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Introductory Lecture Energy landscapes of biomolecular adhesion and receptor anchoring at interfaces explored with dynamic force spectroscopy Evan Evans Departments of Physics and Pathology University of British Columbia V ancouver BC Canada V 6T 1Z1 and Department of Biomedical Engineering Boston University Boston Massachusetts USA 02215 Received 21st December 1998 Beyond covalent connections within protein and lipid molecules weak noncovalent interactions between large molecules govern properties of cellular structure and interfacial adhesion in biology. These bonds and structures have limited lifetimes and so will fail under any level of force if pulled on for the right length of time. As such the strength of interaction is the level of force most likely to disrupt a bond on a particular time scale.For instance strength is zero on time scales longer than the natural lifetime for spontaneous dissociation. On the other hand if driven to unbind or change structure on time scales shorter than needed for diÜusive relaxation strength will reach an adiabatic limit set by the maximum gradient in a potential of mean force. Over the enormous span of time scales between spontaneous dissociation and adiabatic detachment theory predicts that bond breakage under steadily rising force occurs most frequently at a force determined by the rate of loading. Moreover the continuous plot (spectrum) of strength expressed on a scale of log (loading rate) provides a map of the prominent barriers e traversed in the energy landscape along the force-driven pathway and reveals the diÜerences in energy between barriers.Illustrated with results from recent laboratory measurements dynamic strength spectra provide a new view into the inner complexity of receptor»ligand interactions and receptor lipid anchoring. Introduction Well-recognized in biology ligand»receptor interactions are the fundament of nanoscale chemistry in recognition signalling activation regulation and other processes from outside to inside cells. Thus following the advent of atomic force microscopy (AFM) a decade ago,1 it was no surprise that researchers quickly seized the opportunity to test strengths of receptor»ligand bonds. Since then AFM and other sensitive force probes have been used to pull on a variety of molecules embedded in»or adhesively bonded to»surfaces.Applying these techniques experimentalists often imagine that probe force establishes a well-de–ned property of an interaction between molecules. Such expectations originate from the age-old creed of physics which states that strength is the maximum gradient [(dE/dx)max of an interaction potential or energy contour E(x) de–ned along the direction (x) of separation. Hence it is anticipated that detachment forces for diÜerent types of molecular interactions will follow a scale set by the ratio of bond energy to the eÜective Faraday Discuss. 1998 111 1»16 1 range of the interaction (bond length). This seems consistent with the standard model of biochemistry where the scale for bond strength is the free energy *G° reduction when molecules combine in solution as found from the equilibrium ratio keq[Dexp(*G°/kBT )] of bound to free constituents.As such the criterion for a strong bond should be simply a binding energy much larger than the thermal energy per molecule kBT . In marked contrast to these two paradigms we will see that even bonds with binding energies [40 kBT can fail under minuscule forces»more than 100-fold lower than the maximum energy gradient implied by energy/distance. Indeed we will –nd that measurement of molecular detachment force»no matter how precise the technique or how carefully performed»is not in itself a fundamental property of a molecular interaction. So what is the appropriate framework for describing strength of molecular bonds and how can we relate measurements of these forces to nanoscale chemistry ? When we test strength of molecular cohesion or adhesion at surfaces we determine the maximum level of force that a molecular attachment can support at the instant of failure.Unlike intimate covalent connections within protein and lipid molecules biomembrane structure and interfacial adhesion bonds involve noncovalent interactions between large macromolecules which have limited lifetimes and thus will fail under any level of force if pulled on for the right length of time. In other words when we speak of strength we should think of the force that is most likely to disrupt an adhesive bond or structural linkage on a particular time scale. At equilibrium for example bonds dissociate and reform under zero force.Thus an isolated bond has no strength on time scales longer than its natural lifetime ciation. On the other hand if detached within the time needed for diÜusive relaxation over the range of molecular interaction (e.g. x will reach and even exceed the adiabatic limit f=Bo*Eo/xb set by the maximum gradient in a potential of mean force. This is the situation in molecular dynamics (MD) simulations.2,3 t0\1/koff 0 for spontaneous (entropy-driven) dissob[ 1 nm]xb2/D\10~9 s in water) the strength of a bond From the slow limit set by spontaneous transition (from ls to months) to the ultrafast limit set by diÜusive relaxation (\ns) strength is governed by thermally activated kinetics under external force and thus depends on how the force is applied over time.Since application of force always requires a –nite interval of time the simplest way to parameterize the history of loading is to treat force as a ramp in time set by a constant loading rate rf\*f/*t. In fact a ramp of force is what single molecular attachments experience when a force probe and test surface are separated at constant speed (i.e. loading rate\probe stiÜness]speed). Using this parameterization and some nearly sixty year old physics4 for Brownian dynamics of chemical reactions in liquids we have shown that bond dissociation under steadily rising force occurs most frequently at a time determined by the rate of loading.5 Since loading rate is constant the time of dissociation speci–es the most likely rupture force»strength»which has the same dependence on loading rate.Of particular signi–cance the continuous plot of strength expressed on a scale of log (loading rate) maps the e most prominent barriers traversed in the energy landscape to distances along the force-driven pathway and reveals the splitting in energy between barriers. Thus strength vs. log (loading rate) e establishes the basis for a dynamic force spectroscopy (DFS) to probe the inner world of molecular-scale chemistry. Testing bond strength or structural transitions at diÜerent loading rates eÜectively probes the lifetime of a molecular complex under diÜerent levels of force. The experimental challenge is to measure forces over many orders of magnitude in loading rate. This dynamic requirement is dictated by the exponential of the energy diÜerence *E between the highest and lowest barriers divided by thermal energy i.e.exp(*Eb/kBT ) which can be enormous! b The strength spectra to be presented here will show that we can now cover six orders of magnitude in loading rate from \0.1 pN s~1 to D105 pN s~1 with a rather simple dynamic force probe which could be extended to D107 pN s~1 with complementary measurements using other probes. But more important than demonstrations of technique these spectra provide a new level of insight into the complexity of macromolecular interactions and structural linkages. First results from biotin»(strept)avidin6 and carbohydrate»L-selectin7 bond tests will show that a cascade of sharp energy barriers exists in receptor»ligand bonds where each barrier governs strength on a diÜerent time scale.We see then that these bonds cannot be simply idealized by a sole energy barrier and we cannot rely on the classical intuition about kinetics implicit in the detailed balance keq\ kon/koff where kon and koff are constants. Second tests of lipid extraction8 from membranes will show that anchoring of receptors to surface structure plays an important role in adhesion strength and can introduce unexpected transitions in the strengths of receptor»ligand attachments. In other Faraday Discuss. 1998 111 1»16 2 words we should not assume that a structural linkage of several molecules will fail at a speci–c weak connection nor that we can uniquely attach strong or weak labels to bonds in a linkage. Simultaneous kinetics over diÜerent energy landscapes in serial molecular linkages can lead to strength or weakness on diÜerent time scales.Dynamic crossovers in strength switch the site of failure from one location to another. Taken together these insights show that mechanical force can tune and switch time scales for kinetics in biomolecular reactions governed by complex energy landscapes which exposes a potentially new dimension in biochemical regulation and control. Theory of molecular kinetics under force in liquids We begin with an abstract of the physics that underlies the kinetics of bond dissociation and structural transitions in a liquid environment. Developed from Einsteinœs theory of Brownian motion these well-known concepts take advantage of the huge gap in time scale that separates rapid thermal impulses in liquids (\10~12 s) from slow processes in laboratory measurements (e.g.from 10~4 s to min in the case of force probe tests). Three equivalent formulations describe molecular kinetics in an overdamped liquid environment. The –rst is a microscopic perspective where molecules behave as particles with instantaneous positions or states x(t) governed by an overdamped Langevin equation of motion dx/dt\D/kBT [ f[+E]df ] The rate of change in state equals the instantaneous force scaled by the mobility of states or inverse of the damping coefficent c(\kBT /D). The deterministic force ([+E]f ) includes both the local gradient in molecular interaction potential E(x) and the external force f. An uncorrelated random force df from thermal impulses modulates the deterministic force and obeys the —uctuation»dissipation theorem where the integrated square —uctuation in a window of time can be Dexp[[/ df 2dt/(4k modeled as a Gaussian distribution BT )] with variance set by temperature and viscous damping.9 The microscopic physics also de–nes a stochastic process that has become the foundation of an important computational technique»Brownian dynamics or smart Monte Carlo (SMC)10 simulations.In this description the likelihood P(x]*x t]*t o x t) that a state x(t) will evolve to a new state x]*x over a time increment *t is speci–ed by the product of the equilibrium (long-time) Boltzmann weight for the step and a Gaussian weight for dynamics P(x]*x t]*t o x t)DexpM[(*E[f Æ*x)/kBT NexpM[o*x[(D/kBT ) f *t o2/(4D*t)N/(D*t)1@2 Finally on time scales that include many thermal impulses the overdamped dynamics can be cast in a continuum representation where the density of states o(x t) at location x and time t obeys Smoluchowski transport,9 do/dt\[+ Æ J J\D[ f[+E)o/k where the —ux of states BT [+o] re—ects both convection by force and spread by diÜusion.Although each description of the ultrafast kinetics brings to light important features Kramers4 demonstrated that Smoluchowski transport readily predicts the rate of escape from a deeply bound state when a large number of thermally activated steps are needed to pass a barrier in a dissipative environment. Escape from a bound state con–ned by a single barrier Starting far from equilibrium with all states con–ned inside the barrier the kinetics of escape are idealized as a stationary —ux of probability density along a preferential path from the deep energy minimum outward past the barrier via a saddle point in the energy surface.In real molecular interactions there can be many such paths and the paths can map out complex trajectories in con–guration space. However application of an external pulling force acts to select the reaction path which we express by a scalar coordinate x. Assumed to be bounded by steeply rising energy in other directions the energy landscape E(x) along this coordinate is illustrated schematically in Fig. 1(a). Governed by orientation h relative to the microscopic reaction coordinate external force adds a mechanical potential [fx(cos h) that tilts the energy landscape and diminishes the energy barrier E at the transition state (x\x b ts).When the tilted landscape is introduced into the Smoluchowski equation the stationary solution (J\constant in 1-D or\constant/xd~1 in d- Faraday Discuss. 1998 111 1»16 3 Fig. 1 Conceptual energy landscapes for bound states ììcœœ con–ned by sharp activation barriers. Oriented at an angle h to the molecular coordinate x external force f adds a mechanical potential [( f cos h)x that tilts the landscape and lowers the barrier. For sharp barriers the energy contours local to barriers»transition states ìì sœœ»are highly curved and change little in shape or location under force. (a) A single barrier under force. (b) A cascade of barriers under force.The inner barrier emerges to dominate kinetics when the outer barrier is driven below it by PkBT . dimensions) yields a generic expression for rate of escape from bound to unpopulated free states under force,5 c which drives escape. In a harmonic approximation l is derived from curvature koffB(D/lc lts)exp[[Eb( f )/kBT ] D/l The diÜusive nature of kinetics in liquids is embodied in the attempt frequency c lts which is the reciprocal of a characteristic time tD\lc lts(c/kBT ) set by damping and two length scales. The –rst length l represents con–nement in the bound state and de–nes the entropy gradient (do/dxB i l/l c\ c) (d2E/dx2) of the energy landscape local to the minimum i.e. lc\(2pkBT /ic)1@2. The second length c c l is the energy-weighted width of the barrier lts\/ dx exp[*E(x)ts/kBT ] local to the transition ts state x\xts also determined by curvature bB0.1»1 nm.On this scale the rate of escape increases exponentially with force koffB(1/t0) its\(d2E/dx2)ts of the energy landscape i.e. lts\ (2pkBT /its)1@2. Although force can displace and deform the width of the barrier [i.e. (its/2pkBT )1@2Bg( f )] the major impact of force arises in the thermal likelihood of reaching the exp[[E top of the energy barrier b( f )/kBT ]. For a sharp energy barrier the shape and location of the transition state are insensitive to force but force lowers the barrier in proportion to the therx mally averaged projection b\\xts cos h[ i.e. Eb( f )\Eb[fxb . As such thermal activation introduces the characteristic scale for force through the ratio of thermal energy to the distance xb x i.e.fb\kBT /xb which can be surprisingly small since kBT B4.1 pN nm at room temperature and b) as –rst postulated by Bell11 twenty years ago. But in contrast to the resonant frequency exp( f/f of bond excitations described in Bellœs model Kramers showed that the relevant attempt frequency is 1/tD\(ic its)1@2/2pc for overdamped transitions in liquids which is at least 1000-fold slower. With the Arrhenius dependence on initial barrier height and the attempt frequency Kramers classic result for spontaneous escape in the overdamped limit sets the scale for the transition rate i.e. 1/t0\(1/tD)exp([Eb/kBT ). Escape from a bound state con–ned by several barriers Although a naive model of chemical binding the single-sharp barrier model already captures the profound impact of force on thermally activated kinetics i.e.exponential ampli–cation of the forward rate for dissociation (and suppression of backward rate for reassociation) characterized by kBT /xb well below the adiabatic limit[E a small force scale b/xb ! However the energy landscapes of biomolecular bonds are expected to be much more complex because there are many sites of interaction involving large numbers of small molecules. This should produce a rough topography of barriers in an energy landscape and many possible pathways for unbinding. If again conceptualized as precipitous (sharp) energy maxima along a single pathway these prominent barriers are predicted to emerge under increasing force and dominate kinetics in succession as demonstrated by the sketch in Fig.1(b).5 An inner barrier is exposed when the force exceeds a crossover force fcB*Eb/*xb set by the splitting *Eb between barrier heights and separation in projected positions *xb . Depending on the diÜerence in barrier energies the crossovers occur at forces much Faraday Discuss. 1998 111 1»16 4 larger than the local thermal forces given by kBT /xb . Thus marked by these crossovers the kinetic rate constant is predicted to rise in a staircase of force-dependent exponentials that amplify the rate of transition less and less with each increase in thermal force scale. The transition rate for escape past a cascade of n sharp barriers is easily predicted with Kramers»Smoluchowski theory koff( f )B(1/t0)exp( f/f b0)/M1] ; li exp[ f*xb[*Eb]N i?n liBlts/lts 0 [\(its 0/its)1@2] plus diÜerences *xb\xb0[xb and energy *Eb\Eb0[Eb of inner barriers relative to the outermost which at low force begins with the steepest exponential dominated by the outermost barrier.At larger forces the rate crosses over to more shallow exponentials. The transition from one exponential regime to the next depends on the ratio of widths in location barrier as de–ned by Eb0 lts 0 and xb0 . We see then that a major consequence of structured energy landscapes is to make molecular interactions more durable (survive longer) at higher forces. f t[kBT /xb the forward transition Theory of force distributions in probe experiments Even with ultrasensitive probes and high resolution detection tests of molecular detachment yield a spread in force values.To understand the origin of the intrinsic uncertainty in force we have to examine the generic process of bond dissociation in laboratory experiments. Typically a probe decorated with a small amount of ligand»and a substrate studded with speci–c molecular receptors»are repeatedly touched together through steady precision movement to/from contact. If the surfaces are prepared with a sufficiently low density of reactive sites point contacts between the probe tip and the test surface will occasionally result in attachments (e.g. one attachment for every 5»10 touches). Under controlled touch infrequent bonding ensures a high probability of forming single molecular bonds (D95% con–dence for 1 attachment out of 10 touches).An attachment is exposed when the force transducer exhibits an extension or de—ection *x during surface t separation. Identi–ed by rapid recoil at breakage the rupture force is given by the maximum transducer extension * k x i.e. f\kf …*xt where is the spring constant of the transducer. Follow- t f ing many measurements detachment forces are then cumulated into a histogram. The peak in the distribution is the most likely the rupture force which is labelled bond strength. This approach has been reported many times in the literature over the past decade including studies of bond strength using AFM12h16 and other techniques.17,18 The exception to the generic description is that the frequency of attachment in most tests has been one for every touch which represents many molecular bonds and yields broad force distributions.Given that only a single molecular attachment forms on contact the crucial feature of the generic method is that the force experienced by the attachment prior to rupture is not constant but increases in time. This is shown clearly by two traces of attachment force vs. time in Fig. 2 taken from our experiments6 on single receptor»ligand bonds using a biomembrane force probe (BFP).19 In probe tests like these the linear rise of force with time is set by the product of separation speed v and transducer spring constant kf which is called the loading rate rf\kf vt . t (Note if soft structures like long polymers link the bond to a stiÜ probe the loading history can be nonlinear in time.20) Very diÜerent levels of force and time frame characterize the two detachment processes in Fig.2. Comparing these we see then that bond survival and breakage force depend on the rate of loading in reciprocal ways i.e. high speed loading]short lifetime but large detachment force whereas low speed loading]long lifetime but small detachment force which is the direct consequence of thermally activated kinetics. Statistics of transitions under increasing force To analyse bond breakage under steady loading we take advantage of the enormous gap in time (t scale between the ultrafast Brownian diÜusion DB10~10[10~9 s) and the time frame of laboratory experiments (D10~4 s to min). This means that the slowly increasing force in laboratory experiments is essentially stationary on the scale of the ultrafast kinetics.Thus dissociation rate merely becomes a function of the instantaneous force and the distribution of rupture times can be described in the limit of large statistics by a –rst-order (Markov) process with time-dependent rate constants. As force rises above the thermal force scale i.e. r Faraday Discuss. 1998 111 1»16 5 Fig. 2 Testing strength of single molecular attachments with the biomembrane force probe (BFP). The spring component of the BFP is a pressurized membrane capsule.19 Membrane tension sets the force constant kf (force/capsule extension) which is controlled by micropipet suction P and radius Rp kfBP]Rp . Using a red blood cell as the transducer the BFP stiÜness can be selected between 0.1 and 3 pN nm~1 to measure forces from 0.5 to 1000 pN.At the BFP tip a glass microbead of 1»2 lm diameter is glued to the membrane. The probe tip and red cell surfaces are bound covalently with heterobifunctional polyethylene oxide PEG polymers that carry glue components and test ligands.6 The BFP is operated in two orientations (modes) on the stages of inverted microscopes as illustrated by the following examples of fast and slow bond detachment (a) –rst the BFP (on the left) is kept stationary in the horizontal mode and the microbead test surface (on the right) is translated to/from contact with the BFP tip by precision piezo control. Video image processing is used to track the bead as shown by the simulated cursor ; a single high speed (D1000 frames s~1) scan through the center of the bead is used to track de—ection of the transducer (force) on a fast time scale at a resolution of 8»10 nm.Parts (b) and (c) show the BFP tip»substrate separation and force vs. time for rapid bond detachment in the horizontal mode. (b) The test microbead was moved towards the probe tip at a speed of D500 nm s~1. Stopped for D0.5 s after sensing contact at a preset impingement force of D[30 pN the test surface was then retracted at speed of D30 000 nm s~1. (c) Loaded at extremely fast rate the bond held the tip to the surface for D0.003 s (spike in force) and broke at D180 pN as the piezo continued to retract the test surface. The force —uctuations were due to position uncertainties ]BFP stiÜness. (d) In the vertical mode re—ection interference contrast is used to image the BFP tip as it is translated by piezo control along the optical axis to/from contact with a coverglass test surface.Standard video (30 frames s~1) processing of the circular interference pattern reveals elevation of the tip at a resolution of 2»5 nm. Transducer de—ection (force) is obtained from the diÜerence between piezo translation and bead displacement. Parts (e) and (f) show the BFP tip» substrate separation and force vs. time for a slow bond detachment in the vertical mode. (e) The probe was moved towards the coverglass test surface at a speed of D20 nm s~1. After sensing contact at a preset impingement force of D[3 pN the probe was retracted at slow speed of D1 nm s~1. (f ) Loaded at extremely slow rate the bond held the tip to the surface for D24 s and broke at D3 pN as the piezo continued to retract the probe (dashed trajectory).The —uctuations in tip position were due to thermal excitations of the BFP with mean square displacement DkBT /kf . Stretch of the PEG polymers that linked the bond to the glass surfaces is shown by the slight upward movement of the tip (D15 nm) under force prior to detachment. Due to polymer (k below 10 pN s~1 had to be obtained compliance the true loading rate felt by a bond at nominal rates f vt) from the actual force vs. time. Faraday Discuss. 