 119 2563. 33 K. Konishi K. Miyazaki T. Aida and S. Inoue J. Am. Chem. Soc. 1990 112 5639 and references therein. 34 X. Song M. Miura X. Xu K. K. Taylor S. A. Majumder J. D. Hobbs J. Cesarano and J. A. Shelnutt Langmuir 1996 12 2019 and references 35 G. E. Heibel P. Hildebrandt B. Ludvig P. Steinrucke T. Soulimane 36 A. K. Shiemke J. A. Shelnutt and R. A. Scott J. Biol. Chem. 1989 264 37 J. M. Peloquin C. A. Violette H. A. Frank and D. F. Bocian therein. and G. Buse Biochemistry 1993 32 10866. 11236 and references therein. Biochemistry 1990 29 4892. Received 18th July 1997 Accepted 30th September 1997 41 Chemical Society Reviews 1998 volume 27

ISSN:0306-0012    

DOI:10.1039/a827031z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

The biomedical chemistry of technetium and rhenium Jonathan R. Dilwortha and Suzanne J. Parrottb a Inorganic Chemistry Laboratory University of Oxford South Parks Road Oxford UK OX1 3QR b Department of Biological and Chemical Sciences University of Essex Wivenhoe Park Colchester UK CO4 3SQ This review describes recent developments in the chemistry of both first and second generation 99m-technetium-based imaging agents. The material is presented according to the biological target for the agent and where possible actual images are presented to indicate the type of information available to the clinician. Beta emitting isotopes of rhenium offer a possible method for the in situ treatment of cancerous tissue using analogous targeting strategies to those for technetium.Recent developments in the relevant coordination chemistry of rhenium and their extension to in vitro and in vivo studies are presented. 1 Introduction Modern medicine demands progressively more sophisticated methods for the accurate diagnosis of disease states and there is a massive worldwide research effort into developing and improving imaging techniques. Images can be produced either by measuring the absorption of externally applied radiation (e.g. X-ray ultrasound MRI imaging) or by administering a small amount of a radioactive compound and detecting the radiation escaping from the body. All of these techniques enhanced by computerised tomographic methods can produce remarkably high quality images of locations deep inside the body.To an extent these techniques are complementary and the method selected will depend not only on the type of image required but also on other factors such as the availability of equipment. Nuclear medicine has traditionally been favoured for imaging biological function and while some of this area is being taken over by developments in MRI SPECT (single photon emission computerised tomography) and PET (positron emission tomography) remain the methods of choice for imaging low capacity high density receptors. The use of external radiation for treatment of cancer is extremely well developed but now the greater ability to target radiopharmaceuticals has led to the Jon Dilworth graduated from the University of Oxford in 1967 and then studied for a DPhil degree at the Unit of Nitrogen Fixation University of Sussex with Professor Joseph Chatt.After a period on the permanent staff of the Unit he took the Chair of Chemistry at the University of Essex in 1985 and has now taken a post in the Inorganic Chemistry Laboratory University of Oxford (September 1997). His research interests involve the applications of coordination chemistry in biology medicine and catalysis. Outside he hopes one day to master a topspin tennis backhand and a reliable method to get out of bunkers. Suzanne Parrott Jon Dilworth O CH3 N B N O O N Tc O H N H N N O O CI possibility of using b-emitting compounds to deliver radiation in situ to cancer sites. In this review we attempt to present a brief account of the major developments in the use of g-emitting technetium complexes for imaging and recent work directed towards producing b-emitting rhenium compounds for therapeutic use.This is given very much from a chemical perspective although we have wherever possible given examples of the type of diagnostic image that can be produced. The restriction of space means that it cannot be comprehensive and the material has been selected to provide what are hopefully interesting illustrations of the potential scope of the radioactive isotopes of Tc and Re for diagnosis or therapy. Technetium-99m although heavily used (over 90% of all diagnostic nuclear medicine imaging studies carried out worldwide use this isotope) is only one of a range of metallic radionucleides used for medical imaging or therapy but space restricts our coverage here to Tc and Re.There are several reviews available on Tc and Re chemistry1–4 and specialised texts5 and published conference proceedings6 provide more comprehensive surveys of the medical applications of these elements. 2 Technetium The element was first predicted by Mendeleev (ekamanganese number 43) and first isolated by Segr�e and Perrier in 1938. It was separated from a molybdenum target plate that had been bombarded with deuterons in the Berkeley cyclotron. Currently there are no fewer than 20 known isotopes (91Tc–110Tc) and seven nuclear isotopes.7 The 99mTc nuclear isotope is used for medical imaging due to its near ideal nuclear characteristics of a 6 h halflife and g-ray emission energy of 141 keV.The diagram in Fig. 1 shows the radioactive decay series involving the medically important Tc isotopes. The practical use of 99mTc for regular imaging depends totally on the ready availability of Suzanne Parrott received a PhD in Chemistry from the University of Essex in 1993 in the area of rhenium coordination chemistry. After working as a Post-doctoral Fellow at The duPont Merck Pharmaceutical Company in Massachusetts USA she returned to the University of Essex as a Senior Research Officer and is currently a Lecturer in Chemistry at the University. Her research interests include rhenium and technetium coordination chemistry with applications to nuclear medicine.43 Chemical Society Reviews 1998 volume 27 99mTc beta 66 h 88.75% 99Ru gamma 6 h 142 keV 99Mo beta 66 h 12.5% beta 2.14 x 105 y 0.292 meV 99Tc Fig. 1 the isotope using the 99Mo/99mTc generator developed in Brookhaven in the early 1960s. This consists of [99MoO4]2 absorbed at the top of an alumina ion exchange column. The 99Mo decays continuously to 99mTc which can be preferentially eluted with physiological saline (0.15 m NaCl) over a period of 7–10 days. A typical eluate used to prepare an imaging agent will be about 1027 to 1028 m in [TcO4]2. The chemical consequences are that the synthesis of radiopharmaceuticals has to proceed in very dilute aqueous solution directly from [TcO4]2. The very dilute nature of the [99mTcO4]2 solutions means that characterisation of complexes by routine spectroscopic and analytical methods is not possible and HPLC or other chromatographic methods with g detection are virtually the only way to monitor the chemistry.The very long lived b-emitting 99Tc isotope is used (typically on a 10–20 mg of technetium scale) to isolate technetium complexes and characterise them fully using the full range of spectroscopic techniques including X-ray crystallography. HPLC of the 99Tc complexes (UV and b-detection) is then used to infer the structures of the 99mTc analogues. The weak b-emitting properties of the 99Tc isotope means that complexes can be handled safely in conventional glassware with appropriate precautions. With the widespread use of nuclear reactors technetium is no longer a rare element and it has been estimated that 160 000 kg of technetium will in principle be available by the year 2000.It is a remarkable statistic that there is already more of this entirely artificial element available in the world than its stable naturally occurring congener rhenium! Very recently there have been reports of the preparation of the 94Tc isotope by irradiation of 94Mo in a cyclotron. Both the 94mTc and 94gTc isotopes are positron emitters. The ground state isomer also emits g radiation and offers the interesting possibility of also deploying SPECT. The 94mTc isotope has been used for PET imaging of the heart using an isocyanide complex (see section 2.3.2 on heart imaging below).2.1 Imaging techniques for technetium The eluate from the 99mTc generator described above is introduced by syringe via a septum into a vial containing the reagents necessary to produce the imaging agent. After a suitable incubation period the radiopharmaceutical is injected into the patient and after time for biodistribution to occur the image data is collected by a gamma camera equipped with a NaI scintillation detector and photomultiplier system (Fig. 2). The camera is rotated around the patient or a multidetector array is usedo create a tomographic image by use of a sophisticated computerised program which reconstruct the image from a series of projections. A successful imaging agent will generally direct in the order of 1–5% of the injected dose of activity to the target organ the bulk of the remainder generally being excreted via the kidneys.The total radiation dose from a technetium scan is comparable with that from a conventional X-ray. 2.2 Types of technetium imaging agents The first use of technetium for medical imaging was in 1961 and involved the use of [99mTcO4]2 for diagnosis of thyroid disease based on the principle that the pertechnetate anion would behave similarly to iodide known to be taken up by the thyroid. The biodistribution and targeting ability thus depended solely on the size and charge of the complex. This was the first of a Chemical Society Reviews 1998 volume 27 44 Fig. 2 A patient undergoing a technetium scan using a gamma camera. Reproduced with permission from Amersham International.series of the so-called ‘technetium essential’ or first generation agents. These are represented diagrammatically in Fig. 3(A) and such agents have been deployed with great success to image organs such as the heart the brain the kidney and the liver and are discussed in more detail below. However the growing demand for ever more specific agents has prompted the development of second generation agents [Fig. 3(B)]. Here the targeting capability resides in a biologically active molecule (BAM) covalently linked to an appropriate technetium complex. The BAM is typically a small peptide molecule which acts as an agonist or antagonist for a specific receptor site or a monoclonal antibody. The targeting ability of the BAM can be adversely affected by the presence of the technetium complex and the site of attachment to the BAM the size charge and lipophilicity of the conjugate and the length of the covalent linker all need to be optimised for maximum receptor binding.Tc Tc Tc BAM (C) (B) (A) Fig. 3 The ideal situation would be where the outer surfaces of the complex itself contain the groups necessary for receptor binding [Fig. 3(C)]. This approach is far more challenging in terms of the chemistry involved and developments are currently in the early stages. Examples of all three strategies are presented in the discussions of individual imaging agents below. 2.3 First generation technetium imaging agents 2.3.1 Brain imaging The dominant requirement for an agent that will accumulate in the brain is that it is capable of traversing the blood–brain barrier (BBB).Viable complexes must therefore be moderately lipophilic and have an overall neutral charge. Research at the University of Missouri in the 1980s demonstrated that a series of neutral amine–oxime complexes could readily be prepared directly from [TcO4]2 in the presence of SnCl2 as reducing agent. Further development at Amersham International led to the commercially successful Ceretec agent utilising the hexamethylpropyleneamineoxime proligand (HMPAO hexametazime) which loses three protons and forms a neutral square pyramidal TcV mono-oxo complex [Fig. 4(A)].8 The HMPAO O O N N HN N EtO2C CO2Et Tc Tc N N S S O O H (B) (A) Fig.4 derivative was selected from more than 100 structural variants for its optimal biodistribution characteristics. The proligand has two chiral centres and both the d–l and meso-HMPAO have been investigated. The greater effectiveness of the d–l complex is dependent on the formation of a more hydrophilic species once the complex has traversed the BBB which prevents diffusion back out of the brain. The mechanism of this reaction is not clear but glutathione appears to be involved and the complex from d–l proligand is less stable than that from the meso. The Ceretec agent generally has limited stability in solution and considerable effort has been expanded in increasing the lifetime by addition of agents such as CoII. The CoII is rapidly converted into CoIII which is believed to be the active stabilising agent although the exact details of the mechanism are uncertain.In principle the complex from the meso proligand could form two isomeric complexes differing in the orientation of the oxo group relative to the two methyl groups. In practice only the complex with the TcNO group syn to the methyls has been isolated. The TcV complexes of a wide range of bisamidedithiol proligands have been investigated as potential agents for imaging the brain. The ethylenecysteine diester (ECD) complex is commercially available from Dupont as Neurolite. The proligand loses three protons on reaction with [TcO4]2 to give the neutral square pyramidal complex [Fig. 4(B)],9 which readily crosses the BBB.It provides a striking example of the importance of stereochemistry in determining biochemical function. The l–l form of the complex is trapped once across the BBB due to enzymatic hydrolysis of one ester group by an esterase enzyme generating a more hydrophilic complex. The corresponding d–d complex is inert to enzymatic hydrolysis and diffuses back across the BBB. Such enzymatic conversion reactions also occur in the blood during biodistribution and although these impair brain uptake they facilitate clearance from the blood and non-target tissue via the kidneys. These agents actually provide images of regional cerebral blood flow (rCBF) and these are conventionally presented as computer-generated colour pictures such as that in Fig.5 obtained using the Ceretec agent. This represents a tomographic slice through a normal brain (front to back transaxial) with blue–green colours indicating low and orange high Tc concentrations and therefore high rCBF. The advent of advanced computer techniques has permitted the alternative representation of rCBF as three-dimensional surface rendered images where blood flow deficits appear as holes in the surface. Fig. 6 Fig. 5 A transaxial scan of a normal brain using Ceretec. Reproduced with permission from Amersham International. Fig. 6 A three-dimensional surface-rendered SPECT rCBF image for a patient with a stroke in the left upper (parietal) area of the brain. Reproduced with permission from the publishers from M. D. Devous in Clinical SPECT Imaging ed.E. L. Kramer and J. J. Sanger ch. 6 Raven Press Ltd New York 1995. provides a dramatic surface rendered 99Tc-SPECT image for a stroke patient with large zone of depleted blood flow in the left parietal area of the brain. The rCBF is dependent on a wide number of factors other than disease such as anxiety time of day and cognitive involvement and these have to be taken into account in interpreting the images. During epileptic fits there is enhanced rCBF (hyperperfusion) in the site of the EEG abnormality. If EEG is used to monitor the outset of the fit (ictus) then SPECT imaging can be used to image the focus of the abnormality within the brain. The three dimensional surface rendered image in Fig. 7 shows the outline of the area of hyper-perfusion due to the seizure as a 45 Chemical Society Reviews 1998 volume 27 Fig.7 Three dimensional surface-rendered SPECT images for a patient suffering from seizures. The pinkish area shows the area of hyperfusion during the seizure and the upper white area indicated secondary activation of the motor region of the brain. Reproduced with permission from the publishers from M. D. Devous in Clinical SPECT Imaging ed. E. L. Kramer and J. J. Sanger ch. 6 Raven Press Ltd New York 1995. pink solid body within the brain which is delineated in blue mesh. The white areas in the upper portions of the brain are due to the activation of the sensory motor area that accompanies an epileptic seizure. Such images permit precise location of the site of seizure origin.Subsequent surgical partial temporal lobectomy coupled with drug therapy is the best treatment for seizures which are not responsive to drugs alone.10 It has been estimated that in the United States alone there are over 50 000 patients suffering from this type of seizure. Only 1% of these are able to have surgical treatment due to the difficulties of locating the focus of the seizure by techniques such as depth EEG. There are also a number of psychiatric conditions which give rise to characteristic rCBF patterns and SPECT shows promise to be able to assist in precise diagnosis of such disorders. In schizophrenia there is frequently frontal lobe dysfunction which is particularly evident when the patient is carrying out a task requiring cognitive skills.11 The left-hand surface-rendered SPECT HMPAO-99mTc image in Fig.8 is for a schizophrenic Fig. 8 Three dimensional surface-rendered images for a patient suffering from schizophrenia. The left image was taken while the patient was performing a simple number matching exercise; the right shows rCBF during the Wisconsin card sort exercise which normally would enhance perfusion in the frontal areas of the brain. The decreased flow seen for this patient is typical for schizophrenia. Reproduced with permission from the publishers from M. D. Devous in Clinical SPECT Imaging ed. E. L. Kramer and J. J. Sanger ch. 6 Raven Press Ltd New York 1995. patient carrying out a simple task requiring little brain activation. The right-hand image was taken during a Wisconsin Card Sort task which requires intellectual input and would normally result in enhanced perfusion in the frontal lobes.It is characteristic of schizophrenia to observe the decreased perfu- Chemical Society Reviews 1998 volume 27 46 sion in the frontal lobe of the brain. There have also been reports of altered rCBF patterns in cases of depression Alzheimer’s disease and obsessive-compulsive disorder. The use of 99mTc labelled neurotransmitter molecules for the possible diagnosis of psychiatric conditions is reviewed below (section 2.4.2). 2.3.2 Heart imaging Initially it was postulated that lipophilic unipositively charged complexes would accumulate in heart tissue via the Na/K ATPase mechanism as K+ ion mimics.This concept prompted the synthesis of the cationic 99mTc complex [99mTc(dmpe)2Cl2]+ where dmpe = 1,2-bis(dimethylphosphino) ethane (Fig. 9) as a potential myocardial perfusion agent.12 + Me Me Me Me Cl P P Tc P P Cl Me Me Me Me Fig. 9 It was later found that this complex undergoes in vivo reduction to the neutral TcII complex [99mTc(dmpe)2Cl2] which having lost the positive charge has unacceptably fast washout from the heart and accumulates in the liver. Approaches are currently being pursued to lower the susceptibility of the metal ion to reduction which includes evaluation of complexes such as [99mTc(diars)2(SR)2]+ where diars = o-phenylenebis(dimethylarsine) and SR2 = thiolate. The thiolate ligands have been shown to increase the reduction potentials of the TcIII complexes relative to the chlorides.Further development of cationic complexes as myocardial perfusion agents led to the approval and availability of [99mTc(MIBI)6]+ where MIBI is 2-methoxy-2-methylpropylisocyanide Cardiolite which is shown in Fig. 10.13 The O O N N N HN EtO2C CO2Et Tc Tc N N S S O O H (B) (A) Fig. 10 X-ray structure of the tert-butyl isocyanide 99Tc analogue shows an octahedral arrangement for the isocyanide ligands around the central metal core An investigation into the mechanism of uptake has led to the belief that cations such as [Tc(MIBI)6]+ accumulate via a diffusion mechanism and electrostatic binding due to a high mitochondrial membrane potential. The lipophilicity of the complex is known to be important for uptake into the heart.The TcI oxidation state is surprisingly easily accessible directly from pertechnetate as the complex is synthesized by the reaction of 99mTcO42 with [Cu(MIBI)4][BF4] and SnCl2 as reducing agent. The uptake in the human heart is observed to be about 1.5% of the injected dose which slowly decreases to 1% after 4 h. A good organ to background ratio is achieved due to low uptake in the blood lungs liver and spleen. The presence of the alkoxy groups on the monodentate isonitrile ligands is believed to reduce this background activity. 99m The first approved neutral myocardial perfusion agent is Tc-teboroxime (Cardiotec) Fig. 11 which is a member of the BATO class of complexes (BATO—boronic acid adducts of technetium dioximes).The complex has the formula [TcCl(CDO)(CDOH)2BMe] where CDOH2 = cyclohexane-O CH3 N B N O O N Tc N N O H N H O O CI Fig. 11 2Cl2]+ previously discussed. dione dioxime and is prepared by the reaction of 99mTcO42 with a mixture of cyclohexane-1,2-dione dioxime and methyl boronic acid with SnCl2 as a reducing agent. 5 Min after injection 2.2% of the injected dose of the TcIII complex is found to accumulate in the heart via a mechanism which is unknown at this time however the complex exhibits rapid myocardial clearance in normal myocardium. The complex attains a seven coordinate geometry which consists of the three dioxime ligands bound to the TcIII centre via all six nitrogens with one end of the complex capped by a boronic acid derivative.The seventh coordination site is occupied by a chloride ligand.14 Two protons are believed to be shared between the three uncapped oxime ligands. The chloride ligand has been shown to be labile and susceptible to Cl/OH exchange which may be responsible for the initiation of the mechanism for the fast washout. One mechanism for this washout has been suggested which involves in vivo equilibrium between [Tc(OH)(CDO)(CDOH)2BMe] and the cationic complex [Tc(OH2)(CDO)(CDOH)BMe]+. It has been postulated that the neutral complexes may be washed out of the heart and it is the cationic complex which is subsequently retained. This is consistent with the results found with the dmpe complex [Tc(dmpe) A new class of technetium imaging agents containing the 99mTcN2+ core 99mTcN–NOET has been evaluated for use as myocardial imaging agent.In addition to having a new core it also differs from some other heart imaging agents in not carrying a positive charge which confirms that the cationic charge for myocardial perfusion imaging agents is not essential. The exact mechanism by which this neutral complex is accumulated in the heart remains to be determined. An essential feature of the viability of 99mTcN–NOET where NOET is N-ethoxy-N-ethyldithiocarbamate for pharmaceutical use has been the development of a high yield synthesis from methyl-N2-methyldithiocarbazate and 99mTcO42 in the presence of a tertiary phosphine as a reducing agent.15 This generates a nitride intermediate of uncertain structure in high yield and subsequent addition of the dithiocarbamate ligand gives the required complex.The lower charge on the Tc·N2+ core as compared with TcNO3+ means that in complexes with comparable ligands the nitrides will generally be more negatively charged. Some recent comparisons of the biological behaviour of oxo and nitrido complexes of DADS and MAG3 (for structures of these ligands see section 2.3.4) support this view. In human volunteers 99mTcN–NOET showed an uptake in the heart of 4.8–5.2% of the injected dose and slow clearance from normal myocardium. The X-ray crystal structure of the analogous dithiocarbamate complex [99TcN(S2CNEt2)2] Fig. 12 shows the technetium to have a five-coordinate square pyramidal geometry with the nitride ligand in an axial site.S N Tc S S S C C N N Et Et Et Et Fig. 12 Two more recent cationic imaging agents which are now in clinical trials are 99mTc-P53 {a trans-dioxobis[bis(2-ethoxyethyl) phosphino]ethane TcV cation} also known as tetrofosmine or Myoview Fig. 13 and 99mTc-Q12 TechneCard a mixed N2O2-donor Schiff base/phosphine TcIII cation Fig. 14. Both complexes carry a single positive charge and contain coordinated phosphine ligands. + OEt EtO O P P OEt EtO Tc OEt EtO O P P OEt EtO Fig. 13 + PR3 N N Me Me Tc O O Me Me O O Me Me PR3 Me Me R = CH2CH2OMe Fig. 14 Due to the ease of reduction of [Tc(dmpe) The complex [99mTcO2(P53)2]+ Myoview is synthesized via SnCl2 reduction of 99mTcO42 in the presence of the diphosphine ligand P53.In contrast to [TcCl2(dmpe)2]+ Myoview contains the dioxo TcV core which does not undergo in vivo reduction. The complex contains eight alkoxy groups on the bidentate phosphine ligands which help to reduce the background activity in the blood and liver. Uptake of the complex is 1.2% of the injected dose which has slow clearance and reduces to 1% 2 h post injection.16 Myoview rapidly enters the myocardial cells due to its lipophilic properties and the mechanism of uptake is believed to be similar to that of the 99mTc-MIBI complex. The dioxobisdiphosphine complex also exhibits rapid lung and liver clearance. The structure of the 99Tc analogue has been determined to be close to octahedral with the two bidentate ligands in the equatorial plane.A representation of the structure is shown in Fig. 13. Fig. 15 and 16 were produced using Myoview and show a healthy (Fig. 15) and defective heart (Fig. 16). The zone of non-functional heart muscle at the apex of the horseshoe is evident as a dark area. The horseshoe-shaped image is a consequence of the particular cross section of the heart due to the orientation of the scan. 2Cl2]+ 99mTc-Q12 was designed to incorporate fewer phosphine ligands and so reduce the susceptibility of the metal ion to undergo detrimental reduction. The TcIII complex attains an octahedral geometry with the tetradentate ligand occupying the equatorial plane and the two monodentate tertiary phosphines occupying the axial sites.The Schiff base ligand is 1,2-bis-{[(dihydro-2,2,5,5-tetramethylfuran-3(2H)-onato)methylene]amino}ethane which contains a furanone group to aid clearance of the complex from the blood lung and liver to achieve a good background.17 Due to the reducing ability of the phosphine ligands the addition of a separate reducing agent such as SnCl2·2H2O was found not to be necessary. 2.3.3 Liver imaging Technetium(iii) complexes of HIDA [2,6-dimethylphenylcarbamoylmethyl) iminodiacetic acid] derivatives have been shown to be suitable for imaging the hepatobiliary system.21 Currently there are three 99mTc-HIDA analogues which have been approved for this purpose; 99mTc-Lidofenin (TechneScan HIDA) 99mTc-Mebrofenin (Choletec) and 99mTc-Disofenin (Hepatolite).The exact nature of the complexes is uncertain but 47 Chemical Society Reviews 1998 volume 27 Fig. 15 SPECT image of a normal heart taken using Myoview. Reproduced with permission from Amersham International. Fig. 16 SPECT image of a diseased heart taken using Myoview. The diseased regions show as gaps in the horseshoe shape seen for the healthy heart. Reproduced with permission from Amersham International. Fig. 17 shows the proposed structure. The complex is believed to contain two ligands coordinated in an octahedral configuration and bear a single negative charge. _ O O C R1 R2 O O Tc C N CH R 2 R = HN R3 O H H2C R N 2C CH2 C R4 R5 C O O O Lidoferin R1 = CH3 Disoferin R1 = isopropyl Mebroferin R1 = R3 = CH3 R2 = Br Fig.17 Tc-sulfur colloid is also used for liver imaging and is believed to be made up of 99mTc2S7 and colloidal sulfur. The Tc-sulfur colloid is produced by the sodium dithionite reduction of TcO42 in an acidic solution. 80–85% of the colloid is accumulated in the liver via uptake in Kupffer cells by phagocytosis. A normal liver scan taken using the Tc-colloid is shown in Fig. 18. The two images are taken from the front (upper) and side and show uptake of the tracer in the liver and spleen (to the right in upper image). Some uptake in the bone marrow of the spine can also be seen (pale purple in the upper image). Chemical Society Reviews 1998 volume 27 48 Fig.18 Liver SPECT images using Tc-sulfur colloid. The upper scan is taken from the front the lower from the side. The spleen appears as a fainter spot to the right in the upper image and uptake by the bone marrow is evident with a pale purple outline of the spine. Reproduced with permission from Dr S. J. Mather Department of Nuclear Medicine St Bartholomews Hospital London. 2.3.4 Kidney imaging [99mTcO(glucoheptonate)2]2 Glucoscan also known as TechneScan or Glucoheptate is an early kidney imaging agent the precise structure of which is unknown although it is believed to have the five coordinate structure shown in Fig. 19.18 The _ O O O Tc O O O O (CHOH)4 (CHOH)4 CH2OH CH2OH Fig. 19 99m complex is not currently used widely as an imaging agent due to the availability of better alternatives such as ultrasound and CT X-ray imaging.However the complex is regularly used as a precursor for the synthesis of other TcV species via ligand exchange. The complex is synthesized by the reaction of 42 with calcium glucoheptonate in the presence of the TcO reducing agent SnCl2·2H2O. A 99mTc-DMSA complex (DMSA is dimercaptosuccinic acid) has been used to image the kidney for a number of years. The TcIII complex (of unknown structure) is prepared from the reaction of 99mTcO42 with DMSA in the presence of the reducing agent SnCl2·2H2O. Three hours post injection 50% of the injected dose has accumulated in the kidneys and specifically localizes in the proximal convoluted tubule.The TcV complex [TcO(DMSA)2]2 is also known and the complex has three possible conformations syn-endo anti or syn-exo of the carboxylic acid groups with respect to the TcNO core. Fig. 20 shows the syn-endo orientation. The crystal structure of the analogous rhenium complex [ReO(DMSA)2]2 has been determined and displays a square pyramidal geometry of the ligands around the central rhenium atom.20 (see section 3.2.1.2). 