1998 111 1»16 6 on ]0). Thus the likelihood S(t) of remaining in the bound state (escape) rate increases extremely rapidly. Moreover the molecules drift apart faster than diÜusion can recombine them from positions beyond the con–ning barrier so the backward rate for reassociation quickly vanishes (k is dominated by the forward process i.e.dS(t)/dtB[koff(t)S(t) or equivalently S(t)\ exp[[/0?t koff(t@)dt@]. The probability density p(t)\koff(t)S(t) for detachment between times t and t]*t describes the distribution of lifetimes. Since instantaneous force is the product of time and loading rate ( f\rf t) the probability density p( f ) for detachment between forces f and f]*f is given by the distribution of lifetimes p(t),5 p( f )\(1/rf)koff( f )exp[[(1/rf)P koff( f @)df @] 0?f noting the statistical identity p(t)dt\p( f )df. The peak in the distribution of forces de–nes the force f * for most frequent transition which is strength. Analytically the location of a distribution peak is found from dp( f )/df\0 which establishes a transcendental equation that relates the strength f * to loading rate rf [koff]f/f*\rf[d loge(koff)/df ]f/f* Although somewhat forbidding this expression yields a simple result for strength as a function of loading rate in the case of a single sharp energy barrier f *\f loge(rf/rf 0 ) recalling that the rate is modelled by an exponential in force koffB(1/t0)exp( f/fb).Governed by a thermal scale for loading rate rf0\fb/t0 the most likely force»strength»simply shifts upward linearly with the logarithm of loading rate multiplied by the thermal force fb . Similarly the curvature of the distribution local to the peak 1/Df2\[[1/p( f )][d2p( f )/df 2]f/f* can be used to estimate a Gaussian width for uncertainty in the force distribution, f\fb .Hence even without experi- Dynamic force spectroscopy b b Df2\1/M[d loge(koff)/df ]2[[d2 loge(koff)/df2]Nf/f* For a sharp energy barrier this again yields a simple result D mental uncertainty the distribution of forces is broadened by thermal activation (kinetics) ! In the context of experiments the signature of a major sharp barrier is predicted to be a straight line in a plot of most frequent probe force f * vs. log(loading rate) as illustrated in Fig. 3(a). This linear regime can span orders of magnitude in rate as determined by the ratio of barrier energy E 1/tD and height E of the activation barrier. (a) Linear spectrum predicted for a single Fig. 3 Dynamic strength spectra de–ned by most likely bond detachment force f * vs.log (loading rate\ rf/rf 0 ) where the loading rate scale rf0\( fb /tD)exp([Eb/kBT ) is set by thermal force fb\kBT /xb diÜusive e b attempt frequency sharp energy barrier. The logarithmic intercept at zero force (represented by \) is determined by the barrier height and the microscopic diÜusion time loge(rf 0)\[Eb/kBT ]loge( fb /tD). (b) Piece-wise linear spectrum for a cascade of two sharp energy barriers. The abrupt increase in slope from one thermal force scale to the next shows that the outer barrier has been suppressed and that the inner barrier has become the dominant kinetic impedance to detachment [cf. Fig. 1(b)]. The diÜerence between logarithmic intercepts (represented by \) is governed by the splitting in barrier energies and the ratio of thermal force scales loge(* rf 0)B[*Eb/kBT ] *loge( fb ).7 Faraday Discuss. 1998 111 1»16 xb\Sxts cos hT along the direction of force. Moreover to thermal energy k f BT . The slope of this line maps the thermally averaged projection of the microscopic transition state to a distance the logarithmic intercept at zero force re—ects the magnitude of barrier energy as given by b loge(rf 0)\[Eb/kBT ]loge( fb/tD). Setting the scale for loading rate the ratio fb/tD involves the microscopic attempt frequency 1/tD . Assuming that attempt frequency is weakly aÜected by point mutations the simple linear-log behavior exposes a unique opportunity to quantitate the resulting chemical modi–cations in energy and/or location of barriers.Such changes in microscopic properties can be derived from the shift in the logarithmic intercept and/or change in slope *Eb/kBT B [*loge(rf 0)]*loge( fb). Taken together these features demonstrate that the plot of most frequent probe force vs. log(probe loading rate) represents a dynamic spectral image of an activation barrier. [Although unknown attempt frequency can be estimated from the damping factor indicated by MD simulations. Values for damping factor seem to be typically on the order of cB10~8 pN s nm~1 (equivalent to Stokes drag on a 1 nm size sphere in water) e.g. cB2]10~8 pN s nm~1 in simulations of biotin»streptavidin separation2 and cB5]10~8 pN s nm~1 in simulations of lipid extraction from a bilayer.3 Since the product of molecular lengths lc lts should lie in the range D0.01»0.1 nm2 the attempt frequency is expected to be in the range 1/tDB 109»1010 s~1 and the microscopic scale for loading rate in the range fb/tDB1010»1011 pN s~1.The eÜective loading rate in the slowest MD simulations2,3 is even higher P1012 pN s~1.] As described earlier the most idealized view of a complex molecular energy landscape is a cascade of sharp activation barriers which leads to a staircase of exponential increases in the rate constant under force. Using this prescription the most likely force vs. log(loading rate) is predicted to follow a simple spectrum of piece-wise continuous linear regimes with ascending slopes as shown in Fig. 3(b). The abrupt increase in slope from one regime to the next signi–es that an outer barrier has been suppressed by force and that an inner barrier has become the dominant kinetic impedance to escape as sketched in Fig.1(b). These dynamic crossovers occur at somewhat higher forces than the stationary crossovers in rate constant as shown by the analytical approximation f cdynB*Eb/*xb]kBT [loge(xb @ /xb)]/*xb *x where b\xb @ [xb and *Eb\Eb @ [Eb represent adjacent prominent barriers. In contrast to the idealized theory the shape of a strength spectrum could be nonlinear and a challenge to interpret because force can distort physical potentials and molecular structure. Surprisingly the results from recent probe experiments to be shown next yield linear plots for strength vs. log(loading rate) with one or more well-de–ned regimes which allows the spectra to be interpreted in terms of sharp activation barriers.Energy landscapes of receptorñligand bonds Not well-appreciated in biology is that energy landscapes of receptor»ligand bonds can be rugged terrains with more than one prominent activation barrier. The inner barriers are undetectable in test-tube assays but are important since they establish diÜerent time scales for kinetics under force. With two unrelated pairs of molecules we will demonstrate that dynamic force measurements can be used to reveal these hidden barriers. The –rst pair of molecules will be the ligand biotin (a vitamin) and the protein receptor streptavidin (from bacteria) or avidin (a closely similar protein from hen egg white).21 This complex is used widely in biotechnology because it has one of the highest affinity noncovalent bonds in biology with a force-free lifetime on the order of days.22 The second pair of molecules will be a sialylated (carbohydrate) short peptide ligand§ and the Lselectin receptor resident in the outer membrane of blood leukocytes.Although weaker in affinity with a lifetime of D1 s or less the carbohydrate»L-selectin bond plays a crucial role in the initial capture of leukocytes from blood circulation at sites of injury or infection.23 In preparation for both experiments the ligand was covalently anchored to a glass microbead along with a chemical glue for attachment of the bead to the BFP transducer [as noted in Fig. 2(a)]. A similarly pre- § Note the actual ligand used in the tests was a short peptide chimera of the biological molecule called P-selectin glycoprotein ligand (PSGL1) which was constructed by Genetics Institute and obtained through collaboration with Scott Simon at Baylor College of Medicine.The generic label carbohydrate will be used for convenience. Faraday Discuss. 1998 111 1»16 8 Fig. 4 On the left are examples of force histograms taken from tests of single biotin»streptavidin bonds which demonstrate the shift in peak location and increase in width with increase in loading rate (top histogram 0.05 pN s~1 middle histogram 20 pN s~1 bottom histogram 60 000 pN s~1). Superposed on the histograms are Gaussian –ts used to determine the most frequent rupture force»bond strength. Governed ideally by the thermal force fb standard deviations p of the distributions also re—ected uncertainties in posifB[ f b2](kf*x)2](rf*tv)2]1@2.f tion *x and video sampling time *tv i.e. p increased from ^1 pN at As pf the slowest rate to ^60 pN at the fastest rate the standard error in mean force»the uncertainty in strength» ranged from ^0.3 to ^5 pN. On the right are complete dynamic strength spectra for both biotin»streptavidin slopes of the linear regimes seen in the spectra map activation barriers at positions along the direction of force. (open circles) and biotin»avidin (closed triangles) bonds.6 De–ned as thermal energy kBT /distance xb the The common high strength regime in the biotin»streptavidin and biotin»avidin spectra place the innermost x barrier at bB0.12 nm.Separate intermediate strength regimes place the next barrier at xbB0.5 nm for biotin»streptavidin and xbB0.3 nm for biotin»avidin (with a slight reduction in slope below 38 pN suggesting that the biotin»avidin barrier extends to D0.5 nm). Only well-de–ned in the biotin»avidin spectrum a low x strength regime implies a distal barrier at bB3 ( nm. Also marked \AFM) is the biotin»streptavidin strength measured recently by AFM at D105 pN s~1 using a carbon nanotube as the tip.14 This and the earlier measurements of biotin»avidin bond strength13 at loading rates of D6]104 pN s~1 also correlate with the high strength regime shown here. pared microbead was used as the test surface for probing biotin»(strept)avidin bonds whereas a white blood cell (granulocyte) taken from a small blood sample was used as the test surface for probing carbohydrate»L-selectin bonds.kf vt is preselected by setting the transducer force constant k in the range 0.1»3 pN f [Methods In testing molecular bonds the density of reactive sites must be reduced signi–cantly as mentioned earlier so that only 1 out of 7»10 touches results in a molecular attachment. Assumed to be governed by Poisson statistics this ensures that 90»95% of the attachments are single bonds. To obtain strength spectra with the BFP technique detachment forces are measured over a six order of magnitude range in loading rate from 0.05 pN s~1 to 100 000 pN s~1. The loading rate nm v ~1 and the piezo retraction speed in the range 1»30 000 nm s~1 as described in Fig.2. From t thousands of repeated touches at –xed loading rate histograms of detachment forces are compiled at many rates and Gaussian –ts are used to locate the peak in each histogram. These most probable values of force are then plotted as a function of log (loading rate) which yields the dynamic e strength spectrum.] Biotin (strept)avidin bonds Because of high affinity the –rst ligand»receptor pair chosen by researchers for testing with AFM was biotin and streptavidin ; which was soon followed by biotin and avidin.12h14 Deduced from a broad distribution of AFM forces it was concluded that the strength of a biotin»streptavidin 9 Faraday Discuss. 1998 111 1»16 bond lies in a range of D200»300 pN and somewhat lower for biotin»avidin D160 pN. However the examples of force histograms and the strength spectra6 in Fig.4 show that biotin»streptavidin (and biotin»avidin) bond strengths fall continuously from D200 pN to DpN with each decade increase in time scale for rupture from 10~3 to 102 s which clearly demonstrates the thermally activated nature of bond breakage. Moreover distinct linear regimes with abrupt changes in slope imply sharp barriers which can be analysed using the idealized theory.6 First above 85 pN there is a common high strength regime for both biotin»streptavidin and biotin»avidin with a slope of fbB34 pN. This locates a barrier deep in the binding pocket at xbB0.12 nm. Below 85 pN the fbB8 pN slope in the biotin»streptavidin spectrum maps the next activation barrier at xbB0.5 nm whereas the steeper slope fbB13»14 pN between 38 and 85 pN in the biotin»avidin spectrum x indicates that its next barrier maps to bB0.3 nm.Interestingly a slight curvature and reduction in slope between 38 and 11 pN suggests that the barrier in biotin»avidin extends to D0.5 nm. Below 11 pN the biotin»avidin spectrum exhibits a very low strength regime (dashed line) with a slope of fbB1.4 pN that maps to xbB3 nm. A similar low strength regime is indicated by results from the slowest test of biotin»streptavidin bonds; but it was not possible to perform tests at loading rates below 0.05 pN s~1 as needed to verify the existence of this regime. In addition to the map of barrier locations the logarithmic intercepts found by extrapolation of each linear regime to zero force also yield estimates of the energy diÜerences between activation barriers within each landscape as well as energy diÜerences between related barriers of biotin»avidin and biotin» streptavidin landscapes.However instead of discussing barrier heights it is more illuminating to examine how the 1-D map of barrier locations compares with detailed molecular simulations of biotin»(strept)avidin interactions. In separate MD simulations,2 biotin was extracted from a binding pocket of streptavidin and avidin by pulling on the outer end with a pseudo-mechanical spring. Consistent with the numerous bonds to small molecules in the binding pocket simulations yield a —uctuating superposition of many attractions»buÜeted by steric collisions»along the unbinding trajectories.This is shown by a pro–le of instantaneous energy calculated over a slow D500 ps extraction of biotin from avidin [Fig. 5(a) kindly provided by Professor K. Schulten and coworkers Beckman Institute University of Illinois]. Even with the enormous and fast changes in energy simple qualitative features appear in the pro–le that provide important clues to the thermally averaged free energy landscape relevant on laboratory time scales. In particular transition states are readily identi–ed by regions with rari–ed statistics where biotin passes quickly. Taking a simple coarse-grained average over D20 ps windows [Fig. 5(b)] smooths over the strong rapid —uctuations and exposes locations of activation barriers. First within an initial displacement of 0.1»0.2 nm the spring force in the simulations revealed abrupt detachment of biotin from a nest of hydrogen bonds water bridges and nonpolar interactions deep in the binding pocket.Next forces reached maximal Fig. 5 (a) Pro–le of instantaneous energy computed for interaction between biotin and avidin over a halfnanosecond extraction from the binding pocket in the simulations of Israilev et al.2 (kindly provided by Professor K. Schulten and coworkers University of Illinois). Separating regions of rapid intense —uctuations locations of rari–ed statistics coincide with maximal forces in the simulations which signify the presence of transition states. (b) Coarse-grained average over the fast degrees of freedom which yields an approximate potential of mean force.5 Arrows mark barrier locations derived from the intermediate and high strength regimes of the spectrum for biotin»avidin in Fig.4. Faraday Discuss. 1998 111 1»16 10 kBT /distance xb the slope of the high strength regime places the innermost barrier at Fig. 6 On the left are examples of force histograms taken from tests of single carbohydrate (sialylated PSGL1 short peptide chimera)»L-selectin bonds which demonstrate shift in peak location and increase in width with increase in loading rate (top histogram 10 pN s~1 middle histogram 850 pN s~1 bottom histogram 13 000 pN s~1). Superposed on the histograms are Gaussian –ts used to determine the most frequent rupture force» bond strength. On the right is the complete dynamic spectrum of strength vs. log(loading rate).De–ned as thermal energy xbB0.06 nm. The intermediate strength regime places the next barrier at xbB0.3 nm. The low strength regime implies a barrier further out at xbB1.2 nm. values followed by sudden displacements of biotin at a distance of D0.4 nm (and D0.5 nm in the biotin»streptavidin simulation). Finally biotin was observed to still cling to peripheral polar groups at D1.4 nm in the avidin simulation. As labelled in Fig. 5(b) the locations of activation barriers derived from the high and intermediate strength regimes of the laboratory spectra in Fig. 4 correlate well with regions of rari–ed statistics and the qualitative appearance of the energy landscape. The conclusion is that these transition states inferred from the simulations persist on long time scales.However the outer barrier indicated by the low strength regime is 2»3-fold more distant than the last transition state seen in the MD simulation ; this is perhaps due to interactions with the peripheral —exible loops24h27 which border the channel that leads to the binding pocket. More puzzling however extrapolation of the lowest strength regime to zero force implies that bond strength vanishes below a threshold loading rate of D0.0006 pN s~1 for biotin»avidin. In other words the spontaneous oÜ rate would be D1 per hour. This is 50-fold faster than the rate of D1 per 55 hours that we measured for spontaneous dissociation of PEG»biotin from probe tips in free solution and found previously for biotin by others.22 Hence some nontrivial eÜect remains that accounts for the signi–cant increase in rate of dissociation under extremely small forces below 5 pN.CarbohydrateñL-selectin bonds In contrast to the high affinity biotin»(strept)avidin bonds carbohydrate»L-selectin bonds with modest affinity stop white cells at vessel walls in the circulation.23 Numerous bonds to other surface (integrin) receptors then form between the white cell and vessel wall to sustain adhesion and enable subsequent movement into the surrounding tissue. On its initial arrest from the blood —ow the white cell can be subjected to forces of D100 pN in a time frame of milliseconds which implies loading rates of 104»105 pN s~1. With this functional requirement in mind we now examine recent tests of carbohydrate»L-selectin bonds under dynamic loading in probe tests.From the results7 presented in Fig. 6 we again see a sequence of high intermediate and low strength regimes for carbohydrate»L-selectin bonds where strength also falls continuously from 11 Faraday Discuss. 1998 111 1»16 b bB3.4 pN that sets the outermost barrier at xbB1.2 nm. Using the logarithmic intercepts found D200 pN to DpN but over fewer decades in time scale for detachment from 10~3 to 1 s. The high strength regime has a very steep slope of fbB70 pN that maps an inner barrier to a small distance xbB0.06 nm along the direction of force. Although we lack detailed molecular information about L-selectin binding the small value of x seems to imply that the microscopic reaction coordinate deviates signi–cantly from the macroscopic orientation of force.For example if the ligand was bound to the side of the receptor pulling parallel to the axis of the receptor along the surface normal could result in a large orientation angle h relative to the microscopic pathway and weak coupling of force to the energy landscape. Departing from the high strength regime below 70 pN f the intermediate strength regime with a slope of bB13 pN places the next activation barrier at xf bB0.3 nm. Finally below D20 pN the spectrum exhibits a low strength regime with a slope of by extrapolation of each linear regime to zero force the diÜerences in energy between the inner activation barriers are calculated to be only 2»3 kBT . As for biotin»(strept)avidin bonds the innermost barrier deep in the binding pocket provides strength on short time scales (\0.03 s) which is sufficient to meet the functional requirements noted earlier.Even though only 4»6 kBT higher in energy the outermost activation barrier extends the lifetime of the bond almost 100-fold (to D1 s) beyond that set by the innermost barrier. Energy landscapes for anchoring lipids in membranes Lipids and acylated proteins are anchored in bilayers by hydrophobic interactions. The handbook28 correlation for free energy of transfer from aggregates (e.g. micelles or bilayers) to water is a linear proportionality of D1 kBT per aliphatic carbon for lysophosphatidylcholines (PCs) and not quite double D1.7 kBT per carbon for diacylPCs although little evidence exists for diacyl lipids with chain lengths longer than 10»12 carbons.This reinforces the established view that anchoring potential increases with hydrophobic surface area embedded in the bilayer.29 Partition and solubility provide important static-equilibrium assays but represent energetic measures of strong vs. weak anchoring»not strength»which is the force needed to extract a molecule. Based on the hydrophobic energy scale for exposure to water the energy landscape for hydrophobic anchoring in bilayers should simply rise linearly with displacement along the bilayer normal. Treating the embedded molecule as a cylinder with radius rm the surface energy per unit area for creating a water/nonpolar interface and the circumference of the cylinder (i.e. energy/length B30 mJ m~2]2pr or 7 kBT nm~2]2prm) suggest naively that the molecular extraction force should m be a constant set by molecular size fB180 pN]r (nm).Taking a radius D0.5 nm for a lipid the m anchoring force would be D100 pN. On the other hand we will see next that lipids can be extracted from membrane bilayers with forces as small as D1 pN if performed over seconds! Over the range of anchoring strengths between 0 and 100 pN the missing ingredient is thermally activated kinetics. By comparison lipid pull-out forces in MD simulations3 were [200 pN even under the slowest extraction of D10~8 s and increased with speed apparently due to viscous damping. Strength of hydrophobic anchoring in —uid membranes was tested by extraction of single receptor lipids from giant bilayer vesicles prepared with two lipid compositions pure stearoyloleoylphosphatidylcholine (SOPC) (C18 0/1) and a 1 1 mixture of SOPC plus cholesterol (CHOL)» somewhat similar to membranes that encapsulate cells.Doped in the vesicle bilayers at extremely low concentration (\0.0001%) the receptor lipids were a special lipid construct of biotin»PEG» distearoylphosphatidylethanolamine (DSPE) (diC18:0) kindly provided by INEX Pharmaceuticals Burnaby B.C. Canada. Plotted in Fig. 7 we see little structure in the spectra for receptor lipid anchoring and much lower forces compared to the spectra in Figs. 5 and 6 for receptor»ligand bonds.8 Over nearly four orders of magnitude in loading rate only a single linear strength regime is found for extraction of the receptor lipids from SOPC CHOL bilayers.The f low slope of bB2.4 pN places a barrier at a distance xbB1.7 nm along the direction of force. Two linear regimes are found for lipid extraction from pure SOPC bilayers with a modest diÜerence in slopes. The initial slope of fbB3.4 pN locates an outer barrier at xbB1.2 nm and the second slope of fbB6.1 pN implies an inner barrier at xbB0.7 nm. Consistent with the simple concept of hydrophobic interaction the locations of the outermost barriers for both types of bilayers are comparable to (but slightly less than) the hydrophobic half thickness of the bilayer Faraday Discuss. 