99mTc-DTPA DTPA = diethylenetriaminepentaacetic acid has approval for use as a kidney imaging agent. The structure of the 99Tc analogue has not yet been determined and it is unclear at present as to whether the complex contains technetium in the +IV or +V oxidation state. If the complex contains technetium _ O R R S Tc R R S S S syn endo R = COOH Fig.20 in the +IV oxidation state [Fig. 21(A)] the DPTA is proposed to coordinate as a hexadentate ligand or if the correct oxidation state is +V [Fig. 21(B)] the complex is proposed to contain a pentadentate DPTA ligand and the TcNO core. In both cases the complex is likely to have octahedral geometry. The complex is prepared by the reaction of 99mTcO42 with DPTA with SnCl2 acting as a reducing agent. + + O O O N O O N HOOC Tc N COOH N N Tc O O N O O COOH HOOC O O (B) (A) Fig. 21 Fritzberg10 developed the most recent and widely used anionic kidney imaging agent [99mTcO(MAG3)]2 99mTcOmercaptoacetyltriglycine which is shown in Fig. 22.17 [99mTcO(MAG3)]2 contains a free carboxylic acid group which is believed to be necessary for efficient renal excretion.The TcV – O H2C C O N O N C CH2 Tc H2C C N S O CH2 HO C O Fig. 22 complex attains a square pyramidal geometry with the oxo group in the apical position. The structure of the rhenium analogue has been determined. In contrast to the previous kidney imaging agents there is no chiral centre and therefore no problems arise from the existence of isomers.The complex is prepared by the reaction of 99mTcO42 with benzoyl mercaptoacetyltriglycine and the reducing agent SnCl2 when loss of the benzoyl protecting group occurs. The benzoyl protecting group prevents ligand oxidation and therefore increases kit stability and reliability. A few minutes post injection about 1–2% of the injected dose is found in the kidneys.It is the passage into and through the kidneys which provides a measure of renal function. The presence of the thiol group provides additional reducing power to convert TcVII to TcV and assists in the stabilisation of the complex. Current research is directed at variations in the MAG3 ligand by substitution of glycine by l-alanine thereby modifying the renal excretion characteristics. 2.3.5 Bone imaging A series of 99mTc complexes of phosphonate ligands have been developed as bone-imaging agents. The initial developments in this area used pyrophosphate but it was later shown that diphosphonates such as methylenediphosphonate [MDP Fig. 23(A)] gave much improved performance.Typically the agent is prepared by reaction of the [99mTcO4]2 generator eluate with MDP in the presence of SnCl2·2H2O as reductant. The coordination chemistry involved is not simple and the number of species formed is dependent on pH concentration and reductant used. The concentration dependence complicates attempts to characterise the 99mTc complexes by extrapolation from the 99Tc level as the concentrations are hugely different (1028 m for 99mTc 1023–104 m for 99Tc). There is a consensus that the dominant oxidant state for 99mTc/MDP is TcIV and that a mixture of oligomers is formed. At the 99Tc level reaction of [99TcBr6]22 with H4MDP led to the isolation and structural characterisation of a polymeric complex [Fig. 23(B)].22 A hexameric complex has also been isolated and an X-ray structure determination carried out.OH OH O O HO Tc O CH2 O O P O O Tc O O P P HO OH HO O OH P CH2 (B) (A) Fig. 23 The mechanism of absorption on bone is believed to be via co-ordination of the free phosphoryl oxygens to calcium ions on the hydroxyapatite bone surface. Since stressed bone has higher concentrations of calcium ions such areas appear as ‘hot spots’ on the scan. The 99mTc-MDP bone scan in Fig. 24 (front and rear view) gives a clear picture of the skeletal structure with an intense (red) area corresponding to the bladder. The area of increased tracer uptake in the right ankle is caused by arthritis. One of the main uses for 99mTc bone-scans is for cancer patients to identify if there has been metastasis into the bone the metasteses appearing as bright spots on the scan.SPECT 99mTc bone images can in general provide information on lesions which may not be visible by conventional X-ray methods. 99mTc-based scans can also be valuable for diagnosis of problems with joints such as the elbow or knee as it can show up bone damage not immediately visible from MRI imaging. Such images then enable the surgeon to screen for those patients who will benefit most from expensive keyhole type exploratory surgery. 2.4 ‘Second generation’ technetium imaging agents These are classified according to the receptor site or biological function that is targeted. 2.4.1 Steroid receptors About 60–70% of breast tumours are estrogen receptor positive and endocrine therapy with drugs such as tamoxifen is effective in about half of cases with such estrogen receptor positive cancers.If a molecule which binds to such sites could be radiolabelled it would provide a method of monitoring the progress of therapies with agents such as tamoxifen. Most prostate cancers are androgen and progesterone receptor positive and could be imaged with an appropriate labelled receptor hormone. The structures of the three relevant hormones are shown in Fig. 25. The 99mTc labelling of the progesterone receptor has been studied utilising conjugation to N2S2 ligands via a phenyl spacer (Fig. 26).23 The key to success is to find a site of attachment to the steroid which does not impair receptor binding and the 11b site proved to be optimal.The conjugates contain stereoisomers a syn pair and two diastereoisomers and remarkably the syn pair had an affinity for the progesterone receptor 161% of progesterone itself. Although the conjugates showed high binding in vivo studies also showed a high level of non-specific binding. 49 Chemical Society Reviews 1998 volume 27 Fig. 24 Bone SPECT images taken using MDP diphosphonate agent. The skeletal structure shows clearly and the bright red area in the centre is due to accumulation in the bladder. The red area on the right ankle is due to arthritis in the joint. Reproduced with permission from Dr S. J. Mather Department of Nuclear Medicine St Bartholomews Hospital London. OH HO Estradiol O H Dihydrotestosterone Fig.25 Nevertheless the approach is clearly promising although further fine-tuning of biodistribution characteristics is required. An alternative to the pendant receptor ligand approach which was discussed in the introduction above is to integrate the receptor binding sites directly onto the outer periphery of the Tc ligands. A proposed structure for such a complex is shown in Fig. 27(A) and the overall similarity to progesterone is apparent. Chemical Society Reviews 1998 volume 27 50 O O Progesterone OH N S O Tc S OH C CCH3 O Fig. 26 Some initial steps towards producing an analogue of estradiol (Fig. 25) have been made with the synthesis of the complex shown in Fig. 27(B).24 Reaction of a TcV precursor with a 1 1 mixture of the two bidentate N–S ligands leads to a good yield of the mixed complex shown rather than a statistical mixture.The receptor-binding affinity was found to be low as perhaps expected for this initial model but the approach offers interesting possibilities for the future. OH S S O O N Tc Tc N N N HO S S O (B) (A) Fig. 27 2.4.2 Central nervous system (CNS) receptors There are a number of important diseases and psychiatric conditions that are associated with changes in the densities of neurotransmitter receptor sites in the brain benzodiazepene (epilepsy) muscarinic and nicotinic (Alzheimer’s disease) dopaminergic (Parkinson’s disease psychiatric conditions) serotonergic (psychiatric conditions).Most of the initial studies in imaging have used PET but this imaging modality is of limited availability due to the necessity of being close to a cyclotron. The g-emitting iodine-123 has been used for SPECT brain receptor imaging but again this isotope is expensive and unlike 99mTc not widely available. There is currently a worldwide effort directed to producing 99mTc CNS receptor imaging agents via the pendant bioconjugate approach. We give two examples of different approaches to the construction of the conjugate. The molecule ketanserin [Fig. 28(A)] is a potent antagonist for serotonin (5-HT) receptor sites. Detailed biochemical studies have established where modifications can be made without impairing receptor binding and appropriate fragments have been bound via thiolate or isocyanide groups to a TcV oxocore with a tridentate NS2 22 or SS2 22 ligand.25 The structure of an Re analogue [Fig.28(B)] shows the square pyramidal MNO core and the flexible side chain containing the receptor binding sites. Binding studies have been made using rat brain homogenate rich in 5-HT receptor sites and have shown high affinity for the derivative with an OC6H5 group in place of the phenyl group and a isocyanide group to provide ligation to the Tc. The higher affinity of this particular derivative appears to be associated with better ability to traverse the BBB. Cocaine [Fig. 29(A)] and analogues block dopamine transporter sites and iodine-123 substituted derivatives have been explored for the diagnosis of Parkinson’s disease.Linking of the cocaine derivative via the seven-membered ring to a TcV oxo-O H N F N N O O ketanserin (A) N O S S Re S S O S S Tc CH3 CH3 N N N N CO2Me O O C CI Tc-TRODAT (A) (B) Fig. 28 Fig. 29 (B) core via an N2S2 ligand produces a conjugate (‘Tc-TRODAT’) shown in Fig. 29(B). This has produced the first in vivo images of D2 transporter sites in man (Fig. 30) using technetium-99m. Uptake (coloured yellow) in the areas of the brain rich in D2 Fig. 30 A series of SPECT scans taken with 99mTc-TRODAT at the time intervals shown. The initial image accords with normal rCBF and the 60–80 and 120–140 min scans show significant uptake in the regions of the brain rich in dopamine transporter sites as two bright yellow areas in the centre.Reprinted with permission from H. F. Kung H.-J Kim M.-P Kung S. K. Meegala K. Plossl and H.-K Lee Eur. J. Nucl. Med. 1996 23 1527. receptors is evident in the centre of the image taken after 120 min. This exciting advance confirms the viability of the conjugate approach to the 99mTc imaging of CNS receptor sites. 2.4.3 Monoclonal antibodies or antibody fragments Monoclonal antibodies or their fragments the so-called ‘magic bullets’ are potentially ideal vehicles to target radioisotopes to specific sites providing of course the radiolabel can be introduced without interfering with binding to the receptor site. The relatively large size of whole antibodies generally confers undesirably slow biodistribution kinetics for imaging purposes and attention is now directed to antibody fragments [F(ab’)2 Fv Fab’ or Fab] which retain the specific binding characteristics.The use of the fragments also reduces the possibility of immunogenicity and adverse allergic reactions. The crucial aspect of the development of 99mTc labelled antibodies and their fragments is the mode of attachment of the metal and the link must be sufficiently stable to prevent premature release of the radioisotope. The first approach to 99mTc labelling of antibodies involved the reduction of the disulfide groups holding the F(ab’)2 fragments together and binding of the Tc to the resulting SH groups.27 Although attractive in its simplicity the conjugates do not always have high in vivo stability.However Fig. 31 shows a series of images produced by a 99mTc direct labelled antibody PR1A3 for colorectal tumours. The image after 5 min shows uptake mainly in the heart blood pool and the 7 h image additionally shows liver uptake (below the heart from this angle) and slight uptake in the tumour at 4 o’clock. This tumour uptake has increased significantly after 20 h indicating the relatively slow targeting of the monoclonal antibody. Fig. 31 Scans taken with 99mTc labelled PR1A3 monoclonal antibody against colorectal tumours at time intervals shown. The 20 h image shows clear uptake in the tumour as a small orange area at 4 o’clock. Reproduced with permission from Dr S.J. Mather Department of Nuclear Medicine St Bartholomews Hospital London. This has prompted a search for more stable conjugates and variants of the bifunctional chelate approach appear to offer the most promise. Two principal strategies have been adopted. The first (post-formed chelation) involves initial attachment of metal binding groups to the antibody or fragment followed by insertion of the 99mTc. Two of the many examples are shown in Fig. 32. A hydrazine nicotinamide derivative can be bound to lysine groups on an antibody or fragment as shown in Fig. 32(A). The hydrazine group then reacts with [99mTcO4]2 to give an uncharacterised but stable conjugate which may contain TcNN–NH– groups. An alternative elegant route to binding the antibody is via the thiolactone in Fig.32(B) which generates an N2S2 diaminedithiol ligand system which forms a stable neutral square pyramidal TcNO species with [99mTcO4]2.28 51 Chemical Society Reviews 1998 volume 27 O O NH NH2•HCI N N O O AbNH2 O NH NH2•HCI N AbNH (A) Fig. 32 Antibody engineering techniques have also enabled the incorporation of (gly)4cys peptide into single chain antibody proteins. The peptide binds the 99mTc in an analogous fashion to mercaptoacetyltriglycine (MAG3) (see kidney imaging) and provides a stable conjugate. Studies have been made of the protein-coupled fragments of antibodies specific for human ovarian cancer using tumours grafted into mice and significant uptake of 99mTc into the tumour was observed.29 The alternative approach (preformed chelation) requires the initial synthesis of a technetium chelate with an activated ester group and subsequent attachment of the antibody or fragment.This is illustrated in Fig. 33 for a diamidedithiolate ligand with a pendant tetrafluorophenyl (TFP) activated ester. Kits based on this procedure for attaching antibody fragments for targeting melanoma and lung cancer have undergone chemical trials and many other related promising systems are currently in development. F F CO2 NH NH F F S S Tc-gluconate O O Fig. 33 2.4.4 Imaging hypoxia Cells become hypoxic in several disease states. A significant fraction of certain types of tumour are hypoxic (80% for head and neck squamous cell carcinoma) and imaging of such tumours would be of great advantage in devising suitable treatments.Impaired blood flow in the heart gives rise to transient or persistent tissue hypoxia and accurate diagrams of such areas show where medical intervention to restore blood flow would be beneficial. 2-Nitroimidazoles have been shown to be trapped in hypoxic cells due to their reduction to a series of products which either cannot diffuse out or become bound inside the cell. Several groups have investigated the possibility of linking 2-nitroimidazoles to 99mTc chelates for hypoxic site imaging.30 Conjugate A (Fig. 34) shows some promise for hypoxic imaging in vivo but is too lipophilic and clears slowly from background Chemical Society Reviews 1998 volume 27 52 N N O SH S AbNH2 O N N NHAb O SH HS (B) CO2TFP O NH N O O Tc S S AbNH2 CONHAb O NH N O O Tc S S tissue.A more hydrophilic version [Fig. 29(B)] with an oxygen in the backbone is more promising with more rapid liver clearance. N N NO2 O N O N N O N Tc Tc N N N N N N O O O O NO2 H H (B) (A) Fig. 34 A variant on this theme involved conjugation of the 2-nitroimidazole to a amineoxime ligand but with a four- rather than three-carbon backbone (Fig. 35). However a control experiment for hypoxic imaging using the 99mTc complex without the imidazole group showed this to be more effective HN NH N N N N O O NO2 H H Fig. 35 than with the conjugated imidazole. The co-ordination chemistry of the Tc in this type of amine–oxime complex is strongly dependent on the backbone length.With three carbons in the backbone a monooxo complex is formed whereas with five carbons a trans dioxo system is favoured. The four-carbon system apparently undergoes biochemical reduction and is trapped inside the hypoxic cell. This redox behaviour may be associated with labile protic equilibria involving protonation and/or aquation of the oxo-core. This last complex is showing promise as a hypoxic imaging agent in human clinical trials. 2.4.5 Thrombus imaging Current research is being directed at the area of diagnostic agents for imaging thrombi. One group has used the approach of conjugating platelet glycoprotein IIb/IIIa antagonists onto a chelate complex of technetium.The glycoprotein IIb/IIIa complex is expressed on the membrane surface of activated platelets and plays an integral role in platelet aggregation and thrombus formation. Cyclic peptides which incorporate the sequence Arg–Gly–Asp (RGD) have been shown to be high affinity antagonists for the glycoprotein receptor. The glycoprotein IIb/IIIa receptor is expressed only on activated platelets so therefore radiolabelled cyclic IIa/IIIa receptor antagonists were anticipated to bind to only the platelets involved in the thromboembolic event. Fig. 36 shows one example of a technetium complex conjugated to the cyclic glycoprotein IIa/IIIa receptor antagonist. In this case the coordination environment for technetium is based around the N2S2 diamide dithiol chelate ligand with the cyclic peptide conjugated onto the backbone via an active ester.This complex has also been synthesized from the direct reaction of 99mTcO42 with the preformed conjugate ligand which eliminates the synthesis of the active ester complex and subsequent step of peptide conjugation.31 Fig. 37 shows selected images derived from studies of this complex in a dog with implanted deep vein thrombi. The complex was actively incorporated into the growing thrombi with images being clearly detectable within 15 min post-injection. NH O H2N O N H N H N O OH HN O O NH HN O NH N H O O N N O O Tc– O Fig. 36 S 3 Rhenium S An alternative mode of incorporation of the cyclic peptide is via N-hydroxysuccinimidyl hydrazinonicotinate (S-Hynic).32 Synthesis of the conjugate shown in Fig.38 was achieved by the reaction of 99mTcO42 with the Hynic conjugated cyclic peptide in the presence of EDDA (ethylenediaminediacetic acid) and SnCl2·2H2O. The monoanionic ligand X is believed to be chloride and the hydrazine unit is postulated to be coordinated as an isodiazene ligand (NNNHR) as opposed to a diazenide ligand (–NNR). There is a possibility of a number of isomers for this type of complex. Only investigations at the macroscopic level using technetium-99 or rhenium will confirm the structure of the complex and the mode of coordination of Hynic ligand. Another group has also investigated the synthesis of technetium complexes containing receptor binding peptides which bind to the glycoprotein IIb/IIIa receptor.They focused on high affinity peptides containing the receptor binding sequence –Apc–Gly–Asp– where Apc is S-(3-aminopropyl)cysteine. Again a bisamide bisthiol chelator is used to coordinate technetium which was incorporated into the dimeric peptide. The 99mTc complex of this peptide P357 was synthesized at room temperature. This complex has given excellent clinical images of deep vein thrombosis and of pulmonary embolism. 33 The element rhenium (Z = 75) was discovered in 1925 by the Noddacks and is one of the rarest elements occurring naturally as a mixture of two non-radioactive isotopes 185Re (37.4%) and Fig. 37 Images produced by a technetium complex conjugated to a cyclic glycoprotien IIa/IIIa receptor antagonist in a dog with implanted deep vein thrombi.Reprinted with permission from S. Liu D. S. Edwards R. J. Looby M. J. Poirier M. Rajopadhye J. P. Bourque and T. R. Carroll Bioconj. Chem. 1996 7 83. Copyright 1996 American Chemical Society. CON- P P = peptide NH N O O Tc O O MeV g-Energy/ keV 186Re NH Range in tissue/ mm 5 11 N H are summarised in Table 1. 1.07 (71%) 2.1 (100%) 137 (9%) 155 (15%) 90 17 188Re X Fig. 38 187Re (62.6%). The radioactive isotopes of interest in nuclear medicine are 186Re and 188Re the nuclear properties of which Table 1 Radioactive isotopes of rhenium Halflife/ Max. b energy/ h Both isotopes are suitable for therapeutic use by means of b-irradiation.The 5 mm range for 186Re means that it is suitable for small tumours whereas the greater 11 mm range for 188Re is more appropriate for large masses. The selection of isotope is also governed by factors such as halflife and technical aspects of their production. 186Re is generated by neutron radiation of 185Re and there is inevitable contamination with non-radioactive 185Re. On the other hand 188Re is available by radioactive decay of 188W and separable by analogous ion-exchange methods to those used for 99mTc and commercial generators are available. Such a generator with 0.5 Ci of 188W has the potential to provide therapeutic treatments to several hundred patients over its 2–6 month lifetime.The major disadvantage of 188Re for therapeutic applications is the relatively short halflife of 17 h. 3.1 Comparison of the chemistry of technetium and rhenium The ‘lanthanide contraction’ ensures that the complexes of the two elements are very similar in terms of their physical characteristics (size lipophilicity etc). Significantly however rhenium complexes are easier to oxidise (harder to reduce) and more kinetically inert than their technetium analogues. The Chemical Society Reviews 1998 volume 27 53 relative ease of oxidation of rhenium means that in vivo oxidation to [ReO4]2 is common. This can be an advantage in that it provides an ultimate elimination route for the radioactive isotope via the kidneys. 3.2 Rhenium radiopharmaceuticals 3.2.1 ‘Rhenium essential’ agents As discussed in section 2.2 above this is class of therapeutic agents where the biodistribution is determined by the size charge and lipophilicity of the complex.Technetium complexes of this type are used to study major organs and there are few examples of rhenium complexes which have the required specificity to be used for treatment of cancers and other therapies. 3.2.1.1 Agents for the palliation of bone pain Externally applied radiation (sealed source) is widely used for pain relief. However this is difficult to deploy when metastases are widely distributed through the skeletal structure. This has prompted the search for b-emitting radiopharmaceuticals that can be targeted to bone lesions and rhenium is one of several radioactive elements under investigation.Among the more developed agents for palliation of bone pain are rhenium radiopharmaceuticals and are based on diphosphonate ligands. 34 The underpinning co-ordination chemistry is directly analogous to that discussed for 99mTc bone imaging agents above. Typically [186ReO4]2 is incubated at 100 °C for 10 min with diphosphonate (HEDP) in the presence of SnCl2 as reductant giving a 90% labelling yield. As with 99mTc HPLC measurements indicate that a mixture of polymeric complexes are formed but bind preferentially to sites of skeletal damage. The exact mechanism by which bone pain is reduced is still unclear but these agents have proved to be of real benefit in clinical trials.3.2.1.2 Medullary thyroid carcinoma A TcV complex of dimercaptosuccinic acid (DMSA) is used widely as an agent for the imaging of a relatively rare medullary thyroid carcinoma. The 99mTc kidney imaging agent using the same ligand is believed to contain TcIII rather than TcV. The ReV complex has the expected square pyramidal structure with an apical oxo-group (Fig. 39) and exists as a mixture of isomers – O CO2H HO2C S S Re HO CO2H 2C S S Fig. 39 in solution depending on the orientation of the carboxylate groups with respect to the S4 plane. This Re complex displays selective uptake in tumour tissue analogous to that of the Tc species and offers a possibility of therapeutic treatment of this disease.35 3.1.2.3 Monoclonal antibodies and fragments Monoclonal antibodies and their fragments are potentially powerful targeting agents for the therapeutic uses of radionucleides.In principle the methods described in section 2.4.3 above for the attachment of 99mTc can also be used for the radioactive isotopes of Re. The mercaptoacetyltriglycine (MAG3) technetium system is used widely as an imaging agent for renal function (see section 2.3.4. above) forming the familiar square pyramidal oxo complex. This system has been adapted by the attachment of an activated ester group via an amide group (Fig. 40) to permit conjugation to antibodies or fragments as described above. Conjugation of 186Re to a murine (mouse derived) antibody for adenocarcinomas using this technology gave promising results in animals.36 The slower kinetics of complex formation for Re means that the preformed Chemical Society Reviews 1998 volume 27 54 O NH HN O NH O F SH F O O F F Fig.40 chelate approach needs to be used as the conditions for complexation of the Re can denature the antibody. Subsequent developments have included using chimaeric human–mouse antibodies which reduced immunogenicity and biodegradable linker groups which accelerate the excretion of non-targeted radioactivity elsewhere in the body as the Re chelate is released and then eliminated by the kidneys. There have also been studies of the direct labelling of antibodies using the prereduction step to generate free SH groups and subsequent binding to rhenium after stannous chloride reduction of either [186ReO4]2 or [188ReO4]2.As with this approach to labelling antibodies with 99mTc there is a problem with the stability of the conjugates. This is more acute for the therapeutic Re isotopes as it can lead to undesirable radiation doses at locations other than the tumour. 3.2.1.4 Steroids and bioactive peptides Certain tumours (pituitary malignant breast pancreatic) have a large number of receptors for the tetradecapeptide somatostatin and its cyclic analogue octreotide. The latter has disulfide bridges and reduction of this and reaction with 188ReO42/ stannous/citrate provides a direct high yield labelling route. Studies in mice have shown that this is retained on injection in tumours and can induce necrosis of the tumour tissue.Bioconjugates of octreotide using the N2S2 ligand approach described in Section 2.4 have been made for technetium-99m and have been investigated as a potential method for the imaging of tumours. Steroids can be attached to rhenium oxo-complexes of N2S2 ligand systems in a directly analogous fashion to that described for technetium in Section 2.4.1 and show take up in target tissue rich in steroid receptors. This therefore is a promising approach to the delivery of therapeutic radiation to appropriate tumours. An alternative approach to the labelling of steroids involves the attachment of a rhenium cyclopentadienyltricarbonyl unit to the 17 position of estradiol derivatives as shown in Fig. 41.37 OH CICH2 Re(CO)3 HO Fig.41 Surprisingly the derivative (shown) with an 11-chloromethyl group shows higher binding to receptors than the parent steroid. This was attributed to the ability of the acetylene linked Re unit to bend out of the way behind the steroid molecule and an interaction of the CH2Cl group with a Lewis acid group at the receptor site. It has yet to be shown if this interesting approach can be extended to the radioactive isotopes with the requirement to use aqueous [ReO4]2 as the starting point for the chemistry. 4 References Due to the limit on the number of references allowed the authors have quoted some pieces of work without proper acknowledgement. The choice of work referenced is of necessity somewhat arbitrary and will we trust not cause offence to those we appear to have ignored.1 E. Deutsch K. Libson and S. Jurisson Prog. Inorg Chem. 1983 30 75. 2 M. Clarke and L. Podbielski Coord. Chem. Rev. 1987 78 253. 3 F. Tisato F. Refosco and G. Bandoli Coord. Chem. Rev. 1994 135 235. 4 J. R. Dilworth and S. Parrott Directions in Radiopharmaceutical Research and Development ed. S. 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M. Kirschenbaum S. S. Tubeh and R. J. English J. Nucl. Med. 1984 14 K. E. Linder M. F. Malley J. Z. Gongoutas S. E. Unger and 25 1350. A. D.Nunn Inorg. Chem. 1990 29 2428. 15 R. Pasqualini and A. Duatti J. Chem. Soc. Chem. Commun. 1992 1354. 16 J. D. Kelly A. M. Forster B. Higley C. M. Archer F. S. Booker L. R. Canning K. W. Chiu B. Edwards H. K. Gill M. McPartlin K. R. Noyle I. A. Lathan R. D. Pickett A. E. Storey and P. M. Webbon J. Nucl. Med. 1992 24 353. 17 M. A. DeRosch J. W. Brodack G. D. Grummon M. E. Marmian D. L. Nosco K. F. Deutsch and E. A. Deutsch J. Nucl. Med. 1992 22 850. 18 C. E. Costello J. W. Brodack A. G. Jones A. Davison D. L. Johnson S. Kasina and A. R. Fritzberg J. Nucl. Med. 1983 24 353. 19 J. A. Ponto H. M. Chilton and N. E. Watson Pharmaceuticals in Medical Imaging ed. D. P. Swanson H. M. Chilton and J. H. Thrall MacMillan Publishing Co. New York 1990 501.20 J. Singh A. K. Powell S. E. M. Clarke and P. J. Blower J. Chem. Soc. Chem. Commun. 1991 15. 21 A. R. Fritzberg S. Kasina D. Eshima and D. L. Johnson J. Nucl. Med. 1986 27 111. 22 K. Libson E. A. Deutsch and B. Barnett J. Am. Chem. Soc. 1980 102 2476. 23 M. J. Welch J. B. Downer and J. A. Katzenellenbogen Current Directions in Radiopharmaceutical Research and Development ed. S. J. Mather Kluwer Academic Press Netherlands 1996 p.137 and references therein. 24 D. Y. Chi J. P. O’Neil C. J. Anderson M. J. Welch and J. A. Katzenellenbogen J. Med. Chem. 1994 37 928. 25 H. Spies T. Fietz M. Glacer H.-J. Pietsch and B. Johanssen in Technetium and Rhenium Chemistry and Nuclear Medicine ed. M. Nicolini G. Bandoli and U. Mazzi S. G. Editorali Padova Italy 1995 vol. 4 p. 243 and references therein. 26 M. E. Kung H.-J. Kim M.-P. Kung S. K. Meegallu K. Plossl and H.-K. Lee Eur. J. Nucl. Med. 1996 23 1527. 27 A. R. Fritzberg and D. S. Wilbur in Handbook of Targeted Delivery of Imaging Agents ed. V. P. Torchilin CRC Press Inc. New York 1995 p. 83 and references therein. 28 D. J. Hnatowich G. Mardirossan M. Ruscowski M. Fargarasi F. Firzi and P. Winnard J. Nucl. Med. Chem. 1993 34 172. 29 A. J. T. George F. Jamar M.-S. Tai B. T. Heelan G. P. Adams J. E. McCartney L. L. Houston L. M. Weiner H. Opperman A. M. Peters and J. S. Huston Proc. Natl. Acad. Sci. 1995 92 8538. 30 C. M. Archer B. Edwards and N. A. Powell in Current Directions in Radiopharmaceutical Research and Development ed. S. J. Mather Kluwer Academic Press Netherlands 1996 p. 81 and references therein. 31 S. Liu D. S. Edwards R. J. Looby M. J. Poirier M. Rajopadhye J. P. Bourque and T. R. Carroll Bioconj. Chem. 1996 7 203. 32 S. Liu D. S. Edwards R. J. Looby M. J. Poirier M. Rajopadhye J. P. Bourque and T. R. Carroll Bioconj. Chem. 1996 7 83. 33 J. Lister-James W. J. McBride S. Buttram E. R. Civetello L. J. Martel D. A. Pearson D. M. Wilson and R. T. Dean in Technetium and Rhenium in Chemistry and Nuclear Medicine ed. M. Nicolini G. Bandoli and U. Mazzi S. G. Editorali Padova 1994 vol. 3 269. 34 W. A. Volkert and E. A. Deutsch in Advances in Metals in Medicine ed. M. J. Abrams and B. A. Murrer JAI Press USA 1993 p. 115 and references therein. 35 P. J. Blower J. Singh S. E. M. Clarke M. M. Bisundan and M. J. Went J. Nucl. Med. 1990 31 768. 36 A. R. Fritzberg L. M. Gustavson M. D. Hylandes and J. M. Reno in Chemical and Structural Approaches to Rational Drug Design ed. D. B. Weiner and W. B. Williams CRC Press Inc. Boca Raton USA 1994 p. 125 and references therein. 37 S. Top M. Elhafa A. Vessieres J. Quivy J. Vaissermann D. W. Hughes M. J. McGlinchey J. P. Mornon E. Thoreau and G. Jaouen J. Am. Chem. Soc. 1995 117 8372. Received 13th June 1997 Accepted 6th August 1997 55 Chemical Society Reviews 1998 volume 27