1998 111 1»16 12 Fig. 7 On the left are examples of force histograms taken from tests of receptor lipid (biotin»PEG»DSPE) extraction from mixed SOPC CHOL vesicle bilayers which demonstrate shift in peak location and increase in width with increase in loading rate (top histogram 2 pN s~1 middle histogram 200 pN s~1 bottom histogram 5 000 pN s~1).Superposed on the histograms are Gaussian –ts used to determine the most frequent extraction force»anchoring strength. On the right are the complete dynamic spectra of strength vs. log(loading rate) for extraction of receptor lipids from SOPC (closed boxes) and mixed SOPC CHOL bilayers (open circles). De–ned as thermal energy kBT /distance xb the slopes of the initial linear regimes map activation barriers at xbB1.2 nm for extraction from SOPC and xbB1.7 nm for extraction from SOPC CHOL along the direction normal to the bilayer. Not seen in the SOPC CHOL spectrum the break in slope for the SOPC spectrum places a weak inner barrier at xbB0.7 nm.which is increased by cholesterol. Addition of cholesterol to SOPC bilayers increases the outer activation barrier by D2 kBT as shown by the shift between logarithmic intercepts of the initial regimes for SOPC CHOL and SOPC. Quite unexpected the break in slope in the spectrum for SOPC reveals an inner transition state near the middle of the hydrophobic monolayer which appears to be D2 kBT below the outer barrier. Perhaps coincidental the location of this transition state derived from the thermal force scale correlates with the position of the unsaturated bond in the oleoyl chain of SOPC. Completely speculative the split in activation barriers could re—ect an entropic bottle neck as chains transiently pass the average position of the unsaturated group.Very puzzling the bilayer residence time of D0.01 s derived from the logarithmic intercept of these spectra at zero force is much shorter than implied by the lack of perceptible dissociation from an isolated vesicle over the the time scale of 1 h. Without an explanation at present we see again (as for biotin»avidin bonds) that very small forces must strongly aÜect the shape of the soft outer transition state. In any case anchoring of lipids and acylated proteins in bilayers will always be weak unless the molecules are extracted very rapidly from the bilayer. Strong vs. weak bonds in serial linkages For a serial linkage of n identical bonds the rate of breakage under force is simply increased by a factor n koffB(n/t0)exp( f/fb) if the bond kinetics are uncorrelated.This increases the thermal scale for loading rate rf0\nfb/t0 and shifts the strength spectrum along the log (loading rate) axis by a loge(n) which reduces strength at a given loading rate by [fb loge(n). In contrast to a factor of e simple shift along the log(loading rate) axis we expect the strength of a multiple linkage of dissimilar bonds to be limited by the weakest bond and naively that strong vs. weak should be de–ned by the energy barriers sustaining the bonds. However theory shows that this anticipated hierarchy is only correct for some sets of bonds; other sets will exhibit unexpected switching from strong to 13 Faraday Discuss. 1998 111 1»16 weak and vice versa as loading rate increases.In the determination of strong vs. weak at a particular retraction rate the important parameters are both the spontaneous rates of dissociation set by barrier energies and the thermal force scales that characterize e-fold changes in the dissociation rates of bonds under force. Again invoking the simple sharp barrier model we can easily establish a phase diagram [cf. Fig. 8(a)] of the most likely site for breakage in a two-bond linkage. Assuming that both bonds are characterized by the same diÜusive time scale t for simplicity the D rate of uncorrelated breakage is the combined rates for each bond koffB(1/t0)exp( f/fb)M1]exp[[*Eb/kBT ]f*(1/fb)]N 1/ f t and specify the spontaneous rate and thermal force scale of the fast bond (smallest 0 *E and *(1/f represent the diÜerences in barrier energy and *Eb[0) relative to the fast bond.The comwhere barrier energy) as the reference ; b reciprocal thermal force scale for the slow bond (i.e. bined rate and the predicted strength spectrum predict that the fast bond will remain the expected *E weak bond so long as the following inequality holds b/kBT [f*(1/fb). This will always be the case when the thermal force scale for the fast bond is less than the thermal force scale for the slow *(1/f bond [i.e. b)\0 or equivalently *(xb)[0]. On the other hand if the thermal force scale for *E the fast bond is larger then there will be a crossover force where b/kBT Of *(1/fb) ; this strongHweak bond phase boundary is sketched in Fig. 8(a). Now the fast bond will be the strong bond and the slow bond will be the most likely site of failure which is not anticipated in the traditional view.To demonstrate the importance of this concept imagine that the selectin receptor was linked to b ) b ) b Faraday Discuss. 1998 111 1»16 b) a vesicle bilayer by a lipid anchor and then the strengths of carbohydrate ligand bonds to the selectin were probed as in the leukocyte tests. Purely hypothetical Fig. 8(b) shows that at slow loading rates the carbohydrate»selectin bond would most likely detach because lipid anchoring is b[0 characterizes a slow bond relative to a fast bond as de–ned by spontaneous oÜ rates) ; the horizontal b Fig. 8 (a) Phase diagram for de–nition of strongHweak bonds in a serial linkage of two bonds sustained by single sharp energy barriers.The vertical axis is the diÜerence in barrier heights *E for the two bonds (*E axis is the product f*(1/f of applied force f and the diÜerence *(1/f in reciprocal thermal force scales. In the traditional view strong weak equates to slow fast. But for bonds in series this diagram shows that there can be unexpected switching of these attributes under force. (b) Hypothetical strength of carbohydrate bonds to selectins if linked by lipid anchors to membranes. Simultaneous kinetics over diÜerent energy landscapes for the carbohydrate»selectin bond and lipid anchoring predicts a dynamic crossover in site for detachment when pulled on by a probe decorated with the carbohydrate ligand. At slow rates of loading the lipid anchor is stronger than the carbohydrate»selectin bond which is the most likely site of detachment.On the other hand at fast rates of loading the carbohydrate»selectin bond is stronger than lipid anchoring strength which then becomes the most likely site of detachment. 14 stronger. On the other hand under fast loading the carbohydrate»selectin bond becomes strong and the lipid anchor weak by comparison. Hence the lipid-anchored selectin would most likely be pulled out of the membrane by the carbohydrate ligand attached to the probe. In contrast to the image of inner activation barriers in a complex bond we see that the signature of a strong to weak bond metamorphosis in a serial linkage of bonds is an abrupt reduction in slope from one linear strength regime to the next with increase in loading rate.Summary Recent laboratory probe experiments con–rm that bond breakage and molecular detachment occur at forces determined by the loading rate. Measured under steadily rising force over an enormous span of loading rates the spectra of strength vs. log (loading rate) yield images of the e prominent barriers traversed in the energy landscapes along force-driven pathways in unbinding. Simple analysis of the spectra provides a view into the inner complexity of biomolecular interactions and structural cohesion as noted in the following list of highlights. (1) Examining two unrelated receptor»ligand bonds we –nd a similar sequence of linear strength regimes vs. log(loading rate). These regimes reveal a cascade of three activation barriers for both receptor»ligand interactions although quite diÜerent in energy scale.The innermost barrier deep in the binding domain is responsible for the high strength perceived on short time scales and the major portion of total activation energy. The more distal barriers lead to weakness on long time scales but signi–cantly extend bond lifetime in the absence of force. The intriguing question is why did nature structure energy landscapes in receptor»ligand bonds and create a sequence of time scales for ampli–cation of kinetics under force ? Answering this question is likely to introduce a new perspective of biological chemistry. (2) No surprise anchoring strengths of lipids in bilayers are consistent with nearly structureless hydrophobic potentials although small inner barriers do appear in some cases.Most signi–cant the small thermal force scale set by acyl chain length results in very weak anchoring strength unless the molecules are extracted extremely rapidly from the bilayer. Still to be con–rmed integral membrane proteins should be much more strongly anchored to membranes since hydrophilic groups at the interfaces will contribute major activation barriers with large thermal force scales. (3) Dissimilar bonds in a serial linkage can unexpectedly switch from strong to weak and shift the most likely site for failure between bonds as loading rate increases. Such behaviour is not only a major factor in cohesive and adhesive strength but is likely to be important in signalling and regulation of biochemical pathways inside cells.Acknowledgement It is important to credit the individuals who carried out the experiments and developed the instrumentation described in this paper since many of the results are yet to be published. Tests of biotin»(strept)avidin bonds were performed by Andrew Leung (University of British Columbia) and Pierre Nassoy (now at lœInstitut Curie in Paris). Computer control of the biomembrane force probe assembly and video image processing was developed by Ken Ritchie (now at Nagoya University in Japan). Tests of carbohydrate»L-selectin bonds were also performed by Andrew Leung in collaboration with Scott Simon (from Baylor College of Medicine in Houston) and Dan Hammer (from University of Pennsylvania in Philadelphia). Tests of lipid anchoring in bilayer membranes were performed by Florian Ludwig (University of British Columbia).The work was supported by grants HL54700 and HL 31579 from the US National Institutes of Health grant MT7477 from the Medical Research Council of Canada and the Canadian Institute for Advanced Research Program in Science of Soft Surfaces and Interfaces. References 1 G. Binnig C. F. Quate and C. H. Gerber Phys. Rev. L ett. 1986 56 930; B. Drake C. B. Prater A. L. Weisenhorn S. A. C. Gould T. R. Albrecht C. F. Quate D. S. Cannell H. G. Hansma and P. K. Hansma Science 1989 243 1586. 2 H. Grubmuller B. Heymann and P. Tavan Science 1996 271 997; S. Izrailev S. Stepaniants M. Balsera Y. Oono and K. Schulten Biophys. J. 1997 72 1568. 15 Faraday Discuss. 1998 111 1»16 3 S-J.Marrink O. Berger P. Tieleman and F. Jahnig Biophys. J. 1998 74 931. 4 H. A. Kramers Physica (Amsterdam) 1940 7 284; P. Hanggi P. Talkner and M. Borkovec Rev. Mod. Phys. 1990 62 251. 5 E. Evans and K. Ritchie Biophys. J. 1997 72 1541. 6 R. Merkel P. Nassoy A. Leung K. Ritchie and E. Evans Nature (L ondon) 1999 397 50. 7 S. Simon A. Leung D. Hammer and E. Evans to be submitted. 8 F. Ludwig and E. Evans to be submitted. 9 M. Doi and S. F. Edwards T he T heory of Polymer Dynamics Clarendon Press Oxford 1986; N. G. van Kampen Stochastic Processes in Physics and Chemistry North-Holland Amsterdam 1981. 10 P. J. Rossky J. D. Doll and H. L. Friedman J. Chem. Phys. 1978 69 4628. 11 G. I. Bell Science 1978 200 618. 12 G. U. Lee D. A. Kidwell and R. J. Colton L angmuir 1994 10 354. 13 E-L. Florin V. T. Moy and H. E. Gaub Science 1994 264 415. 14 V. T. Moy E-L. Florin and H. E. Gaub Science 1994 264 257. 15 P. Hinterdorfer W. Baumgartner H. J. Gruber K. Schilcher and H. Schindler Proc. Natl. Acad. Sci. USA 1996 93 3477. 16 S. S. Wong E. Joselevich A. T. Woolley C. L. Cheung and C. M. Lieber Nature (L ondon) 1998 394 52. 17 S. P. Tha J. Shuster and H. L. Goldsmith Biophys. J. 1986 50 117. 18 E. Evans D. Berk and A. Leung Biophys. J. 1991 59 838. 19 E. Evans K. Ritchie and R. Merkel Biophys. J. 1995 68 2580. 20 E. Evans and K. Ritchie Biophys. J. 1999 in press. 21 N. M. Green Adv. Protein Chem. 1975 29 85. 22 A. Chilkoti and P. S. Stayton J. Am. Chem. Soc. 1995 117 10622. 23 R. Alon D. A. Hammer and T. A. Springer Nature (L ondon) 1995 374 539; K. D. Puri S. Chen and T. A. Springer Nature (L ondon) 1998 392 930. 24 P. C. Weber D. H. Ohlendorf J. J. Wendoloski and F. R. Salemme Science 1989 243 85. 25 O. Livnah E. A. Bayer M. Wilchek and J. L. Sussman Proc. Natl. Acad. Sci. USA 1993 90 5076. 26 S. Freitag I. Le Trong L. Klumb P. S. Stayton and R. E. Stenkamp Protein Sci. 1997 6 1157. 27 V. Chu S. Freitag I. Le Trong R. E. Stenkamp and P. S. Stayton Protein Sci. 1998 7 848. 28 D. Marsh Handbook of L ipid Bilayers CRC Press Boca Raton FL. 1990 p. 275»280. 29 C. Tanford T he Hydrophobic EÜect Formation of Micelles and Biological Membranes John Wiley and Sons New York NY 1973. Paper 8/09884K Faraday Discuss. 1998 111 1»16 16

ISSN:1359-6640    

DOI:10.1039/a809884k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Collective membrane motions of high and low amplitude studied by dynamic light scattering and micro-interferometry Rainer Hirn,a Thomas M. Bayerl,*a Joachim O. Raé dlerb and Erich Sackmannb a Universitaé t W ué rzburg Physikalisches Institut EP-5 97074 Wué rzburg Germany b T echnische Universitaé t Mué nchen Physik Department E22 85747 Garching Germany Receiøed 9th October 1998 Undulations of lipid bilayers were experimentally studied for the two limiting cases of high and weak lateral tension using two well established model systems freely suspended planar lipid bilayers so-called black lipid membranes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in changes of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BLM) thus requiring diÜerent physical methods for their detection.We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. The dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250\q/cm~1\35 000 cm~1. This enabled the observation of collective membrane modes in two regimes the oscillating one at low q and the overdamped regime at high q. The transition between both regimes at the bifurcation point is rather abrupt and depends on the lateral tension of the bilayer as is demonstrated by comparing the dispersion curves of pure lipid and of lipid»cholestrol BLMs over the same q-range.The DLS measurements allowed a critical test of a hydrodynamic theory of the dispersion behaviour of membrane collective modes under tension. The DLS measurements are compared with RICM results of undulatory excitations of giant vesicles weakly adhering to substrates in the 10~6»2.5]10~7 m wavelength regime and at low frequencies (0.1»25 Hz). Experimental evidence for the strong decrease in the relaxation rate by the hydrodynamic coupling of the membrane with the wall is established. Introduction Collective motions (undulations) of —uid membranes are crucial for their mutual interaction at the nm lengthscale and thus for the swelling of lipids.1 The measurement of these motions provides not only a deeper understanding of membrane micromechanic properties down to the molecular scale but also sheds some light on the processes which are essential for the formation of contacts between cells and between cells and solid surfaces.In the low-frequency regime (up to 100 Hz) time-resolved interferometric microscopy is the method of choice to study collective motions of membranes of whole cells and of some microorganisms in great detail. On the other hand incoherent neutron scattering has been successfully employed for the measurement of collective membrane modes in the THz range.2 However there is quite a lack of experimental approaches for their study in length and frequency regimes which are commonly denoted as the mesoscopic range (kHz»MHz). So far only solid-state NMR techniques have been successful in detecting collective modes in this range,3 but their application is 17 Faraday Discuss.1998 111 17»30 limited by sensitivity sample geometry narrow accessible frequency range and NMR data analysis is hampered by the constraint that one-dimensional solid-state NMR measures temporal correlations only and not spatial correlations. The availability of a technique which can bridge the wide frequency gap of ca. six orders of magnitude between the well established low-frequency regime and the highest accessible frequency range would be extremely helpful in the testing of a number of theories based on membrane microelasticity as well as for sorting out the essential collective modes for each time domain. DLS has been used previously to measure collective motions of BLM and it demonstrated successfully the viscoelastic behaviour of such membranes4 but their usefulness was limited by the narrow range of accessible undulation modes from 600 to 1 800 cm~1.Here we report the –rst results of DLS measurements on BLMs obtained over a much wider wavevector range from 250 to 35 000 cm~1 (1.8\j/lm\250) and a timescale of four orders of magnitude (from 6]10~3 to 6]10~7 s) which is sufficient for a critical testing of theoretical predictions. A transition from single to two-exponential relaxation is encountered at high tension which exhibits much higher frequencies in the vicinity of the bifurcation point. The experiments in this new frequency regime enable one to study the eÜects of proteins peptides and polymers on collective membrane dynamics.Since BLMs exhibit generally a high lateral tension their undulations are dominated by lateral forces resulting in very low undulation amplitudes. On the other hand LUV show only negligible or very weak tension and thus a diÜerent undulatory behaviour with high undulation amplitudes.5 The latter is constrained when the LUV comes into close contact with a wall. As a result a repulsive pressure is established which is gradually reduced with increasing lateral tension. A comparison between tension dominated BLMs and rather tensionless LUV thus allows a critical test of theoretical predictions on membrane micromechanics in the limit of two extremes. We have additionally performed time-resolved re—ection interference contrast microscopy (RICM) of LUV close to a solid surface.The results show that the relaxation modes are dominated by the hydrodynamic interaction between the LUV and the wall. (1) Theory of DLS of membranes under high tension One of the –rst comprehensive theoretical treatments of the dispersion behaviour resulting from collective motions of thin elastic membranes and the consequences for its dynamic light scattering properties was published by Kramer.6 The basic assumptions of this theory were The membrane elastic properties can be described by compression- and shear moduli and a membrane tension. All hydrodynamic equations (i.e. Navier»Stokes) are linear. The —uid symmetrically surrounding the membrane is incompressible. At the membrane/—uid interface the velocities of the two media are the same.Fluid velocity becomes zero at large distances from the membrane. The wavelength of the collective modes is large compared to the membrane thickness and small compared to its diameter. For a membrane of isotropic molecules separating two compartments of the same liquid transverse shear was identi–ed as the only DLS sensitive mode. Here the molecules perform out-ofplane motions only leading to an eÜective —uctuation of the membrane area. The dispersion relation of this mode is 2mou2]cq3(q[m)\0 where m\(q2[iou/g)1@2 q\2n/j is the scattering vector u\u0[iC is the complex frequency consisting of the eigenfrequency u and the damping constant C. o and g are the density and 0[iuc@ is a complex tension with the real part being the membrane c\c viscosity of the —uid and 0 tension and c@ being the surface viscosity.A plot of f and C vs. q of eqn. (1) using tensions typical for a free planar bilayer (BLM) shows and the faster by C 0 the following features (Fig. 1) which were discussed in greater detail by Kramer6 and later in the work of Earnshaw:4 For transverse shear there exists an oscillating and an overdamped regime in q the mesoscopic range and the transition between them at 0B5000 cm~1 is abrupt (bifurcation point). Above this q value the single damping constant C of the oscillating regime is replaced by 0 two exponential decays describing two overdamped modes with the slower one characterized by C 2 . No experimental veri–cation of this transition has been provided yet.For 1 Faraday Discuss. 1998 111 17»30 18 c0\1 mN m~1 Fig. 1 Theoretical curve according to Kramerœs theory [eqn. (1)] with membrane tension —uid density o\1 mg ml~1 and —uid viscosity g\1 mPs at diÜerent surface viscosities c@. increasing q-values above the bifurcation point the slower mode C approaches the value m\c0/c@ 1 asymptotically while the faster mode C merges at the same value of m with the bulk mode having 2 the dispersion relation (2) iu[gq2/o\0 Moreover this bulk mode is insensitive to DLS and therefore the faster mode disappears at q-values beyond m. By considering the anisotropy of lipids in a BLM an additional splay mode coupled to the transverse shear mode was found.7 The coupling strength between the two modes scales with the total free energy change a single lipid undergoes when its molecular director gets tilted by a certain angle away from that of its neighbour.This leads to a modi–cation of the membrane tension in eqn. (1) now becoming an eÜective tension (3) c0eff which includes the curvature energy i c0eff\c0]iq2 0 Hence at higher q the bending modulus i will dominate undulation behaviour over the membrane tension c (Fig. 2). In terms of the eÜective undulation amplitude ueff of a membrane of area A and driven by a thermal energy kT this corresponds to the Helfrich1 equation (4) Sueff 2 (q)T\ A(cq2]iq4) kT For standard BLM parameters (c0\1 mN m~1 i\10~19 J BLMdiameter\3.5 mm) eqn. (4) u gives effB0.09 ” at q\1000 cm~1 and a maximum amplitude of ueffB10 ” is obtained by summing the contributions over a q range limited by the cut-oÜs 1/membrane thickness and 1/membrane diameter.The reason for these tiny amplitudes compared to those reported below for LUV is the dominance of tension for BLMs while the —ickering systems are tension free and thus i dominated. For BLMs the eÜect of i is negligible up to very high q values of say q\300 000 cm~1 (j\200 nm) where the contribution from i amounts to 1/10 of that of the tension term. Nevertheless it has been shown experimentally4 that DLS is sensitive even to these very small amplitudes exhibited by BLMs in the q-range 600 to 1800 cm~1. 19 Faraday Discuss. 1998 111 17»30 1 Fig. 2 c Comparison of Kramerœs [eqn. (1)](2)] and Fanœs dispersion theory [eqn. (1)] for two pairs of and 0 i values.For the sake of clarity only the slow damping modes C are shown. The curves with the increasing slopes at higher q-values (curves 1 and 2) correspond to Fanœs theory. The upper two lines were calculated for c0\1 mN m~1 and i\0 (curve 3) or i\10~19 J (curve 2). The lower two lines are for tension c0\0.1 mN m~1 and i\0 (curve 4) or i\50]10~19 J (curve 1). However to obtain information about the surface viscosity c@ requires DLS measurements over a q-range that extends well into the overdamped regime since the dependence of C and of the 0\u0/2n on c@ is almost negligible in the oscillating regime (Fig. 1). Similarly a f eigenfrequency 0 representative test of eqn. (1) and (2) is impossible without measurements covering q in both the oscillating and overdamped regimes.(5) (6) (7) Theoretical predictions for —accid vesicles near a wall One of the most striking aspects of bending excitations of —accid membranes close to a wall is the strong repulsive interaction pressure pD(kB T )/ih0 3 arising from the entropy loss of a membrane con–ned to an average membrane»wall distance h0 . In the presence of membrane tension however the steric repulsion is strongly suppressed and decays exponentially pDexp([ch0 2/kB T ). For a weakly adhering liposome the entropic pressure is balanced by an attractive wall interaction. In this case the external forces experienced by the membrane can be described by an eÜective harmonic interaction at the equilibrium spacing h from the wall 0 V (h)\1/2V A(h[h0)2 where V A is the second derivative of V (h) at the equilibrium distance h0 .Since we are mainly interested in small-wavelength excitations (q~1) the undulation-induced dynamic surface roughness can be analysed in terms of plane waves h(r t)\; hq(t)expMiqrN q where q is the undulation wave vector. The amplitudes are determined by the equipartition theorem hq2\ kB T L2Miq4]cq2]V AN Faraday Discuss. 1998 111 17»30 20 where E(q)\iq4]cq2]V A is the energy of the Fourier mode q and i and c are again the membrane bending modulus and tension respectively. L2 is the system size over which the Fourier transformation was carried out. The dynamics of the randomly excited bending modes is characterized in terms of the time correlation function of the Fourier components which are given by Gq(t)\Shq(t)hq(0)T\kq2 expM[C(q)tN (8) with a relaxation rate C(q) to be discussed.In most LUV experiments we deal with membranes at weak tension (cB10~6 mN m~1). This situation has been treated theoretically in great detail by Seifert.8 One has to consider three situations (1) At low wave vectors (and thus low frequencies) the local density —uctuations within each monolayer are rapidly equilibrated by lateral diÜusion and the bending excitations are damped solely by coupling to the bending-induced hydrodynamic —ow in the surrounding —uid. (2) At increasing q the equilibration of the local density —uctuations is impeded by the friction between the monolayers which is characterized by a friction coefficient b\gm/dm where g is the m 2D membrane viscosity and d is the monolayer thickness.The bending modulus is renormalized m by the coupling of the bending excitations and the lateral density —uctuations to (9) i*\i]2dmK where K is the membrane modulus of isotropic compression. (3) At very high frequencies and q-vectors the damping is –nally determined by the in-plane shear deformation which is not considered here. Due to the interplay of bending and density —uctuations the membrane dynamics (of free and adherent vesicles) is determined by a low-frequency mode and a high-frequency mode. The relaxation of the latter is determined by monolayer friction. The cross-over wave vector between the two regimes is (10) q12\2gK/bi*B107 m~1 The number on the right-hand side holds for the data summarized in Fig.3. In the presence of a wall the situation is more complex. A third cross-over wave vector has to be considered since long wavelength excitations are impeded by the interaction of the membrane k F m m and are de–ned in eqn. (14) and (15). Note that at q\m both decay rates scale as C P q2. 0 h0\1 nm c\10~6 J m~2 Fig. 3 Theoretical dispersion relation of decay rates of bending excitation of adherent membrane as calculated by Kraus and Seifert.9 For weak adhesion corresponding to weak tension the two lowest frequency modes (out of three) are shown. C is the decay rate of the slow bending mode with rapid equilibration of local W density —uctuations within monolayers. C is the decay rate of the mode controlled by mutual friction between monolayers.The curves have been calculated for the following values of the parameters 0 V A\4]106 J m~4 b\5]108 J s m~4 g\10~3 J s m~3 K\0.1 J m~2 i\0.5]10~19 J. 21 Faraday Discuss. 1998 111 17»30 with the hard wall. Due to the symmetry break induced by the wall there are two density modes (besides the bending mode) For negligible tension the undulations are determined by the wall at qPmk~1\(V A/i)1@4\2]108 m~1 while for membranes under tension the wall becomes eÜective at qPmc~1\(V A/c)1@2\106 m~1 The transition between the bending and the tension dominated regimes occurs at wave vectors (11) (12) (13) q\m\(c/i)1@2\3]106 m~1 The RICM experiments were performed at a tension cB10~6 N m~1 and the range of wave vectors studied was 0.6]106\q/m~1\4]106.We can thus assume that the tension can still be considered as weak. Summing up the above conditions let us conclude that there should be two modes accessible by microinterferometry A low-frequency long-wavelength mode CW which is aÜected by the presence of the wall (being at distance h0) but which is slow enough to allow for the equilibration of the monolayer density —uctuation and which is not aÜected by monolayer friction. Its damping constant is (14) CW(q)\(V Ah0 3 q2]ch0 3 q4)/12g q@(mi~1 h0~1)B107 m~1 and is thus observable by the This mode dominates the regime of microinterferometric technique. For bending modulus q nm h0~1AqAm~1 the damping constant is dominated by the C(q)\ih0 3 q6/12g which is however not observable by our technique since max\5]106 m~1.With the data summarized in Fig. 3 one –nds for qB106 m~1 and h0B40 CWB0.02 s~1 corresponding to a relaxation time qB50 s. The second interesting mode CF is controlled by the friction between the monolayers. It decays with a rate (15) CF\Kq2i/2bi* By inserting the data from Fig. 3 one can estimate a wave vector qB106 m~1 a decay constant CFB20»200 s~1 or a relaxation time q\0.05»0.005 s. A completely diÜerent type of behaviour is expected for permeable membranes as shown by Prost et al.10 The undulations can relax by permeation of water through the bilayer. This mechanism is expected to dominate over the hydrodynamic process for wave vectors q@q*\(jp g)1@2 ~3@2 where j is the water permeability of the bilayer.It is of the order jpB10~6 h0 m2 s kg~1 and p 0B40 nm one obtains for q@3]105 m~1. h The decay rate is (16) m~1) s C ~1 and in the case of bending dominated membranes C Cp\jp(cq2]iq4) For q\106 m~1 one expects in the case of tension dominated membranes (c[2]10~6 N pB0.1 pB1 s~1. Materials and methods Substances Cholesterol and n-decane 99]% were purchased from Sigma-Aldrich (Steinheim Germany) and the n-decane was further puri–ed by passing it through an alumina column until all coloured impurities were removed. The phospholipid 1,2-di-elaidoyl-sn-3-glycerophosphocholine (DEPC) was purchased from Avanti Polar Lipids Inc. (Alabaster AL USA) and was used without further puri–cation. All light scattering experiments were performed in 20 mM Hepes buÜer (Life Technologies Ltd.Daisley UK) containing 50 mM KCl (Fluka Chemie AG Buchs Switzerland) and the water used was from a Milli-Q puri–cation unit (Millipore Corp. Bedford USA). The buÜer was –ltered through a sterile –lter of 0.1 lm pore size (Millipore Corp. Bedford USA) and Faraday Discuss. 1998 111 17»30 22 degassed prior to its use in the scattering cell. Microinterferometric measurements used SOPC from Avanti Polar Lipids in Millipore water. Membrane preparation The scattering cell was a standard rectangular glass cuvette of 40]10]10 mm3 (Hellma GmbH & Co. Mué lheim Germany) separated into two compartments by a diagonally inserted Te—on wall (thickness 2 mm)11 with a circular aperture of 4.5 mm in the centre. Over this hole a 25 lm thick Te—on foil was spanned using Te—on glue (primer 770 and bonder 406 from Loctite GmbH Mué nchen Germany) which itself featured a hole of 3.5 mm diameter.The latter hole was punched with extreme care to ensure its rim was as smooth as possible (roughness was below the optical resolution limit of a light microscope). This smoothness together with the use of a very thin foil ensures the BLM long-time stability (up to 5 days) and prevents positional —uctuations of the BLM as a whole. The scattering cell was placed within a metal cell holder the temperature of which (22 °C for all measurements) was controlled by a water-bath thermostat. Before BLM preparation the scattering cell was –lled with buÜer solution to a level of ca. 3 mm above the upper edge of the Te—on wall so that both compartments were connected by the water.BLMs were prepared by sliding a Te—on loop containing the –lm-forming solution over the hole.12 Prior to this the hole was pretreated by spreading a methanol solution of 2 wt.% lipid onto it followed by methanol removal by drying in air.8 The term lipid refers to pure DEPC for BLM sample 1 and to a mixture of DEPC with 30 mol% cholesterol for BLM sample 2. The –lm-forming solution was n-decane containing 1 wt.% lipid. BLM formation was con–rmed by the formation of a sharp spot of specular re—ected incident laser light. A rectangular Te—on pressure cap was then carefully lowered from the top into the scattering cell down to the water level thereby eliminating any air bubbles and eÜectively damping out all surface capillary waves of the water which might couple with the BLM below.Dynamic light scattering (DLS) Experimental set-up. The scattering geometry of our set-up is shown in Fig. 4. Note that undulations like the transverse shear mode represent plane waves u(r t) with (17) u(r t)\u0 e~i(qr`ut) and can spread in all directions perpendicular to the membrane normal thus giving rise to both in-plane and out-of-plane scattering of the incident light vector ki . Thus in a plane perpendicular to the membrane normal at the in—ection point all possible q-vectors belonging to the same undulation mode are con–ned within a circle. However owing to the inclination of the detector i k (scattering angle 45°) the inplane wavevector q the in-plane scattered vector ks (with quasi the ki i.e.quasi elastic light scattering) the out-of-plane wave vector q@ and the out-of-plane Fig. 4 Schematic depiction of the scattering geometry with the incoming vector k and specular re—ected vector same length as ir scattered vector ks @ . 23 Faraday Discuss. 1998 111 17»30 ir plane by 45° with respect to the membrane normal (i.e. the plane perpendicular to the specular re—ected vector k k ir) the region of scattered vectors at the detector site can be approximated to s the –rst order by an elliptical aperture with an aspect ratio of 1.41 in front of the detector and with k going through its centre. The DLS set-up was mounted on a 3]2]0.2 m3 (200 kg weight) laser table (Melles Griot) supported by four air damping modules for mechanical shock protection.A 4 W argon ion laser (Coherent Inc. Santa Clara CA USA) was used operating at 457.9 nm in TEM mode with a 00 maximum power of 150 mW. The beam was focused onto the BLM at an incident angle of 0.11° by a lens of f\50 cm to obtain high q-resolution giving an illuminated BLM spot of 160 lm. Its polarisation was adjusted parallel to the membrane plane to give the highest amount of scattering intensity. At a re—ection angle of 45° and at a distance of 30 cm from the BLM the photomultiplier (PM) was arranged at a goniometer arm. Two pinholes between BLM and PM one right after the BLM (£\700 lm) and one in front of the PM (£\80 lm) were used for q-range selection. For measurements at q[3400 cm~1 the PM pinhole was replaced by a vertical slit aperture (2000]200 lm2) to allow additionally the detection of out-of-plane scattering giving an increase in PM signal by a factor of 80 without any reduction in signal quality compared to the use of two pinholes.The slit aperture represents an approximation of the above-mentioned elliptical shape of the out-of-plane scattering at detector site by simply rotating the PM with respect to the BLM centre thus replacing the ellipse by its tangent in each point. The error made by this approach compared to the use of diÜerent elliptical pinholes for each q-value is 3% at q\3400 cm~1. The PM signal was preampli–ed and the heterodyne autocorrelation function calculated in 388 channels using an ALV 3000 correlator (ALV GmbH Langen Germany).The time increments used were in the range 0.1 to 15 ls. Here the diÜuse scattering arising from the molecular roughness of the membrane was used as local oscillator for the heterodyne mode. Data analysis. For the oscillating regime the theoretically expected autocorrelation function2 (18) G0 @ (t)\a]b cos(u0 @ t]U) e~\0t]ct was –tted to the experimental data. Here ct is a linear baseline correction and U is a phase factor. Corrections of the above autocorrelation function at small q for instrumental eÜects as suggested in ref. 4 were not performed since deviations from eqn. (5) were completely negligible for q[400 cm~1. At the transition point (bifurcation point) between the oscillating and the overdamped regime and within a narrow q-range above it (250»400 cm~1 depending on the membrane used) the data were –tted according to (19) Gt(t)\a](b]ct) e~Ct]dt Hence in this regime both overdamped modes C and C were generally detectable.In the over- 1 2 damped regime where two damping modes are expected according to eqn. (1) a function of the form (20) Gd(t)\a]b e~\1t]c e~\2t]dt was –tted to the data again including a linear baseline correction dt. However beyond C2\c0/c the fast overdamped mode becomes undetectable and the remaining slow mode C can be –tted by 1 a single exponential. Examples of representative data sets for q-values in the oscillating and the overdamped regime together with the –tting results according to eqn. (18) and (2) are shown in Fig. 5. Microinterferometric technique The microinterferometric technique developed for the analysis of undulatory excitations of weakly adherent vesicles (i.e.weak lateral tension) has been described previously by Raé dler et al.13 The vesicles are observed by RICM enabling local measurements of the distance between substrate Faraday Discuss. 1998 111 17»30 24 Fig. 5 Examples of typical correlation functions G(t) measured in the damped and in the overdamped regime for a free planar bilayer (BLM). The full lines represent –ts to the data according to eqn. (5) and (7). To distinguish the –t from the data more clearly not all data points are represented by a circle. Note that the overdamped dataset is –tted only with a mono-exponential function. and vesicle with high precision (^1 nm). The method is illustrated in Fig.6(a). The adhering liposome shows interference fringes at the contact rim where the contour bends away from the surface with a –nite contact angle. The —at centre part of the liposome appears dark and exhibits dynamical intensity —uctuations due to height —uctuations. The heights h(x y) are obtained from the digitized interference intensities by inverse cosine transform using a Pixelpipeline frame grabber (Perceptics) and NIH image software. A one-dimensional contour line hx\Sh(x y)Ty was evaluated by averaging over the width of a –nite stripe as shown in Fig. 6(b). The height pro–le hx along the stripe with length L was numerically Fourier transformed and the resulting one- G dimensional modes correlated in time qx(t)\Sh 8 qx(t)/h 8 qx(0)T.In this case the correlation function Gqx(t) is given by (21) Gqx(t)\PdqyShqx qy 2 TexpM[C(qx qy)tN Note that the correlation function Gqx(t) exhibits a superposition of relaxation modes qy which is a result of the data processing due to the integration over the –nite width of the evaluated stripe.9 25 Faraday Discuss. 1998 111 17»30 Fig. 6 (a) Analysis of undulations of a —accid vesicle near a transparent substrate by re—ection interference contrast microscopy. The image is generated by interference of light re—ected from the substrate and the adhering body respectively. For contrast enhancement the glass substrate is covered by an MgF –lm. (b) Snapshot of weakly adherent vesicle. The leopard-like pattern is due to —uctuations of the distance h(x y) 2 between substrate and vesicle.Bright areas correspond to large and dark areas to small values of h(x y). The white frame shows the stripe over which a one-dimensional time correlation analysis was carried out. f0(q) and the damping C(q) as shown in Fig. 7A. For the overdamped data set shown in Results BLMS measured by DLS Typical autocorrelation functions measured by DLS at selected q values corresponding to the damped and to the overdamped regime of the transverse-splay mode of BLM (3.5 mm diameter) of DEPC are shown in Fig. 5. The data were –tted according to eqn. (6) and (8) giving the mode frequency Fig. 5 a single exponential corresponds to the slow mode C1. However in the vicinity of the bifurcation point only two-exponential –ts [eqn. (7)] gave satisfactory results.This clearly indicates that around the bifurcation point the fast mode C is indeed detectable while it decreases 2 rapidly and merges with the non-detectable bulk mode [eqn. (2)] at higher q. This behaviour strongly suggests a non-negligible eÜect of the surface viscosity c being not less than 10~7 mN s m~1. The data in Fig 7A are compared with best –ts according to hydrodynamic theory [eqn. (1) full lines in Fig. 7A] to obtain the average lateral tension c and the shear interfacial viscosity c@ 0 acting in the normal direction of the membrane. It is obvious that the agreement between experiment and theory is excellent over almost 3 orders of magnitude of q. The predicted transition from the damped to the overdamped case (cf. Fig. 1) is clearly observed experimentally above q\3300 cm~1.Minor deviations of f0(q) from the theory at lowest measurable q in Fig. 5A can be ascribed to a slight average overall equilibrium deformation of the BLM giving rise to some diÜuse re—ection of the incident laser light at the lowest values of q. Moreover at q\400 cm~1 there could be a non-negligible contribution arising from the (Gaussian) instrumental function of the set-up. 0\0.42 mN m~1 From the –ts to the data in Fig. 7A an average value of the lateral tension c with a maximum deviation of ^0.03 mN m~1 is obtained while the viscosity c@ was in the range 2]10~7 mN s. The rather weak in—uence of c@ seems justi–ed considering that the BLM is not expected to exhibit any signi–cant frictional drag between its two constituent monolayers owing to the presence of retained solvent (decane) between them.The decane retained in the BLM increases the internal volume thereby eÜectively reducing the van der Waals interactions between adjacent lipid tails and between the two monolayers. The maximum thickness of this decane layer is ca. 2 nm.11 26 Faraday Discuss. 1998 111 17»30 Fig. 7 Mode frequency f0\u0/2n and damping C vs. mode wave vector q of a free planar bilayer (BLM) of DEPC (A) and of DEPC with 30 mol% cholesterol (B) calculated from the autocorrelation functions measured at the corresponding q-values. The full lines represents the theoretical prediction with the average lateral tension c and the viscosity c@ (see text) as parameters. 0 In a second experiment we have studied the eÜect of cholesterol on the collective modes of a BLM as measured by DLS.This steroid is well known from a number of bilayer studies to cause a stiÜening of the —uid bilayer and a drastic increase in its molecular order. Fig. 7B shows results for f0(q) and C(q) for the case of a DEPC»cholesterol (30 mol%) BLM for a comparable q range as for the pure DEPC BLM from Fig. 7A. The eÜect of cholesterol manifests itself by a signi–cant f increase in the transition of 0(q) from the damped to the overdamped case and a corresponding change in the damping C(q). Fits of the data to eqn. (1) now give an average lateral tension of c0\1.55 mN m~1 with a maximum deviation of ^0.07 mN m~1 while c@ is within the same order of magnitude as for pure DEPC.LUV measured by RICM h Fig. 8 shows the relaxation measurements of the bending oscillations for a vesicle weakly adhering to the substrate. In a previous analysis by Raé dler et al.13 the equilibrium contour and the static —uctuations of adhering vesicles were evaluated. Under the given conditions it was shown that the equilibrium distance 0B30 nm and the membrane tension cB10~6 J m~2. Here we evaluated 27 Faraday Discuss. 1998 111 17»30 (1)\0.67]106 m~1. Fig. 8 Time correlation of the Fourier modes hqx of the one-dimensional intensity pro–le h shown in Fig. 0\30 nm c\10~6 J m~2 h 6(b). All relaxation curves are –tted using eqn. (14) and eqn. (21) with –xed values x and V AB5]108 J m~4. The data are in agreement with hydrodynamic modes CW which are slowed down by the presence of the wall.The semi-logarithmic presentation of the same data (inset) shows the predicted multi-exponential decay. The –rst mode (=) corresponds to a wave vector qx the time correlation functions by numerically –tting eqn. (21) to a set of time correlation functions. We tested the diÜerent models for relaxation rate C. We found the hydrodynamically damped modes eqn. (14) –tted best. As shown in Fig. 8 the relaxation of all –ve modes can be simultah neously –tted with one set of constants 0\30 nm c\10~6 J m~2 g\10~3 J s m~3 and V AB5]108 J m~4. The parameter V A was left variable to improve the individual –ts but the resulting values did not scatter by more than 15%. The fact that the data show indeed a superposition of relaxation modes is demonstrated in the semi-logarithmic plot (inset in Fig.8) which shows clearly deviations from a straight line at low q. Hence the data are in good agreement with the slow mode C described by Kraus and Seifert.9 The fast frictional mode C W F on the other hand was not observed even though the theoretical cross-over should have allowed its detection. We have to assume that this mode is too fast to be detected by video microscopy. In fact even the fourth and –fth mode of the hydrodynamic damping in Fig. 8 are at the signal-to-noise detection limit. It must be mentioned that the data can be brought to reasonable agreement with the dispersion relation predicted by the permeation model. In this case the parameters c\10~6 J m~2 and jB10~6 m2 s kg~1 obtained best simultaneous –ts.However the –ts did not follow the multiexponential decay as seen in the semi-logarithmic plot shown in Fig. 8. q Discussion We have combined two physical techniques and two model systems to study undulations in two limiting cases high tension (BLM) and weak tension (LUV). The data shown in Fig. 7 provide for the –rst time the viscoelastic dispersion behaviour of a transverse shear mode of a BLM over an exceedingly wide (mesoscopic) q-range. For comparison previously reported measurements by this method were limited to maxB1800 cm~1 while for the present work we have qmaxB35 000 Faraday Discuss. 1998 111 17»30 28 cm~1. This allows the experimental observation of the transition from the oscillatory or damped to the overdamped regime of the transverse shear mode and thus oÜers a critical test of the validity of the hydrodynamic theory suggested previously for such modes.The general agreement of our DLS data (Fig. 7) with the Kramer theory [eqn. (1)] is quite excellent and thus provides strong support for its validity within the mesoscopic q range covered by our experiments. In particular we can draw the following conclusions (1) The dispersion behaviour of the BLMs studied is clearly dominated by the lateral tension only at highest q values might there be some minor contributions arising from membrane bending rigidity. (2) Accordingly the transverse shear interfacial viscosity c@ is nearly negligible over the q-range considered most likely re—ecting the fact that qA1/d in the mesoscopic q range (d being the bilayer thickness).(3) In contrast to the theoretically predicted existence of two overdamped modes our DLS data show in the overdamped regime with the exception of the vicinity of the bifurcation point only one mode which is compatible with the slower of the two predicted ones. Considering the changes in the DEPC-BLM dispersion behaviour upon the addition of cholesterol (cf. Fig. 7B) at an amount (30 mol%) that causes in a —uid bilayer the creation of the so-called liquid-ordered (l0) phase we can conclude that this system shows an approximately three-fold higher lateral tension c while the transverse shear interfacial viscosity c@ remains nearly 0 negligible. The increase in c is probably caused by the homogeneous distribution of the hydro- 0 phobic cholesterol over the bilayer in the l -phase.The tails of DEPC molecules adjacent to the 0 stiÜ steroid body are forced into a state of higher molecular order thereby reducing their area per molecule projected in the normal direction and thus giving rise to an increase in c0 . Note that DEPC is a synthetic lipid which can pack rather densely because it exhibits just one trans-double bond in each acyl chain. A surprising result is that in spite of the excellent agreement between experiment and theory only one of the two theoretically predicted overdamped modes C and C 1 2 [eqn. (20)] seems to be detectable by DLS. While C describes the slow recovery of the system 1 driven by tension and viscosity C is supposedly driven by the inertia of the system arising within 2 the approximations of the theory from the —uid surrounding the BLM.Hence while C is readily 1 detectable in our experiment with the data following the predicted course up to the qmax limit relaxation via the C -process is only seen in the region of the bifurcation point. A rationale for this 2 behaviour is not given here but there is room for some speculation. One possible reason could be that the occurrence of surface viscosity c@ drives the fast mode C into a merger with the bulk 2 mode which in turn is not detectable by DLS. The interferometric measurements are limited to a smaller q-range but are unique in the analysis of membranes interacting with solid supports. We provided quantitative evidence for hydrodynamic coupling between the undulating membrane and the wall.The hydrodynamic mode was found to be in better agreement than the permeation mode. However discrimination between the modes is difficult since in both cases the dominating terms scale as CDq2. It will be necessary for future measurements to extend the q-range of the interferometric microscopy using ultra-fast cameras and two-dimensional image processing. In this case the fast frictional mode might become visible. On the other hand the fast modes are even more likely to be biased by water permeation. Conclusions The dynamic undulations of lipid bilayers reveal a hierachy of relaxation mechanisms if measured over a wide range of wave vectors. These relaxation modes are sensitive to the bilayer tension the elastic bending modulus as well as the local hydrodynamic environment in case of membranes close to a solid wall.The study of multi-component membranes where the dominating relaxation parameters can be controlled by incorporation of steroids transmembrane pores or short membrane binding peptides will further elucidate the dynamics of membranes. Acknowledgements The authors R.H. and T.M.B. are indebted to Professor Lorenz Kramer (Universitaé t Bayreuth) for many helpful discussions. 29 Faraday Discuss. 1998 111 17»30 Paper 8/07883A References 1 W. Helfrich and R-M. Servuss Il Nuovo Climento 1984 3D 137. 2 W. PfeiÜer S. Koé nig J. F. Legrand T. Bayerl and E. Sackmann Europhys. L ett. 1993 23 457. 3 C. Dolainsky A. Mops and T. Bayerl J. Chem. Phys. 1993 98 1712. 4 J. F. Crilly and J. C. Earnshaw Biophys. J. 1983 41 197. 5 W. Haé ckl U. Seifert and E. Sackmann J. Phys. France 1997 7 1141. 6 L. Kramer J. Chem. Phys. 1971 55 2097. 7 C. Fan J. Colloid. Interface Sci. 1973 44 369. 8 U. Seifert Adv. Phys. 1997 46 13. 9 M. Kraus and U. Seifert J. Phys. France 1994 4 1117. 10 J. Prost and R. Bruinsma Eur. Phys. J. B 1998 1 465. 11 J. Dilger and R. Benz J. Membrane Biol. 1985 85 181. 12 S. White Biophys. J. 1978 23 337. 13 J. Raé dler T. Feder H. Strey and E. Sackmann Phys. Rev. E 1995 51 4526. Faraday Discuss. 1998 111 17»30 30