ISSN:0306-0012    

DOI:10.1039/a827043z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Aspects of weak interactions Dudley H. Williams* and Martin S. Westwell† Cambridge Centre for Molecular Recognition University Chemical Laboratory Lensfield Road Cambridge UK CB2 1EW Weak interactions such as those non-covalent interactions that occur in biological systems are less well characterised than their strong covalent counterparts. Here we discuss associations between two or more molecules and consider the effect of interactions with solvent molecules (particularly water) and changes in the internal structure of the associating molecules on binding. We go on to discuss some of the progress that has been made in the estimation of binding constants. 1 Introduction In this article we use the definition that ‘weak interactions’ are those involving bonds which are comparable to thermal energies.In such systems equilibrium constants between two possible states can frequently be varied (often drastically) in the temperature range 0–100 °C. The study of weak interactions is a topic of great current interest because it is these non-covalent interactions which determine the stability for example of DNA duplexes of the folded states of proteins of enzyme–substrate complexes and of ligand–receptor interactions. A deeper understanding of weak interactions is highly desirable not simply because of the intellectual drive to understand the above systems but also because of the wish of the pharmaceutical industry to ‘rationally’ design new drugs. We largely discuss an approach developed in our laboratory which builds on the earlier work of others (particularly Jencks).1,2 We first discuss associations where in the ideal case the associating entities do not interact strongly with solvent nor do they change their structures significantly upon association.Second we go on to consider associations in polar media particularly water and where the associating molecules may modify their internal structures upon binding. Lastly we discuss some of the progress that has been made in the estimation of binding constants. † Current Address Dyson Perrins Laboratory University of Oxford South Parks Road Oxford UK OX1 3QR. Martin Westwell Dudley Williams Dudley Williams has bachelor and PhD degrees in chemistry and organic chemistry respectively from the University of Leeds in 1958 and 1961.He subsequently studied as post-doctoral fellow at Stanford University from 1961–64. Since 1964 he has worked at the University of Cambridge where he is a Fellow of Churchill College and Professor of Biological Chemistry. He was elected a Fellow of the Royal Society in 1983. 2 Model 1 Where A and B do not interact strongly with solvent nor do they extensively modify their structures upon association Perhaps the most commonly used expression for the representation of a reversible association is eqn. (1). (1) A + B"A·B This kind of formalism has arisen because it is very useful in considering the association of for example two water molecules to give a hydrogen bonded dimer.To a good approximation each water molecule (one designated A and the other B) retains the same internal structure in the dimer as in the dissociated state. Thus as a useful approximation the bonding between A and B in A·B can be represented as a property of the interface between A and B. The approximation works well because the bonds within A and B are strong compared to the bonding between A and B. In such circumstances we have previously argued3,4 that the restriction of motion which occurs when the association of eqn. (1) takes place (measured in terms TDS where T is the temperature 298 K and DS is the overall loss of entropy of translation and rotation) will be related to the exothermicity of the association (DH) by a curve of the general form shown in Fig.1. The direction of curvature arises because motional restriction reaches a limit of TDS Å 50 to 60 kJ mol21 for the immobilisation of a molecule of mass of ca. 100–300 at room temperature and this limiting entropy loss is approached much before covalent bond strengths are reached. The general shape of this curve has subsequently received theoretical support,5 and we reproduce here experimental data for associations occurring in methylene chloride solution (Fig. 2).4 As a good approximation the species involved in these associations do not interact strongly with the solvent nor do they modify their structures on association. Since DG = 2RTlnK = DH 2 TDS the direction of curvature satisfies the requirement that in these simple systems the equilibrium constant for association will increase with increasing exothermicity of association.More importantly the curve qualitatively emphasises how the adverse entropy of Martin Westwell was born in 1971 in Lancashire. He obtained his undergraduate and doctoral degrees from Churchill College Cambridge University. After a further year working in the Chemistry Department with Professor Williams Dr Westwell has moved to Oxford University where he is the Glaxo Junior Research Fellow in Biological or Medical Sciences at Lincoln College. 57 Chemical Society Reviews 1998 volume 27 Fig. 1 The general form of the extent of the exothermicity of association (DH°) A + B?A·B as a function of the entropic cost (DS°) at a temperature of 298 K.There is a limit in the price in entropy to be paid (due to loss of translational and rotational freedom) and this limit is approached before covalent bond strengths are approached. Fig. 2 Enthalpy (DH°) vs. entropy (298DS°) for the association of macrocycles with neutral molecules in dichloromethane. Data collected by Izatt et al.39 Very weak exothermic associations have favourable entropies due to desolvation [an effect which is absent for an idealised non-polar solvent (or gas phase) (Fig. 1)]. association increases gradually as a function of increased bonding of the associating entities i.e. how increased bonding gradually restricts the dynamic motion in A·B. 3 Problems in trying to obtain the free energy of binding of specific groups in networks of weak interactions Much effort has gone into attempts to estimate free energies of binding for common types of weak interactions [e.g.the hydrophobic effect (Å22) amide–amide hydrogen bonds and salt bridges] as they occur as parts of a network of weak interactions. To understand the problems inherent in such an Chemical Society Reviews 1998 volume 27 58 approach let us first consider binding in a non-polar solvent where there are no internal rotations to be restricted on binding. A favourable free energy contribution to binding (DGp) can occur for any pair of electrostatic interactions (e.g. amide– amide hydrogen bond formation) found in the binding site. It is possible to consider the sum of all such pairs of interactions (SDGp).These binding terms are opposed by the entropic cost of reducing the overall motion of the ligand when it binds to its receptor. This is described as DGt+r the adverse free energy change due to loss of entropy of translation and overall rotation. Thus if the observed free energy of binding is DGobs an attempted partitioning can consider the approximation eqn. (2). (2) DGobs = DGt+r + SDGp The reason for believing that eqn. (2) might serve as a useful approximation derives from a limiting case considered in 1981 by Jencks.2 Consider that two species X and Y can associate separately as at left or connected together by a strain free connection as at right into two distinct binding sites of a receptor (Scheme 1). In the ideal and limiting case X Y and X–Y all lose all their translational and (overall rather than internal) rotational entropy on binding to the receptor; this corresponds to the value of TDS Å 50 to 60 kJ mol21 for immobilisation referred to above.Thus in applying eqn. (2) to all three possible binding events (of X Y and X–Y to the receptor) the adverse value of DGt+r would be the same 50 to 60 kJ mol21 in all three cases. Scheme 1 Let us hypothetically (and unreasonably—see later) select the binding constants of X and Y as KX = KY = 103 dm3 mol21 and take the loss of entropy as the limit DGt+r Å 57 kJ mol21 (because each 5.7 kJ mol21 opposes binding by a factor of 10 at room temperature). The price in entropy only has to be paid once in the binding of X Y or X–Y.Applying eqn. (2) the intrinsic binding affinity (the limiting DGp value defined by Jencks2 as the binding affinity expressed when association occurs without adverse entropy) of both X and Y can be derived. When X (or Y) binds alone to the receptor we know that DGobs Å 217.1 kJ mol21 (K = 103 dm3 mol21) and DGt+r Å + 57.0 kJ mol21 such that eqn. (3) holds. DGp(X) = DGobs2D Gt+r = 217.1257 = 274.1 kJ mol21 (3) Now if we consider the binding of X–Y the lost entropy is again in the limit DGt+r Å + 57.0 kJ mol21 and the hypothetical DGobs from eqn. (2) is given by eqn. (4). DGobs(X–Y) = DGt + r + SDGp = D Gt+r = + DGp(X) + DGp(Y) (4) = + 57.0274.1274.1 = 291.2 kJ mol21 Thus if KX = KY = 103 dm3 mol21 then from the above analysis KX–Y Å 1016 dm3 mol21.Although this example illustrates the principle of an extreme case it is also clear that it involves an assumption which is physically completely unrealistic. Any association with K = 103 dm3 mol21 would not occur with complete loss of translational and rotational entropy but rather with only partial loss of this entropy. The contribution of Fig. 1 towards an understanding of the problem is that a semi-quantitated form would allow a crude guideline to the entropic cost (due to loss of translational and rotational entropy) as a function of the strength of the electrostatic interaction formed. That is for a gas phase association the horizontal axis of Fig. 1 is the DGt+r term and the vertical axis is the DGp (or SDGp) term (although not the limiting DGp term as defined by Jencks).We see how in qualitative terms the term opposing association (DGt+r) by restricting motion is played off against the term promoting association [DGp (or SDGp)] by favourable bonding interactions. The gas phase description is carried over to the case of association in non-polar solvents as a useful approximation. Since even a non-polar solvent will always interact finitely with the associating species that are dissolved in it the nature of the approximation is exposed by the fact that the plot does not pass through the origin in Fig. 2. Where X and Y associate with very low exothermicity the adverse DGt+r term for the association is very small and is more than offset by the favourable entropy of release of solvent.The curve therefore tails into the TDS > 0 of Fig. 2 very weakly exothermic associations occurring with a net favourable entropy. Let us now consider some physically plausible cases in terms of the entropy–enthalpy compensation curve. In the situation we consider first (solid lines in Fig. 3) the simplifying assumption is made that the exothermicity of the association of X–Y with the receptor is simply the sum of the exothermicities when X and Y bind separately (DHX–Y = DHX + DHY). Suppose X alone binds with an exothermicity of 20 kJ mol21 and Y with an exothermicity of 50 kJ mol21. On the basis of Fig. 3 the respective adverse TDSt+r terms would be approximately 20 and 38 kJ mol21 giving rise to DGX and DGY of 0 and 12 kJ mol21 (KX = 1 and KY = 1.3 3 102 dm3 mol21 at 298 K).Taking the exothermicity of binding of X–Y as 70 kJ mol21 from Fig. 3 the cost in TDSt+r is 46 kJ mol21 giving DGX–Y as 24 kJ mol21 (KX–Y = 1.6 3 104 dm3 mol21). Thus in this case of X binding with a relatively small exothermicity and Y binding with a moderate exothermicity (case 1 Table 1) the enhancement of binding constant of X–Y relative to its separate components (expressed as KX–Y/KXKY) is ca. 102. In contrast if the exothermicities of binding of X and Y are respectively 50 and 70 kJ mol21 then the respective TDSt+r values for X and Y are estimated as 38 and 46 kJ mol21 giving rise to DG DG X and Y of 12 and 24 kJ mol21 (KX = 1.3 3 102 and KY = 1.6 3 104 dm3 mol21 at 298 K); X–Y binds with an exothermicity of 120 kJ mol21 and an adverse TDSt+r value of 50 kJ mol21 (TDSt+r is approaching its limit) giving DGX–Y as 70 kJ mol21 (KX–Y = 1.9 3 1012 dm3 mol21).In this case of X binding with Fig. 3 Entropy–enthalpy compensation curve illustrating the enthalpy and related entropy values for the association of X and Y with a substrate where DHX = 20 kJ mol21 DHY = 50 kJ mol21 DHX–Y = 70 kJ mol21 (solid lines). When X and Y are tethered together the binding of X will enhance the binding of Y and vice versa. This is represented by the dashed lines. a moderate exothermicity and Y binding with a larger exothermicity (case 3) the enhancement of binding constant of X–Y relative to its separate components (expressed as KX–Y/ KXKY) is ca.9 3 105 dm3 mol21. The most important of these data along with the analysis of one where the X and Y exothermicities are respectively small and large (case 2) are summarised in Table 1. Whatever a more precise form of Fig. 3 may be given only the generality that more exothermic interactions approach a limiting cost in entropy it is seen that this expression of cooperativity (the classical chelate effect) is greatest where each of the associations of X and Y are quite strongly exothermic. (It is for this reason that the hypothesis KX = KY = 103 dm3 mol21 made at the beginning of this section for the purposes of illustration of an intrinsic binding affinity is an unreasonable one if X and Y lose essentially all their entropy in binding to a receptor the binding into the receptor sites would have to be very strong and KX and KY would have to much greater than 103 dm3 mol21).Table 1 First approximation for estimating some hypothetical relative magnitudes of chelate effects as a function of exothermicities of association K K X–Y b Y b X a DHY a KX b Case DH KX–Y/ KXKY c 1 1 2 3 220 220 250 1.3 3 102 1.6 3 104 1.2 3 102 1 1.6 3 104 2.3 3 107 1.4 3 103 1.3 3 102 1.6 3 104 1.9 3 1012 9.0 3 105 250 270 270 a kJ mol21. b dm3 mol21. c mol dm23. We can in fact make a simple refinement of the situation considered thus far. The consequences of this refinement will be considered for case 1 in Table 1. For each of the X and Y interactions only a part of the total theoretical maximum translational and rotational entropy which could be lost is lost (cf.Fig. 3). This is because in a weak interaction the theoretical maximum bonding (which could be expressed at 0 K) is not expressed at room temperature due to the opposing entropic advantage of residual motion resulting in an average position in the enthalpic well corresponding to an exothermicity of 20 kJ mol21 when X binds alone and 50 kJ mol21 when Y binds alone. However if the restriction of motion is aided by a neighbouring exothermic interaction then the average position in the enthalpic well will correspond to some larger exothermicity. This is because the binding of the X part of X–Y is aided by the binding of the Y part of X–Y and vice versa.Thus in the general and real case it is to be expected that the exothermicity of binding of X–Y will be greater than the sum of the exothermicities with which X and Y separately bind. The effect of this second cooperative factor which is quite distinct from that described to give the numbers in Table 1 is illustrated by a hypothetical example in Fig. 3. The solid horizontal lines indicate the enthalpies of association of X Y and X–Y (20 50 and 70 kJ mol21 respectively) for case 1 (Table 1) analysed with the previous simple assumption that the exothermicity of association of X–Y is equal to the sum of X and Y. The dotted lines indicate the corresponding analysis allowing for the effect now being considered whereby X–Y binds more exothermically than the sum of its parts since in X–Y X helps to anchor Y and Y helps to anchor X.The increase in binding energy of X–Y due to this effect is DDH 2 298DDS (Fig. 3). Fig. 3 has been drawn to illustrate the physically likely consequence that a weakly exothermic interaction (of X) is likely to be strengthened more by the assistance of a stronger adjacent interaction (of Y) than vice versa. The importance of this last analysis is in a case where X and Y bind in a strain free manner as X–Y through a direct connection of X and Y. The enhancement of binding should occur not simply through a classical entropy driven chelate effect but also as a consequence of an improved enthalpy of binding. It is this latter consequence that precludes the derivation of free energies of binding which are character- 59 Chemical Society Reviews 1998 volume 27 istic of common functional groups even in relatively simple systems.The binding energy obtainable from any specified weak interaction will always be context dependent. In sections 5 and 6 we present experimental data to support this view. 4 Model 2 Where associations occur in water Having considered weak associations occurring where the interaction with solvent is relatively weak we now turn to the more complex situation of associations in water. Much effort has gone into attempts to estimate free energies of binding for common types of weak interactions [e.g. the hydrophobic effect (Å22),6–9 amide–amide hydrogen bonds10–16 and salt bridges10,17–19] as they occur as parts of a network of weak interactions in water as solvent.For convenience we may consider two kinds of models in this area. First those systems where it might be a useful approximation to consider that the associating entities A and B retain their structures in the associated state except insofar as there might be restrictions of internal rotations of A and/or B. Second where A and B do not retain their internal structures in the same form; we reserve considerations of these cases until section 6. In the former case eqn. (5) would form a simple extension of eqn. (2) for strain-free systems eqn. (5). DGobs = DGt+r +DGr +DGh + SDGp (5) Here the contributions which promote binding are the familiar term SDGp and the new term DGh the contribution from the hydrophobic effect which corresponds to the favourable free energy of binding arising from the removal of hydrocarbon surface area from water upon association.It is a term which is conveniently separated from other binding terms for two reasons. First at room temperature it is essentially purely an entropy term—favourable because when hydrocarbon surface area is removed from exposure to water the water structure becomes more disordered.20 Second its magnitude is proportional to the surface area of hydrocarbon removed from exposure to water.6–9 This surface area is frequently conveniently measured from modelling studies. The terms which oppose binding are the familiar DGt+r term and the new term DGr due to loss of entropy associated with the restriction of any internal rotations which may occur on binding.If this loss of t+r entropy is DSr then at 298 K DGr = 2298DSr. A problem in applying eqn. (5) will presumably derive from the same source as in applying eqn. (2). For example if we add a polar group into a binding site and then measure the change in DGobs we will in the general case not obtain a true DGp value because the addition of the polar group increases the adjacent interactions in a manner analogous to the arguments already presented for the case in non-polar solvents. It is true that the addition of the polar group will also increase the adverse DG term but the crux of Fig. 1 and 3 is that the benefit to binding of the extra bonding will outweigh the extra cost in entropy.The key conclusion is that deletions (including mutation studies on proteins) of groups which contribute to binding in strain-free systems will normally be expected to give DGp values which are too large. In the following section we present data for binding in water to support this conclusion. 5 Attempts to obtain individual group contributions to binding and cooperativity at an interface In the early 1990s we selected glycopeptide antibiotics of the vancomycin group in their binding of bacterial cell-wall peptide analogues as a vehicle to test the application of the approximation represented by eqn. (5). Since we now know in what way this approximation is likely to err what have we been able to learn from the studies? Since the study involved restriction of internal rotations within the bacterial cell-wall peptide analogues upon binding to the antibiotics we required guides to DGr.This parameter was taken to lie in the range 2–5 kJ mol21; the lower end of this Chemical Society Reviews 1998 volume 27 60 range came from the restriction of internal rotations for the formation of crystals from the liquid state,6,21 and the upper end from the restriction of internal rotations in the formation of small rings from linear hydrocarbons.22 The smaller values (2–3 kJ mol21) presumably reflect the larger residual motions in crystals compared to tight ring structures and therefore are probably more appropriate to the associations commonly found in biology. We were able to delete methyl groups and amide–amide hydrogen bonds in the antibiotic binding sites and so evaluate the apparent binding energies associated with these entities.Our first attempts carried out the partitioning incorrectly and gave amide–amide hydrogen bond strengths which were far too high (ca. 20 kJ mol21 in water).23,24 A more appropriate partitioning gave these bond strengths in the range 0–7 kJ mol21 and a hydrophobic effect of 0.20 kJ mol21 Å22 (of hydrocarbon buried from water on binding).6 Interestingly the values are in good accord with the apparent binding energies obtained by protein engineering experiments (1–8 kJ mol21 and a hydrophobic effect of 0.23 kJ mol21 Å22).7 Despite this agreement the arguments presented in sections 2 and 3 suggest that these values should be larger than the true local binding energies.We now present two of our most recent studies which support this conclusion. O H O– CH3 O– H3C N H3C O NHAc O H O N H3C CH3 H H3C H N H (i) We have examined binding of the ligands 1 to 4 into the binding site of vancomycin group antibiotics. As the network of interactions which increases along the series 1 to 4 is extended (to the left as displayed in Scheme 2) the strength of the hydrogen bond between the carboxylate oxygen atom of the ligands and the antibiotic NH (designated w2) gradually increases.25 Thus the carboxylate anion is bound more strongly into the pocket which receives it as it is aided in this binding by adjacent interactions which help to restrict the ligand motion.This result indicates that if amide–amide hydrogen bond free energies are inferred from free energies of binding in the series 1 ? 2 ? 3 then the derived hydrogen bond strengths will be too large for the addition of these hydrogen bonds to the network increases the strengths of adjacent interactions. (ii) We have also examined the ligand series 3 5 6 and 7. In every case it is observed that a change in the ligand of a Gly O H O H N H3C N H N-AcDA 2 N H O H H H H O H H O acetate 1 H3C O Di- N-AcKDADA 4 O– N H N-AcGDA 5 H3C N CH3 H O O H O N-AcDAG 6 H O H N CH3 O– N CH3 H O O O H N-AcDADA 3 H CH3 O– H CH3 O– O H N O– N H H O H N-AcGG 7 NHAc CH O H O 3 N N N O – w2 H O H CH O H 3 N H N Di- N-Ac-KDADA H O H N C N antibiotic binding pocket Scheme 2 to an Ala increases the strength of the hydrogen bond from carboxylate oxygen to w2 the antibiotic NH.26 Thus the apparent increased binding energies from the Gly ? Ala ‘mutations’ must reflect in part a contribution from the increase in strength of this adjacent hydrogen bond.This cooperative strengthening of adjacent interactions can account for the fact that the values cited earlier in this section for the hydrophobic effect (0.20 and 0.23 kJ mol21 Å22)7,21 are larger than the solvent transfer value9 (0.125 kJ mol21 Å22 which cannot benefit from the type of cooperativity described).While it is of course also possible that the conformational bias of the Alacontaining peptides (relative to Gly-containing) might improve binding site affinity and so contribute to the strengthening of the adjacent hydrogen bond the larger apparent value of the hydrophobic effect from the ‘binding site’ vs. ‘solvent transfer’ experiments is understandable from a common viewpoint the addition of binding affinity or the restriction of motion at one point in the binding site can improve the binding affinity in an adjacent site. In summary real systems will always have residual motion. The addition of an extra interaction will typically (in a strainfree system) reduce adjacent motions and improve the free energy of binding of adjacent groups.Therefore attempts to estimate the binding affinities of specified groups will in strain free systems tend to give values that are too large. This consequence has been exemplified for the hydrophobic effect above and also implies that the apparent amide–amide bond strengths in water of 0–8 kJ mol21 are probably benefiting from other cooperative interactions and are therefore likely to be over-estimates. This conclusion is consistent with recent calculations which suggest that the enthalpy of formation of peptide hydrogen bonds is close to zero.27 6 Model 3 Binding where A and B can adjust their structures on association In the light of the formalism of eqn. (1) it has perhaps been inevitable that many studies of weak interactions have sought the origins of experimental binding energies by an examination of the interactions of A and B with solvent and of the interface between these two entities.This approach will be invalid if A and B change their internal structures upon association. Therefore if one or more of the associating components is a folded polypeptide (essentially all biological receptors) or a polymer of DNA or RNA which is involved in duplex or folded structures then the binding energy of the two components cannot reliably be sought at the binding interface even after consideration of the energetics of desolvation of this interface. If we temporarily ignore the interactions with solvent then in such cases a more appropriate form of the equilibrium constant would be given by eqn.(6). (6) A + B"AA·BA Eqn. (6) recognises that once A and B have associated then typically they no longer exist. Rather they have been replaced by modified entities AA and BA. Crucially the formalism of eqn. (6) emphasises that the binding energy between the two entities that come together is not simply a property of the interface between them but also is dependent upon the modifications of the internal structures of A and B (A ? AA and B ? BA). Although it might be argued that so much is self-evident reference to the literature indicates that this is so for many authors but equally not so for many others. It is a common practice to rationalise the observed binding energy between A and B by examination of the interface between them and to ignore the consequences of the changes A?AA and/or B?BA.These changes may at one extreme take the form of obvious structural modifications but at the other extreme may in principle involve essentially no structural reorganisation but simply a ‘tightening’ (or a ‘loosening’) of the internal structure of A when it is modified to AA (or of B when it is modified to BA). Where these consequences can be considered it may be possible to make semi-quantitative adjustments for the reorganisation B?BA where B is a small substrate but so far as we are aware never for A ? AA where A is a large receptor. Calculations might attempt to account for the free energy change and even to include the effect of solvent but the forcefields currently in use are not sufficiently accurate to give reliable free energy changes for systems involving large receptors.‡ So the binding affinities of greatest interest cannot be readily understood in molecular terms.We present below a relatively simple example of this complication. 7 Cooperativity beyond the interface While some glycopeptide antibiotics show no measurable propensity to dimerise (e.g. teicoplanin) some do strongly (e.g. eremomycin).29 In general the antibiotics dimerise more strongly in the presence of bacterial cell-wall mucopeptide precursor analogues than in their absence.29 For example the dimer of the glycopeptide antibiotic eremomycin has Kdim = 3 3 106 dm3 mol21 in the absence of di-N-acetyl-Lys-d-Alad-Ala but Kdim = 3 3108 dm3 mol21 in its presence.It follows from a thermodynamic cycle that di-N-acetyl-Lys-d-Ala-d-Ala is bound by a factor of 10 more strongly by the dimer than by the monomer (each site of the dimer binds cell-wall analogue with the same affinity.30) The basis for at least part of this cooperativity can be seen from the structure of the ligand-bound dimer [Fig. 4(a)]. The antibiotics showing the largest dimerisation constants have in addition to the four hydrogen bonds at the dimer interface [heavy dashed lines in Fig. 4(a)] two additional hydrogen bonds. These two bonds are from the alkylammonium ions of the amino sugars (which are unique to the strongly dimerising antibiotics) to amide carbonyl groups in the other half of the dimer [Fig.4(b)]. The alkyl ammonium ions one at each end of the head-to-tail dimer are brought into the proximity of the cell-wall analogue carboxylate anion [one in each half of the dimer; Fig. 4(b)]. The resulting Coulombic attraction can tighten binding at both the ligand–antibiotic and dimer interfaces (Fig. 4). One way of looking at this cooperativity is that strong dimer formation also makes for the formation of a salt bridge which is mediated through an intervening amide bond [arrowed in Fig. 4(b)]. The evolution of this sophisticated interaction suggests that dimers may work more efficiently than monomers in antibacterial action and indeed this has been shown to be the case.31 However in the present context the relevant questions are (i) in the promotion of the dimerisation constant of eremomycin from Kdim = 3 3 106 to Kdim = 3 3 108 dm3 mol21 by ligand can we infer anything useful about the origin of the extra binding energy? ‡ The difference in binding affinities (DDG) of two closely related ligands to a common receptor of moderate size had been calculated with impressive accuracy,28 but this accuracy is only possible because relatively large systematic errors in the calculation are removed by difference.61 Chemical Society Reviews 1998 volume 27 Fig. 4 (a) The structure of the antibiotic dimer (backbone only) bound to ligand (N-acetyl-d-Ala-d-Ala). The hydrogen bonds at the dimer interface are represented as broad dashed lines and those in the ligand binding pocket as narrower dashed lines.(b) The strongly dimerising antibiotics have an amino sugar on residue 6 which has a Coulombic interaction with the carboxylate of the ligand. This is essentially a salt bridge mediated by the peptide bond between residues 2 and 3. (ii) similarly in the promotion of the binding of di-N-acetyl- Lys-d-Ala-d-Ala to the dimer over the monomer by a factor of 10 can we infer anything useful about the origin of the extra binding energy? These questions have not yet been addressed by experiment. Yet what seems to be a physically reasonable model representing dimer by 8 ligand-bound monomer by 9 and ligand-bound dimer by 10 gives plausible insights (in these diagrams the + signs represent the ammonium ions of the amino sugar and the 2 signs represent the negative charge of the carboxylate ion of the cell-wall analogue ligand).Since there is little doubt that at least part of the cooperativity is due to the Coulombic attraction between these two opposite charges the models 8 to 10 suggest the following. In the case of question (i) part of the extra stability of the dimer structure 10 over the dimer structure 8 lies in the stronger binding of the ligand in 10 relative to its binding in 9. That is when two of the entities shown in 9 come together to give one of 10 then compared to the formation of 8 from two antibiotic monomers part of the increased binding energy will come from Chemical Society Reviews 1998 volume 27 62 the fact that ligand is bound more tightly in 10 than 9.In other words in the increase of the dimerisation constant by a factor of 100 some of the favourable free energy should come from the ‘tightening’ of the ligand–antibiotic interface upon dimerisation. The Coulombic attractions which are unique to 10 (and indicated by double-headed arrows in this structure) can be expected to lead to strengthening of the weak interactions at all three interfaces present in 10. It is for this reason that we have schematically inferred bond-shortening at all three interfaces in 10 relative to 8 and 9. In the case of question (ii) the analysis is of course equally relevant in illustrating how the increased affinity of the ligand for the dimeric receptor (10) over the monomeric receptor (8) cannot be simply ascribed to the strengthening of the weak interactions at the ligand–receptor interface in 10 relative to 9.It will in part be also due to the strengthening of the weak interactions at the dimer interface. These models illustrate in a simple way the potential origins of binding affinity which is remote from the binding interface. In large systems this makes the prediction of binding constants more difficult because interactions remote from the binding site may significantly contribute to the overall binding constant. 8 Can binding affinities be reliably predicted? The extensive and subtle changes which can affect binding affinities as outlined in the preceding sections suggest that relatively accurate de novo predictions of equilibrium constants for associations of extended networks are not likely to be achieved in the general case in the near future.Despite the problems in the search for a solution that is universally valid the use of eqn. (5)21,32 has recently been extended by Bohm and with a surprisingly good outcome for a limited data set.33 In this work the four terms (DGt+r + DGr + DGh + SDGp) of eqn. (5) are used but the last of these (for binding electrostatic attractions) is sub-divided into two terms—one for formally uncharged hydrogen bonds (DGhb) and a second for a polar interaction involving a charged entity (DGionic). A set of 45 interactions of experimentally known binding constants was then considered where ligands of relatively small molecular mass (66 to 1047) interact with proteins.Since the modified form of eqn. (5) has only five unknowns and the 45 interactions involve different combinations of these unknowns average values for the five parameters can be obtained. From this training set the average contribution from a neutral hydrogen bond of ideal geometry was 24.7 kJ mol21 from the corresponding ionic interaction was 28.3 kJ mol21 from the hydrophobic effect was 20.17 kJ mol21 Å22 and from the restriction of a rotatable bond was +1.4 kJ mol21. The average opposition to binding from DGt+r was +5.4 kJ mol21. This energy function reproduced the binding constants of the training set (which experimentally range from 40 to 2.5 3 1013 dm3 mol21) with a standard deviation of 7.9 kJ mol21 corresponding to 1.4 orders of magnitude of binding affinity.These results are approximations which may be very useful and merit some comment. First the values obtained for the polar interactions are in good agreement with those obtained by other approaches,11–13,16 although for the reasons presented earlier these should perhaps best be regarded as apparent binding energies12,34,35 rather than as localised bond energies. Second, the value for the hydrophobic effect is in reasonable accord with that from other work being intermediate between that from solvent transfer experiments (20.12 kJ mol21 Å22)9 and from the deletion of methyl groups in binding sites (20.20 to 20.23 kJ mol21 Å22).7,21 Third the cost of restricting a rotatable bond is somewhat less than that found for the melting of crystals (2 to 3 kJ mol21),6 and may reflect the fact that rotations are somewhat less restricted in these binding sites than they are in crystals.Fourth the average value of DGt+r of +5.4 kJ mol21 is remarkably small. It represents only about one tenth of the maximum theoretical entropy loss (corresponding to complete immobilisation of the ligand). If it is assumed that this estimate is realistic (Bohm notes that this parameter is the most uncertain of all those derived) then it may reflect small average exothermicities of association of these ligands to their protein receptors in water—a suggestion which seems physically plausible. Last it is noteworthy that one of the largest errors in the estimated binding constants is found for the streptavidin– biotin interaction (Kcalc = 1010.75 vs.Kexp = 1013.4 dm3 mol21). In a sense this is remarkably good agreement for such high affinity binding when computed by a simple method. But it should be noted that this measure of agreement was only obtained by regarding the ureido group of biotin as a charged entity (following the suggestion of Weber et al.),36 despite the fact that it is formally uncharged. This is perhaps an attempt to rationalise the origins of binding energy at an interface where in fact the origins are probably much more complex in this case.37 Despite such problems the application of eqn. (5) in the approach of Bohm gives impressively accurate predictions overall and as a pragmatic and simple approach it has much to commend it.It will be of great interest to see how it performs with a wider set of associations or possibly also after further refinement. Another promising approach to predict the binding affinity of novel ligands for receptors of known three-dimensional structure is currently being explored.38 This method known as VALIDATE gives an absolute average error of only 1.45 log units between experimental and computed binding constants for 11 thermolysin inhibitors which were not part of the training set (though other thermolysin–ligand interactions were in the training set). Additionally for a set of 14 inhibitors where neither the ligands nor the specific receptors were included in the training set the technique gave estimated binding constants that had an absolute average error of only 0.68 log units relative to the experimental values.Both of the above semi-empirical methods may lack an accurate physical description of binding processes but their importance lies in the reasonable success of their predictions. Conclusions In studying weak interactions we have considered the play-off or compensation between electrostatic bonding and the restriction in motion. These two terms form the basis of our attempt for gas phase interactions and those occurring in non-polar solvents to factorise the observed free energy of binding into its component parts. For associations in water we have developed an approach where the hydrophobic effect and internal rotations in the associating molecules are taken into account such that some prediction of binding constants and their component parts can be made.Further we note that in biological systems cooperativity both at the binding interface and remote from it can contribute significantly to the observed free energy; see also ref. 37. That is the sum of the isolated component parts is less than the whole observed binding energy. Because the entropic cost of an association is variable this complication cannot be removed by simply trying to factor out the entropic term. Motion and bonding are intrinsically linked. Despite these complications current semi-empirical approaches for the estimation of binding affinities are giving promising results. Weak interactions may still be ill understood but they are giving up their secrets.10 Acknowledgements We thank the EPSRC BBSRC and Glaxo Research and Development for financial support. Lilly Laboratories Indianapolis are thanked for the exchange of information and useful discussions regarding the vancomycin group antibiotics. 11 References 1 W. P. Jencks Adv. Enzymol. 1975 43 219. 2 W. P. Jencks Proc. Natl. Acad. Sci. USA 1981 78 4046. 3 M. S. Searle M. S. Westwell and D. H. Williams J. Chem. Soc. Perkin Trans. 2 1995 141. 4 M. S. Westwell J. Klein and D. H. Williams J. Phys. Chem. 1996 100 16000. 5 J. D. Dunitz Chem. Biol. 1995 2 709. 6 M. S. Searle and D. H. Williams J. Am. Chem. Soc. 1992 114 7 L. Serrano J.-L. Neira J. Sancho and A. R. Fersht Nature 1992 356 8 K. A.Sharp A. Nicholls R. Friedman and B. Honig Biochemistry 10690. 453. 1991 30 9686. 9 R. L. Baldwin Proc. Natl. Acad. Sci. USA 1986 82 8069. 10 A. Fersht Enzyme Structure and Mechanism W H Freeman and Company New York 2nd edn. 1985. 11 A. R. Fersht Trends Biochem. Sci. 1987 12 301. 12 A. R. Fersht J.-P. Shi J. Knill-Jones D. M. Lowe A. J. Wilkinson D. M. Blow P. Brick P. Carter M. M. Y. Waye and G. Winter Nature 1985 314 235. 13 B. A. Shirley P. Stanssens U. Hahn and C. N. Pace Biochemistry 1992 31 725. 14 R. L. Baldwin Trends Biochem. Sci. 1989 14 291. 15 K. A. Dill Biochemistry 1990 29 7133. 16 D. H. Williams M. S. Searle M. S. Westwell U. Gerhard and S. E. Holroyd Phil. Trans. R. Soc. London A 1993 345 11. 17 A. R. Fersht J. Mol. Biol.1972 64 497. 18 M. F. Perutz Nature 1970 228 726. 19 A. Bierzynski P. S. Kim and R. L. Baldwin Proc. Natl. Acad. Sci. USA 1982 79 2470. 20 N. Muller Acc. Chem. Res. 1990 23 23. 21 M. S. Searle D. H. Williams and U. Gerhard J. Am. Chem. Soc. 1992 114 10697. 22 M. I. Page and W. P. Jencks Proc. Natl. Acad. Sci. USA 1971 68 1678. 23 D. H. Williams Aldrichim. Acta 1991 24 71. 24 A. J. Doig and D. H. Williams J. Am. Chem. Soc. 1992 114 338. 25 P. Groves M. S. Searle M. S. Westwell and D. H. Williams J. Chem. Soc. Chem. Commun. 1994 1519. 26 G. J. Sharman M. S. Searle B. Benhamu P. Groves and D. H. Williams Angew. Chem. 1995 34 1483. 27 M. J. Sippl J. Mol. Biol. 1996 260 664. 28 W. C. Still and R. Liu Phil. Trans. R. Soc. London A 1993 345 97. 29 J. P. Mackay U. Gerhard D. A. Beauregard M. S. Westwell M. S. Searle and D. H. Williams J. Am. Chem. Soc. 1994 116 4581. 30 G. Batta M. F. Cristofaro G. J. Sharman and D. H. Williams Chem. Commun. 1996 101. 31 D. A. Beauregard D. H. Williams M. N. Gwynn and D. J. Knowles Antimicrob. Agents Chemother. 1995 39 781. 32 P. R. Andrews D. J. Craik and J. L. Martin J. Med. Chem. 1984 27 1648. 33 H.-J. Bohm J. Comp.-Aided Mol. Design 1994 8 243. 34 A. E. Mark and W. F. van Gunsteren J. Mol. Biol. 1994 240 167. 35 D. H. Williams M. S. Searle M. S. Westwell J. P. Mackay P. Groves and D. A. Beauregard Chemtracts 1994 7 133. 36 P. C. Weber J. J. Wendoloski M. W. Pantoliano and F. R. Salemme J. Am. Chem. Soc. 1992 114 3197. 37 D. H. Williams and M. S. Westwell Chem. Biol. 1996 3 695. 38 R. D. Head M. L. Smythe T. I. Oprea C. L. Waller S. M. Green and G. R. Marshall J. Am. Chem. Soc. 1996 118 3959. 39 R. M. Izatt J. S. Bradshaw K. Pawlak R. L. Bruening and B. J. Tarbet Chem. Rev. 1992 92 1261. Received 10th July 1997 Accepted 15th September 1997 63 Chemical Society Reviews 1998 volume 27

ISSN:0306-0012    

DOI:10.1039/a827057z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Covalency in semiconductor quantum dots James R. Heath and Joseph J. Shiang UCLA Department of Chemistry and Biochemistry 405 Hilgard Avenue Los Angeles California 90095-1569 USA Chemical schemes for the preparation of direct band-gap semiconductor quantum dots have advanced rapidly over the past few years. It is now possible to prepare a variety of III–V semiconductors with a finite size (InP InAs GaAs etc.) and compare their size-dependent properties with the well studied II–VI class of quantum dots (ZnS CdS CdSe etc.). In this Review various physical properties of semiconductor quantum dots are presented within a discussion framework of lattice covalency. Included in the Review are discussions of the various chemical synthetic routes for making the particles as well the electronic structure and the electronic dynamics of nanocrystals.1 Introduction Crystalline solids are typically characterized by various physical properties such as melting point conductivity color etc. However each of these physical properties has a characteristic length scale generally of the order of 10–100 nm or so. If the physical limits of the crystal are reduced below the characteristic length scale for a particular property then that property becomes ‘size confined’. In this case the property is no longer just a function of the chemical and structural nature of the material but also of the size and shape of the crystal. In the past decade the modification of materials characteristics by size and shape control has been demonstrated for a host of technologically important metallic and semiconducting solids.1 Although all crystalline materials can in principle exhibit finitesize effects perhaps the most spectacular examples of size-confinement have been observed in nanocrystals (or quantum dots) of inorganic semiconducting solids.The number of size-dependent phenomena that have been observed in James R. Heath is Professor of Chemistry at UCLA. As a graduate student in 1985 he was a co-discoverer of the fullerenes along with Richard Smalley Robert Curl and Sir Harold Kroto and Sean O’Brien. He received his PhD in 1988 and took a Miller Fellowship at UC Berkeley. At Berkeley he worked with Richard Saykally developing spectroscopic techniques for probing the structures of bare clusters of refractory elements.In 1991 he joined the research staff at IBM Watson Labs and in 1994 he moved to UCLA taking the position of assistant professor. He was promoted to Associate Professor in 1996 and Professor in 1997. His current research interests center on building nanoscale architectures for computational applications. James R. Heath semiconductor quantum dots include light emission from Si nanocrystals and porous Si,2 broad band-gap tunability in the direct gap semiconducting nanoparticles,3 and size-dependent structural phase diagrams and melting points.4 This remarkable range of size-dependent properties that are available from chemically similar systems have made semiconducting nanoparticles attractive candidates for many technological applications.For example nanocrystals can be size-tuned to behave as wavelength specific photoemitters or photoabsorbers for light emitting diode (LED) or photovoltaic applications respectively. 1 Another potential application takes advantage of the fact that the energies of the photo-generated positive and negative charge carriers within a particle are also size-tunable. Thus various applications in which particles are used as energy selective photo-oxidative or -reductive catalysts have been explored.5 Other possible applications are related to electrically charging nanocrystals. The energy to electrically charge a nanocrystal is size-dependent and for particles less than 10 nm in size this charging energy is substantially larger than kT at room temperature.Thus several groups have explored using nanocrystals as single-electron capacitive or switching devices. 6 At this date however large-scale applications of semiconducting nanocrystals have yet to emerge and the study of these materials is still most appropriately referred to as ‘nanoscience’ rather than ‘nano-technology’. The chemical techniques for controlling nanocrystal size and the accompanying development of a physical picture of finite size effects began in the early 1980s with Brus’ pioneering work on solution-phase synthesized II–VI quantum dots.7 Since that time a steady series of advances in the chemical preparation of these nanocrystals have made II–VI nanocrystals the prototypes for the investigation of finite size effects.The wide variety of II–VI nanocrystals available has enabled Joseph Shiang received his bachelor’s degree from the California Institute of Technology in 1988. He received his PhD (Chemistry) in 1994 from the University of California Berkeley while working under the direction of A. Paul Alivisatos and was a Postdoctoral associate at the University of California Los Angeles from 1995 to 1997. His scientific interests are in the use of spectroscopic methods and in particular their application to nanoscale systems. He is currently at the Center for Ultrafast Optical Sciences at the University of Michigan Ann Arbor. Joseph Shiang 65 Chemical Society Reviews 1998 volume 27 researchers to establish periodic trends within the II and VI columns of the periodic table (i.e.ZnSe vs. CdTe). Until recently it has not been possible to explore periodic trends within rows of the periodic table. Moving along rows within the periodic table (toward III–V or Group IV materials) increases the covalent character of the chemical bonding and thus should have profound impact on size-dependent properties. In the past few years synthetic schemes have been developed for fabricating Group IV (Si,2,8 Ge9) and III–V (InP,10,11 InAs,12 GaP,10 GaAs13) semiconducting materials in finite size. Of these two classes of materials the III–Vs are the most closely related to the II–VIs. Like the II–VIs most III–Vs are direct gap semiconductors. They are also characterized by tetrahedral bonding geometries (wurtzite or zinc blende crystal habits) and the chemical nature of the unit cell has both ionic and covalent bonding contributions.The availability of high-quality sizeselected III–V nanocrystals has enabled various chemical and physical characterizations of finite-size effects in these materials. The purpose of this review is to use the language of lattice covalency to compare the size-dependent properties of a typical III–V nanocrystal system InP with those of the best understood II–VI nanocrystal systems CdSe and/or CdS. In Section 2 we present a basic picture of the electronic structure of a nanocrystal emphasizing briefly the similarities and the differences that exist between various semiconductors.In Section 3 we will show that the covalency of the nanocrystal lattice is reflected in the relative level of difficulty in preparing high quality size-selected nanocrystals of these materials. In Section 4 we discuss how lattice covalency affects the various electronic length scales in these quantum dots including the size of the electron and hole wavefunctions and the length and time scales associated with electron-phonon coupling and short time-scale excited electronic state dephasing. In Section 5 we discuss how lattice covalency is reflected in the nature of observed surface electronic states and how this impacts long time-scale excited electronic state relaxation processes. Finally recent developments related to the inorganic passivation of the surface states for both types of nanocrystals will be discussed.2 Electronic wavefunctions in semiconductor nanocrystals Excellent articles concerning the size-dependence of the electronic states in semiconductor nanocrystals have appeared in the recent literature,1,14,15 and so only a very brief discussion will be presented here with a focus on the importance of covalent interactions in quantum size effects. Further details will be provided as need in Sections 4 and 5. A reasonable picture of finite size effects in semiconductor nanocrystals can be gained by considering the energy band representation of a simple one-electron one-dimensional semiconductor such as that shown in Fig. 1. Each unit cell is represented by a lattice constant a and contains a single pzorbital with a unit cell energy of E.The energy level diagram is essentially that expected from Huckel theory where b is the electronic overlap integral between adjacent unit cells. The lowest energy state will correspond to the situation in which there exists a bonding interaction between neighboring unit cells. A cartoon diagram is shown at the bottom right of Fig. 1. Note that all of the p-orbitals have the same orientation with respect to one another—i.e. in ‘real’ space’ they are all ‘in phase’. In reciprocal or k-space this corresponds to the k = 0 state and it has an energy E 2 b. At the bottom right of Fig. 1 is shown a representation in which each unit cell has a net antibonding interaction with each of its neighbours and the phase of any given p-orbital is rotated by 180° (or p) with respect to the neighboring unit cells.In k-space this corresponds to k = p/a and the energy of this state equals E + b. The imposition of finite size modifies this picture in several ways. First the continuous curve drawn in Fig. 1 implies an infinite 1D solid. In any finite solid the curved line correlating the wavevector k with energy is non-continuous only containing as Chemical Society Reviews 1998 volume 27 66 Fig. 1 Band structure of a one dimensional solid containing only a single p-bonding p-orbital. The energy width of the band is a measurement of the amount of covalent bonding in the unit cell. For a bulk crystal the energy band is continuous but for a finite-sized (nano)crystal the band is discrete containing only as many points as there are unit cells in the crystal.An example of this is shown in the circular inset. The wavefunctions near k = 0 are the largest wavefunctions in this solid and are therefore first removed by the imposition of finite size. many points as there are unit cells in the solid. Second the largest wavefunctions are those near k = 0 and in a finite sized crystal such wavefunctions will no longer fit within the physical dimensions of the crystallite. Thus as the particle size is reduced the distribution of energy levels with respect to k becomes discrete and the k-values corresponding to the largest size wavefunctions (near k = 0) are removed first. If the semiconductor were a direct-gap material then the lowest energy allowed transition would be at k = 0 from the top of the valence band (VB) to the bottom of the conduction band (CB).In a tetrahedral semiconductor the orbitals at the top of the valence band are the p-orbitals which are threefold degenerate at k = 0. The s-orbital is at the bottom of the conduction band. The imposition of finite size would affect the lowest energy optical transitions by moving them to higher energy (by removal of the lowest k states). This is shown in the circular inset of Fig. 1. In addition rather than an absorption spectrum that is a continuum above the band gap energy the absorption spectrum would now be composed of discrete states since k itself is discrete. Relevant to this review is the issue of how increased covalent bonding manifests itself in finite size effects.Of primary importance is the width of the energy bands ( = 2b in Fig. 1) a quantity which is directly proportional to the amount of covalent bonding in the solid. Strongly ionic solids are characterized by flat (and narrow width) energy bands. This translates into a large uncertainty in k with respect to energy and therefore localized electronic wavefunctions. Covalent solids on the other hand are characterized by broad highly curved energy bands and delocalized wavefunctions. A measurement of the curvature of an energy band and thus a measurement of the amount of delocalization of the corresponding wavefunctions is the effective mass m* (= h2/8p2ba2) of electrons (or holes) in those states where h is Planck’s constant and a is the unit cell lattice constant.16 For a few II–VI and III–V direct gap semiconductors the ratio of m* to the mass of an electron me is CdS ( = 0.20) CdSe ( = 0.13) InP ( = 0.13) and InAs (0.039).17 To some extent lighter carrier masses also correlate to narrower energy gaps and the energy gaps for these same four semiconductors are CdS ( = 2.56 eV), CdSe ( = 1.84 eV) InP ( = 1.42 eV) and InAs ( = 0.42 eV).17 Parameters which are of equal importance to m* in considering finite size effects and which are even more strongly correlated to the amount of covalent bonding in the lattice are the static and optical relative permittivity (e0 and eH ).eH describes the response of the lattice valence electrons to an applied electric field.e0 describes the response of both the valence electrons and the cationic and anionic cores and is thus larger than eH. For a homoatomic material where all nuclei have same effective charge the two constants are the same. In homoatomic metals where bonding is completely covalent and electrons are delocalized e is of course inifinite. In large band gap insulators on the other hand eH is typically < 6 or so indicating that charge carriers are poorly screened from the ionic cores. The tendency of eH to increase with increasing covalency is demonstrated by considering a series of solids ranging from I–VII to Group IV materials CuCl ( = 5.7) CdS ( = 5.2) CdSe ( = 5.8) InP ( = 9.6) InAs ( = 12.3) and Ge ( = 16.0).17 H.In a direct gap semiconductor the first excited state is an electron–hole pair or an exciton which has a radius that from a simple Bohr atom picture is given by rexc = {e*/(m*/me)}ao where ao is the Bohr radius.18 The relative permittivity here is denoted as e* because in general one cannot use either e0 or eH directly. In an exciton the electron and the hole are screened from one another by both the lattice cores and the valence electrons. If an exciton is small (the highly ionic I–VIIs fall into this category) then the charge carriers move much faster than the characteristic vibrational frequency of the lattice and only the valence electrons contibute to the screening. Thus e* = e However in more covalent materials (such as III–Vs) the exciton is relatively large and is screened by both the ionic cores and the valence electrons and thus e* = e0.II–VI semiconductors are intermediate and e* has contributions from both constants. The upshot of this argument is that III–Vs are characterized by much larger exciton radii than the more ionic II–VIs. For example in CdSe rexc = 35 Å and in InP rexc = 70 Å. The exciton radius is the most important length scale in determining at what particle diameter ‘finite size’ effects become important. When the size of the crystallite lattice is decreased below the exciton radius quantum size effects appear in the room temperature electronic and optical properties. Other length scales such as the respective sizes of the negative and positive charge carrier wavefunctions become important at smaller sizes.According to the simple physical models presented in this section the degree of covalent bonding in the semiconductor lattice determines both the onset and the magnitude of finite size effects. Thus for the semiconducting materials discussed here finite size effects should be more pronounced in III–V nanocrystals. 3 Synthesis of II–VIs and III–Vs There are many ways to produce semiconductor quantum dots ranging from gas-phase photolysis/thermolysis of inorganic precursors,2,8 to growing quantum dot ‘islands’ via heteroepitaxy, 19 to inorganic and organo-metallic solution-phase synthetic schemes for producing quantum dot colloids.1 These various approaches all have advantages.The gas-phase techniques are very general and the heteroepitaxial techniques are relatively consistent with semiconductor processing technology. The colloidal syntheses produce particles that are most amenable to the various characterization techniques familar to most chemists. In addition the highest quality particles in terms of narrow size distribution and defect-free crystal structures have been produced by these solution-phase routes. It is these particles that we will focus on here. The ideal nanocrystal synthesis produces particles that are soluble monodisperse and characterized by a (chemically controllable) narrow size distribution. In 1951 Reiss showed that in order to produce a narrow size distribution of a given type of particle it was necessary to temporally separate the particle nucleation and growth steps.20 He showed that if the nucleation step could be carried out within some discrete time window dt and the particles then grown by a diffusion-controlled growth mechanism for a given time tg then the width of the particle size distribution would be determined by both dt and tg.If narrow distributions are required for very small particles then it is necessary to keep dt as short as possible. Although this concept is easy to visualize it can be very difficult to put into practice. Murray et al. showed that for the II–VI class of nanocrystals nucleation and growth could be temporally separated and that very narrow distributions of monodisperse nanocrystals could be readily prepared in the size range from 15–150 Å.3 From a chemical point of view it is the nature of the II–VI nanocrystal precursors that allow for such a reaction scheme.In a typical synthesis bare ions or atoms are directly reacted with each other at high temperature ( ~ 350 °C) via rapid injection of one reagent [e.g. Me2Cd dissolved in trioctylphosphine (TOP)] into a flask containing a hot (300 °C) solution of equimolar amounts of another (e.g. a solution of Se metal dissolved in a TOP– trioctylphosphine oxide (TOPO) solution). Both of the solvents TOP and TOPO are coordinating solvents that are stable to very high temperatures. Particle nucleation is initiated immediately upon injection. During the injection process sufficient volume (of the Me2Cd–TOP solution) is added such that the temperature of the reacting mixture drops below 180 °C thereby stopping particle nucleation within seconds.The flask is slowly raised to above 200 °C to allow for particle growth. After a predetermined time period the reaction is quenched and the product nanocrystals are precipitated and purified. This reaction or variations thereof can be utilized to produce extremely narrow distributions of many II–VI nanocrystals. Subsequent size-narrowing techniques such as solvent pair precipitation can be utilized to further narrow the product size distribution to the point where all particles are virtually identical when viewed by transmission electron microscopy (TEM). In a successively more covalent series of materials it becomes increasingly difficult to separate the nucleation and growth processes.This is due both to the nature of the reagents used to make the particles and to the nature of the particles themselves. For the III–V and Group IV systems bare atoms or ions are not chemically stable species and so the reactions to produce nanocrystals must be carried out with strongly complexed precursors. Because of this particle nucleation and growth are both high temperature processes and it is difficult to separate the two. For the II–VI III–V and Group IV semiconductors the ground state structure is crystalline. However for the more covalent of these materials amorphous structural phases become increasingly important and are typically formed more easily at lower temperatures.Consider for example the series of semiconductors CdS InP and Ge. It is possible to grow CdS nanocrystals at room temperature21 (although not by the method described above) and only moderate annealing temperatures are required to produce high quality nanocrystals. InP nanocrystals however require temperatures in the range of 250 °C or so and such temperatures must be maintained for a several day annealing period to obtain high quality nanocrystals. Finally the temperatures and times required to make Ge nanocrystals are near 300 °C for several days with most of this time again devoted to crystallite annealing. Because of the high temperatures and long times required for producing III–V nanocrystals it is not possible to obtain a narrow distribution from a single synthesis and particles sizes must be selected after the synthesis is complete.Nevertheless certain similarities do exist between III–V and II–VI synthetic schemes. Most notably is the use of the coordinating solvent TOPO in concert with the dehalosylation reactions developed by Wells’ group.22 In a typical synthesis a TOPO–InClx complex is prepared by heating a flask of InCl3 dissolved in TOPO at 100 °C for several hours. An equal molar amount of 67 Chemical Society Reviews 1998 volume 27 tris(trimethylsilyl)phosphine is injected and the temperature is slowly raised to ~ 260 °C and maintained at that temperature for several days. The temperature of the flask is lowered to 100 °C and an excess of an alkylamine (dodecylamine for example) is injected.The final product consists of InP nanocrystals capped with a mixture of TOPO and alkylamine and characterized by a size distribution ranging from 15–50 nm. All steps are carried out using Schlenk lines and other standard airless procedures. The final product InP nanocrystals may be size separated by dissolving the particles into toluene. A series of steps are then carried out in which small aliquots of methanol or acetone are added to the stirring toluene/particle solution and the solution is filtered. The dried precipitate is then redissolved in a non-polar organic solvent such as hexane or toluene. The largest particles precipitate from the solution first. In this manner up to 40 spectroscopically unique (as measured by the optical absorption edge) size distributions may be extracted from a single synthesis.11 Several UV–VIS spectra taken from a much larger precipitation series from a single reaction are shown in Fig.2. Particle sizes as measured by various microscopic and diffraction techniques are indicated next to a couple of the absorption curves. Fig. 2 Absorption spectra of a series of InP nanocrystal colloids size selected from a single synthetic product mixture. Adapted from Ref. 11. 