ISSN:1359-6640    

DOI:10.1039/a807883a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Trapping of short-lived intermediates in phospholipid phase transitions The La* phase Peter Laggner Heinz Amenitsch Manfred Kriechbaum Georg Pabst and Michael Rappolt Institute of Biophysics and X-ray Structure Research Austrian Academy of Sciences Steyrergasse 17 A-8010 Graz Austria Receiøed 12th August 1998 a Time-resolved small-angle X-ray diÜraction of liquid-crystalline phospholipid»water systems under temperature or pressure jump conditions has demonstrated the existence of an ordered intermediate L phase with a sub-second lifetime designated as the L*-phase. The lamellar repeat spacing is universally 0.3 nm smaller than that of the parent phase irrespective of the lipid composition and of the jump conditions provided that the jump leads to a net volume expansion of the phase.The presence of salts most notably LiCl leads to a prolongation of the lifetime. The results suggest a non-monotonic potential function for the interbilayer water thickness. a Faraday Discuss. 1998 111 31»40 I. Introduction Among the manifold molecular interactions which govern the intra- and intercellular communication in biomembranes the supramolecular structure and dynamics of lipid constituents deserves particular attention as they provide the structural variability and controlled one- or twodimensional —uidity essential for membrane function. The wide chemical diversity of membrane lipid constituents e.g. phospho- or glycolipids with varying hydrocarbon chain lengths saturation or branching sterols confers a powerful mechanism for the cell to control the physico-chemical properties mechanical electrostatic and chemical of membranes.1h4 A crucial point in the discussion of supramolecular lipid aggregates is their cooperativity,5 i.e.the fact that properties and mechanisms are governed by the collective behaviour of cooperative units rather than by individual molecules. In the extreme this leads to the concept of generic physical properties and interactions of bilayer membranes where the individual molecular structure plays only a subordinate role.6,7 One of the most interesting features of phospholipid»water systems is their polymorphism i.e. the existence of various ordered crystalline gel or liquid-crystalline phases.8 In the transitions between these phases the above-mentioned supramolecular nature becomes particularly apparent.9 Most transition processes are highly cooperative such that several hundreds of individual lipid molecules transit simultaneously and discontinuously from one phase to the other.It has long been an open question how one ordered phase changes into another with diÜerent geometry without disruption of lattice order.10 This problem is schematically illustrated in Fig. 1 for the simplest case of a one-dimensional lattice as given in principle by a multilamellar liposome structure. It seems that for highly cooperative transitions between two ordered phases with a minimisation of structural disruption and disorder there can only be highly symmetric and localised transition mechanisms as de–ned by the martensitic transition type originally de–ned for metallurgical phases11,12 [Fig.1(b)]. 31 Fig. 1 Scheme of two alternative transition mechanisms in lamellar phases. (a) Transition mechanism with zones of disorder resulting in a loss of coherence. (b) Transition mechanism with minimal loss of order and coherence (martensitic transition). In addition to the concept of cooperativity the non-equilibrium nature of transition processes gains relevance in the discussion of supramolecular processes and transitions. Molecular dynamics diÜusivity —exibility are normally treated as equilibrium phenomena.13,14 Close to equilibrium at minor elevations from the thermodynamic equilibrium potential well the kinetic and mechanistic behaviour can still be described classically by single-exponential energy and entropy terms.However under strong jump conditions at large elevations from the equilibrium potential trough the system may respond non-linearly no longer able to be described by single exponential terms.15,16 The simplest consequence of this notion is that transition processes may follow diÜerent pathways depending on whether they are guided in a slow isothermal or in a fast adiabatic fashion. Structural intermediates of diÜerent lifetime play a signi–cant role in this concept. Such intermediates have been postulated and de–ned some time ago in studies of membrane fusion by several groups.17v19 By time-resolved X-ray diÜraction experiments on various diÜerent lipid phases such intermediates have been demonstrated.20v22 Although the results have made it possible to propose hypothetical models it has not been possible so far to provide a sound structural analysis of these intermediates to good resolution.a In the present work this jump»relaxation approach has been focussed on intermediates within one thermodynamic equilibrium phase region the L -phase of phosphatidylcholines. The experiments performed with advanced jump»relaxation techniques and synchrotron X-ray diÜraction lead to the notion of a modulated potential versus bilayer-distance function and a universal thickness increment for the interbilayer water space which closely corresponds to the molecular dimensions of water. II. Materials and methods Sample preparation 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) egg-yolk phosphatidylcholine (EYPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3- phosphoethanolamine (POPE) were purchased from Avanti Polar Lipids Birmingham Alabama and used without further puri–cation.Multilamellar liposomes were prepared by dispersing weighed amounts of dry lipids typically 20»30 wt.% in bidistilled water and in solutions of 0.1 and 0.3 M LiCl respectively. To ensure complete hydration the lipid dispersions were incubated for ca. 1»2 h at least 10 °C above the main transition temperature and thereafter vigorously vortexed under a N atmosphere to prevent oxidation. Aqueous dispersions of these lipids dis- 2 played narrow cooperative melting transitions within the limits of published values,23 thus proving that the lipid purity corresponded to the claimed one of 99%.Faraday Discuss. 1998 111 31»40 32 Fig. 2 Schematic view of the set-up for T- and P-jump relaxation experiments. Throughout fast time-resolved X-ray diÜraction recordings aqueous suspensions can be studied far from the equilibrium situation. An IR laser with a pulse-characteristic of 1»2 J ms~1 provides T-jumps up to 20 °C and with an in-house-built pressure cell P-jumps up to 3 kbar can be carried out. Experimental protocol Fast time-resolved X-ray diÜraction experiments on lipid dispersions were carried out at the Austrian small-angle X-ray scattering station at ELETTRA Trieste.24v26 During rapid excitation of the lipid»water systems using temperature- (T-) and pressure (P-) -jump techniques the relaxation processes were monitored with a millisecond time resolution (Fig.2). For temperature-jump (T-jump) experiments the lipid dispersions were sealed in a thin-walled 1 mm diameter Mark capillary held in a steel cuvette which provides good thermal contact to the Peltier heating unit. T-jumps were generated with an erbium-glass laser27 with an pulse length of 2 ms and a maximum emitted energy of 4 J. Barotropic phase transitions were investigated with a high-pressure X-ray cell28 using jump amplitudes up to 3 kbar (0.3 GPa) within 10 ms. In particular P-jump induced phase transitions of the phospholipid DOPE within the temperature region 5»70°C and pressure region 1»3000 bar were performed. Data analysis The raw data of the time-resolved experiments were normalised for the integration time of each time-frame.Each small-angle X-ray diÜraction pattern was analysed by –tting the –rst-order Bragg re—ections using a least-squares method based on the Levenberg»Marquardt algorithm. Depending on the number of given phases the model function was given by one or the sum of two Lorentzians respectively. The lamellar repeat distances were determined from the corresponding peak positions. The relaxation kinetics of the d spacings are best described by a doubleexponential model (1) B B 0[A expA[ q t [B expA[ q t d(t)\d B A For comparison single and triple exponential models were also checked but have been proven by statistical tests (Variance-analysis F-Test) not to describe the relaxation kinetics as well as the two-component model.33 Faraday Discuss. 1998 111 31»40 a III. Results The intermediate La*-phase in diÜerent transitions The observation of a thin ordered lamellar structure which we denote as the L*-phase as a transient intermediate has been made in diÜerent jump»relaxation experiments on various lipid classes. In the following these shall be described separately. Faraday Discuss. 1998 111 31»40 34 a The La«La*«La transition (single phase). Fig. 3 shows the results of a typical experiment in a which a phosphatidylcholine lipid has been subjected to a T-jump starting from the single L phase which it attains under equilibrium conditions at the starting temperature. The jump amplitude was ca.15 °C and the maximum temperature reached was well within the L -phase region. In the jump experiment the d value –rst decreases discontinuously by ca. 0.3 nm and then relaxes back within less than 15 s to the equilibrium spacing. The minimum d spacing with a value of Fig. 3 L a»L a *»L a transition in POPC (20 wt.%) induced by a 15 °C(2 ms)~1T-jump (initial temperature 30 °C). (a) A series of time-sliced diÜraction patterns shows the temporal development of the –rst-order Bragg peaks (maximum resolution 5 ms). Raw data are given. (b) The evolution of the d spacings in fully hydrated POPC is displayed. Each single diÜraction pattern was –tted by a Lorentzian distribution ([). The line gives the best –t to the relaxation model of the L a * phase [eqn.(1)]. For comparison the temperature dependence of the lamellar repeat distance d of POPC under equilibrium conditions is depicted in the insert. ca. 6.2 nm is clearly thinner than the corresponding lattice parameter under near-equilibrium conditions which even at a temperature of 70 °C does not drop below 6.37 nm [see insert Fig. 3(b)]. Similar La ]La*]La experiments have been performed with other phosphatidylcholine and ethanolamine lipids and the resulting parameters are listed in Table 1. The result that with all lipids studied the non-equilibrium decrease in d value is ca. 0.3 nm irrespective of the hydrocarbon-chain or head-group composition emerges as a salient result. The La«La*«La-transition in two coexisting phases. Alkali-metal ions especially Li` can induce an equilibrium phase separation of two coexisting L -phases.29 These phases of which the structural nature and the origin of their coexistence is not yet quite clear diÜer in d spacings by ca.0.6 nm. It was of particular interest therefore to examine their behaviour under nonequilibrium jump conditions. Fig. 4 shows that both phases as demonstrated by the time-course of the lamellar repeat spacings respond in a parallel fashion to the T-jump and relax with similar kinetics. As with the single-phase transitions presented above the discontinuous changes in d spacing amount to ca. 0.3 nm and the relaxation times are of the order of 8»15 s i.e. still faster than the thermal equilibration within the sample cell. However the thermal equilibration does in—uence the relaxation times thus the relative behaviour is more relevant than the absolute values in the discussion.a Lipid a/degrees d /nm 0 T /°C f T /°C i 18 18 16 6.53 6.43 6.55 45 76.5 31.5 POPC DPPC EYPC POPE 0.1 M KCl 0.3 M LiCl 0.1 M LiCl 0.1 M MgCl2 a Table 2 The –tting parameters of the relaxation curves of POPC according to eqn. (1) phase 2 phase 1 phase 1 phase 1 phase 2 phase 1 no salt a In view of the dramatic eÜects of Li on the L -phase structure it is necessary to evaluate also the eÜects of other alkali-metal ions. This has not yet been done comprehensively due to the excessive requirements on synchrotron beam-time. Some interesting features of salt eÜects are already visible from Table 2 which summarises the results from all jump»relaxation experiments so far performed in the presence of salts.The general feature of the parameters obtained is the Table 1 Summary of laser T-jump experiments in the liquidcrystalline phase of various phospholipids *d/nm 0.31 0.32 0.26 30 70 25 30 36.5 5.37 0.37 21 All samples were equilibrated in the liquid-crystalline phase at the initial temperature T and jumps of 15 and 6.5 °C respectively were performed. The d spacing of the L phase d reduces i 0[*d. The theoreti- d directly after the laser-pulse to the value 0 cal declination angle a between the parent (L and nascentL a ) ( a *) phase respectively is described through eqn. (2). 0.06 0.09 0.06 0.06 0.10 0.09 0.13 A/s 0.5 0.8 0.6 0.3 0.3 1.0 0.5 0.23 0.25 0.23 0.27 0.27 0.29 0.18 3.1 8.3 8.1 15.8 14. 1 6.9 6.5 5.89 6.37 5.95 6.37 6.07 6.73 6.53 A/nm q B/nm q d /nm B/s 0 *d/nm 0.29 0.33 0.37 0.38 0.31 0.31 Increasing LiCl concentration results in longer lifetimes of the intermediate phase L (see A]qB). The a * kinetics of the salt-induced phases 1 and 2 respectively show similar relaxation behaviour. Independent from the salt concentration the d spacings of the liquid crystalline phases always decrease by ca. 0.3 nm directly after the laser pulse (*d\A]B). The errors in the parameters are of the order of the last digit given. 0. q 32 Faraday Discuss. 1998 111 31»40 35 Fig. 4 L a»L a *»L a transition in POPC (20 wt.% in 0.1 M LiCl) induced by a 15 °C(2 ms)~1 T-jump (initial temperature 30 °C).(a) A series of time-resolved diÜraction patterns given in the form of a contour plot demonstrates the kinetics of the salt-induced L phases (maximum resolution 5 ms). (b) The d spaca1 ings for L a1 and L a2 determined from the –tted Bragg-peak positions (sum of two Lorenztians). Both phases and L a2 exhibit similar temporal and structural behaviour (see also Table 2). prolongation of the lifetimes of the intermediates by alkali-metal salts. The prolongation factors reach a value of 5 for the addition of 0.3 M LiCl. a a a The La«La*«HII transition of phosphatidyl-ethanolamines pressure jumps. The existence of an intermediate thin L*-phase has been previously observed by T-jump experiments.20 This intermediate structure provides the contact conditions of opposing bilayers necessary for fusion and the formation of tubular structures.By P-jump experiments this result has been fully veri–ed in all details [Fig. 5(a)]. The initial fast step of the transition is the formation of a thin L*-phase which disappears in parallel with the formation of the H -phase taking comparatively long times of II several seconds to develop fully. In contrast to the T-jump technique the P-jump technique has the advantage of allowing jumps in either direction. This makes it possible to investigate the reversibility of the transitions and their pathways. Fig. 5(b) shows the pressure-drop experiment from the H phase –rst into the L -phase II and then onwards into the L phase.Two observations are particularly noteworthy. First the transition from the hexagonal phase into the lamellar phase proceeds without intermediates i.e. b Faraday Discuss. 1998 111 31»40 36 a a * Hphase. (b) II b a b a Fig. 5 Time-resolved X-ray diÜractograms of fully hydrated DOPE exposed to pressure-jumps recorded with a time resolution of 5 50 and 500 ms for each frame respectively. (a) Starting from the lamellar L phase at high pressure (T \41 °C and p\2300»155 bar) the lattice transforms immediately after a depressurizing jump into the intermediate lamellar L phase which coexists then with the emerging hexagonal Starting in the hexagonal H phase at low pressure the lattice transforms after a P-jump (T \20 °C and II p\1»2940 bar) immediately into the lamellar L phase.After 500 ms the –nal L phase begins to appear growing at the expense of the coexisting L phase until the phase transformation into L is completed. The lattice spacing of all phases remains constant over the entire time interval. b a a a there appears no L*-phase in this direction. Second the L -and L -phases coexist over a long period unlike in the other direction where the two phases coexist only for the period of the temperature jump i.e. 1»2 ms as described previously.30 No thin intermediate L*-phase can be detected in either direction of the La%Lb transition. a a a IV. Discussion a The programme under which the present investigation was performed is primarily aimed at the exploration of methods for prolonging the lifetimes of structural intermediates in phospholipid phase transitions.A bene–t from such an achievement could be the better structural description of the intermediates because longer lifetimes would lead to better precision of diÜraction data. Also there are likely to be biomedical bene–ts from such results since the development of agents modulating the dynamics of membrane transformations such as fusion is likely to play an important role in many medical applications e.g. liposome-based gene therapy fertility modulation or percutaneous drug applications to name but a few.31 Strong interest in such intermediates comes also from the –eld of nanomaterial research,32 where such structures could serve as templates for new materials which cannot be obtained under equilibrium conditions.The main discovery made in this search for trapped intermediates is the demonstration of a discrete ordered transition state the L*-phase which occurs rapidly and cooperatively upon a T- or P-jump from the normal equilibrium L -phase. This phase is always characterised by a ca. 0.3 nm lower d spacing than the parent phase independent of the hydrocarbon chain composition of the phosphatidylcholine species ; it also occurs transiently during the P- or T-jump from the L into the H phase. These two facts the constant *d and the composition independence II particularly that of hydrocarbon chain composition are taken to indicate that the La ]La* transition involves primarily a change in interbilayer water thickness.Indirect support for this idea comes from the fact that the same seems to be the case in the thin L phases found in the presence of LiCl (also separated by ca. 0.3 nm29) where the Fourier analysis of the coexisting diÜraction patterns of normal and thinner phases results in an invariant bilayer thickness (paper in preparation). It remains uncertain whether the transient L*-phase in the jump experiments is indeed the same as one of the equilibrium structures in the presence of LiCl. However it is tempting to assume that the two represent the same discrete secondary minimum in the hydration separation of bilayers and that the constant *d of ca. 0.3 nm relates to a change in water thickness a Faraday Discuss. 1998 111 31»40 37 by one molecular layer.Since there is presently no evidence for a diÜerent interpretation e.g. in terms of a discrete thinning of the hydrocarbon chain thickness or a change in head-group conformation we adhere to the hypothesis of a discontinuous hydration change. Two questions follow immediately from this hypothesis –rst how is the quasi-immediate thinning of the interbilayer water space achieved while the lattice order is fully preserved ? Second how does the intermediate L lattice return into the equilibrium a* La structure ? A tentative answer to the –rst question is presented by the martensitic lattice-disclination mechanism as shown schematically in Fig. 1(b). The transition would be localised in a discontinuous transition plane linking the parent to the nascent phase and moving rapidly with the speed of sound through the liposome.At the transition plane the two lattice planes would be disclinated at the angle a [Fig. 1(b)] which is given simply by the cosine relation between parent and nascent d spacings (2) a\arccosAd[*d \arccosA1[ *d d d B B where *d\0.3 nm. This mechanism results in a minimum disruption of lattice order and involves as a diÜusion component only the rapid movement of the transition plane thus providing for maximum transition speed. It should be emphasised that this behaviour is indeed demonstrated by Fig. 3(a) where the Bragg peaks immediately after the jump are very sharp and no intermediate disordering can be observed. What happens to the water where does it disappear to while the bilayer separation decreases ? The relative decrease in water layer thickness amounts to ca.15% assuming a value of 2 nm for the water layer in the fully hydrated L structure.33 A transient increase in water density for the lifetime of the intermediate (of the order of 0.1 s) seems unlikely considering the typical ps relaxation times of water. An efflux from the liposome structure into the excess water phase through transient defects in the lamellar lattice might be more plausible but it is again the observation of the very sharp Bragg re—ections right after the jump which indicates that such defects are not increased and suggests that this is not a likely mechanism. Another possibility would be an increase in bilayer surface area by ca. 15% and a concomitant reduction in water layer thickness thus conserving the interbilayer water volume.As a consequence the molar phospholipid volume would have to increase by the same proportion to conserve the bilayer thickness implying an increase in lateral headgroup separation of 7%. Perhaps the simplest way to dispose of the water is through the formation of localized ìì lentils œœ or cavities which would not gravely perturb the multibilayer order (Fig. 6). This would avoid the necessity of increasing the molecular surface area to conserve the bilayer thickness. Faraday Discuss. 1998 111 31»40 a While the formation mechanism of the intermediate L lattice appears to be best described by a* the discontinuous martensitic mechanism sketched in Fig. 1(b) the return to the equilibrium La structure follows a diÜerent pathway.As Fig. 3(b) and the decay parameters in Table 2 show this follows slow (on the experimental timescale) bi-exponential kinetics and most signi–cantly passes through a relatively disordered lattice situation as indicated by the broadening and decrease in intensity of the Bragg peaks only to increase again with complete re-equilibration. This can be interpreted qualitatively in terms of a model as shown in Fig. 1(a) where zones of disorder link the parent thin lattice with the nascent thicker one. The process could be analysed in terms of a nucleation-and-growth mechanism but a quantitative evaluation would require more detailed information on the morphology of the liposome particles during the process and is beyond the scope of the present work.The results of the T-jump experiments in the presence of LiCl (Fig. 4) indicate that the La*- lattice is not a limiting one to smaller thicknesses. There the initial equilibrium structure is Fig. 6 Schematic view of ìì lentils œœ of bulk water in multilamellar liposomes. 38 a* already separated into three coexisting phases with discrete d spacings diÜering by ca. 0.3 nm.29 The thickest and thinnest ones respectively diÜering in d spacing by ca. 0.6 nm are the dominant ones. The T-jump experiment shows an essentially identical behaviour for these two coexisting phases. Again immediately after the jump a discontinuous shift of both Bragg peaks to smaller (by 0.3 nm) d spacings is followed by a slow continuous return to the equilibrium situation.We assume that both parent phase structures have the normal molar phospholipid surface areas of approximately 0.63 nm2.34 The jump-induced thinning of the water-layer thickness could be again compensated by a transient increase in surface area or by the formation of ìì lentils œœ. The parallel return kinetics suggest the same mechanism and thermodynamic driving force for the two phases. The universality of the 0.3 nm increment in the jump response of hydrated phospholipids is further con–rmed by the behaviour of ethanolamine phospholipids in the La ]HII transition in a pressure-release-jump experiment [Fig. 5(a)]. There again the intermediate L phase shows a d spacing which is by the same amount thinner as was observed in all other cases so far investigated.An analogous observation has been made previously by a T-jump experiment. Hence *d seems to be largely independent of the chemical structure of the phospholipid and also of the physical nature of the jump as long as it leads to an overall volume expansion. In the pressurising jump through the HII ]La transition on the other hand no intermediate was observed [Fig. 5(b)]. a Having thus established the model for the L*-phase it remains to be examined what are the reasons for its formation. Our previous results of an LiCl-induced L -phase separation in liquidcrystalline phosphatidylcholines29 have suggested that there exists a modulation in the interbilayer separation potential with secondary minima at distances of ca.one molecular water diameter. The present results seem to support this notion although they are derived from a strongly non-equilibrium approach. This is in contrast to the hitherto generally held belief of a smooth continuous potential function with only one potential well at the equilibrium separation. 35 Indeed none of the studies on the hydration dependence of the lamellar repeat spacing has shown any sign of secondary minima.36h38 The reason might simply lie in the dynamic undulations of the individual bilayers such that at thermal equilibrium all states diÜering in energies by a few kT only are simultaneously occupied and the Bragg peaks represent the average bilayer separation. The simultaneous energy supplied by the T-jump would lead to a uniform occupation of the secondary potential minimum and hence to the observed sharp Bragg peak of the L phase.a a* The interbilayer separation in the L phase is ca. 2 nm corresponding hypothetically to 7 water layers if one layer is ca. 0.3 nm thick. Despite the highly dynamic state of the water between the bilayers it is obvious that the restrictions to mobility are much more stringent in the direction normal to the bilayer plane than in the parallel direction. These restrictions together with a modulated interaction potential between opposing bilayers would become more pronounced the closer the bilayers approach simply from a steric consideration of the discrete water molecule dimensions despite the —exibility and vertical ììbobbingœœ motions of individual phospholipids.The fact that the interbilayer water spacing decreases with temperature even isothermally and not just in the adiabatic jump experiments shows that the water volume expands thermally in the two dimensions of the bilayer plane. This creates an increased interface area which is counteracted by the manifold intermolecular phospholipid interactions the water bridges in the backbone region and by the hydrophobic eÜect which however decreases with temperature. We want to speculate therefore that the 0.3 nm increment is indeed the average thickness of one water layer in the sense that the interbilayer water space as it decreases under non-equilibrium conditions tends to prefer integral multiples of this thickness. Concerning the underlying intention to explore ways to prolong and eventually trap ordered Faraday Discuss.1998 111 31»40 a intermediate structures the present experiments with salts hold some promise although the obtained changes in relaxation times are not as dramatic as one would wish. The most important observation is that the salts used do not change the 0.3 nm increment by which the d spacings vary under jump conditions. The mechanism by which they prolong the lifetimes of the intermediates are still unclear and therefore no predictions in the direction of more potent trapping agents can be made. However since we assume as has been elaborated above that the cause for the discrete intermediate formation is the water structure it can be speculated that cosmotropic rather than chaotropic agents might be suitable candidates.If one considers the results obtained with 39 LiCl under equilibrium conditions,29 with Li` being at the cosmotropic end of the lyotropic series of monovalent cations this speculation gains in substance. Acknowledgements This work has been supported by the ììElettra-Projectœœ of the Austrian Academy of Sciences. M. Rappolt is the recipient of a long-term grant from the European Commission under the programme ììTraining and Mobility of Researchersœœ [Contract no. SMT4-CT97-9024(DG12-CZJU)]. Phasenué bergaé nge und Kritische Phaé nomene F. Vieweg & Sohn Braunschweig Paper 8/06384B References 1 M. Bloom and O. G. Mouritsen in Structure and Dynamics of Membranes ed. R. Lipowsky and E. Sackmann Elsevier Amsterdam 1995 p.65. 2 M. Bloom E. Evans and O. G. Mouritsen Q. Rev. Biophys. 1991 24 293. 3 E. Sackmann in ref. 1 p. 1. 4 P. Kinnunen Chem. Phys. L ipids 1991 57 375. 5 R. Biltonen J. Chem. T hermodyn. 1990 22 1. 6 R. Lipowski in Festkoé rperprobleme Advances in Solid State Physics ed. U. Roessler Vieweg Braunschweig/Wiesbaden 1992 vol. 32 p. 19. 7 W. Helfrich in ref. 1 p. 691. 8 J. M. Seddon and G. Cevc in Phospholipids Handbook ed. G. Cevc Marcel Dekker New York 1993 p. 403. 9 D. Marsh Chem. Phys. L ipids 1991 57 109. 10 P. Laggner and M. Kriechbaum Chem. Phys. L ipids 1991 57 121. 11 J. W. Christian in Physical Metallurgy ed. R. W. Cahn North-Holland Amsterdam 1970 p. 471. 12 Z. Nishiyama Martensitic T ransformations Material Science Series ed.M. Fine M. Meshii and C. Wayman Academic Press New York 1978. 13 A. Blume in ref. 8 p. 455. 14 P. F. F. Almeida and W. L. C. Vaz in ref. 1 p. 305. 15 W. Gebhardt and U. Krey 1980. 16 D. Lou J. Casas-Vasquez and G. Lebon Extended Irreversible T hermodynamics Springer Berlin 1993. 17 P. R. Cullis M. J. Hope B. deCruijÜ A. Verkleij and C. P. S. Tilcock in Phospholipids and Cellular Recognition ed. J. F. Kuo CRC Press Boca Raton 1985 p. 1. 18 D. P. Siegel Biophys. J. 1986 49 1155. 19 D. P. Siegel Biophys. J. 1986 49 1171. 20 P. Laggner M. Kriechbaum and G.Rapp J. Appl. Crystallogr. 1991 24 836. 21 P. Laggner J. Phys. IV 1993 3 259. 22 M. Rappolt PhD Thesis Hamburg 1995. 23 M. CaÜrey L ipid T hermotropic Phase T ransition Database (L IPIDAT 2) NIST 1994.24 H. Amenitsch S. BernstorÜ M. Kriechbaum D. Lombardo H. Mio M. Rappolt and P. Laggner J. Appl. Crystallogr. 1997 30 872. 25 H. Amenitsch S. BernstorÜ and P. Laggner Rev. Sci. Instrum. 1995 66 1624. 26 S. BernstorÜ H. Amenitsch and P. Laggner J. Synchrotron Rad. 1998 5 1215. 27 G. Rapp M. Rappolt and P. Laggner Prog. Colloid Polym. Sci. 1993 93 25. 28 K. Pressl M. Kriechbaum M. Steinhart and P. Laggner Rev. Sci. Instrum. 1997 68 4588. 29 M. Rappolt K. Pressl G. Pabst and P. Laggner Biochim. Biophys. Acta 1998 1372 389. 30 M. Kriechbaum G. Rapp J. Hendrix and P. Laggner Rev. Sci. Instrum. 1989 60 2541. 31 D. D. Lasic L iposomes From Physics to Applications Elsevier Amsterdam 1993. 32 S. Mann Nature (L ondon) 1988 332 119. 33 J. F. Nagle R. Zhang S. Tristram-Nagle W. Sun H. I. Petrache and R. M. Suter Biophys. J. 1996 70 1419. 34 S. Tristram-Nagle H. I. Petrache and J. F. Nagle Biophys. J. 1998 75 917. 35 J. Israelachvili in Intermolecular and Surface Forces Academic Press London 1992 p. 176. 36 R. P. Rand and V. A. Parsegian Biochim. Biophys. Acta 1989 988 351. 37 T. J. McIntosh and S. A. Simon Biochemistry 1993 32 8374. 38 B. Koenig H. H. Strey and K. Gawrisch Biophys. J. 1997 73 1954. Faraday Discuss. 1998 111 31»40 40

ISSN:1359-6640    

DOI:10.1039/a806384b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Sensing isothermal changes in the lateral pressure in model membranes using di-pyrenyl phosphatidylcholine Richard H. Templer,* SaÜron J. Castle A. Rachael Curran Garry Rumbles and David R. Klug T he Department of Chemistry Imperial College L ondon UK SW 7 2AY Received 17th August 1998 In this work we present data from a homologous series of di-pyrenyl phosphatidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangential to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer —uorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity the relative intensity of the excimer to monomer signal g is a measure of the pressure.We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chains having 4 6 8 and 10 carbon atoms and then measured g in fully hydrated bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine (DOPC and DOPE respectively). Although the resolution of our measurements of lateral pressure as a function of distance into the monolayer was limited we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE composition and hence the desire for interfacial curvature we observed as expected that the net excimer signal increased.However this net increase was apparently brought about by a transfer of pressure from the region around the glycerol backbone to the region near the chain ends with the lateral pressure dropping above the cis double bond but increasing at a greater rate beyond the double bond. (1) gc\12i(c1]c2[2c0)2]iG c1 c2 Introduction In an amphiphilic bilayer the back to back monolayers are restrained to lie —at even though each will in general possess an intrinsic desire for interfacial curvature. The curvature elasticity1 of the monolayer may be expressed in terms of the bending energy per unit area gc 0 2 . where c c and are the principal curvatures of the region of interest c0 is called the spontaneous 1 2 curvature of the interface i is the mean curvature modulus and i is the Gaussian curvature G modulus.For the case where the monolayer is held —at the stored curvature elastic energy is simply 2ic This is precisely the case for cell membranes so we would anticipate that the amphiphilic fabric of the cell membrane would be storing a curvature elastic energy in proportion to i and c0 and it 41 Faraday Discuss. 1998 111 41»53 has been suggested that this stored energy may play a role in modulating the behaviour of membrane proteins.2,3 Evidence that this is the case has been building steadily,4h8 but the correlation between protein activity and the physical state of the membrane has for the most part remained qualitative.In part this has been because our knowledge of i and c for most amphiphilic lipid 0 systems remains sketchy. However even if we had a better and more reliable catalogue of values of i and c we would have to accept that these two parameters are a rather crude attempt to 0 characterise the physical state of the amphiphilic membrane. The connection between the stored curvature elastic energy and a more complete description of the physical state of the monolayer is in principle relatively straightforward. The desire for monolayer curvature arises because of the diÜerential distribution of lateral pressure Fig. 1. At the polar/apolar interface there is a strong inward pressure as the system attempts to limit the contact between water and hydrocarbon.This is resisted in the chain region where there is a strong outward pressure due to thermally driven collisions between chains. In the headgroup region there may also be positive contributions to the lateral pressure from steric hydrational and charge eÜects but also negative contributions may occur if there is direct hydrogen bonding between headgroups. Although the integral of the lateral pressures for the monolayer must necessarily come to zero the –rst moment of the lateral pressure will in general be non-zero. It is this torque tension which represents the monolayerœs frustrated desire for interfacial curvature and it is related to i and c0 by (2) 0 Pl n(z)z dz\2ic 0 0 where l is the monolayerœs thickness9 and we use the convention that c is negative when the 0 monolayer wishes to bend towards the water.Clearly n(z) is the most complete description of the average lateral forces exerted by the lipid membrane on proteins inserted into the bilayer. Unfortunately measurements of c i and cannot uniquely de–ne n(z). In this work we have therefore attempted to take the alternative approach of trying to measure n(z) directly. This is as far as we are aware the –rst attempt to do this that has been reported in the literature. We have restricted ourselves to probing the pressure in the chain region. We have done this for a number of reasons. Statistical mechanical calculations of the lateral pressure pro–les in the chain regions are possible because of the relatively simple nature of the interactions between —uid chains.10h12 This is not true for the headgroup and interfacial regions.Fluorescent probe molecules tend to be relatively bulky and this militates against their use either in the headgroup region or the interface where it is already known that very small changes can have profound eÜects on phase behaviour.3,13 The evidence from the eÜect of small amounts of hydrophobic additives in bilayers indicates that the eÜect on the phase behaviour of having the probe in the —uid chain region would be signi–cantly less.3 Finally since the chains form an extended region we are most likely to be able to extract some of the details of the shape of the pressure pro–le here. Fig. 1 Distribution of lateral pressure within a —at monolayer. The isotropic lateral pressure n can be plotted as a function of depth into the monolayer z.Faraday Discuss. 1998 111 41»53 42 Being an extended region of positive lateral pressure we would expect the magnitude of the pressure and hence the magnitude of variations to be less than in the headgroup and interfacial regions. We may estimate what the magnitude of n(z) might be for the relatively well characterised lipid system we have used in this work; dioleoylphosphatidylcholine (DOPC) and dioleoylphosc phatidylethanolamine (DOPE). For DOPC and DOPE at 25 °C we have 0\[1/90 Aé ~1 and [1/56 Aé ~1 and i\9 kT and 12 kT respectively. The determination of the torque tension is independent of the origin of the integration and hence we choose to place this at the headgroup region so that the torque tension arises predominantly from the pressure in the chain region.We will assume that n(z) in the chain region is constant (\n6 c) and hence that n6 c\[4ic0/l2. Since l is approximately 18 Aé in the case of DOPC and DOPE the average pressure in the chain region for —at monolayers of DOPC and DOPE will be 5 and 10 MPa respectively. These estimates are of the same order of magnitude as the theoretical calculations of Szleifer and co-workers on saturated chains.10 The experiments we present here examine the changes in the lateral pressure pro–le in the chain region of DOPC/DOPE membranes as a function of the lipid composition. Our probes will therefore reside in a milieu where the pressures are at least one order of magnitude greater than atmospheric and where we can vary this pressure by a factor of two by changing the relative composition of DOPC and DOPE in excess water.Our report is concerned with the measurement of n(z) in the —uid lamellar (La) phase which means that we will only brie—y report on our measurements at high DOPE contents where the inverse hexagonal phase is found. We chose to use dipyPCs Fig. 2 to make —uorescence measurements of lateral pressure. In particular we were aiming to make use of intra-molecular pyrene excimer formation as a measure of the lateral pressure. Excimers will occur when a pyrene moiety in its excited state comes into close proximity (B3.5 Aé ) and in the correct orientation (face to face) to a ground state pyrene and they form an excited dimer or excimer.14h19 Using a model in which pyrene re-orientation must occur after aggregation Cheng and co-workers have shown that aggregation is the rate limiting step in excimer formation in dipyPCs.20 Since the frequency with which aggregates form will be proportional to the lateral pressure their –ndings imply that the excimer signal in turn will be Fig.2 The 10dipyPC —uorescent probe molecule. 43 Faraday Discuss. 1998 111 41»53 proportional to the lateral pressure. In practice it is more reliable to measure the ratio of the excimer to the monomer —uorescence intensity g. Although it has not been stated there are in fact a number of reports which show evidence that g is a measure of lateral pressure. In the –rst reports of the synthesis and —uorescence of dipyPC [1,2-bis(1-pyrenyldecanoyl)-L-a-phosphatidylcholine or 10dipyPC] Sunamoto and co-workers showed that low concentrations of 10dipyPC in dipalmitoylphosphatidylcholine and egg lecithin bilayers exhibited increases in g with the addition of cholesterol or increases in temperature.21,22 Both are known to increase the monolayer torque tension.Cheng and co-workers used 1,2-bis(1- pyrenylmyristoyl)-L-a-phosphatidylcholine (14dipyPC) to examine lipids in both the lamellar and inverse hexagonal phases.23 Both g and the monomer lifetime were found to be sensitive to the temperature induced lamellar to inverse hexagonal phase transition in DOPE. Butko and Cheng24 used 14dipyPC to examine mixed dilinoleoylphosphatidylethanolamine/palmitoyloleoylphosphatidylcholine mesophases.Using Birksœ description of the kinetics for excimer formation,14 Fig. 3 they showed that g can be related to the rate constants by (3) g\ (kfD/kfM)kDMC (kD]kMD) For dipyPCs at low –xed concentration the ratio kfD/kfM the ratio between the radiative decay parameters of the excimer and monomer and (kD]kMD)~1 the —uorescence lifetime of the excimer were found to be independent of temperature. Hence g was simply proportional to kDM the rate constant of excimer formation. They used this to determine the activation energy for excimer formation which they found to be lower in the L phase. Again this is consistent with the a increased lateral pressures in the L phase. Sassaroli and co-workers25 found that g for 10dipyPC a decreases with applied hydrostatic pressure in DOPC.They calculated that the activation energy for excimer formation decreased with increasing pressure in agreement with Butko and Chengœs –ndings. Cheng and co-workers have also used dipyrenylphosphatidylethanolamine dipyPE probe molecules.20 In these studies they found that g for 4dipyPE was always greater than g for 10dipyPE at any given DOPC/DOPE composition and temperature. This is consistent with the theoretical calculations of the lateral pressure pro–le where the lateral pressure is always highest nearest the polar/apolar interface.10 Although the data on dipyPCs in bilayers have been the subject of intense analysis there has been as far as we are aware only one mention in the literature of lateral pressures being the source of variation in the photophysical properties of dipyPC.The authors26 have in fact attempted to determine the variation in c for DOPC/DOPE mixtures in a complex analysis of their 0 spectroscopic data. In doing so they have unwittingly made use of the fact that i for DOPC and DOPE are so close that almost all of the variation in torque tension is due to the changes in c0 .27 fD k kfM k and k are the iM kDM is the second-order rate constant for excimer iD Fig. 3 Birksœ kinetic scheme for excimer formation. M is the monomer M* the excited monomer and MM* the excited dimer. and are the monomer and excimer radiative decay parameters monomer and excimer non-radiative decay parameters from the dissociation of the excimer. formation and C is the pyrene concentration.kMD is the rate constant for the regeneration of excited monomer Faraday Discuss. 1998 111 41»53 44 With these encouraging data we have set about to record how the lateral pressure pro–le in the chain region changes as a function of composition in DOPC/DOPE mixtures. and aluminium chloride (approximately 0.2 and 0.4 mol respectively) Materials and methods Materials and synthesis DOPC and DOPE were purchased from Avanti Polar Lipids (Alabaster Alabama USA). They had a stated purity [99% which we con–rmed by thin layer chromatography (TLC) before and after use. We found no degradation of these components in our experiments. Vauhkonen and Somerharju28 have shown that the maximum excimer —uorescence intensity is achieved where the pyrenyl moieties are appended to acyl chains of equal length.DipyPCs having pyrenyl moieties attached to acyl chains with 4 and 10 carbon atoms (4dipyPC and 10dipyPC) were purchased from Molecular Probes Inc. (Eugene Oregon USA). They had a stated purity [99% which we con–rmed with high pressure liquid chromatography (HPLC). Since dipyPCs having chain lengths of 6 and 8 (6 and 8dipyPC) were not available commercially we synthesised these ourselves. Their production was broken into two major parts the synthesis of the pyrenyl fatty acids 6-(pyren-1-yl)hexanoic acid and 8-(pyren-1-yl)octanoic acid ; and their subsequent acylation onto glycerophosphocholine. The –rst part of the synthesis was carried out in our laboratory using a modi–cation of the method described by Sunamoto and co-workers,21 whilst the –nal stage of the synthesis was carried out for us by Dr D.M. Phillips of Lipid Products Ltd. (Nut–eld Surrey UK) using an adaptation of the method used to synthesise 4dipyPC.29 2)nCO2H Making the pyrenyl fatty acid proceeds via a three step synthesis. 0.25 mol EtO C(CH 2 (n\6 or 8) and 0.5 mol thionyl chloride were mixed in 400 ml of low boiling point petroleum ether and left at room temperature until no solid monoethyl adipate remained about 16 h. The petroleum ether and much of the thionyl chloride was removed on a rotary evaporator using toluene to form an azeotropic mixture with the thionyl chloride. The monoethyl adipoyl chloride or monoethyl suberoyl chloride were then distilled under vacuum at 66»70 °C 2 mmHg. The yield varied from 70»90%.The EtO C(CH 2 2)nCOCl were mixed with 65 ml of dichloromethane and placed in an ice bath. Approximately 0.2 mol of pyrene in 225 ml of dichloromethane was added dropwise to the mixture and stirred for 16 h. Iced water (190 ml) was added slowly with stirring to the orange reaction mixture causing a vigorous exothermic reaction. After 16 h of stirring the dark green dichloromethane rich layer was removed and dried with sodium sulfate before removal of the dichloromethane on a rotary evaporator. The remaining product was recrystallised from methanol at a yield of 60»80% with respect to pyrene. Approximately 0.05 mol of 1-(5-carbethoxy-1-oxapentanyl)pyrene or 1-(7-carbethoxy-1-oxaheptanyl) pyrene 0.18 mol of potassium hydroxide and 0.17 mol of hydrazine hydrate were mixed with 210 ml of diethylene glycol and re—uxed at 162»178 °C for 4 h.After re—uxing the excess hydrazine hydrate was removed by distillation at about 100 °C. The remaining mixture was cooled to room temperature and 250 ml water was added dropwise with stirring. The mixture was allowed to stir for 12 h. We found difficulties in obtaining a pure precipitate of the pyrenyl fatty acids. In the case of the 6-(pyren-1-yl)hexanoic acid we ground the product mixture and stirred with several washings of acetone to dissolve it. The addition of hexane was then used to make the acid precipitate. Mass spectroscopy infra-red spectroscopy elemental analysis and TLC indicated that although not pure the sample was predominantly 6-(pyren-1-yl)hexanoic acid.In the case of the 8-(pyren-1-yl)- octanoic acid the potassium salt of the acid had already precipitated out of the reaction mixture. The salt was –nely ground and vigorously stirred with a solution of hydrochloric acid and water for two days resulting in a beige solid which was mostly 8-(pyren-1-yl)octanoic acid. In this case the precipitate was additionally analysed by NMR and though not pure was apparently predominantly composed of 8-(pyren-1-yl)octanoic acid. It was assumed that the 6-(pyren-1-yl)hexanoic acid was 66% pure and was treated with 1,1@-carbonyldiimidazole in dry chloroform for 26 h at 30 °C. After concentrating the solution it 45 Faraday Discuss. 