4 The first excited electronic state short time scale dynamics In Section 2 a simple description for size confined wave functions in semiconductor nanocrystals was presented and the quantization of the energy level spacing with respect to wavevector (k) was discussed.An implication of this picture is that the band gap absorption oscillator strength for the bulk solid is collapsed into a few discrete transitions. In sufficiently narrow distributions of II–VI quantum dots such discrete transitions have been observed and more recently spectroscopically assigned.23 The nature of the lowest lying band gap transitions in finite sized solids is particularly important for photonics-related applications and much effort has been expended toward trying to measure the natural linewidths and Chemical Society Reviews 1998 volume 27 68 short-time scale relaxation dynamics in semiconducting nanocrystals. In this section of the review we present a brief overview of this picture and we pay special attention to how the nature of covalent vs.ionic lattice interactions affects short-time scale dynamics. Section 5 will deal with longer-time scale carrier relaxation processes. In a bulk (direct-gap) semiconducting solid optical excitation across the band gap produces two charge carriers of opposite sign an electron (e2) and a hole (h+). At finite temperature the dominant relaxation mechanism of the initially prepared state is provided by interactions between the charge carriers and lattice vibrations. These lattice vibrations are described in the next paragraph. Such a scattering mechanism is intrinsic to the material while other secondary processes such as scattering at defects or dopant sites are extrinsic. With the imposition of finite size new intrinsic and extrinsic mechanisms can become important.For example charge carrier scattering at the particle boundaries is an intrinsic mechanism and is of course expected to exhibit a particle size dependence. Charge carrier scattering or trapping by surface states or nanocrystal defects are extrinsic mechanisms which can potentially be removed by appropriate nanocrystal chemical (surface) passivation and crystallite annealing. Over the past few years several groups have addressed both theoretically24 and experimentally excited state charge-carrier dynamics on short time scales. Experimentally probes such as temporal25 and spectral hole burning single particle spectroscopy,26 femtosecond two- and three-pulse photon-echo experiments,27 and resonance Raman spectroscopy,28 have provided much information about the various fast relaxation processes.This is a very complex topic and not completely understood. Only a brief discussion of the points most salient to the degree of crystallite lattice covalency will be discussed here. The bulk solid state analogues of molecular vibrations are lattice phonons. These phonons come in two flavors—low frequency acoustic modes ( < 50 cm21) and higher frequency optical modes (300–800 cm21). Both the acoustic and optical phonons can be classified by two types of lattice motion. Longtitudinal modes (LO and LA modes for optical and acoustic phonons respectively) are symmetric breathing type motions. Transverse modes (TO and TA) are twisting-like motions.As is the case with molecules these vibrations can couple to excited electronic states and such coupling can provide mechanisms for excited electronic state relaxation. The coupling of these various types of modes to excited state charge carriers is not equivalent. It turns out that the primary mechanism for the initial relaxation (dephasing) of an excited charge carrier is through the longtitudinal acoustic (LA) phonons mediated through what is known as deformation potential coupling. When a charge carrier is placed on a lattice site there is an accompanying dilation of the lattice around the charge carrier. This dilation affects the electronic wavefunction overlap between adjacent atoms in the neighborhood of the charge and thus alters the energy levels of the corresponding electronic states.This coupling between lattice distortion and electron energy levels is deformation potential coupling and it is mediated by the LA phonons. The following analogy is useful when extending this picture of electron–phonon coupling in semiconductor nanocrystals. When an electron is excited across the band-gap it is moved from a bonding to an anti-bonding orbital and thus one net bond is broken. This ‘broken bond’ however is distributed over several unit cells (the exciton volume). With the imposition of finite size however the broken bond becomes more localized and so the accompanying lattice dilatation has a larger amplitude. Alivisatos et al. have developed a mathematical formalism for relating deformation potential coupling to a finite sized particle.25 This picture relies heavily on the elastic constants of a sphere and as will be discussed below the covalency of the lattice enters into this formalism through these constants.If the ground and excited electronic states are assumed to have harmonic oscillator potential forms then the offset between the lowest energy vibrational states (n = 0) of the two wells is given by Dac where the ‘ac’ subscript implies that the vibrational levels correspond to acoustic phonons. Dac is given by (1) Dac = [0.972 (De2Dh)2] [pR3 C11 hwac]21 where De 2 Dh is the acoustic mode deformation potential and C11 is the elastic constant and R is the radius of the crystallite. The acoustic mode frequency is given by wac = (zac/R)ns where n designed to make the acoustic mode wavefunction vanish at the crystallite boundary.29 Combining this expression for wac with eqn.1 one finds a net R22 dependence in the offset D temperatures eqn. 1 leads to the following expression for the excited state dephasing rate:30 s is the sound velocity and zac is a mathematical root ac. At high bT t2)/h} (2) F(t) = exp{pw D2 k In eqn. 2 T is temperature kb is Boltzmann’s constant and h is Planck’s constant. The degree of covalency within the semiconductor quantum dot enters into this rate expression in a few ways. One might expect based on its physical description that the deformation potential is strongly coupled to covalency. In fact Harrison has shown that within a tight binding picture of the band structure a formalism based on electronic overlap between nearest neighbors in the crystal can be used to determine the deformation potential.31 However he also points out that while such an approach yields reasonable answers there is no clear correlation between the deformation potential and covalency probably because of the difficulty in obtaining accurate experimental values of the deformation potential.However the C11 elastic constant is a measurement that relates how volume changes in the unit cell alter the electronic energy of the solid. It is therefore a sensitive function of covalency and can be accurately measured and calculated. Experimentally determined values of C11 for various semiconductors are (in 1011 dynes cm22) Si (16.6) InP (10.11) CdS (8.3) and CdSe (7.41).17 This value enters into the rate equation (2) as exp(C1122).Arguments based on lattice covalency will thus predict that (at finite temperatures) the more covalent the nanocrystal lattice the faster the excited state dephasing time. In all cases the dephasing times should shorten substantially as particle size is decreased. Note that the scattering of charge carriers off the nanocrystal surface is an important additional dephasing process for all particles independent of the nature of the lattice. In Figs. 3 and 4 we present two complementary pieces of experimental data in which the excited state dephasing rate is measured as a function temperature for a single size (Fig.3) and as a function of size for a single temperature (Fig. 4). Fig. 3 (from Ref. 32) represents a direct measurement of this rate as probed by femtosecond 3-pulse photon echo experiments on 29 Å InP and CdSe nanocrystals.27 Also included in Fig. 3 is a curve based on the intrinsic effects accounted for by eqn. 3 to model both the InP and CdSe data. Fig. 4 (from Ref. 28) represents an indirect probe of the same process. Here the ratios of the experimentally determined Raman scattering cross sections for the LO and TO modes of InP nanocrystals are presented as a function of particle size. It is not intuitively obvious that the resonance Raman experiment should measure the same dephasing dynamics that are measured by the timedomain experiments.It turns out that the LO and TO modes are coupled to the excited state potential surface in different manners and are 90° out of phase. The TO mode intensity is maximum at the instant of electronic excitation. The LO mode has zero intensity at the moment of optical excitation and reaches a maximum at one half the LO mode period. Because of this the ratio of the two observed phonon intensities serves as an indicator of the short time-scale exciton dephasing dynamics. For short dephasing lifetimes the LO TO intensity ratio is small. In Fig. 4 a theoretical calculation (not a fit) of the Fig. 3 The temperature dependence of the dephasing rate for 29 Å InP and CdSe nanocrystals as measured by femtosecond photon echo experiments.Adapted from Reference 32. resonance Raman intensity ratios in which eqn. 2 is explicitly included is presented with the data. Both the experiment and the theory indicate that the LO mode is suppressed in small size implying that the dephasing lifetime shortens with decreasing particle size. Although the TO phonon is not observed in finite sized CdSe nanocrystals resonance Raman LO phonon overtone spectra of CdSe quantum dots has been correlated with exciton dephasing dynamics in that system as well.27 Those experiments provide further supporting evidence for the picture presented here. 5 The first excited electronic state long time scale dynamics In Section 4 short time-scale exciton dynamics were dephasing processes that could be discussed largely in terms of the intrinsic properties of nanocrystals such as the particle size the deformation potential and the elastic constants.For these same particles the long-time scale exciton recombination kinetics are dominated by extrinsic properties of the particles such as Fig. 4 The ratio of the LO TO phonon intensities (circles) as a function of particle size. The line is a time-dependent quantum mechanical calculation that utilizes the experimentally determined dephasing rate (from Fig. 3) together with a model describing the size-dependence of electron–phonon coupling in semiconductor quantum dots. This experiment is the frequencyspace analogue of the data shown in Fig. 3 although plotted as a function of size rather than temperature.Adapted from Reference 28. 69 Chemical Society Reviews 1998 volume 27 surface passivation. In large part this is due to the fact that the synthetic techniques discussed in Section 3 are non-ideal. The solubility requirement imposed upon these nanocrystals means that bulky organic ligands are used to passivate the nanocrystal surface. The bulkiness of the ligand is important for solubility but it also leads to a particle surface that is incompletely passivated. The SiO2 passivated Si nanocrystals produced by Brus’ group have largely circumvented this problem although the size distribution of those particles is not near the state-ofthe-art for II–VIs or even III–Vs. Furthermore the very recent development of inorganic passivation techniques is beginning to improve the properties of both II–VI and III–V nanocrystals and such techniques will be briefly discussed at the end of this section.However for the particles that are discussed in this review the surface passivation is incomplete and this has a primary affect on the room-temperature exciton recombination kinetics. The electron energy level spectrum of III–V and II–VI particles can be modelled as a three-level system such as that presented in Fig. 5. Although the details of exciton recombination kinetics are complex and have not been completely worked out a few basic facts have emerged.33,34,35 Photoexcitation initially places an electron in an (interior) conduction Fig. 5 The three-level system used to describe exciton recombination kinetics in semiconductor nanocrystals.All radiative pathways are indicated by straight arrows and all non-radiative pathways are indicated by curved arrows. The surface trapping process (indicated by the rate constant kt dominates the observed room temperature photophysics. band (CB) state. At temperatures near 0 K CB-edge luminescence provides the primary mechanism for exciton recombination. Trapping of the excited state electron to a surface state is a slightly activated process and at some finite temperature such trapping begins to dominate the photophysics. Above a few degrees kelvin the surface states provide a reservoir for nearly all subsequent band-edge or surface state emission processes. Band-edge emission then proceeds through an activated ‘detrapping’ mechanism which returns an electron from a surface state back to the CB edge.Thus the nature of the surface states can play a determining role in the observed photoluminescence efficiency at room temperature. The activation barriers for detrapping are determined largely by the depth of the surface states with respect to the CB. These barriers are much greater than the barriers for trapping. This means that band edge luminescence quantum yields will decrease exponentially with increasing depth of the surface state energy levels. It is here that issues relevant to covalency play an important role. Increased ionic bonding character within a bulk crystal leads to successively shallower surface traps. For the Chemical Society Reviews 1998 volume 27 70 relatively ionic II–VI class of materials the case of surface states on bulk ZnO has been well studied.According to Luth,36 the presence of the surface (termination of the bulk periodic potential) and the effects of surface reconstruction are only weak perturbations when compared to the strong ionic forces which dominate the bonding. Thus the surface states remain very shallow. By measuring the temperature dependence and temporal decay curves of both the band-edge luminescence and surface state luminescence in semiconductor quantum dots it is possible to measure the activation barrier to surface state detrapping and thus obtain estimates for the depth of surface states in such systems. The reported values for the depth of the surface states do indeed exhibit a strong dependence on the covalency of the lattice.For 32 Å CdSe nanocrystals surface trap depths are about 0.25 kJ mol21. For similar sized InP nanocrystals (30 Å) we have found trap depths a full factor of 20 larger (6.3 kJ mol21). The size dependence of the depth of the surface states depends on the coupling of the surface states to the bulk electronic wave functions. Weaker coupling leads to a stronger size dependence as the conduction band shifts to higher energy (with decreasing size) the surface states remain behind and thus become deeper traps with decreasing particle size. As particle size increases from 30 Å to 49 Å the trap depth in InP nanocrystals is observed to decrease to approximately 3.6 kJ mol21.In the introduction of this review the potential of using semiconductor quantum dots for photonics-based devices was discussed. Such applications typically require high quantum yields for photo- or electro-luminescence efficiency. At first thought one might expect high luminescence efficiencies from semiconductor quantum dots the overlap between the electron and hole wavefunctions should be excellent especially for the smallest particles. However surface trapping of the carriers greatly reduces photoluminescence quantum yields such that at room temperature the observed yields for II–VIs are typically only a few percent. For III–Vs the situation is much worse— quantum yields are reduced to only a few hundredths of a percent. One of the most exciting advances in semiconductor quantum dot syntheses has been the very recent development of inorganic passivation techniques.Although several variations on this theme exist of particular importance for various photonicsbased applications are structures which are characterized by the energy level diagram shown in Fig. 6. The diagram and the cartoon representation of a nanocrystal are intended to describe a quantum dot in which a higher bandgap semiconductor has been grown epitaxially on the surface. Because of the similarities between this system and the well known quantum well systems (GaAs–AlxGa(12x)As for example) this type of quantum dot is often called a quantum dot/quantum well (QD/ QW). This quantum dot system has in principle all of the surface passivation problems that organically passivated quantum dots have.However separating the low-band gap nanocrystal core from the surface states is a higher band gap semiconductor shell. This high-band gap shell effectively cuts off the radiationless processes that lead to surface trapping and the lowest energy recombination pathway is not radiationless relaxation from a surface state but rather charge-carrier recombination within the nanocrystal core accompanied by photoemission. Examples of such systems include CdSe on ZnS,37,38 or CdS on InP. Reports of room temperature quantum yields nearing unity have appeared in the recent literature for II– VI on II–VI QD/QWs. 6 Conclusions Over the last few years synthetic techniques for producing nearly any type of semiconductor quantum dot have been reported.The properties of these structures are complex depending on the size shape and stoichiometry of the Fig. 6 Energy level diagram of a quantum dot/quantum well system in which a high band semiconductor material has been grown epitaxially onto the surface of a lower band gap semiconductor quantum dot. The higher band gap material forces exciton recombination to occur in the core of the nanocrystal thus resulting in dramatic increases in photoluminescence efficiency. nanocrystal. The richness of the available physical and chemical properties have made quantum dots appealing candidates for a variety of applications and thus it has become important to develop a self-consistent physical picture which can explain and predict those properties.The II–VI class of nanocrystals have been the prototypes for the investigation of size-dependent phenomena and a detailed picture capable of describing and predicting such phenomena has emerged over the past dozen years or so. With the more recent development of chemical syntheses for producing size-selected III–V nanocrystals it has become possible to carry out detailed comparisons of the sizedependent properties of one class of particles with another. In this Review the properties of II–VI and III–V nanocrystals have been rationalized in terms of lattice covalency. While the physical description of III–V nanocrystals is far from complete arguments based on lattice covalency go a long way toward explaining differences observed with respect to particle synthesis electronic structure and electronic dynamics.7 Acknowledgements Much of the work described here was carried out by an excellent group of postdocs and graduate students including Sang-Ho Kim Rolf Wolters and Dr Caroline Arnold. Furthermore a long-time collaboration with the group of Professor Paul Alivisatos is gratefully acknowledged. The work was funded by the NSF-NYI program a David and Lucille Packard Fellowship a Dreyfuss Fellowship and through the Lawrence Berkeley Laboraty Molecular Design Institute supported by the Office of Naval Research Order No. N00014-95-F-0099 and by the Director Offrice of Energy Research Office of Basic Energy Research Division of Materials Sciences of the US Dept.Of Energy under Contract No. DE-AC03 76SF00098. 8 References 1 For recent reviews see (a) A. P. Alivisatos J. Phys. Chem. 1996 100 13 226; (b) M. G. Bawendi M. L. Steigerwald and L. E. Brus Ann. Rev. Phys. Chem. 1990 41 477. 2 L. E. Brus J. Phys. Chem. 1994 98 3575. 3 C. B. Murray D. J. Norris and M. G. Bawendi J. Am. Chem. Soc. 1993 115 8706. 4 S. H. Tolbert and A. P. Alivisatos Ann. Rev. Phys. Chem. 1995 46 595. 5 M. R. Hoffman S. T. Martin W. Choi and D. W. Bahnemann Chem. Rev. 1995 95 69. 6 See for example G. Markovich D. V. Leff S. W. Chung and J. R. Heath Appl. Phys. Lett. 1997 70 3107 and references therein. 7 N. Chestnoy T. D. Harris R. Hull and L. E. Brus J. Phys. Chem. 1986 90 3393.8 P. E. Batson and J. R. Heath Phys. Rev. Lett. 1993 71 911. 9 (a) J. R. Heath J. J. Shiang and A. P. Alivisatos J. Chem. Phys. 1994 101 1607; (b) J. R. Heath and F. K. LeGoues Chem. Phys. Lett. 1993 208 263. 10 O. I. Micic J. R. Sprague C. J. Curtis K. M. Jones J. L. Machol A. J. Notik H. Giessen B. Fluegel G. Mohs and N. Peyghambarian J. Phys. Chem. 1995 99 7754. 11 A. A. Guzelian J. E. B. Katari A. V. Kadavanich U. Banin K. Hamad E. Juban A. P. Alivisatos R. H. Wolters C. C. Arnold and J. R. Heath 15 L. E. Brus A. L. Efros and T.Itoh J. Lumin. 1996 70 R7. J. Phys. Chem. 1996 100 7212. 12 A. A. Guzelian U. Banin A. V. Kadavanich X. Peng and A. P. Alivisatos Appl. Phys. Lett. 1996 69 1432. 13 M. A. Olshavsky A. N. Goldstein and A. P. Alivisatos J.Am. Chem. Soc. 1990 112 9438. 14 N. A. Hill and K. B. Whaley Chem. Phys. 1996 210 117. 16 A derivation of m* in terms of b is presented in L. Brus New J. Chem. 1987 11 123. 17 Landolt-Bornstein New Series ed. K. H. Hellwege Group III vol. 17a 1982 Springer-Verlag Berlin. 18 J. I. Pankove in Optical Processes in Semiconductors 1987 Dover New York p. 9. 19 G. Medeiros-Ribeiro F. G. Pikus P. M. Petroff and A. L. Efros Phys. Rev. B 1997 55 1568. 20 H. Reiss J. Chem. Phys. 1951 19 482. 21 H. Weller Angew. Chem. Int. Ed. Engl. 1996 35 1079. 22 R. L. Wells S. R. Aubuchon S. S. Kher M. S. Lube and P. S. White Chem. Mater. 1995 7 793. 23 A. I. Ekimov F. Hache M. C. Schanne-Klein D. Ricard C. Flytzanis I. A. Kudryavsev T. V. Yazeva A. V.Rodina and AI. L. Efros J. Opt. Soc. Am. B 1993 10,. 100. 24 T. Takagahara Phys. Rev. Lett. 1993 71 3577. 25 A. P. Alivisatos A. L. Harris N. J. Levinos M. L. Steigerwald and L. E. Brus J. Chem. Phys. 1988 89 4001. 26 M. Nirmal B. O. Dabbousi M. G. Bawendi J. J. Macklin J. K. Trautman T. D. Harris and L. E. Brus Nature 1996 383 802. 27 D. Mittleman R. W. Schoevlein J. J. Shiang V. L. Colvin A. P. Alivisatos and C. V. Shzuk Phys. Rev. B 1994 49 14 435. 28 J. J. Shiang R. H. Wolters and J. R. Heath J. Chem. Phys. 1997 106 8981. 29 S. Nomura and T. Kobayashi Solid State Commun. 1992 82 335. 30 R. A. Harris R. A. Mathies and W. T. Pollard J. Chem. Phys. 1986 85 3744. 31 An excellent book which relates covalent bonding to the properties of bulk crystalline semiconductors is W. A. Harrison Electronic Structure and the Properties of Solids 1989 Dover London. 32 U. Banin G. Cerullo A. A. Guzelian C. J. Bardeen A. P. A. Alivisatos and C. V. Shzuk Phys. Rev. B 1997 55 7059. 33 A. Eychmuller A. Hasslebarth L. Katsikas and H. Weller J. Lumin. 1991 48 745. 34 (a) M. G. Bawendi W. L. Wilson L. Rothbert P. J. Carroll T. J. Jedju M. L. Steigerwald and L. E. Brus Phys. Rev. Lett. 1990 65 1623; (b) M. G. Bawendi P. J. Carroll W. L. Wilson and L. E. Brus J. Chem. Phys. 1992 96 946. 35 See S.-H. Kim R. H. Wolters and J. R. Heath J. Chem. Phys. 1996 105 7957 and references therein. 36 H. Luth Surfaces and Interfaces of Solids 1993 Springer New York. 37 A. R. Kortan R. Hull R. L. Opila M. G. Bawendi M. L. Steigerwald P. J. Carroll and L. E. Brus J. Am. Chem. Soc. 1990 112 1327. 38 M. Danek K. F. Jensen C. B. Murray and M. G. Bawendi Chem. Mater. 1996 8 73. Received 21st May 1997 Accepted 24th September 1997 71 Chemical Society Reviews 1998 volume 27

ISSN:0306-0012    

DOI:10.1039/a827065z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Equilibrium frozen excess and volumetric properties of dilute solutions Michael J. Blandamer Department of Chemistry The University Leicester UK LE1 7RH The properties of aqueous solutions can be analysed in several ways leading to the identification of for example frozen equilibrium relaxational complex sophisticated delphic isodelphic ideal and excess contributions. These terms are examined and illustrated in terms of increasing orders of partial differentials of Gibbs energies volumes expansions and compressions. 1 Introduction The volume of a closed system is an important and indeed comprehensible thermodynamic function of state defined,1a for example for a single phase by the set of independent variables temperature T pressure p and amount of each chemical substance j nj; eqn.(1-1); equations are numbered according to the section in which they appear. V = V[T p ni] (1-1) We note that temperature T and pressure p are intensive variables. With reference to eqn. (1-1) the symbol ni represents the set describing the amounts of all chemical substances in the system. Chemists are challenged by eqn. (1-1) in several interesting ways. There is the problem of understanding the contributions made to volume V by each chemical substance j in the system. One way of tackling this problem probes the change in volume dV when dnj moles of substance j are added leading to the definition of a partial molar volume Vj. The consequence of switching from a closed to an open system in this formalism is not discussed here although it is an interesting point.1b For the most part we confine our attention to aqueous solutions prepared by adding nj moles of substance j (e.g.urea) to n1 moles of liquid water. We also confine our attention to solutes which do not undergo solvolysis reactions. In order to probe therefore the role of solute–solute solute–solvent and solvent– Professor Michael J. Blandamer graduated from the University of Southampton with BSc and PhD degrees in 1961. Following post-doctoral research at NRC in Ottawa (Canada) he joined the staff at the University of Leicester where he was appointed to a Personal Chair in 1990. He is Visiting Professor in the Department of Organic and Molecular Inorganic Chemistry at the University of Groningen The Netherlands.His research interests concern the thermodynamic and kinetic properties of solutes in aqueous solutions. Otherwise he is learning to play the piano the plan being to attain a standard which is likely to be a pale shadow of that demonstrated by the late Thelonius Monk. solvent interactions in determining the volume V(aq) of a solution a useful procedure identifies the corresponding volume V(aq;id) in the event that the properties of this solution are ideal. The difference [V(aq) 2 V(aq;id)] defines an excess volume. In the context again of eqn. (1-1) another challenge concerns understanding the dependences of volume V on temperature and on pressure. These dependences are described in terms of expansion E compression K expansivity a and compressibility k.Closely linked to this new set of properties are the corresponding instantaneous (frozen) and equilibrium properties. 2 Gibbs energies The set of independent variables [T p ni] used in eqn. (1-1) also defines the function of state called the Gibbs energy which for systems at fixed T and p is the thermodynamic potential. For a closed system at equilibrium the Gibbs energy is a minimum. Chemistry is based on the assumption that across the whole range of possible compositions (and organisations—see below) the minimum in Gibbs energy G is unique.2 (All experience based on experiment supports this assumption.) If the state defined by eqn. (1-1) is not at this minimum spontaneous chemical reaction/reorganisation takes place driven by the affinity for spontaneous change A the product of A and extent of chemical reaction being positive;1c A dx ! 0 De Donder’s inequality where A = 2 (dG/dx)T,p.Conceptually we freeze-frame the thermodynamic state defined by eqn. (1-1) in by definition state (I) where the affinity for spontaneous change is A(I) and the composition is x(I). Again conceptually we perturb the system into a nearby state by changes in pressure dp temperature dT and amount of substance j dnj. There are two interesting constraints which we impose on this perturbation. (A) Perturbation is to a near neighbouring state such that affinity A remains constant. (B) Perturbation is to a near neighbouring state at constant extent of reaction; i.e.at constant composition/organisation where x is constant. We are interested in the differential change in for example volume V under these two conditions. In fact we direct our attention to equilibrium states perturbed such that either (a) A = Aeq = 0 or (b) x = xeq. The keyword in the above two paragraphs is spontaneous. In a typical kinetics experiment ni moles of reactants are added to an aqueous solution at time t = 0 (at fixed T and p). Spontaneous chemical reaction driven by the affinity for change leads the system to a minimum in Gibbs energy. The rate at which this process occurs is examined using the formalism of chemical kinetics. Similarly when an aliquot of a solution containing micelles formed by an ionic surfactant is injected into water the micellar aggregates break up as the system spontaneously moves towards a minimum in Gibbs energy.3 3 Partial molar volumes The distinction between the two types of perturbation is illustrated using the following examples.A given closed 73 Chemical Society Reviews 1998 volume 27 1 eq aqueous solution of benzoic acid at specified temperature and pressure (close to ambient) contains at equilibrium (G = minimum A = 0 and x = xeq) n moles of water neq (PhCOOH) moles of benzoic acid neq (H+) moles of hydrogen ions and neq (PhCOO2) moles of benzoate anions. Then the analogue of eqn. (1-1) has the following form. eq,neq(PhCOO2),neq(H+)] V = V[T,p,n1 (3-1) This volume V is readily measured by direct measurements of density r and mass w.To this system we rapidly add dn(PhCOOH) moles of benzoic acid with consequent change in volume from V to (V + dV). The two limiting conditions identified in the previous section are now considered. The system is frozen such that the amount of benzoic acid in the new system is [neq(PhCOOH) + dn(PhCOOH)]. In other words the change in volume dV occurs at fixed extent of reaction xeq the perturbation being characterised by [dV/dn(PhCOOH)]T,p,xeq. The latter quantity is the frozen partial molar volume of PhCOOH in this aqueous solution; if j · PhCOOH the defined quantity is Vj,T,p,xeq (aq). By adding dn(PhCOOH) moles of benzoic acid the system is taken away from a minimum in G and the affinity for spontaneous acid dissociation increased.Spontaneous (and in this case fast) chemical reaction allows the system to regain after time Dt an equilibrium state where A is zero. So an alternative name for the frozen partial molar volume is an instantaneous partial molar volume. In the other class of perturbations the system responds such that in the new state chemical equilibrium is established the amounts of benzoate and hydrogen ions increasing. The increase in amount of PhCOOH in the solution when dn(PhCOOH) moles are added is moderated. [Moderation is not however universally true despite the widespread application of Le Chatelier’s principle—see ref. 1(d)]. In other words the change in volume dV occurs at constant affinity A actually at ‘A = 0’ so that the perturbation is characterized by the partial quantity [dV/dn(PhCOOH)]T,p,A = 0.This is the equilibrium partial molar volume of PhCOOH in this aqueous solution. One more example is relevant. Consider an aqueous solution at specified T and p prepared using n1 moles of liquid water (l) u moles of urea(s). The volume of this system is specified and n by eqn. (3-2); cf. eqn. (1-1). (3-2) V = V[T,p,n1 nu]eq The superscript ‘eq’ (plus conditions that G = minimum and A = 0) indicates that the organisation of the solution described by xeq characterising water–water water–urea and urea–urea interactions is unique to the system defined by eqn. (3-2). Conceptually we add dnu moles of urea producing a change in volume dV. The frozen (or instantaneous) partial molar volume of urea (dV/dnj)T,p,n1,xeq describes the change in volume where these intermolecular interactions remain unchanged.In contrast the equilibrium partial molar volume (dV/dnj)T,p,n1,A = 0 characterises the change in volume urea–urea urea–water and water–water interactions changing in order to hold the system at equilibrium where ‘A = 0’; see for example data in ref. 4. In the latter case although the Gibbs energies are different before and after addition of dnj moles of urea the Gibbs energies are at minima in both states. The above argument centres on volumes and partial molar volumes. But the question is raised as to which thermodynamic variables and related partial molar properties need be classified along similar lines namely frozen (instantaneous) and equilibrium.4 Gibbs energies and potentials Returning to eqn. (1-1) we replace variable V by the Gibbs energy G; eqn. (4-1). (4-1) G = G [T,p,ni] We assert that eqn. (4-1) is valid over the range of compositions/ organisations described by the variable x. Chemical Society Reviews 1998 volume 27 74 The system described by eqn. (4-1) is perturbed by a change in pressure dp. Two limiting perturbations of the Gibbs energy are envisaged; (i) at constant affinity A and (ii) at constant x linked by the following calculus operation. At constant temperature eqn. (4-2) holds. p p p æ è ç ¶G ¶ A Fig. 1 Functions of state ø ÷ ö = æ è ç ¶Gö ø ÷ - æ è ç ¶Aö æè ç ¶x öø ÷ æ è ç ¶Gö ø ÷ ¶ ø ÷ ¶ ¶ ¶ i A nj G nj A ¶G æ x ¶A p x x The latter equation applied to the state where G is a minimum (i.e.at equilibrium) and ‘A = 0’. But1e the affinity A equals T . In other words at equilibrium (dG/dx)T,p is zero. T equals volume V. G ¶nj 2(dG/dx) Moreover the partial differential (dG/dp) Hence eqn. (4-3). V(A = 0) = (¶G/¶p)T,A = 0 = (¶G/¶p)T,xeq = V(xeq) (4-3) Therefore the equilibrium and frozen volumes are the same which is not unexpected because volume V is a function of state being a property which is not path dependent; ‘volume’ is ‘volume’. Similar transformations [cf. eqn. (4-2)] with respect to the dependences on temperature of G and G/T (at fixed pressure) confirm that (a) S(A = 0) = S(xeq) and (b) H(A = 0) = H(xeq); thus entropy S and enthalpy H are functions of state;4 Fig.1. An important first differential of the Gibbs energy G is with respect to the amount of substance j namely (dG/dnj) at fixed T p and ni ­ j. This important partial differential is the chemical potential of chemical substance j. We return to eqn. (4-1) and consider a freeze-frame description of the system at defined T and p prepared with composition n0; the superscript ‘0’ refers to time zero. Spontaneous chemical reaction occurs driven by the affinity for spontaneous change. We freeze-frame the system where the composition is x and the affinity equals A. This system is perturbed by adding dnj moles of substance j one of the i-substances. The analogue of eqn. (4-2) has the following form (at defined T and p) eqn.(4-4). æ ¶ ö è ç ¶ ø ÷ æ ¶ è ç ¶ ö ø ÷ - ö ø ÷ = ¶ æ è ç We consider the case where the system being perturbed was at equilibrium where ‘A = 0’ G is a minimum and significantly (dG/dx) is zero. Therefore eqn. (4-5) holds. mj(A = 0) = (¶G/¶nj)A = 0 = (¶G/¶nj)xeq = mj(xeq) (4-5) In other words the chemical potential mj for substance j in this system at ‘A = 0’ equals the chemical potential mj for the condition x = xeq. Thus frozen (instantaneous) and equilibrium chemical potentials are equal placing chemical potentials on a par with the function of state H V and S in the thermodynamic hierarchy these being first derivatives of the Gibbs energy; Fig. 1. Hammett reached the same conclusion by noting in his terms that ‘sophisticated’ and ‘primitive’ chemical potentials5 are equal.In the context of describing the properties of solutions Grunwald uses the terms6 isodelphic to describe the partial molar properties of substance j when addition of dnj moles of solute j does not change the organisation of the solvent network; lyodelphic describes the difference between isodelphic and equilibrium partial molar properties. The lyodelphic contribution to the chemical potential of solute j is zero [cf. eqn. (4-4)]. In other words the equilibrium chemical potential of solute j is equal to the isodelphic chemical potential.7 In a similar context Ben-Naim8 uses the terms (4-2) x A p p (4-4) è ç ¶x ¶ ö æ ø ÷ è ç ö ø ÷ nj x x ‘freeze’ and ‘release’ in treating the thermodynamics of aggregation.5 Chemical equilibria Using an approach based on Henry’s Law the chemical potential of a neutral solute j [e.g. urea(s)] in aqueous solution is related to the molality mj using eqn. (5-1) where gj is the activity coefficient; m0 = 1 mol kg21. mj(aq;T;p) = mj(aq;id;mj = m0;T;p) + RTln(mjgj/m0) (5-1) By definition limit (mj ? 0; mi ? 0) gj equals 1.0 at all temperatures and pressures; mj(aq;id;mj = m0;T;p) is the chemical potential of solute j in an ideal solution (gj = 1.0) having unit molality. Consider an aqueous solution prepared using n0 Granted that mj(aq;T;p) is the same for frozen xeq and equilibrium properties the question arises—does this same condition apply to mj #(aq;T;p) [·mj(aq;id;mj = m0;T;p)]? x moles of chemical substance X where again superscript ‘0’ indicates at time zero.Experimental evidence indicates that two chemical substances X(aq) and Y(aq) exist in chemical equilibrium (at G = minimum where A = 0 at defined T and p). Such experimental evidence often arises because the two solutes have quite different UV–VIS absorption spectra. At equilibrium eqn. (5-2) holds. (5-2) meq X (aq) = mY eq(aq) Eqn. (5-2) offers the link with the argument developed in conjunction with the chemical potential of substance X. For this aqueous solution we would record the same increment in Gibbs energy G when dnX moles of substance X are added irrespective of whether only substance X is present in the solution or substance X is in equilibrium with substance Y.Using Hammett’s terminology5 the primitive and sophisticated chemical potentials of solute X are identical. Using the primitive description of the above system the chemical potential of substance X(aq) is related to the composition of the solution using the following equation [cf. eqn. (5-1)] for fixed T and p eqn. (5-3). m Y X X(aq) = mX #(aq;prim) + RTln[m0 XgX(prm)/m0] (5-3) Using the sophisticated description of the solution wherein the composition is described by molalities meq(soph) and meq- (soph) combination of eqn. (5-1) and (5-2) yields eqn. (5-4) for the solution at fixed T and p. X m X #(aq;soph) + RTln[meq(soph)g X eq(soph)/m0] = m # Y(aq;soph) + RTln[mY eq(soph)g r eq/m0] eq(soph) (soph)g X X eq Y K = m (soph)g Y eq(soph)/meq (5-4) By definition equilibirum constant K is given by eqn.(5-5). (5-5) = {1 + X X X X X Y But m0 = [mY eq(soph) + meq(soph)] so that m0 [Kg X eq(soph)/g eq(soph)]} meq(soph)g eq(soph). Then using the formulations for the chemical potential of substance X in solution we obtain eqn. (5-6). X mX (5-6) # (aq;prim) = m# (aq;soph) 2RTln[{1 + [Kg eq(soph)/g eq(soph)]}/gIRX(prim)] Y X In the limit that the solutions have ideal properties under both descriptions we obtain eqn. (5-7). (5-7) #(aq;prim) = m#(aq;soph)2RTln[1 + K] X mX Therefore the reference chemical potentials for a given solute are not identical. Consequently all other reference partial molar quantities (e.g.limiting partial molar volumes and limiting partial molar enthalpies) characterising solutes in sophisticated and primitive descriptions also differ. 6 Limiting partial molar properties and excess properties According to eqn. (5-1) the chemical potential of solute j in a solution molality mj is related to the activity coefficient gj and j a reference chemical potential m#(aq) at fixed temperature and pressure. In the event that the solution is ideal such that there are no solute–solute interactions gj = 1.0 and the chemical potential for solute j mj(aq;id) is given by eqn. (6-1). (6-1) mj(aq;id) = mj #(aq) + RTln (mj/m0) Interestingly in the limit that mj tends to zero mj(aq;id) [and mj(aq) for real solutions] tends to minus infinity.9 This means that solute j is increasingly stabilised as the solution becomes more dilute.This is the thermodynamic reason for the problems faced by industries which require solvents having very high purity. To remove the last trace of solute presents an awesome task. Eqn. (5-1) and (6-1) are important equations because they provide the basis for equations which describe the dependences on composition of other partial molar properties. For example the partial molar volume Vj(aq) of solute j is given by the partial derivative (¶mj/¶p)T. Thus from eqn. (5-1) for simple solute j we obtain eqn. (6-2). (6-2) V j j j(aq) = Vj #(aq) + RT(¶lngj/¶p)T Therefore from the definition of gj limit (mj?0) Vj(aq) equals V#(aq) which we may also write as VH(aq) the limiting partial molar volume.We make one further point in this connection because it is always advisable to examine these and comparable equations in terms of what happens in certain limits.9 For example the partial molar entropy Sj of solute j is given by the partial derivative 2(¶mj/¶T)p. Then eqn. (6-3) holds. (6-3) Sj(aq) = Sj #(aq)2Rln (mjgj/m0)2RT(¶lngj/¶T)p j Therefore from the definition of gj limit (mj?0) Sj(aq) equals + H. In other words the term SH(aq) has no practical meaning whereas Vj H(aq) does. As preparation for some of the subject matter discussed in the following sections we set down the basis of a definition for the excess Gibbs energy of an aqueous solution containing the single solute chemical substance j.For a solution with ideal properties (at fixed T and p) the chemical potential of solute j is given by eqn. (5-1) with gj = 1.0. Then eqn. (6-4) holds. (6-4) E(aq) = mj(aq)2mj(aq;id) = RTlngj j (6-5) m1(aq) = m1 *(†)2fRTM1mj (6-6) 7 Volumes V A V x ¶ (7-1) m For the solvent water in this solution the chemical potential m1(aq) is related to mj and the practical osmotic coefficient f using eqn. (6-5) where m1 *(†) is the chemical potential of water at the same time T and p eqn. (6-5) applies. For an ideal solution f = 1 and gj = 1. The excess Gibbs energy is defined by eqn. (6-6) for a solution prepared using 1 kg of solvent and mj moles of solvent. GE = mjRT[12f + lngj] GE is an intensive property of the solution because it refers to a thermodynamic property of a solution prepared using a fixed mass of solvent.The link between the dependence of f and gj on molality mj is provided by the Gibbs–Duhem equation. We consider a solution prepared using n In Section 3 we commented on the significance of equilibrium and frozen partial molar properties using volumetric properties of solutions as examples. Here we take up the story again but develop the argument along the lines given in Section 5. The broad sweep of our analysis is set out in Fig. 2 starting with the function of state volume V. j moles of substance 1 moles of water (†). The analogue of eqn. j (e.g. urea) and n (4-2) describes the change in volume dV when dnj moles of chemical substance j are added to the solution (i) at constant affinity A and (ii) at constant extent of reaction x both at constant T p and n1 eqn.(7-1). V j ¶ ¶A ¶ ¶n ¶ ¶n ¶ ¶n ¶x j j ö ø ÷ æ è ç ö ø ÷ ö æ ø ÷ è ç ö æ ø ÷ - è ç ö æ ø ÷ = è ç A x x nj nj æ è ç Chemical Society Reviews 1998 volume 27 75 Fig. 2 n But (¶V/¶x) j is not zero and so the equilibrium partial molar volume of substance j Vj(A = 0) is not equal to the frozen partial molar volume Vj(xeq). We recall our discussion in Section 2 of the volumetric properties of urea(aq). The volume of an aqueous solution comprising n1 moles of water and nj moles of solute j is related to the equilibrium partial molar volumes Vj(aq) and V1(aq) using eqn.(7-2). (7-2) V(aq) = n1V1(aq) + njVj(aq) However an alternative form of eqn. (7-2) defines V(aq) in terms of the apparent molar volume10 f(Vj) eqn. (7-3). (7-3) (7-4) V(aq) = n1V1 *(†) + njf(Vj) If the properties of the solution are ideal eqn. (7-4). V(aq;id) = n1V1 *(†) + njf(Vj)H Here f(Vj)H [ = Vj H(aq)] is the limiting (indinite dilution) partial molar volume of solute j. Another form of eqn. (7-3) describes the volume of a solution prepared using 1 kg of water. So we have eqn. (7-5) where eqn. (7-6) holds. (7-5) (7-6) V(aq) = (1/M1)V1 *(†) + mjf(Vj) V(aq;id) = (1/M1)V1 *(†) + mjf(Vj)H Therefore the excess volume VE is given by eqn. (7-7). (7-7) VE(aq) = mj[f(Vj)2f(Vj)H] An advantage of the latter equation is that VE is an intensive variable.Eqn. (7-3) forms the basis of the experimental j). The mass of the aqueous solution w determination of f(V equals (w1 + wj) or (w1 + nj Mj) where Mj is the molar mass of the solute. The densities of the solution and the solvent are r(aq) 1 *(†) respectively. Hence11a eqn. (7-8) [ = w/V(aq)] and r holds. (7-8) f(Vj) = (mj)21[r(aq)212r1 *(†)21] + (Mj/r) Eqn. (7-8) does not determine the dependence of f(Vj) on mj; the dependence is characteristic of the solution. In fact the form of Chemical Society Reviews 1998 volume 27 76 2 this dependence12 is not defined by thermodynamics although in many cases the dependence of f(Vj) on mj for dilute solutions is linear.[For salt solutions the Debye–H�uckel limiting law prompts analysis in the form of a dependence of f(Vj) on (mj)1.] 