1998 111 41»53 was used to acylate glycerophosphocholine vacuum dried onto a viscose support material at an assumed molar ratio of 6-(pyren-1-yl)hexanoylimidazole glycerophosphocholine of 4 1.The acylation was carried out with stirring at 40 °C for 93 h. The dark orange solution was then chromatographed on Davisil (a silicilic acid) eluting with binary and ternary chloroform methanol water solvent systems. Nitrogen bubblers were used in the receivers. The products were monitored and detected by TLC in chloroform methanol water 69 27 4. The disubstituted phosphatidylcholine ran to R 0.35 the monosubstituted phosphatidylcholine ran to R 0.16. The 6dipyPC F F was not completely pure by TLC with –ve spots seen on the plate which could not be separated by the column. A small amount of the mono-acylated product was also produced.With the assumption that the 8-(pyren-1-yl)octanoic acid was 80% pure the same synthesis was F used to produce 8dipyPC. The reaction mixture was chromatographed as before showing a major spot at R 0.31 due to 8dipyPC and a small contaminant running just ahead. A small amount of F the monosubstituted product was also formed with a R of about 0.11. To purify these products we used preparative HPLC. A solvent system of chloroform methanol water (60 40 7) was used with a silica column (250 mm]9.6 mm). The absorption detector was set to 345 nm and the —ow rate used was 1 cm3 min~1. Approximately 0.2 mg of crude product was put on to the column. 8dipyPC showed a single peak after treatment (purer than the 4 and 10dipyPC) but 6dipyPC showed a secondary contaminant peak.Fractions of each of the peaks were taken and examined under UV light for —uorescence. Only the major fraction exhibited —uorescence and fast atom bombardment mass spectroscopy con–rmed that this was 6dipyPC. In subsequent experiments we determined that the contamination was caused by exposure to the UV light during HPLC. We were able to obtain the same eÜects in all of the dipyPCs but the 6dipyPC appeared to be peculiarly sensitive to such degradation. Other work with pyrene probes suggests that the use of HPLC grade chloroform stabilised with amylenes causes degradation in the presence of light but that reagent grade chloroform does not have the same eÜect.30 The 6dipyPC used for all subsequent work was that obtained after puri–cation by HPLC which we estimate to be better than 90% purity.More importantly perhaps the —uorescence arises entirely from 6dipyPC. All samples were dissolved in reagent grade chloroform stored at [23 °C in the dark and handled in darkroom lighting conditions to avoid light induced sample degradation. Sample preparation and instrumentation Samples were prepared by freeze drying the individual components i.e. DOPC DOPE and dipyPC. With the assumption that each component was in fact dihydrate the requisite masses of DOPC and DOPE were then weighed under a dry nitrogen atmosphere and co-dissolved in cyclohexane. The dipyPC was also dissolved in cyclohexane at a known concentration and a sufficient quantity added such that the –nal concentration of dipyPC to total lipid was 0.1 mol%.In agreement with previous work,20v26,28 we found that at this concentration virtually no intermolecular excimers form. The resulting mixture was then freeze dried and mixed with an excess of fresh triply distilled deionised water. We did not use de-oxygenated water since separate experiments indicated that this did not have a detectable eÜect on the pyrene —uorescence. The mixture was thoroughly homogenised by centrifuging it up and down in a sealed tube. A small amount of sample was then scooped out and placed between two 1 mm thick quartz glass slides separated by a 100 lm thick silicone rubber gasket and sheared until the sample appeared clear and exhibited little if any birefringent textures. The slide was then sealed up along its edges with a high temperature adhesive tape and the —uorescence measurements taken.The silicone rubber gave no —uorescence signal and was not hygroscopic. These samples were robust giving identical results after 24 h and being heatable to 80 °C. In our initial experiments we did not shear the sample to surface align the liquid crystalline phase. We found that as a consequence the intensity of the excimer and monomer —uorescence varied widely with time. We assume that this occurred because defect structures created during homogenisation were being slowly annealed and that the lateral pressures in such highly curved regions were quite diÜerent from —at ones. By aligning the mesophase the number of defects was lowered to levels Faraday Discuss. 1998 111 41»53 46 where we were unable to detect any variation in the intensity of the excimer and monomer —uorescence over a period of 1 h.Most previous studies of dipyPC —uorescence in amphiphilic bilayers have been done with vesicles. These are convenient since they can be made into clear solutions but variations in the vesicle diameter osmotically induced tension31 and fusion events can all aÜect the lateral pressure. For the —uorescence measurements reported here we used a SPEX Fluoromax photon counting —uorimeter running DM3000FL software (Spex Industries Inc. Edison New Jersey USA). Excitation was at 345 nm and the emission spectrum was recorded between 370 and 600 nm in 0.5 nm increments integrating at each wavelength for 0.1 s (the excitation and emission bandwidth were both 0.4 nm).All spectra were corrected for variations in detector response with wavelength. The —at samples were held in a temperature controlled cell at an angle of 22° to the incoming excitation beam. This ensured that virtually no scattered excitation light entered the emission monochromator. Sample temperature during these measurements was controlled to within ^0.03 °C by a thermoelectric heater of our own design. A drift in temperature of 0.1 °C resulted in a 2% change in excimer and monomer —uorescence intensities which were easily detected. In Fig. 4 we show a typical example of the emission spectra from dipyPC in DOPC/DOPE bilayers. There are two sharp and intense monomeric signals at 377 and 397 nm whilst the broad excimer emission peaks at 477 nm.By converting the spectra to energy scales it is possible to –t the excimer peak to a Gaussian curve and thence determine the ratio of the excimer to monomer signal by integrating the remaining monomer signal and the extracted excimer signal. However we found no advantage in doing this over measuring the intensity of the 377 nm peak and the excimer peak intensity. We did not use the 397 nm peak because our signal deconvolution indicated that there was signi–cant overlap between it and the excimer emission. No matter which technique we used we found that sample to sample reproducibility in the measurement of g was ^5%. Results In Fig. 5 we present our measurements of g at 25 °C in DOPC/DOPE membranes in the presence of excess water. Up to 84 mol% DOPE in the binary lipid mixture is in the L phase.Over a a period of several days at room temperature the system will convert to an inverse bicontinuous Fig. 4 Fluorescence emission spectrum from 4dipyPC in the L phase excited at 345 nm. The sharp peaks in a the UV end of the spectrum are due to monomeric pyrene emission whereas the broad peak in the visible region is from pyrene excimers. 47 Faraday Discuss. 1998 111 41»53 Fig. 5 Variation in g at 25 °C as a function of DOPC/DOPE composition in the L phase in excess water. a The excimer to monomer ratio has been measured using 4- 6- 8- and 10dipyPC (Ö ),K L and respectively). Linear –ts to the data are shown and the coefficients are listed in Table 1. cubic phase in the region of the phase boundary.32 Independent checks with polarising microscopy and small angle X-ray diÜraction indicated that this had not occurred for our samples in the region of the phase boundary.We observe both increasing and decreasing signals in the lamellar phase as we increase the amount of DOPE. We have –tted the variation in g to linear functions which are shown in Fig. 5 and reported in Table 1. Cheng and co-workers have also measured g in 4- and 10dipyPC in DOPC/DOPE membranes at 20 and 30 °C.20 In their measurements they used the intensities at 392 and 475 nm to determine g. These are peculiar values since 392 nm does not correspond to a maxima but sits on the rising edge of the second most intense monomer peak. Analysing our results at the same wavelengths we obtain good agreement as far as the rate of change of g with respect to composition is concerned but the absolute values of g disagree by approximately 6%.In fact there is a considerable degree of variation between diÜerent groups in the absolute value of g measured. We have found that the ratio is aÜected by the sample preparation used. For example we were able to achieve as much as a factor of 2 diÜerence in the value of g depending on whether Table 1 Fitting coefficients for the variation of g as a function of DOPC/DOPE composition at 25 °C in the L phase in excess water (see Fig. 5)a a Intercept dipyPC chainlength Gradient/10~3 4 6 8 10 [1.2^0.2 [0.55^0.15 0.5^0.2 1.3^0.1 1.143^0.009 0.788^0.008 0.59^0.01 0.693^0.008 a The data were –tted using the Marquardt» Levenberg algorithm implemented on Kaleidagraph.Faraday Discuss. 1998 111 41»53 48 we aligned our samples between the glass slides or not. Consistent sample preparation methods are therefore critical to obtaining a self consistent set of data. Returning to Fig. 5 one might at –rst glance expect the lateral pressures and hence g to rise at all depths as we increase the monolayer torque tension by adding DOPE. In fact if we sum g at all dipyPC chain lengths we do –nd this Fig. 6. What our measurements indicate is that whilst g is dropping for the 4- and 6dipyPC probes it is rising more steeply for 8- and 10dipyPC. Since we know that DOPE increases the torque tension in DOPC/DOPE bilayers by reducing the average molecular cross-sectional area (72 Aé 2 for DOPC (see Fig.7) and 64.5 Aé 2 at 75 mol% DOPE33) we should similarly see a drop in g for 4dipyPC if we dehydrate DOPC bilayers since this also reduces the average molecular cross-section and hence the net lateral pressure. The results of such an experiment are shown in Fig. 7 where we observe a halving in g for a reduction in molecular area from 74 to 68 because these measurements were made directly on the X-ray capillary samples used to determine molecular dimensions which are not surface aligned.) Aé 2. (The diÜerences in the absolute values of g compared to Fig. 5 have arisen These measurements therefore indicate that in DOPC/DOPE bilayers isothermal increases in torque tension involve not only a net increase in the lateral pressure but also a transfer of pressure towards the chain ends.In Fig. 8 we have sequentially plotted the pro–les of g calculated from our –t to the data in Fig. 5 at 0 40 and 80 mol% DOPE. The transfer of pressure to chain ends is clear and our results indicate that this occurs across a minimum in g. If one assumes that the dipyPC and the DOPC/ DOPE chains contract by the same proportion upon chain melting then this minimum is in the neighbourhood of the carbon double bond (the pyrene moieties span the C9»C15 positions of DOPC and DOPE when they are in the all-trans conformation). Such a dip in the lateral pressure has not been predicted in the case of saturated chains but recent calculations for unsaturated chains show a dip in the order parameter at the position of the double bond.34 This lends credence to the idea that the dip in the pro–le of g is due to a dip in the lateral pressure.We have measured g in the inverse hexagonal phase as a means of providing us with further evidence that we are recording changes in the lateral pressure pro–le. We would expect that the lateral pressure and hence g would drop at all points between the pivotal surface and the chain ends. Using Chen and Randœs data on the pivotal surface in DOPE27 and Reiss-Husson and Luzzatiœs data for the Fig. 6 Variation in the total excimer to monomer signal gnet at 25 °C as a function of DOPC/DOPE composition in excess water. The total excimer to monomer signal is found by summing the individual values of g from 4- 6- 8- and 10dipyPC. 49 Faraday Discuss.1998 111 41»53 Fig. 7 Changes in unit cell dimensions (L) ( and g K) measured with 4dipyPC in DOPC as a function of water composition. Using Luzzatiœs approach36 we have determined the excess water volume fraction 0.42 and hence the average molecular cross-sectional area in excess water 72 Aé 2 for a molecular volume of DOPC of 1292 Aé 3.27 The samples contained 0.1 mol% 4dipyPC but this area is in agreement with measurements on pure DOPC by Gruner and co-workers.13 The same samples were used within two days of being made to measure g. At the lowest water composition we calculate that the molecular area has shrunk to 68 Aé 2 and g has declined by a factor of 2. volume of hydrocarbon moieties in —uid chains,35 we calculate that the pivotal surface for DOPE is located between C1 and C2 position on the hydrocarbon chain.Hence g should drop for all of our probes upon entering the inverse hexagonal phase and indeed this is what we –nd with g being 0.94 0.62 0.53 and 0.73 for 4- 6- 8- and 10dipyPC in DOPE at 25 °C. Furthermore the Fig. 8 ììLateral pressure pro–leœœ in the excess water L phase for three DOPC/DOPE compositions at 25 °C. a The excimer to monomer ratio for 4- 6- 8- and 10dipyPC are represented by bars of decreasing darkness. The error bars have been determined from the –t to the data in Fig. 5. Faraday Discuss. 1998 111 41»53 50 (Ö ),K L and respectively). The gradients dg/dT for 4- 6- 8- and 10dipyPC are Fig. 9 Response of dipyPC excimer formation to temperature for 80 mol% DOPE. g is plotted for 4- 6- 8- and 10dipyPC 0.051^0.003 0.040^0.001 0.035^0.001 and 0.045^0.003 respectively.calculations of Szleifer and co-workers10,11 show that the lateral pressure is always highest somewhere around the C3 position on the hydrocarbon chain and drops as one moves to the chain terminus. These observations are compelling evidence that although the pro–les of Fig. 8 do not give absolute values of n or z their qualitative appearance is consistent with the lateral pressure distribution of an oleoyl chain. Discussion Although the results we have reported only provide us with a qualitative picture of the lateral pressure pro–le in part of the chain region they tell us a number of interesting things. The –rst of these has already been stated and that is that although the net lateral chain pressure increases as we increase the monolayer torque tension it does so by decreasing the pressure near the head of the chains but increasing the pressure more rapidly beyond the cis double bond.This transfer of lateral pressure away from the region of the pivotal plane increases the torque tension more rapidly than simply increasing the lateral pressure uniformly along the chain. The increase in monolayer torque tension is brought about by the fact that adding DOPE to DOPC results in a reduction in the average molecular cross-sectional area. Clearly if the average area per molecule is reduced so is the exposure of hydrocarbon to water and hence the tension (negative pressure) at the polar/apolar interface will fall.Since the pressure in the chain region has increased this must mean that there has been a negative contribution to the lateral pressure in the headgroup region (the total lateral pressure must sum to zero). From this we deduce that phosphatidylethanolamine headgroups impart a negative pressure that is they exert an attractive force on phosphatidylcholine and -ethanolamine headgroups. We hypothesise that this attractive force is the result of direct hydrogen bond formation between headgroups. The pull between headgroups further increases the desire for the monolayer to bend. In the case of fully saturated chains Szleifer et al.10 –nd a single maximum in the lateral pressure. The position of this maximum relative to the length of the molecule moves towards the pivotal interface as the area is reduced.Since it is unlikely that we are collecting any data on the lateral pressure above the C4 position it is possible that something similar is occurring with the unsaturated oleoyl chains. Furthermore Szleifer and co-workersœ calculations indicate that as the 51 Faraday Discuss. 1998 111 41»53 area is reduced the rate of growth of the lateral pressure is most rapid near the chain ends. This is entirely consistent with our measurements. The feature which does not agree is that there is a drop in lateral pressure ; this is not seen in calculations on saturated chains. We do not believe that the drop we observe is an experimental artefact for example caused by the relatively bulky pyrene moieties having their motion hindered at shorter chainlengths.Given that g is approximately two times greater for 4dipyPC than any of the other probes it would seem it has little difficulty in forming excimers. Furthermore we –nd that g for 4- and 6dipyPC increases rapidly if we raise the temperature Fig. 9. This is in agreement with theoretical expectations that the pressure should scale with temperature as long as the cross-sectional area remains constant (for these small changes in temperature the area changes will not be as great as those imparted by changing the lipid composition). Further the fact that in this case g for 4- and 6dipyPC is seen to increase as we increase the torque tension contradicts the notion that some experimental complication is the cause of the decrease in g for 4- and 6dipyPC when we change the lipid composition.On balance we therefore conclude that the lateral pressure really does drop along some parts of the oleoyl chain as the cross-sectional area per chain is reduced and this remains to be explained theoretically. Conclusions This study has revealed a number of qualitative features of the changes in the lateral pressure pro–le brought about by an isothermal increase in the monolayer torque tension. The results indicate that DOPE drives mixed DOPC/DOPE monolayers into the inverse hexagonal phase by drawing headgroups into closer proximity. This in turn increases the total lateral pressure in the chain region and this is apparently brought about by the transfer of lateral pressure away from the region close to the pivotal surface to the opposite side of the cis double bond.Such subtle re-arrangements of the diÜerential lateral pressure are lost when we use the more familiar curvature elastic parameterisation of the torque tension. It is quite possible that two membranes may have identical torque tensions but distinctly diÜerent lateral pressure pro–les. Furthermore this work implies that the way in which the lateral pressure pro–le in such membranes changes under identical variations in the environmental conditions is likely to be diÜerent. It would be surprising if there were not some membrane proteins which were able to sense such subtle material diÜerences. The possible oversimpli–cation of correlating membrane protein properties with curvature elastic parameters makes the search for a lateral pressure probe worthwhile.However any further advances in our measurement and understanding of the micro-mechanics of the monolayer will require that we obtain quantitative measurements of the lateral pressure. To calibrate the probes is experimentally challenging. The rate of change of g with pressure could presumably be found by measurement upon application of an external pressure. However for this to be meaningful it would be necessary to do this in the L phase since in a solvent excimers can occur with chain a conformations which are simply not possible at an interface. The more vexed question is how one might –nd the value of g at zero lateral pressure. Put another way this would require one to measure g in the limit that the interfacial area per molecule tends to in–nity.Of course one cannot do this and nor could one extrapolate a series of measurements to zero pressure since the lateral pressure has yet to be established. It seems to us that a more realistic alternative is to calibrate measurements of g against a model system for which the lateral pressure has been determined from the statistical mechanics. This might also be used to calibrate the region of space probed by the excimer sensor. In the event that we were able to do such a thing there would still remain concerns about the perturbative eÜect of using a relatively bulky probe moiety such as pyrene on the lateral pressure. In this regard alternative smaller excimer forming moieties such as naphthalene might be better alternatives to pyrene.Finally it should be noted that we estimated a two fold change in the average chain lateral pressure between DOPC and DOPE. At least for the section of the chain that we have examined g changes by no more than a factor of 1.2 suggesting that we are making our measurements in the presence of a sizeable zero pressure excimer signal. It would seem probable that probes such as dipyPC will always suÜer from this. Faraday Discuss. 1998 111 41»53 52 Acknowledgements We would like to thank Dr D. M. Phillips of Lipid Products Ltd. for his help and advice. This work was supported by an IMPEL Fellowship to S.J.C. and Royal Society University Research Fellowships to R.H.T. and D.R.K. References 1 W. Helfrich Z. Naturforsch.1973 28c 693. 2 S. M. Gruner Proc. Natl. Acad. Sci. USA 1985 82 3665. 3 J. M. Seddon Biochim. Biophys. Acta 1990 1031 1. 4 G. Lindblom A. Wieslander M. Sjoelund G. Wikander and A. Wieslander Biochemistry 1986 25 7502. 5 S. L. Keller S. M. Bezrukov S. M. Gruner M. W. Tate I. Vodyanoy and V. A. Parsegian Biophys. J. 1993 65 23. 6 R. M. Epand Chem. Phys. L ipids 1996 81 101. 7 P. J. Booth M. L. Riley S. L. Flitsch R. H. Templer A. Farooq A. R. Curran N. Chadborn and P. Wright Biochemistry 1997 36 197. 8 G. S. Attard R. H. Templer W. S. Smith A. N. Hunt and S. Jackowski Nature 1999 submitted; G. S. Attard W. S. Smith R. H. Templer A. N. Hunt and S. Jackowski Biochem. Soc. T rans. 1998 26 5230. 9 W. Helfrich in Physics of defects ed. R. Balian M. Kleç man and J.P. Poirier North-Holland Amsterdam 1981 p. 715. 10 I. Szleifer A. Benshaul and W. M. Gelbart J. Phys. Chem. 1990 94 5081. 11 I. Szleifer D. Kramer A. Benshaul W. M. Gelbart and S. A. Safran J. Chem. Phys. 1990 92 6800. 12 I. Szleifer D. Kramer A. Benshaul D. Roux and W. M. Gelbart Phys. Rev. L ett. 1988 60 1966. 13 S. M. Gruner M. W. Tate G. L. Kirk P. T. C. So D. C. Turner D. T. Keane C. P. S. Tilcock and P. R. Cullis Biochemistry 1988 27 2853. 14 J. P. Birks Photophysics of aromatic molecules Wiley London 1970. 15 J. B. Birks and L. G. Christophorou Proc. R. Soc. L ondon Ser. A 1963 274 552. 16 J. B. Birks D. J. Dyson and I. H. Munro Proc. R. Soc. L ondon Ser. A 1963 275 575. 17 J. B. Birks and L. G. Christophorou Proc. R. Soc. L ondon Ser. A 1964 277 571.18 J. B. Birks D. J. Dyson and T. A. King Proc. R. Soc. L ondon Ser. A 1964 277 270. 19 J. B. Birks M. D. Lumb and I. H. Munro Proc. R. Soc. L ondon Ser. A 1964 280 289. 20 K. H. Cheng L. Ruymgaart L-I. Liu P. Somerharju and I. P. Sugaç r Biophys. J. 1994 67 914. 21 J. Sunamoto H. Kondo T. Nomura and H. Okamoto J. Am. Chem. Soc. 1980 102 1146. 22 J. Sunamoto T. Nomura and H. Okamoto Bull. Chem. Soc. Jpn. 1980 53 2768. 23 K. H. Cheng S-Y. Chen P. Butko B. W. Van Der Meer and P. Somerharju Biophys. Chem. 1991 39 137. 24 P. Butko and K. H. Cheng Chem. Phys. L ipids 1992 62 39. 25 M. Sassaroli M. Vaukhonen P. Somerharju and S. Scarlata Biophys. J. 1993 64 137. 26 S-Y. Chen K. H. Cheng and B. W. Van Der Meer Biochemistry 1992 31 3759. 27 Z. Chen and R. P. Rand Biophys. J. 1997 73 267. 28 M. Vaukhonen and P. Somerharju Chem. Phys. L ipids 1990 52 207. 29 H. S. Henderson and P. N. Rauk Anal. Biochem. 1981 116 553. 30 G. Rumbles unpublished data. 31 J. Y. A. Lehtonen and P. K. J. Kinnunen Biophys. J. 1981 66 1981. 32 K. H. Madan MSc thesis Imperial College London University 1991. 33 R. P. Rand N. L. Fuller S. M. Gruner and V. A. Parsegian Biochemistry 1990 29 76. 34 A. Ben-Shaul personal communication. 35 F. Reiss-Husson and V. Luzzati J. Phys. Chem. 1964 68 3504. 36 V. Luzzati and F. Husson J. Cell. Biol. 1962 12 207. Paper 8/06472E 53 Faraday Discuss. 1998 111 41»53