8 Isochoric conditions An interesting set of independent variables defines the Helmholtz energy of a system F; cf. eqn. (1-1) eqn. (8-1). (8-1) F = F[T,V,ni] Then all spontaneous processes under isochoric–isothermal conditions lower spontaneously the Helmholtz energy of a closed system. We do not develop this point further except to note that in contrast to the set of independent variables given in eqn. (1-1) and (4-1) the set in eqn. (8-1) involves two extensive variables volume and amounts ni. The isochoric condition has aroused controversy13–16 in analysis of kinetic data with respect to the calculation of isochoric activation parameters.14,17,18 The controversy centres on answers to the simple question—which volume is held constant?13 The issue remains unresolved.9 Isobaric expansions and isobaric expansibilities We return to a consideration of thermodynamic variables defined by the set of independent variables specified in eqn. (1-1) and (4-1). The volume of an aqueous solution having defined composition (e.g. n1 moles of water and nj moles of solute j) depends on temperature at fixed pressure. There are two limiting ways in which the volume of this solution may change as a result of a change in temperature; (a) at constant affinity A and (b) at constant extent of reaction/organisatn x. These two limiting changes are related (at defined T and p) eqn. (9-1).(9-1) A T T æ V ö ¶ è ç ¶T A T x x T æ ¶ ¶ ö ¶ ö æ ø ÷ = æè ç ¶ V öø ÷ - æè ç ¶ Aöø ÷è ç ¶ x ø ÷ è ç V ø ÷ ¶ ¶x In particular case of a system at equilibrium we identify two limiting expansions; the equilibrium isobaric expansion Ep(A = 0) [ = (¶V/¶T)p;A = 0] and the frozen isobaric expansion Ep(xeq) [ = (¶V/¶T)p;xeq]. Further for the condition ‘A = 0’ then (¶V/¶T)p;x equals1f T (¶H/¶x)T,p. Hence eqn. (9-2). Ep(A = 0) = Ep(xeq)2T21 (¶x/¶A)T,p (¶V/¶x)T,p (dH/dz)T,p (9-2) Equilibrium isobaric thermal expansions of aqueous solutions can be directly measured dilatometrically.19 The analogue of eqn. (7-1) in which enthalpy H replaces volume V is a key equation with respect to the temperature-jump fast reaction technique.20 Although at equilibrium (¶A/¶x)T;p is negative the sign of the product (¶V/¶x)T;p (¶H/¶x)T;p is not predetermined.Therefore the sign of Ep(A = 0) is not fixed. In fact for water below 277 K at ambient pressure the temperature of maximum density (TMD) Ep(A = 0) is negative;21 a similar feature is shown for many aqueous solutions22,23 in the region of 277 K. As written in eqn. (9-2) Ep(A = 0) and Ep(xeq) are extensive variables but not functions of state because they characterise pathways. We divide by the volume an extensive function of state (see above) to define an isobaric equilibrium expansibility a(A = 0) [ = V21 Ep(A = 0)] and an isobaric frozen expansivity a(xeq) [ = V21 Ep(xeq)] two volume intensive properties of a solution eqn.(9-3). a(A = 0) = a(xeq)2(VT)21(¶x/¶A)T,p(¶V/¶x)T,p(¶H/¶x)T,p (9-3) In these terms a(xeq) represents the volumetric response of a system to a thermal shock as the temperature is increased in an infinitesimal time. The extended product term describes the relaxation of the system24 to the state characterised by a(A = 0). For the aqueous solution described in eqn. (7-4) the isobaric thermal expansion is described by eqn. (9-4). N 0) E (A 1 p = = æè ç ¶V öø ÷ ¶T P;A=0 = = m EEp 1)E1 *(†;A = 0) + mjf(Ej)H j[f(Ej)2f(Ej)H] j[f(Vj)/V(aq)][f(Vj)]21f(Ej) ap(A = 0)V = n1V1 *(†)a1 *(A = 0) + njf(Ej) ö ø ÷ We define an apparent equilibrium molar isobaric expansion of solute j f(Ej) by the partial differential (¶f(Vj)/¶T)p;A = 0 an intensive property of solute j.Similarly for the solvent water E1 *(†;A = 0) equals [¶V1 *(†)/¶T]p;A = 0. Hence for a solution prepared using 1 kg of solvent where both E1 *(†;A = 0) and f(Ej) are intensive variables we have eqn. (9-5). Ep(A = 0;aq;w1 = 1 kg) = (1/M1)E1 *(†;A = 0) + mjf(Ej) (9-5) For the corresponding ideal solution we have eqn. (9-6). Ep(A = 0;aq;w1 = 1 kg;id) = (1/M The equilibrium partial molar expansion of solute j Ej(aq) and of solvent water E1(aq) are defined by [¶Vj(aq)/¶T]p and [¶V1(aq)/ ¶T]p. These quantities are normally calculated from measured dependences of Vj(aq) and V1(aq) [cf. eqn. (7-8) using f(Vj)] on composition at a series of fixed temperatures. For urea(aq) f(Ej) and f(Ej)H are positive4 at ambient pressure over the range 0 @ mj @10.0 mol kg21.For 2-methylpropan-2-ol(aq) in very dilute solutions,12 the dependence of Ej(aq) on mj and temperature is complicated. The second differential [¶2VH(aq)/¶T2]p has been used to classify solutes on the basis of their effect on water– water interactions.25 An (intensive—based on fixed mass of solvent) excess equilibrium isobaric expansion EE is defined by eqn. (9-7); cf. eqn. (6-6). EEp (A = 0) = Ep(A = 0;aq;w1 = 1 kg) 2Ep(A = 0;w1 = 1 kg;aq;id) Therefore eqn. (9-8) holds. In other words the excess expansion E (A = 0) is a welldefined property given by the product of solute molality mj and a difference in real and ideal apparent molar expansions. However a similar clear definition does not emerge if we turn our attention to expansibilities.The starting point is the definition given above for aT(A = 0) [ = V21Ep(A = 0)]. Then using eqn. (9-4) we have eqn. (9-9). ap(A = 0) = n1[V1 *(†)/V(aq)][V1 *(†)]21E* p(†;A = 0) + n If for the pure solvent ap *(A = 0) equals [V1 *(†)]21Ep *(A = 0;†)] then the isobaric expansibility of the solution ap *(A = 0) is given by eqn. (9-10). ap(A = 0) = n1[V1 *(†)/V(aq)]ap *(†) + [nj/V(aq)]f(Ej) (9-10) The property of the solution ap(A = 0) is given by an equation which only partly resembles eqn. (7-3) with the added complexity of a volumetric ratio [V1 *(†)/V(aq)]. There is no obvious quantity which could be described as an apparent molar isobaric expansivity of the solute j.Consequently there is no obvious route leading in an elegant manner to an excess isobaric expansibility of the solution; cf. eqn. (9-8). We end this section by returning to eqn. (9-9) written as eqn. (9-11). The latter equation forms the starting point for the derivation of eqn. (9-12) which shows how f(Ej) is calculated using measured p(A = 0) for a solution molality mj. a f(Ej) = [mjr(aq)r1 *(†)]21 {[r1 *(†)ap(aq;A = 0)] 2[r(aq)a1 *(†;A = 0)]} + [a(aq)Mj/r(aq)] This equation closely resembles eqn. (7-9) and is a member of the same family of volumetric equations (see below). (V ) f ¶ (l) j * 1 + nj ¶V ¶T ¶T æ è ç p;A+0 P;A=0 j æ è ç p Ep ö ø ÷ (9-4) (9-6) (9-7) (9-8) (9-9) (9-11) (9-12) 10 Isothermal compression and compressibility In addition to isobaric expansion the other major component of Fig.2 describes compressions and compressibilities of solutions. There are two limiting ways in which the volume of a solution may change as a result of a change in pressure (at fixed temperature); (i) at constant affinity A and (ii) at constant extent of reaction/organisation x. Thus at fixed temperature V (10-1) ¶A ¶p ¶p æ ¶V è ç ¶ p p A x p The partial differential (¶A/¶p)T;x equals1g 2(¶V/¶x)T;p. In the case where the system was at equilibrium the partial differential 2(¶V/¶p)T;A = 0 is the equilibrium isothermal compression KT(A = 0) whereas 2(¶V/¶p)T;xeq is the frozen compression K K (10-2) k (10-5) x T(xeq).Hence from eqn. (10-1) we have eqn. (10-2). T(A = 0) = KT(xeq)2(¶x/¶A)T;p eq [¶V/¶x)T;p eq ]2 The compressions KT(A = 0) and KT(xeq) are extensive properties of a solution. The volume intensive properties are isothermal compressibilities kT(A = 0) and kT(xeq) defined by eqn. (10-3) and (10-4). T(A = 0) = 2(1/V)(¶V/¶p)T;A = 0 = + V21KT(A = 0) (10-3) (10-4) T;A=0 ÷ è æ ¶V*(l)ö = -n ç j (10-6) x ø ÷ ö = æ ¶V ö ø ÷ - æ ¶Aö æ ¶ ö æ ¶ ö è ç ø ÷ è ç è ç ø ÷ è ç ø ÷ ¶x 1 1 ¶p ø T;A 0 = = k T(xeq) = 2(1/V)(¶V/¶p)T;xeq = + V21KT(xeq) Then from eqn. (10-2) we have (10-5). kT(A = 0) = kT(xeq)2(¶x/¶A)T;p eq [(¶V/¶x)T;p eq ]2 But (¶A/¶x)T;p is negative for all stable phases.Hence irrespective of the volume of reaction (¶V/¶x)eq T;p kT(A = 0) is always greater than kT(xeq). In fact eqn. (10-5) is the key equation for the pressure-jump fast reaction technique,20 the second (large) term on the right-hand side of eqn. (10-5) being the relaxational term. For an aqueous solution the isothermal equilibrium com- T(A = 0) is related to the isothermal differential of pression K eqn. (7-3) with respect to pressure eqn. (10-6). K (A T = 0) = -( ¶V / ¶p) K = m E T - nj T (10-9) (10-10) æ ¶f(V )ö è ç¶p ø ÷ T;A 0 Thus KT(A = 0) is an extensive property of a solution. It is convenient to define an equilibrium apparent molar isothermal compression f(KTj) { = 2(¶f(Vj)/¶p)T;A = 0}.For an aqueous solution prepared using 1 kg of solvent we relate the intensive compression KT(A = 0;aq;w1 = 1 kg) to the composition using eqn. (10-7). KT(A = 0;aq;w1 = 1 kg) = (1/M1)K* 1T(†;A = 0) + mjf(KTj) (10-7) For the solution whose properties are ideal eqn. (10-8) applies. KT(A = 0;aq;w1 = 1 kg) = (1/M1)K* 1T(†;A = 0) + mjf(KTj)H (10-8) Then the excess compression KE is defined by eqn. (10-9). j[f(KTj)2f(KTj)H] Interestingly the form of eqn. (10-9) resembles those for the excess volumes [eqn. (7-8)] and excess isobaric expansions [eqn. (9-8)]. The general form of eqn. (10-7) describes the extensive compression of a solution containing nj moles of solute j and n1 moles of water eqn. (10-10). KT(A = 0) = n1K1T(†;A = 0) + njf(KTj) Calculation of f(Ej) from compressibilities uses eqn.(10-11) which is derived in a manner analogous to that used to obtain eqn. (9-12). Chemical Society Reviews 1998 volume 27 77 f(KTj) = [mjr(aq)r1 *(†)]21{[r1 *(†)kT(aq;A = 0)] 2[r(aq)k1 *(†);A = 0)]} + [kT(aq);A = 0)Mj/r(aq)] (10-11) Further just as for expansibilities an elegant equation similar to that used for excess volumes [eqn. (7-8)] cannot be used to define an excess isothermal compressibility. Similarly there is no analogue of a partial molar volume which can be identified as a partial molar compressibility. Direct measurement of isothermal compressibilities of aqueous systems is nevertheless not straightforward bearing in mind that k(aq) depends on composition temperature and pressure.11 Isentropic compression and isentropic compressibilities Data describing equilibrium isothermal compressibilities of aqueous solutions are not extensive. It may at first sight seem surprising therefore that information concerning equilibrium isentropic compressibilities (i.e. compression at constant entropy kS) is more extensive. The reason for this state of affairs is the Newton–La Place equation relating isentropic compressibility to the density of a solution r and the velocity of sound u in the solution; kS = (u2r)21. If the frequency of the sound wave is low (e.g. in the MHz range) the calculated quantity is the equilibrium isentropic compressibility kS(A = 0). In other words each microscopic volume of a solution is compressed at constant entropy.Moreover these compressibilities can be precisely measured.26,27 The isentropic condition raises problems from the standpoints of both thermodynamic theory and the interpretation of derived parameters. From the outset we have to change the basis of the thermodynamic treatment. Thus in reviewing equilibrium isobaric expansions Ep(A = 0) and isothermal compressions KT(A = 0) the analysis developed in a straightforward manner from the function of state called volume defined using eqn. (1-1). In the latter case our interests centred on the T–p composition domain for which the thermodynamic potential function is the Gibbs energy. In order to understand the significance of isentropic compressibilities kS and isentropic compressions KS we switch interests into the S-p-composition domain.This is not a trivial switch. All spontaneous processes in closed systems at fixed entropy and pressure lower the enthalpy H of a system such that thermodynamic equilibrium corresponds to a minimum in enthalpy where the affinity for spontaneous change is zero. Thus the enthalpy of a closed system is defined1e by eqn. (11-1). (11-1) H = H [S p x] The volume of the system is given by the partial differential (¶H/¶p)S;x. The analogue of eqn. (1-1) is eqn. (11-2). (11-2) V = V [S p ni] In other words we have switched the set of independent variables defining the volume from [T p ni] to [S p ni] a switch from ‘Lewisian’ to ‘non-Lewisian’ independent variables. 28 The key point to note is that in one set T and p are both intensive variables whereas in the other set [cf.eqn. (11-1) and (11-2)] the independent variable S is extensive making two extensive variables in this definition. This contrast between the two sets is not trivial indicating that the choice of independent variables is more than a matter of convenience. Two equilibrium quantities are of interest in this section; (i) the equilibrium isentropic compression KS(A = 0) and (ii) the equilibrium isentropic compressibility kS(A = 0). The corresponding instantaneous properties are KS(xeq) and kS(xeq). The difference between these equilibrium and instantaneous properties is at the heart of the ultrasonic fast reaction technique.20,29 At low rates of compression (i.e.change in pressure) at constant entropy solvent–solvent solvent–solute and solute–solute interactions within each microscopic volume of an aqueous solution change in order to keep the system at a minimum in the enthalpy. The differential dependence of volume V on pressure at constant temperature and affinity is related to the differential Chemical Society Reviews 1998 volume 27 78 dependence of volume V on pressure at constant entropy and affinity using eqn. (11-3). V T S æ V (11-3) ¶ ¶T ¶ ¶p p ¶S ¶p ö ø ÷ ¶ ö ø ÷ æ è ç ö ø ÷ - æ è ç ö ø ÷ ö ø ÷ p;A p;A T;A T;A S;A æ è ç T;A = 0 (11-8) = æ V ¶ è ç p(aq;A = 0)V(aq)]2T/Cp(A = 0;aq)} (11-9) ¶ è ç¶ We use eqn. (11-3) with the constraint that constant affinity A is at ‘A = 0’.The last partial derivative in eqn. (11-3) is therefore the isobaric equilibrium expansion Ep(A = 0); cf. eqn. (9-2). A Maxwell relationship1g shows that (¶S/¶p) equals 2(¶V/¶T)p;A = 0 which is 2Ep(A = 0). Further the partial differential (¶S/¶T)p;A = 0 equals1h the ratio of the equilibrium isobaric heat capacity to the temperature Cp(A = 0)/T. Moreover Ep(A = 0) equals ap(A = 0)V; cf. eqn. (9-3). Therefore,30 KS(A = 0) = KT(A = 0)2{[ap(A = 0)V]2T/Cp(A = 0)} (11-4) Thus by definition the equilibrium isentropic compression KS(A = 0) equals 2(¶V/¶p)S;A = 0. In eqn. (11-4) KS(A = 0) KT(A = 0) V and Cp(A = 0) are extensive properties of the solution. The ratio [Cp(A = 0)/V] is the equilibrium isobaric heat capacity per unit volume of the solution s(A = 0).Then we have eqn. (11-5). KS(A = 0) = KT(A = 0)2{[ap(A = 0)]2VT/s(A = 0)} (11-5) Similarly in terms of isentropic compressibilities eqn. (11-6). kS(A = 0) = kT(A = 0)2{[ap(A = 0)]2T/s(A = 0)} (11-6) In the next stage we return to an equation for the volume of an aqueous solution prepared using n1 moles of solvent water and nj moles of solute j; eqn. (7-3). The isobaric compression KT(A = 0) is given by eqn. (10-6) in terms of the equilibrium isobaric compression of the solvent K* 1T(†) and an equilibrium partial differential isothermal dependence of f(Vj) on pressure. Thus we have eqn. (11-7). KT(aq;A = 0) = n1K* 1T(†;A = 0)2nj[¶f(Vj)/¶p]T;A = 0 (11-7) The task at this stage is to write down a satisfactory equation for the isentropic equilibrium compression of the solution.In the context of treating the thermodynamic properties of binary liquid mixtures the way forward was signalled by Benson and Kumaran,31 by Reis28 and by Douh�eret Moreau and Viallard32 particularly in developing equations describing excess compressibilities and excess compressions. Here we comment on the equilibrium isentropic compressions of aqueous lutions. Thus it follows from eqn. (11-4) that for the pure solvent water eqn. (11-8). K* 1T(†;A = 0) = K* S1(†;A = 0) + {[a* 1p(†;a = 0)V* 1(†)]2T/C* p1(†;A = 0)} For the solution according to eqn. (11-4) we have eqn. (11-9). KT(aq;A = 0) = KS(aq;A = 0) + {[a In eqn.(11-8) K* 1T(†;A = 0) and K* S1(†;A = 0) refer to a mole of liquid water whereas in eqn. (11-9) V(aq) and Cp(A = 0;aq) are extensive properties of the solution described in eqn. (11-7) having compression KT(aq;A = 0). The analytical problem is highlighted by eqn. (11-8) and (11-9). The isentropic condition on K* S1(†;A = 0) is understandable in terms of the condition associated with compression by the sound wave. The isothermal conditions on K* 1T(†;A = 0) and KT(aq;A = 0) are understandable in terms of the isothermal condition associated with measuring the dependence of volumes on pressure. Thus we can arrange experimentally for these two temperatures to be the same in order to compare the isothermal compressions of water and an aqueous solution having defined molality.Unfortunately we cannot ensure in a comparison of K* 1S(A = 0) for water and KS(aq;A = 0) for an aqueous solution that the two S entropies are the same. In each case the compression is isentropic (cf. the Newton–Laplace equation) but we cannot be sure that the properties of solution and solvent are compared at the same entropy. Furthermore in examining the dependence of KS(aq;A = 0) on the composition of a solution (e.g. on molality of solute j) we cannot be sure that comparisons can be made of the properties of these solutions at the same entropy. There is merit in defining an apparent equilibrium isentropic compression of solute j f(K j) in terms of 2[¶f(Vj)/¶p]S;A = 0. Similarly f(KTj) = 2[(¶f(Vj)/dp]T;A = 0.Granted therefore that experiment yields kS(aq;A = 0) for an aqueous solution (at defined T and p) and k* S1(†;A = 0) for the pure solvent water eqn. (11-10) yields the apparent molar isentropic compression of the solute f(KSj). (11-10) f(KSj) = [1/mjr* 1(†)][kS(aq;A = 0)2k* S1(†;A = 0)] + kS(aq;A = 0)f(Vj) Indeed this is a classic equation used by many authors who cite as the key reference the monograph by Harned and Owen,12b confidence being boosted by the strong similarity with equations for f(Vj) f(Epj) and f(Kpj) as described in the previous sections. So common practice has been to use eqn. (11-10) as a method of determining the apparent property of f(KSj) from measured kS(aq;A = 0) for a solution molality mj. S Then the patterns which emerge are discussed in the conceptually simpler context of isothermal compression.Lara and Desnoyers note33 that for 2-butoxyethanol(aq) at 298 K ‘isothermal and isentropic compressibilities are quite similar and reflect the same kind of interactions’. Franks and coworkers34 discuss the dependence of f(Kj) on composition and structure for various sugars in aqueous solutions in terms of solute-hydration and solute–solute interactions although the measured quantity was f(K j). In the context of aqueous salt solutions for example Criss and co-workers35 compare f(KSi)H for ion i with the trend predicted by the Born equation for the isothermal property. Quite generally therefore measured isentropic properties are understood in terms of models based on isothermal properties.This approach has obvious practical merit. Nevertheless this author has reservations concerning what seems a somewhat cavalier approach to the isentropic condition. Perhaps there is a need for a new approach to the task of understanding the significance of isentropic compressibilities of solution. 12 Acknowledgements I thank Professors H. Høiland (University of Bergen) and J. B. F. N. Engberts (University of Groningen) for valuable discussion. Also my long-suffering Secretary Vikki who has seen this same paper (with alterations) more times than she cares to remember. 13 References 1 I. Prigogine and R. Defay Chemical Thermodynamics trans. D. H. Everett Longmans Green London 1954 (a) p. 3; (b) p. 67; (c) p.38; (d) p. 266; (e) p. 52; (f) p. 59; (g) p. 54; (h) p. 48. 2 F. Van Zeggeren and S. H. Storey The Computation of Chemical Equilibria Cambridge University Press Cambridge 1970. 3 J. Bach M. J. Blandamer J. Burgess P. M. Cullis L. G. Soldi K. Bijma J. B. F. N. Engberts P. A. Kooreman A. Kacperska K. C. Rao and M. C. S. Subha J. Chem. Soc. Faraday Trans. 1995 91 1229. 4 R. H. Stokes Aust. J. Chem. 1967 20 2087. 5 L. P. Hammett Physical Organic Chemistry McGraw-Hill New York 1970 2nd edn. p. 16. 6 E. Grunwald J. Am. Chem. Soc. 1984 106 5414; 1986 108 1361; 5726. 7 M. J. Blandamer J. Burgess A. W. Hakin and J. M. W. Scott Water and Aqueous Solutions ed. G. W. Neilson and J. E. Enderby Colston Papers No. 37 Adam Hilger Bristol 1986. 8 A.Ben-Naim Hydrophobic Interactions Plenum Press New York 1980 p. 130. 9 J. E. Garrod and T. M. Herrington J. Chem. Educ. 1969 46 165. 10 See for example G. N. Lewis and M. Randall Thermodynamics revised by K. S. Pitzer and L. Brewer McGraw-Hill New York 1961 2nd edn. 11 H. S. Harned and B. B. Owen The Physical Chemistry of Electrolytic Solutions Reinhold New York 1958 3rd edn. (a) p. 358; (b) p. 376. 12 F. Franks and H. T. Smith Trans. Faraday Soc. 1968 64 2962. 13 M. J. Blandamer J. Burgess B. Clark R. E. Robertson and J. M. W. Scott J. Chem. Soc. Faraday Trans. 1 1985 81 11. 14 J. R. Haak J. B. F. N. Engberts and M. J. Blandamer J. Am. Chem. Soc. 1985 107 6031. 15 P. G. Wright J. Chem. Soc. Faraday Trans. 1 1986 82 2557. 16 L. M. P. C. Albuquerque and J. C. R. Reis J. Chem. Soc. Faraday Trans. 1 1989 85 202. 17 M. G. Evans and M. Polanyi Trans. Faraday Soc. 1935 31 875. 18 E. Whalley J. Chem. Soc. Faraday Trans. 1 1987 83 2901. 19 J. L. Neal and D. A. I. Goring J. Phys. Chem. 1970 74 658. 20 E. Caldin Fast Reactions in Solution Blackwell Oxford 1964. 21 M. J. Blandamer J. Burgess and A. W. Hakin J. Chem. Soc. Faraday Trans. 1 1987 83 1783. 22 F. Franks and B. Watson Trans. Faraday Soc. 1967 63 329. 23 D. D. Macdonald and J. B. Hyne Can. J. Chem. 1976 54 3073. 24 See discussion by C. M. Davis and J. Jarzynski Water and Aqueous Solutions ed. R. A. Horne Wiley-Interscience New York 1972 ch. 10. 25 L. G. Hepler Can. J. Chem. 1969 47 4613. 26 R. Garnsey R. J. Boe R. Mahoney and T. A. Litovitz J. Chem. Phys. 1969 50 5222. 27 H. Høiland and E. Vikingstad J. Chem. Soc. Faraday Trans. 1 1976 72 1441. 28 J. C. R. Reis J. Chem. Soc. Faraday Trans. 2 1982 78 1595. 29 M. J. Blandamer Introduction to Chemical Ultrasonics Academic Press London 1973. 30 G. Douh�eret and M. I. Davis Chem. Soc. Rev. 1993 43. 31 G. C. Benson and M. K. Kumaran J. Chem. Thermodyn. 1983 15 799. 32 G. Douh�eret C. Moreau and A. Viallard Fluid Phase Equilibria 1985 22 277 289; 1986 26 221. 33 J. Lara and J. E. Desnoyers J. Soln. Chem. 1981 10 465. 34 F. Franks J. R. Ravenhill and D. S. Reid J. Soln. Chem. 1972 1 3. 35 J. I. Lankford W. T. Holladay and C. M. Criss J. Soln. Chem. 1984 13 699. Received 20th June 1997 Accepted 5th August 1997 79 Chemical Society Reviews 1998 volum

ISSN:0306-0012    

DOI:10.1039/a827073z

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

New synthetic methods via radical cation fragmentation Mariella Mella Maurizio Fagnoni Mauro Freccero Elisa Fasani and Angelo Albini Department of Organic Chemistry The University v. Taramelli 10 27100 Pavia Italy. Fax 39-382-507323. E-mail albini@chifis.unipv.it Photoinduced single electron transfer followed by radical cation cleavage is a selective method for generating radicals and cations from unconventional substrates under unparalleled mild conditions. The versatility of the method and its synthetic significance are demonstrated through the discussion of selected examples involving alkyl radicals as the key species such as nucleophilic aromatic substitution addition to unsatured esters nitriles and ketones and reduction as well as of an example of addition to cations.1 An unconventional synthetic method The basis of organic synthesis is the formation of the carbon– carbon bond. Available methods are based upon active species Mariella Mella born 1961 graduated at Pavia where she now continues her research both in organic photochemistry with particular attention to the development of new synthetic strategies based on photoinduced SET and in structure determination via NMR. Maurizio Fagnoni born 1968 graduated at Pavia with a thesis on photo-SET induced carbon–carbon bond forming reactions and is now research assistant at Istituto Ronzoni Milan working on the synthesis of polysaccharides. Mauro Freccero born 1966 graduated in Pavia and after a post-doctoral stay at the Dublin City University returned to Pavia as a research assistant working on the mechanism of pericyclic reactions.Maurizio Fagnoni Elisa Fasani Mauro Freccero RX A* hn RX•+ R• A A• – Y R H+ M • Y R H Y such as carbanions carbocations or radicals in turn prepared through hetero- or homo-lytic cleavage of a bond. This introduces an intrinsic limitation as only relatively weak bonds e.g. C–I undergo homolytic cleavage under convenient conditions while heterolytic cleavage requires the use of an aggressive reagent. For example a strong base must be used for abstracting a proton (or another electrofugal group). Organic molecules derive their stability from being closed shell species; thus another way of inducing fragmentation is to weaken first a molecule by subtracting or adding an electron.The resulting odd-electron species (a radical ion) is strongly destabilized and often fragments [e.g. eqn. (1)]. 2e (1) R–X —? R–X.+?R.+X+ Elisa Fasani born 1952 graduated at Pavia where she is now a member of the Faculty. Her research interest centres on photochemistry both for synthetic application and with regard to photodegradation mechanisms of drugs dyes and pollutants. Angelo Albini born 1946 graduated at Pavia where he is now professor of chemistry after a period at the University of Torino. He carried out postdoctoral work at the Max-Planck- Institut at M�ulheim and has been a visiting professor at the Universities of Western Ontario and Odense. He was recipient of the Federchimica Prize 1990.His research interest is in organic photochemistry. Angelo Albini Mariella Mella 81 Chemical Society Reviews 1998 volume 27 How can such a redox activation be carried out? One may look to electrochemistry or to the use of inorganic oxidants or reductants (which often operate via atom transfer not electron transfer however) certainly not to oxidation or reduction by another organic molecule. Undergraduate students may sometimes be puzzled by the fact that redox reactions [e.g. eqn. (2)] are one of the main topics in inorganic chemistry courses while they never hear of redox reactions between organic molecules. This is because inorganic ions mostly have several closely spaced redox states while in organic molecules the HOMO– LUMO gap is too large and thus with a very few exceptions single electron transfer (SET) between any pair of organic molecules [e.g.eqn. (3)] would be largely endothermic. (2) Mm+ + Nn+?M(m21)+ + N(n+1)+ A + R–X?A.2 + R–X.+ (3) However precisely because of the HOMO–LUMO gap being so large the situation is reversed with electronically excited states and if SET is a rare event in ground state organic chemistry it is quite common in photochemistry. Indeed the method discussed below is based on the activation of a substrate by photoinduced single electron transfer (SET) to an acceptor A [see eqn. (3)] followed by fragmentation of the thus formed radical cations giving the carbon centred radicals or carbocations. It will be shown that 4 Photochemistry is a convenient method for generating radical cations in solution.4 Under such conditions strong bonds (e.g. C–H C–C) are selectively cleaved and in this way radicals and cations are generated from unconventional precursors under unparalleled mild conditions. 4 The thus generated intermediates in particular carboncentred radicals can be trapped for synthetically useful reactions via C–C bond formation. 2 Principles of the method 2.1 Photosensitised redox processes Electronically excited states while widely differing in their electronic structure and thus in their chemical reactivity share one common feature viz. they are both stronger oxidants and stronger reductants than the corresponding ground states. This is because they can both easily donate the electron promoted in a vacant orbital and accept an electron in the vacancy created in an occupied orbital.1 The realisation that such is the case was attained first in photophysical studies but somewhat later at the beginning of the 1970s it was clearly demonstrated that some photochemical reactions involve SET as the primary step [eqn.(6)] and the bond-making and bond-breaking steps take place at the radical ion stage [eqn. (7)] rather than directly on the excited state surface [eqn. (5)].1 (4) (5) (6) (7) A + hn?A* A*?Products A* + D?A.2 + D.+ A.2 and/or D.+?Products This area has expanded in the last two decades,2–11 and the underlying motivation has changed. While it remains interesting to demonstrate that a specific photochemical reaction occurs via electron transfer the use of photoinduced SET [eqns.(4) (6)] as an efficient and versatile method for generating radical ions has a much more general interest. The other methods available either have only a spectroscopic interest as in the case of ionisation in the gas phase or have intrinsic limitations as with radiolysis (non-specific) or electrochemistry (a conducting salt is required SET occurs at the electrode surface and is influenced by absorption phenomena). Chemical Society Reviews 1998 volume 27 82 In contrast photochemistry allows us to carry out a redox process between organic molecules [eqn. (3)] in homogeneous solution. Therefore this is often the method of choice for the unambiguous characterisation of radical ions and the assessment of their reactivity e.g.through time-resolved spectroscopy. Such information may be used for supporting the intervention of a redox step in chemical or enzyme induced (or enzyme mimetic) reactions. More importantly photoinduced SET makes radical ions useful to the synthetic chemist. In fact these are generated directly in organic solution in a way not requiring the addition of aggressive reagents or of inorganic salts and allowing a large choice in the experimental conditions. This gives further possibilities for controlling the course of the reaction so that one can use ‘synthetic’ know-how and intuition for exploiting the chemistry of these unusual species without bothering about the generation of the key intermediates which does not significantly limit the choice of experimental parameters.Furthermore since excited states are extremely strong oxidants the choice of oxidizable substrates by photoinduced SET is much larger than with thermal methods as will be shown below with examples involving aliphatic radical cations generated from poor donors for which non-photochemical methods are difficult to devise. In theollowing part of this section the conditions for generating radicals and cations by this method are discussed while their synthetic uses are discusssed in section 3. 2.2 Generation of radical ions Several conditions must be met in order that a photoinitiated SET process occurs efficiently. The first one is that the SET step is exothemic or at least thermoneutral.The reduction potential of a molecule in an excited state is raised by an amount corresponding to the excitation energy viz. by 2 to 4 eV. (8) E(A*/A.2) = E(A/A.2) + Eexc This is a dramatic change and poor electron acceptors in the ground state become extremely strong oxidants in the excited state far superior to available inorganic oxidants. Ground state organic molecules all have largely negative reduction potentials with only very good acceptors e.g. chloranil approaching E(A/A.2) = 0 vs. SCE. However as Scheme 1 shows excited states all have largely positive reduction potentials. ‘Moderate’ photoexcited acceptors such as aromatic ketones [E(A*/ A.2) = 1 to 1.6 V vs. SCE] oxidise relatively good donors such as amines E(R3N.+/R3N ca.1.2 V). These are comparable to ‘strong’ inorganic oxidants [e.g. E(CeIII/CeVI) = 1.28 V]. However more effective photoexcited acceptors such as quinones heterocyclic (e.g. pyrrolinium and pyrylium) salts and aromatic nitriles have E(A*/A.2) > 2 or even > 3 V and thus oxidise even weak donors. In recent years the research has progressed upward along the y axis of Scheme 1. The initial results were obtained with moderate donors such as alkenes,12 aromatic hydrocarbons13 and stannanes. Later the study was extended to compounds which would usually be regarded as oxidation resistant (and correctly so if only thermal oxidants are considered) such as aliphatic ketals14 and silanes15 and even aliphatic hydrocarbons.16 It was found that the corresponding radical cations were efficiently generated provided that the photosensitiser was chosen in such a way that SET was exothermic (see Scheme 1). A peculiarity of photoinduced SET is that the oxidant is the in situ generated short-lived excited state (t @ 1028 s for singlet excited states and << 1026 s for triplet states) the steady state concentration of which is very low. This has two important consequences. First the redox process is selective and little affected by impurities since it is a bimolecular process between two species one of which is present at a very low concentration. Therefore the rate will be significant only when the rate constant is high (ket see Scheme 2 approaches the diffusion A*/A• – CN NC Cl CN NC 3 Cl CN CN CN Ph 2 Ered/V vs.SCE CN CN CN PhCOCF3 PhCOPh 1 Scheme 1 controlled value ca. 1010 dm3 mol21 s21 when the process is exothermic) and only for compounds present at a large enough concentration (on the other hand using the reagent at a high concentration is desirable from the preparative point of view). Second further SET steps involving the excited state and shortlived intermediates forming in the course of the reaction are usually too slow to matter. This is particularly important for alkyl radicals since these have a low Eox and are often further oxidised when generated by means of inorganic reagents obviously added at a large initial concentration or by electrochemical methods (see section 3.3).Thus photoinduced reactions usually involve a single SET step while thermal reaction often leads to different products resulting from further oxidation of an intermediate. A* + RX ket A• – + RX • + hn kbet A + RX Scheme 2 2.3 Fragmentation of radical cations That radical cations fragment is well known being the principle of mass spectroscopy. When generated in the gas phase and with excess energy (usually 70 eV electron impact is used) these species undergo a variety of fragmentation processes. This is of course desirable in that case since it gives more hints for structural identification. However when radical cations are generated in solution by a mild method a single mode of fragmentation is usually operating and gives selectively a radical and a cation.Again this is due to kinetic selection. Photoinduced SET leads to a radical ion pair. Thus the thermodynamically favoured back electron transfer regenerating the reagents in their ground state (kbet > 108 dm3 mol21 s21)11 always D• +/D O RH Cl O O Cl O R2O R4Si Ph BF4 – Ph PhCH3 O+ Sn(But)4 R3N kcl R • + X + competes with any chemical reaction of the radical ions (Scheme 2) and introduces a second requirement for the success of the overall process. Fortunately most radical cations react at a rate comparable with kbet since ionisation strongly destabilizes the molecule. The reaction may be a nonfragmentative process such as a rearrangement or an addition reaction; as an example with alkenes and dienes single electron oxidation may be considered as a more powerful method for obtaining the Umpolung of the molecule than e.g.complexation with Lewis acids and leads to very fast ionic addition or cycloaddition (e.g. see Scheme 3).12,17 ref. 12 • + A• – A A* A• – Scheme 3 More often cleavage to yield a neutral radical and a charged fragment dominates (see kcl Scheme 2). This may be expected since SET injects a large amount of energy into the substrate ca. 1–3 eV 22 to 70 kcal mol21 (1 cal = 4.184 J) approaching the order of magnitude of chemical bond energy. This depends on which bond is cleaved and the labilization occurring upon ionisation can be evaluated through the appropriate thermochemical cycle (Scheme 4) which leads to eqn.(9).2,4,8,18 R • + X + E(X•/X +) R–X • + R • + X • E(R–X/R–X • +) BDE(R–X) R–X BDE(R–X • + ) = BDE(R–X) -[E(R–X/R–X • + ) - E(X • /X +)] (9) Scheme 4 Notice that the quantity indicated in square brackets is always positive since radicals are oxidised at a lower potential than neutral molecules and thus BDE(R–X.+) < BDE(R–X). In other words all bonds are weakened. However the effect is different for different bonds since it depends both on its strength [the BDE(R–X) term] and on how good an electrofugal group is X [E(X./X+)]. If the second term has a small positive or even better a negative value bond weakening is substantial. Indeed such thermochemical calculations show that the barrier for dissociation of some radical cations is reduced to a few kcal mol21 and only in that case can fragmentation compete with back electron transfer (see below for further limitations).Some knowledge has now accumulated about radical cations fragmentation in solution and the main characteristics can be summarized as follows. 4 Fragmentation in solution is often efficient. As seen above efficiency is limited by competition with back electron transfer. Actual quantum yields Freact Å kcl/(kcl + kbet) range from less than 1% to ca. 40%. Obviously the most synthetically interesting cases are those at the higher limit of this interval and there is quite a number of them. 4 While homolytic cleavage in neutral molecules is restricted to weak bonds e.g.carbon–iodine fragmentation of radical ions has a more extensive scope. Certainly a relatively weak bond such as the C–Sn bond cleaves upon ionisation and formation of carbon centred radicals by destannylation of aliphatic organostannanes by photoinduced (as well as thermal) SET works well.9,19,20 However fragmentation of strong s bonds (C–C C–H) can also be obtained. In particular this applies to weak donors the more difficult is the substrate to oxidise [the larger is the E(R–X/R–X.+) term] the more energy is accumulated in the radical cation and thus the easier will it be to cleave a strong bond. 83 Chemical Society Reviews 1998 volume 27 (a) E(X •/X+)/ V vs. SCE R–X • + R• + X • + (b) log kcl 7 6 5 4 p-MeOC6H4CH2X•+ p-MeOC6H4CH2 • + X + 4 Fragmentation is highly selective.Furthermore the preferred fragmentation can be predicted. A first indication is given by eqn. (9) and in particular by the electrofugacity scale of leaving cations the more shifted towards negative potentials is E(X·/X+) (see Scheme 5(a)] the more weakened is the R–X bond. Deprotonation is predicted to be a facile process despite the strength of the C–H bond since in acetonitrile where the experiments referred to here are usually carried out the redox pair H/H+ equilibrium has a very negative potential,21 and desilylation comes near to it [Scheme 5(a)]. Deprotonation is slower than expected from the thermochemical calculations apparently because there is a ‘kinetic overhead’.This corresponds to the relevant internal and external reorganisation energy involved in the transfer of a proton to the solvent from a sC–H bond non-polarised in the starting radical cation (since the radical cation in general has lower-lying MOs where the charge resides). This process is fast when assisted by a nucleophile which may be an added species e.g. an alcohol or the radical anion when this has a nucleophilic character (e.g. in the case of a ketone).6,22,23 Reorganisation is less expensive when the electrofugal group is larger or more delocalised. Acetonitrile often the solvent used in such a reaction is a sufficiently good nucleophile for assisting the detachment of a trialkylsilyl cation. With a non- Chemical Society Reviews 1998 volume 27 84 H+ –2 Me3Si+ –1 MeOCH2 + 0 2 + O + O PhCH PhCMe2 + MeO CH + 2 Me3Si+ Me O + O CO2 + H+ H O + O + CMe2OMe H+ Scheme 5 cl (Me3Si+) > 10 kcl (CO2 + H+) > 10 kcl (H+).nucleophilic radical anion (e.g. an aromatic nitrile) in acetonitrile the experimental electrofugacity scale places the silyl group in the first position.24 As an example the rates of fragmentation measured for a-substituted p-methoxybenzyl radical cations are reported in Scheme 5(b).25 The rate constants change greatly with different substrates; in particular all reactions will be much faster with less stabilized radical cations. However the order of the leaving groups remains the same and Scheme 5(b) reasonably represents an ‘electrofugacity’ scale for radical cations in acetonitrile.What is important for synthetic purposes is that leaving groups are well differentiated in this scale and indeed observed fragmentations are > 90% selective. As an example with xylyl radical cations of the type XCH2C6H4CH2Y.+ the rates of cleavage of group X+ to yield a benzyl radical are in the order k 4 Selectivity is also observed within a given type of bond. As eqn. (6) shows a BDE difference in the neutral substrate translates without change in the radical cation. Thus all other factors being equal the rate of fragmentation follows the order of homolytic strength. As an example with adamantane deprotonation from the radical cation occurs selectively ( Å 100 1) from the bridgehead (tertiary) position (Scheme 6).16 Likewise with ketals or silanes alkyl radicals are generated with marked ( Å 10 times) tertiary > secondary > primary selectivity.• + H p Ka–13 O Me • + But O O Me • + Ph O O H • + H O Me Me • + Si But Me 3 Synthetic use of radical cation fragmentation It has been shown above that heterolytic cleavage of an organic molecule is efficiently obtained under unparalleled mild conditions through photoinduced single electron oxidation and selective fragmentation of the radical cation. Unusual precursors (p s or n donors) can thus be used for generating neutral radicals and cations. As an example alkyl radicals were conveniently and selectively generated from aliphatic stannanes silanes or silyl ethers (via cleavage of a carbon–metal bond),13 aliphatic ketals (C–C cleavage),14 carboxylic acids (deprotonation followed by CO2 loss),26 or alkanes (C–H deprotonation),16 provided that in each case the appropriate photochemical oxidant is chosen (see Scheme 6).This versatility should be useful for synthetic applications since one should be able to choose the most convenient functionality as the precursor of the radical in view of other groups present in the donor and of the structure of the electron acceptor as well as of the desired radical trap. • + H+ ref. 16 Me + But • O + O ref. 14 Ph + Me• O + O H+ + H O • O Me ref. 15 Me Si+ + But • Me Scheme 6 Me – CH O + O O Me •+ O 3 • H2O Me O O TCB* TCB • – HO – CH3 • OH O O Me O O O O O O Me 34% OH O O O Me O O Scheme 7 3.1 Nucleophilic addition onto the cation Both species generated the cation and the radical can be exploited.The cation is trapped by a nucleophile e.g. by moisture present in the solvent (in most cases acetonitrile) or an added alcohol. Photosensitized fragmentation of an aliphatic acetal gives an a,a-dioxy carbocation and an ortho-acid (or ester) from it (Scheme 7).4 This realizes a mild conversion of a ketone to an ester function. The intramolecular application of such a scheme is of some interest in view of the possible selectivity. A representative case is that of 1,2;5,6-di- O-isopropylidene-d-mannitol diacetonide (Scheme 7) where the cation formed from the fragmentation undergoes selective addition by the hydroxy group in position 4 to give a bicylic orthoester (the process can be repeated to give a bis orthoester).This reaction can be considered as a method for protecting group exchange and has been extended to some carbohydrate derivatives.27 3.2 Radical addition to the acceptor aromatics ketones From the point of view of synthetic planning the most appealing side of the above method is certainly the generation of carbon-centred radicals and thus the use of photoinduced SET for carbon–carbon bond forming reactions through this path. Generation of radicals through oxidative procedures is obviously largely precedented.However comparison with reported thermal reactions shows that photochemical initiation is much broader with respect to the radical precursors. Thus thermal oxidants such as MnIII or CeIV oxidize only good donors such as conjugated alkenes enamines and (tautomeric) enols,28 while excited states have a more positive Ered and also oxidize poor donors including alkanes. Photochemical sensitization has further important differences; as it appears from Scheme 2 in this method radicals are generated in the presence of radical anions and of course coupling between two odd-electron species is fast. When aromatic nitriles are used as the acceptors the corresponding radical anions are persistent non-nucleophilic O Me O OH Me Me OH O O O O OH + Me OH O O O Me O Me Me O 44% TCB* – CH TCB • – 3 • + Me ref.27 species which can build up to a relatively high steady state concentration and these couple with the alkyl radical (Scheme 8).29 This gives an unconventional method for aromatic alkylation.30 Benzonitrile—and better polycyanobenzene which are stronger acceptors—undergoes alkylation when irradiated in the presence of a variety of substrates ranging from tert-butyl esters26 to siloxanes,6 ethers,31 alkylaromatics,13,32 and alkanes.16 The reaction proceeds as shown in Scheme 8 and addition of the alkyl radical to the radical anion is regioselective (the position with the largest spin in the radical anion is always attacked independently of whether it is substituted or not).As a result substitution of an alkyl for a cyano group takes place with o- and p-dicyanobenzene as well as with tetracyanobenzene while alkylation at an unsubstituted position occurs with m-dicyano and 1,3,5-tricyanobenzene.13 When cyanated naphthalenes are used the reaction proceeds in the same way up to the anion adduct but this protonates instead of rearomatizing by cyanide ion loss and gives an alkyldihydro derivative (such compounds undergo easy base-catalysed dehydrocyanation however).30 Free radicals have a significant lifetime since radical–radical anion coupling is a second-order process involving two species both at a low steady-state concentration.When using rearranging radicals (‘radical clocks’) we found that the radical incorporated is partially rearranged to an extent which depends on the substrate chosen and the medium used (e.g. added salts; see the cyclopropylmethyl derivative in Scheme 8).33 With ketones related reductive alkylations have been observed but at the moment the process is limited to benzylic donors where the heterocoupling (Scheme 9) is accompanied by homocoupling to give ArACH2CH2ArA.23 3.3 Sensitized radical addition to C–C multiple bond The above direct alkylation of the acceptor may be of some interest e.g. alkylated aromatic nitriles are intermediates for the synthesis of phthalogenines. However a more general issue uses photoinduced SET and radical cation fragmentation for Chemical Society Reviews 1998 volume 27 O OH O Me ref.4 85 AZ* + RX NC AZ • – + RX •+ NC R• + X+ NC AZ • – AZR – NC –Z – AR NC AZR – + H+ ARZH ArCOR* + Ar¢CH2SiMe3 ArCOR •– + Ar¢CH2SiMe3 •+ COCF3 ArCOR •– + Ar¢CH2• ArC(O–)RCH2Ar¢ ArC(OH)RCH2Ar¢ generating the radicals and uses these for alkylating an added substrate limiting the role of the acceptor to that of a regenerated photosensitizer rather than that of a reagent. For such intermolecular trapping two further conditions must be met besides those mentioned above. First the radical must diffuse out of the cage and live long enough as a ‘free’ species to be trapped by an added reagent rather than coupling in the cage with the acceptor radical anion.That radicals do diffuse is Chemical Society Reviews 1998 volume 27 86 CN Me3Si-C5H11 CN CN Me3Si-O-SiMe3 CN O CN O CN CN PhCH2SiMe3 CN Scheme 8 CH2SiMe3 + OMe Scheme 9 indicated by the above ‘radical clocks’ experiments where it is shown that radical–radical anion coupling at least in part involves diffusion and re-encounter.33 In view of the nucleophilic character of alkyl radicals one may expect that carrying out the irradiation in the presence of an electron-withdrawing substituted alkene radicals may be trapped and indeed photosensitized addition to such substrates occurs satisfactorily under appropriate conditions. Second the radical anion of the CN C5H11 ref.13 NC CN 62% Me NC ref. 13 CN NC 90% NC NC CN 45% + ref. 33 NC 50% NC CN CN CN CH2Ph CH2Ph + CN CN 30% 60% ref. 30 OH OMe CH2 CF3 ref. 23 59% acceptor must be re-oxidized to the starting material at some stage of the process in order to participate in further sensitization cycles. The conditions for obtaining such photosensitized addition (see Schemes 10 and 11) have been explored. The probability that the radical escapes coupling with the radical anion of the acceptor and is trapped by the alkene depends on the structure of the acceptor of the donor (in particular of the electrofugal group in it) and on the structure of both radical and trapping olefin. Thus e.g. triplet state acceptors such as aromatic esters work better in this reaction than singlet state acceptors such as aromatic nitriles since diffusion of the radical ions is faster in the first case; alkyl radicals produced by C–C cleavage in ketals or C–Sn cleavage in stannanes are easier to trap than radicals arising from the deprotonation of alkane radical cations; more stabilised tertiary radicals are better trapped than primary radicals; the trapping ability of mono- and di-substituted olefins depends on the balance between electronic activation and steric hindrance.34,35 AZ* + RX Y A Z • – + RX •+ Y a Y • Y X + + R• R b – A Scheme 10 The following course of the reaction is characterized as it is typical of this kind of chemistry by the copresence of several radical species.Trapping of the originally formed (‘educt’) radical by the alkene leads to a new (‘adduct’) radical. This in turn interacts with A.2. The electron withdrawing group makes the adduct radical more stable and more easily reducible than the educt radical and two paths are possible it can be reduced by A.2 [Scheme 10(a)] or add to it (path b).34,35 In the latter case which has been studied with aromatic nitriles as the acceptor both alkylation and arylation of the double bond occur and the final product results from a threecomponent combination with formation of two C–C bonds (Scheme 11).35 Y Y R• • R AZ • – Y R –A Z O But NC CN Me O NC CN NC CN NC Scheme 11 In the other instance the second SET closes the cycle regenerating the sensitiser (typically turnover numbers of 30 to + AZ – R R Z CN But CN ref.35 42% 50 are observed). The combination of the two redox steps establishes a photosensitised radical addition where the educt radical is generated through an oxidation step (in this case it is the reduction potential of the excited acceptor which matters) and the adduct radical is converted to the final product via a reduction step (and in this case it is the reduction potential of the ground state acceptor that matters) (see Scheme 12). Since one can usually choose between different acceptors where Ered(A) and E reaction either way towards the three-component addition [Scheme 10(a)] or towards the photosensitised radical addition (path b).The latter process is the more interesting one from the preparative point of view and can be satisfactorily carried out with a variety of C–C multiple bonds provided that the acceptor–donor combination is properly chosen.27,34 red(A*) change in a different way it is possible to drive the This photosensitized addition has been tested with several alkenes the relative reactivity of which is the same as that observed in conventional ‘free’ radical alkylation. As for alkynes those with two activating substituents in positions 1,2 react and the reaction cleanly stops at the double bond level due to the increased steric hindrance in the product.27 A further convenient alkylation is that of ‘push–pull’ alkenes.27 Radical benzylation can be obtained starting directly from the hydrocarbons.In some cases addition of benzyl radicals to the alkene is inefficient since p-interaction between donor and acceptor slows down diffusion out of cage. However this can be overcome by adding a protic cosolvent which weakens the complex and allows us to obtain the efficient benzylation of alkenes.27 It is too early to estimate the synthetic potential of this method in comparison to classic free radical alkylation. Some key points should be stressed however. First new radical precursors are used which pertain to neither of the categories classically used viz. neither have a homolytically weak bond nor are good donors (such as could be oxidized thermally). Second all of these reactions are carried out by simple irradiation in neat acetonitrile under neutral conditions with the photosensitizer (typically 1023–1022 mol dm23) as the only added (and recovered) chemical.Third this method differs from thermal redox initiated reactions in that the radical adduct is not oxidized as usually happens in that case but rather reduced as has been shown above and thus the final products are different. A likely limitation is that precisely because excited states are such strong oxidants the method may be scarcely tolerant of other functions present in the substrates which may undergo competitive redox processes. 3.4 Radical addition to a,b-unsatured ketones Differently from the previously considered unsaturated esters nitriles and sulfones a,b-unsaturated ketones absorb efficiently in the near UV and intersystem cross to the corresponding triplet states.These are reduced by good donors such as amines and Mariano has demonstrated that this can be exploited for obtaining a convenient aminoalkylation.6 On the other hand adding an unsubstituted alkyl radical through a similar path poses a problem since suitable precursors e.g. tetraalkylstannanes are too weak donors for reducing these triplets. This limitation is overcome in a system where the first step is energy transfer to an additive which has similar triplet energy to—but is a better electron acceptor than—the unsatured ketone such as a pyromellitate ester.27 This exploits photoinduced SET in a different cycle (see Scheme 13) showing how photosensitization can be conveniently tuned by taking into account both energy and redox potential of excited states.In this way an efficient alkylation is obtained with both cyclic and acyclic ketones provided that they are not b,b-disubstituted (since these are too hindered). 3.5 Radical reduction When easily reducible (e.g. benzyl) radicals are generated by this method reverse SET from A.2 takes place and is followed 87 Chemical Society Reviews 1998 volume 27 examples RX A* hn RX•+ CN Bu4Sn TMPM hn R• A A• – Y R H+ Me • Y SMe R ButSnMe3 H TCB hn SO2Ph Y CH3OOC ButSnMe3 COOCH3 CH3OOC DCN/BP hn But CH3OOC o o Ph But COOCH3 CH3OOC TCB hn E and Z CH3OOC COOCH3 CH3OOC PhMe DCN hn PhCH2 Scheme 12 O hn O O 3* O H + R A O A3* • A • – R O R • O Scheme 13 O RX obtained [eqn.(10)].3,32 O ArCR (10) 4 Conclusion and outlook Recent work has shown that probably contrary to what most chemists think radical cations are not only the key intermediates in mass spectroscopy but can also be taken down from the gas phase to solution and they are useful synthetic intermediates. Photoinduced SET is a convenient method for preparing such species characterized by the mild conditions under which it is carried out and the great potential enabling oxidation even of very weak donors. Radical cations often undergo a selective (and predictable) cleavage and this is a new RX • + by protonation to yield a hydrocarbon.In this way hydrolysis of substituted bibenzyl and some related derivatives have been hn Sens,2e 2CR2ArA —————? ArCR2 . + ArACR2 + +e + H2O —————? ArCR2H + ArACR2OH Alkyl radicals are less easily reduced than benzyl radicals and in this case electron transfer from A2· does not take place. However when the bond fragmented in the radical cation is part of a ring a distonic radical cation is formed. Such an intermediate is longer lived and the radical centre while hindered toward addition is activated toward hydrogen abstraction. Thus reduction of the radical centre takes place via atom transfer rather than electron transfer while the cationic centre undergoes nucleophilic addition (cf. section 3.1).The final result is hydrolysis (or solvolysis) of an unactivated C–C bond under mild conditions (Scheme 14). Interestingly with suitable substrates hydrogen transfer is stereoselective.14,27 Chemical Society Reviews 1998 volume 27 88 CN Bu 50% ref. 34 SMe But Me SO2Ph 50% ref. 27 COOCH3 ref. 35 85% But COOCH3 ref. 27 40% COOCH3 80% ref. 27 O SnBu4 TMPM hn But 60% ref. 27 O SnBu4 TMPM hn But 67% TCB hn O HO RSH ref. 14 O 70% Scheme 14 method for preparing radicals and carbocations from unconventional substrates. In the third section of this review we have demonstrated that this cleavage can be exploited for synthetic purposes in particular for carbon–carbon forming reaction with neutral radicals as the key intermediates.The complex sequence involved differs largely from thermal methods via radicals. The final output both in terms of efficiency and product selectivity depends on several competitions at various stages of the process which have been rationalised and can be controlled. In a sense the present stage of the study of radical ions recalls the development of homolytic radical chemistry where synthetic applications were greatly accelerated only after the underlying mechanism was understood. We believe that the results obtained so far give only a hint of the synthetic possibilities offered by radical ions chemistry. In particular the scope of photochemical SET is larger than its chemical counterpart and there are more possibilities to control the process by changing the conditions.Several efficient and selective reactions have been developed. Apart from this the fact that photoinduced SET is essentially independent of experimental conditions enables us to obtain a more complete knowledge of radical ions chemistry. This offers new elements (and bona fide probes) for evaluating the role of electron transfer in thermal and biological processes. 5 Acknowledgement The part of the work reported above which was done in the authors’ laboratory was sponsored by CNR and MURST Rome. 6 References 1 Photoinduced Electron Transfer ed. M. A. Fox and M. Chanon Elsevier Amsterdam 1988. 2 A. Albini M. Mella and M. Freccero Tetrahedron 1994 50 575. 3 E.Albrecht J. Averdung E. W. Bischof A. Heidbronner T. Kirschberg A. F. Mueller and J. Mattay J. Photochem. Photobiol. A Chem. 1994 82 219. 4 P. Maslak Top. Curr. Chem. 1993 168 1. 5 F. D. Saeva Top. Curr. Chem. 1991 156 59. 6 U. C. Yoon and P. S. Mariano Acc. Chem. Res. 1992 25 233. 7 E. R. Gaillard and D. G. Whitten Acc. Chem. Res. 1996 29 292. 8 R. Popielartz and D. R. Arnold J. Am. Chem. Soc. 1990 112 3068. 9 K. Mizuno and Y. Otsuji Top. Curr. Chem. 1994 169 301. 10 F. D. Lewis Acc. Chem. Res. 1986 19 401. 11 I. R. Gould and S. Farid Acc. Chem. Res. 1996 29 520. 12 M. Mella E. Fasani and A. Albini Tetrahedron 1991 47 3137; R. Torriani M. Mella E. Fasani and A. Albini Tetrahedron 1997 53 2573. 13 N. d’Alessandro E. Fasani M. Mella and A.Albini J. Chem. Soc. Perkin Trans. 2 1991 1977 and references therein. 14 M. Mella E. Fasani and A. Albini J. Org. Chem. 1992 57 3051; M. Mella M. Freccero and A. Albini J. Org. Chem. 1994 59 1047. 15 M. Mella N. d’Alessandro M. Freccero and A. Albini J. Chem. Soc. Perkin Trans. 2 1993 515. 16 M. Mella M. Freccero and A. Albini Tetrahedron 1996 52 5533; M. Mella M. Freccero T. Soldi E. Fasani and A. Albini J. Org. Chem. 1996 61 1413. 17 N. L. Bauld D. J. Bellville K. T. Harirchian K. T. Lorenz R. A. Pabon D. W. Reynolds D. D. Wirth H. S. Chiou and B. K. Marsch Acc. Chem. Res. 1987 20 371. 18 D. D. M. Wayner D. J. McPhee and D. Griller J. Am. Chem. Soc. 1988 110 132. 20 S. Fukuzumi and J. K. Kochi J. Org. Chem. 1980 45 2654. 21 V. D. Parker J. Am. Chem. Soc. 1992 114 7458 and 1993 115 1201. 22 X. Zhang S. R. Yeh M. Hong M. Freccero A. Albini A. D. E. Falvey and P. S. Mariano J. Am. Chem. Soc. 1994 116 5503 and references therein. 23 L. Cermenati M. Freccero P. Venturello and A. Albini J. Am. Chem. Soc. 1995 117 7869. 24 N. d’Alessandro A. Albini and P. S. Mariano J. Org. Chem. 1993 58 937; E. Fasani N. d’Alessandro A. Albini and P. S. Mariano J. Org. Chem. 1994 59 1047. 25 M. Freccero A. C. Pratt C. Long and A. Albini J. Am. Chem. Soc. in the press. 26 E. Fasani D. Peverali and A. Albini Tetrahedron Lett. 1994 35 9275. 27 Unpublished results from the authors’ laboratory. 28 P. I. Dalko Tetrahedron 1995 51 7579. 29 M. Freccero M. Mella and A. Albini Tetrahedron 1994 50 6401 and references therein. 30 A. Albini E. Fasani and M. Mella Top. Curr. Chem. 1993 168 143. 31 E. Fasani M. Mella and A. Albini J. Chem. Soc. Perkin Trans. 2 1995 449. 32 A. Sulpizio A. Albini N. d’Alessandro E. Fasani and S. Pietra J. Am. Chem. Soc. 1989 111 5773; L. Bardi E. Fasani and A. Albini J. Chem. Soc. Perkin Trans. 1 1994 545. 33 M. Fagnoni M. Mella and A. Albini Tetrahedron 1994 50 6401. 34 M. Fagnoni M. Mella and A. Albini J. Am. Chem. Soc. 1995 117 7877; M. Fagnoni M. Mella and A. Albini Tetrahedron 1995 51 859. 35 M. Mella M. Fagnoni and A. Albini J. Org. Chem. 1994 59 5614. Received 10th April 1997 Accepted 6th August 1997 89 Chemical Society Reviews 1998 volume 27

ISSN:0306-0012    

DOI:10.1039/a827081z

出版商:RSC    

年代:1998

数据来源: RSC