ISSN:1359-6640    

DOI:10.1039/a806472e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Interfacial enzyme activation non-lamellar phase formation and membrane fusion. Is there a conducting thread? Feç lix M. Gon8 i,* Gorka Basaç n 8 ez M. Begon8 a Ruiz-Argué ello and Alicia Alonso Grupo Biomembranas (Unidad Asociada al C.S.I.C.) Departamento de Bioquïç mica Universidad del Païç s V asco Aptdo. 644 48080 Bilbao Spain. E-mail gbpgourf=lg.ehu.es Receiøed 11th August 1998 Previous studies from this laboratory have shown that the enzymic generation of diacylglycerol in bilayers by phospholipase C may lead to membrane fusion through the formation of transient non-lamellar lipidic intermediates. The present paper intends to explore the correlations existing among the three main processes involved namely (a) the induction (or inhibition) of lamellar-to-non-lamellar phase transitions in lipid mixtures through the addition of small (\5 mol%) proportions of other lipids (b) the promotion by the latter lipids of fusion in otherwise stable phospholipid vesicles (large unilamellar liposomes) under conditions leading to inverted hexagonal/inverted cubic phase formation in bulk lipid systems and (c) the modulation by the same small proportions of lipids of phospholipase C hydrolysis of phosphatidylcholine in liposome bilayers.It is concluded that phospholipase C may give rise to non-lamellar lipidic structures that in turn permit liposomal fusion to occur but neither enzyme activity is directly modulated by non-lamellar phase formation nor will whatever kind of enzyme-induced non-lamellar structure give rise to fusion.Moreover only under certain kinetic conditions will the enzyme give rise to the organization of non-lamellar structures that are conducive to the fusion event. 1 Introduction The present view of biomembranes as essentially dynamic structures has been very fruitful in the molecular understanding of many cellular processes. One of the most important membrane-based cellular events is membrane fusion. Membrane fusion is relevant in a wide variety of physiologic (organelle biogenesis cell secretion acrosomic reaction) and pathologic (in—uenza HIV viral infections) phenomena that take place in all eukaryotic cells. Although in the cell environment membrane fusion is a carefully regulated process occurring as a result of the concerted action of numerous proteins fusion consists essentially of the merging and reorganization of two lipid bilayers i.e.it is mainly a lipid event although its regulation is protein-dependent. The central role of lipids in a virus-cell membrane event has been put forward recently by Zimmerberg and co-workers.1 A number of years ago our group introduced a model system for the study of membrane fusion namely the fusion of large unilamellar phospholipid vesicles induced by the catalytic action of phospholipase C.2,3 Among its merits are that fusion is induced by a catalytic agent (in previous models fusogens were added at stoichiometric ratios) that the system is simple enough to allow 55 Faraday Discuss. 1998 111 55»68 detailed structural studies of the lipid components that the fusogen is an enzyme thus susceptible to regulation and that phospholipase C has been claimed to be involved in physiological membrane fusion processes.4,5 That the catalytic action of phospholipase C and not the mere presence of the enzyme molecule is responsible for the fusion event became clear from our early studies.2 Further data6,7 have helped to understand the mechanism of this enzyme-induced membrane fusion process.The enzyme appears to play two roles (a) the rapid localized and asymmetric production of diacylglycerol (the end-product of the enzyme reaction on phospholipids) acts as trigger for the fusion event,6,8 (b) the generation of diacylglycerol in sufficient amounts permits the formation of nonlamellar transient structures that are essential intermediates in the fusion process.7 Consequently a strong relationship apparently exists between phospholipase C activity membrane fusion and the generation of non-lamellar structures.This is exempli–ed by the experiments shown in Fig. 1. The –gure combines kinetic (panels A and B) and equilibrium (panel C) data. In Fig. 1A,B large unilamellar vesicles consisting of egg phosphatidylcholine (PC) egg phosphatidylethanolamine (PE) and cholesterol (Ch) at a 2 1 1 molar ratio are treated with phospholipase C. This is the ììcontrolœœ experiment as described by Nieva et al.2 Fig. 1A shows the progress of hydrolytic activity with time and Fig. 1B depicts the parallel (with a few seconds delay) mixing of vesicular aqueous contents measured with water-soluble —uorescent probes.Fig. 1A,B also describes the eÜect of small amounts of additives in the control lipid mixture namely 2 mol% squalene (a linear Fig. 1 Phospholipase C-induced liposomal fusion. (A) Enzyme hydrolytic activity (B) vesicle fusion measured as contents mixing. Curve C control (liposomal composition PC PE Ch 2 1 1 mol ratio) ; curve SQ ]2 mol% squalene; curve LPC ]5 mol% lysophosphatidylcholine. (C) Lamellar-to-non-lamellar phase transition detected as an increase in TMA-DPH —uorescence anisotropy. Curve C control (liposomal composition PC PE Ch diacylglycerol 47 23 25 5 mol ratio) ; curve SQ ]2 mol% squalene; curve LPC ]5 mol% lysophosphatidylcholine. Faraday Discuss. 1998 111 55»68 56 hydrocarbon precursor of cholesterol) or 5 mol% lysophosphatidylcholine (an analogue of PC lacking the sn[2 fatty acid).These additives are seen to have a clear eÜect stimulatory and inhibitory respectively on the enzyme phosphohydrolase activity (panel A) and even more marked on vesicle fusion (panel B). Fig. 1C corresponds to a diÜerent type of experiment whose foundations and methodology have been described in detail elsewhere.9 Essentially the —uorescence polarization of a probe (TMA-DPH) included in a hydrated lipid mixture is measured as a function of temperature. The increase in polarization (anisotropy) under these conditions is related to the transition from a lamellar to a non-lamellar (presumably cubic) phase.7,9 The lipid composition is intended to mimic the ììcontrolœœ mixture of Fig. 1A,B (PC PE Ch 2 1 1) once the enzyme has converted 5% of the phospholipids into diacylglycerol.Previous studies6 have shown that this is the minimum amount of diacylglycerol that allows fusion to occur. Thus the control mixture in Fig. 1C is PC PE Ch diacylglycerol (47 23 25 5 mole ratio). The phase transition occurs at a temperature of ca. 63 °C. When 2 mol% squalene is added to the lipid mixture the lamellar-tonon-lamellar transition is clearly facilitated with a decrease in the midpoint transition temperature of B12 °C. The opposite is found with the enzyme- and fusion-inhibiting lysoPC addition of 5 mol% to the lipid mixture stabilizes the lamellar phase increasing the transition temperature by B6 °C. The eÜects of squalene and lysoPC are dose-dependent (data not shown).Similar eÜects are caused by hexadecane or arachidonic acid among the positive eÜectors and by palmitoylcarnitine or gangliosides among the inhibitors (data not shown). Thus a clear correlation appears to exist between phospholipase C activity vesicle fusion and the formation of non-lamellar structures in this PC PE Ch system. The correctness of this assertion and its applicability to other lipid mixtures will be examined in the sections to follow. In this analysis the three phenomena under study namely interfacial enzyme activity formation of non-lamellar structures and fusion of lipid vesicles will be confronted with each other. 2 Enzyme activity and vesicle fusion This aspect is examined –rst because the correlation between both phenomena is very clear both qualitatively and quantitatively for every enzyme activity-induced fusion system studied to date.Most of the available data correspond to the phospholipase C-induced fusion of PC PE Ch (2 1 1) vesicles described by Nieva et al.2 Fusion occurs as a result of enzyme activity neither heat-inactivated enzyme nor enzyme incubated with the speci–c inhibitor o-phenanthroline can induce fusion. The presence of diacylglycerol per se when this lipid is added to the lipid mixture prior to liposome formation does not allow vesicle aggregation or fusion either the resulting vesicles are stable for days (see ref. 2 and 6 for experimental details). Moreover addition of heatinactivated or o-phenanthroline-treated enzyme to vesicles containing up to 20% diacylglycerol does not lead to fusion either (G.Basaç n8 ez unpublished data). Quantitative variations either positive or negative in enzyme activity lead to parallel increases or decreases in the vesicle fusion rates. Some examples are summarized in Table 1. The potencies of the various additives diÜer considerably. So do their chemical structures. However in all cases an increase or decrease in phospholipase C hydrolytic rate leads to a corresponding increase or decrease in vesicle fusion rate. The changes in both phenomena are not of the same order of magnitude small changes either positive or negative in enzyme activity are ampli–ed when vesicle fusion is measured for reasons that will be discussed in the next section. All eÜects described in Table 1 are dose-dependent (data not shown).Enzyme activity has also been modi–ed by other procedures such as changes in enzyme concentration temperature or speci–c inhibitors. Lowering the temperature from 37 to 25 °C or decreasing the enzyme concentration below 0.16 U ml~1 (the standard concentration in our experiments) diminish considerably the rate of diacylglycerol production and correspondingly the rate of vesicle contents mixing (fusion).8,11 In spite of this very good correlation between enzyme activity and fusion special situations exist at very low and very high enzyme activities. The –rst description of phospholipase C-induced liposomal fusion2 included an experiment in which fusion was measured as a function of enzyme concentration (rate of phospholipid hydrolysis).An optimum enzyme concentration was found for 57 Faraday Discuss. 1998 111 55»68 Table 1 Parallel changes in phospholipase C hydrolytic activity and rate of phospholipase C-induced liposomes fusion (content mixing) as a result of small changes in vesicle lipid compositiona reference fusion rate hydrolysis rate % additive Lipid composition 100 32 100 21 1 4 4 \1 10 215 108 125 80 10 2 10 11 11 11 12 271 1 122 101 119 88 1 11 1 11 232 22 18 87 01 1 1 2 2 5 3 5 5 5 control ]GM3 ganglioside ]GM1 ganglioside ]GT1b ganglioside ]hexadecane ]squalene ]PEG-PE ]arachidic acid ]arachidonic acid ]lysoPC ]palmitoylcarnitine a Data are expressed as percentages with respect to a control lipid mixture consisting of PC PE Ch (2 1 1 mole ratio).the vesicle concentration used in those measurements while both above and below certain values fusion was virtually abolished. The lack of vesicle contents mixing at high enzyme concentrations was explained years later,7 when it was found that the bicontinuous cubic structure that would allow intervesicular mixing of aqueous contents could only be formed within certain limits of diacylglycerol concentration namely between B5 and 20 mol% at 37 °C.6 Beyond a certain enzyme activity the upper limit of diacylglycerol concentration was reached before any signi–cant fusion could be detected. The reason why low enzyme activities never lead to vesicle fusion even after incubation times that allow the formation of appropriate concentrations of diacylglycerol (i.e.between 5 and 20%) is of a kinetic nature. The phenomenon is clearly shown in Fig. 2. In which phospholipase C activity and vesicle fusion are measured in the presence of increasing concentration of the speci–c enzyme inhibitor o-phenanthroline. Both phenomena decrease notoriously in the presence of inhibitor. However they do not change in parallel as soon as the enzyme activity decreases below 25% of the native value fusion is virtually abolished. Our interpretation of this phenomenon is that as stated in the Introduction one of the essential roles of the enzyme (perhaps the most Fig. 2 Percent inhibition by o-phenanthroline of phospholipase C activity (Ö) and of liposomal fusion measured as contents mixing (L).Faraday Discuss. 1998 111 55»68 58 essential one) is to act as a trigger for the fusion process. The enzyme triggers fusion by producing a high local diacylglycerol concentration asymmetrically (the enzyme is present only on one side of the bilayer) and in a short time. The latter point is essential to overcome the spontaneous diÜusion of diacylglycerol in the membrane that will act against the formation of a diacylglycerolrich patch in turn a hot point for vesicle aggregation.13 Low enzyme concentration leads naturally to low rates of diacylglycerol production that cannot compete with the surface dilution rates of the lipid.14 A good relationship between enzyme concentration and fusion rate has been observed by Ruiz- Argué ello et al.8 in a diÜerent system namely the fusion of PC PE Ch sphingomyelin (1 1 1 1 mol ratio) vesicles in the presence of both phospholipase C and sphingomyelinase.The fusion rates increase linearly when the enzyme concentrations vary from 0.4 U ml~1 each to 1.6 U ml~1 each. In summary in systems showing enzyme-induced vesicle fusion the rate of fusion increases with enzyme activity within a certain range of activities beyond which either kinetic or thermodynamic reasons prevent the formation of the structural intermediates that are required for fusion to occur. In other words certain phosphohydrolases catalyze vesicle fusion if and when they allow the formation of fusion structural intermediates.The nature of these intermediates and their relationship to the fusion event are discussed in the section that follows. 3 Non-lamellar phase formation and vesicle fusion The earliest indications of the existence of a ìì structural intermediateœœ in phospholipase C-induced liposomal fusion arose from the observations by Nieva et al.6 of a precise range of diacylglycerol concentrations outside of which no fusion was detected. It was hypothesized that an intermediate of a given lipid composition was involved at least transiently in the fusion event. This idea received considerable experimental support from our 31P-NMR and X-ray diÜraction studies,7 and was further reinforced by its accommodation within the so-called ìì stalk hypothesisœœ of membrane fusion.15,16 The stalk is proposed to be a semitoroidal lipidic structure having a negative curvature (the convention is followed that the curvature of a monolayer in the inverted hexagonal H phase is negative) that would allow the merger of the closest (cis) lea—ets of apposed mem- II branes.17 Moreover the transient formation of non-bilayer structural intermediates is an unavoidable requirement of membrane fusion.It is also an essential tenet of the stalk hypothesis since the stalk itself is a non-bilayer structure in which the monolayers have a negative curvature such as seen in inverted lipid phases H hexagonal or Q cubic. ììNon-lamellarœœ has been equated in practice to II II ììreversed hexagonalœœ in the context of membrane fusion,16,18 although isotropic 31P NMR signals which may be compatible with among others inverted cubic phases have also been associated with fusion intermediates.19h22 Siegel and Epand23 have recently suggested that TMC (trans-monolayer contacts) intermediates play a role in lamellar-to-non-lamellar phase transitions and that they can either rupture to form fusion pores that modulate transitions to Q inverted II cubic phases or assemble into bundles of H inverted hexagonal phase tubes.Nieva et al.7 showed II a direct correlation between bilayer compositions and temperature giving optimum fusion and those leading to the formation of an ìì isotropic œœ component which was identi–ed with a bicontinuous inverted cubic phase Q224 by X-ray diÜraction (Fig. 3). Both the stalk and the pore as predicted by the modi–ed stalk theory have geometries that can be related to that of the Q224 phase.In our previous studies of fusion inhibition by positive-curvature lipids ganglioside and poly-(ethylene glycol)-modi–ed PE,10,12 a good correlation has been shown between the inhibitory eÜects of those lipids and the increased temperatures in the corresponding lamellar-to-non-lamellar transitions. This point has also been explored using a —uorescence polarization technique.9 The eÜects of a variety of single-chain lipids on the lamellar-to-non-lamellar (isotropic Q224) phase transition of a PC PE Ch diacylglycerol (50 25 25 5 mol ratio) mixture have been studied by —uorescence polarization (Fig. 1C) and 31P NMR.11 A very good correlation is observed between the modi–cation of phase transition temperature and fusion activity.Squalene and arachidonic acid which were found to enhance lipid and content mixing are seen to facilitate the lamellar»isotropic transition and the opposite occurs with the positive-curvature lipid lysoPC. Arachidic acid was virtually neutral both with respect to fusion and with respect to phase transition. These results are in obvious agreement with the stalk model. Faraday Discuss. 1998 111 55»68 59 Fig. 3 Relationship between rates of vesicle aggregation rates of vesicle fusion (content mixing) and lamellarto-non-lamellar transitions. The maximum rates of phospholipase C-induced vesicle aggregation (Ö) and fusion (L) are plotted as a function of temperature. Aggregation and lipid mixing change in parallel in this system.The vesicle composition was PC PE Ch (2 1 1 mole ratio). The vertical lines correspond to the (Ton) ( and completion T temperatures of the onset —uid lamellar to the Q224 bicontinuous cubic phase co) L a transition of a PC PE Ch diacylglycerol (47 23 25 5) mixture. Data are redrawn from Nieva et al.6,7 Previously published data on phospholipase C-induced liposomal fusion can be reinterpreted in the light of the predictions of the modi–ed stalk theory. Siegel16 suggests that when the lipid in the bilayers is very close to the Th H lamellar-to-hexagonal transition temperature stalks may form II phase precursors and any TMCs that form should have a tendency to radially expand decreasing the driving force for fusion pore formation.However the expanded TMC would make a large comparatively stable lipid connection between opposed bilayers which would promote extensive lipid mixing. It has been observed that the content mixing rate18,24h26 often increases with tem- Fig. 4 A pseudo-phase diagram of PC PE Ch DG in excess water constructed from 31P-NMR data. L lamellar ; H hexagonal; I isotropic. In parentheses the nature of the cubic phases as identi–ed by the X-ray scattering experiments (sample concentration 50% w/w). The shaded area corresponds to the region of temperature and composition at which optimum liposome fusion induced by phospholipase C is observed. Taken from Nieva et al.7 Faraday Discuss. 1998 111 55»68 60 perature and goes through a maximum at T BTh ,18,24h26 decreasing thereafter while the lipid mixing rate increases monotonically.Combining our data on phospholipase-induced fusion as a function of temperature6 with those on the phase behaviour of our lipid mixtures,7 we can show in Fig. 4 the correlation that is found in our case for fusion rates and transition temperatures. The –gure includes data of vesicle aggregation rates as a function of temperature (under those conditions aggregation and lipid mixing always go in parallel) data of fusion (content mixing) rates as a function of temperature and two vertical lines marked T on and Tco respectively corresponding to the onset and completion temperatures of the lamellar»Q224 cubic transition of a PC PE Ch diacylglycerol (47 23 25 5) mixture (Fig. 3). This mixture was selected because 5 mol% diacylglycerol is the minimum amount of this lipid that allows fusion to occur.6 The mixture does not undergo a direct lamellar»cubic transition but instead lamellar hexagonal and T cubic phases appear to coexist between Tco .7 Fusion and phase behaviour studies are not and on strictly comparable since they consist of kinetic and equilibrium measurements respectively.Still it can be seen in Fig. 4 that vesicle aggregation increases monotonically with temperature while content mixing has a maximum in the temperature region corresponding to the lamellar-to-nonlamellar (in our case cubic) transition in agreement with the above-mentioned predictions and observations. Thus the structural ììfusion-intermediateœœ whose existence was predicted from our kinetic studies6 corresponds probably to the stalk»TMC»pore.4 Enzyme activity and non-lamellar phase formation After having discussed the strict cause»eÜect relationships existing between enzyme activity and fusion and between non-lamellar phase formation and fusion we shall deal with the third side of the triangle namely the connection between enzyme activity and the formation of non-lamellar phases. One aspect of this problem i.e. whether enzyme activity is responsible for the lamellar-tonon-lamellar transitions is rather straightforward. In the phospholipase C-induced vesicle fusion system,3 as well as in the sphingomyelinase-based systems,8,27 it is obvious that the enzymes are instrumental in modifying the chemical composition of the bilayer so that new equilibrium conditions settle in and a phase transition ensues.For example at 37 °C the equilibrium phase structure of PC PE Ch (50 25 25 mol ratio) in excess water is lamellar but when 10% of the phospholipid has been converted into diacylglycerol and the new composition is PC PE Ch diacylglycerol (43 22 25 10) then the predominant phase structure is nonlamellar (HII]QII).7 It is thus clear that in these systems the non-lamellar phases appear precisely as a result of the enzyme activity. However what about the reverse question ? Do lamellar-to-non-lamellar transitions somehow modify the activity of interfacial enzymes such as phospholipase C? The answer to this is less clear and will require the analysis of certain peculiarities of phospholipase C.Phospholipase C like many other lipases is a ììsolubleœœ (i.e. non-membrane-bound) enzyme whose substrate is in the solid phase. This is at variance with most enzyme reactions in which the substrate is in aqueous solution. Also at variance with classical or soluble enzyme kinetics maximum enzyme rates are not the initial enzyme rates. They do not occur as soon as enzyme and substrate are mixed but rather after a latency period or lag time.28 The origin of the lag time in the phospholipase C reaction has been examined by us recently.13,29 The enzyme starts its catalytic work as soon as it meets the substrate only it does it at a very low rate. The low-rate regime continues until a given proportion of diacylglycerol (B10 mol% when the substrate is pure PC)13 is formed.Then a ììburstœœ of activity occurs accompanied by vesicle aggregation.13,28 Considering that diacylglycerol is notorious for its ability to induce negative curvature in monolayers thus facilitating lamellar-to-non-lamellar phase transitions,21,24 it is tempting to assume that a relationship exists between non-lamellar phases and enzyme activation. Indeed this has been proposed by some authors.30 At –rst sight however our experimental data do not support this hypothesis 31P-NMR studies of mixtures containing egg PC and egg diacylglycerol (80 20 mol ratio) that allow full phospholipase C activity show spectra compatible with purely lamellar structures in the 20»50 °C temperature range (G. Basaç n8 ez unpublished data). In a more recent series of studies,28 phospholipase C activity was tested against an extensive series of binary mixtures of egg PC with either PE Ch or SM.Of these lipids PE and Ch are known to facilitate non-lamellar phase formation while SM is a stabilizer of the lamellar phase. It was found that in Faraday Discuss. 1998 111 55»68 61 Fig. 5 The eÜect of small amounts of other mostly non-substrate lipids on phospholipase C hydrolysis of egg PC in the form of large unilamellar vesicles. The time course of the reaction is followed as an increase in turbidity (absorbance at 405 nm). (A) Linear single-chain lipids. C control (pure egg PC); SQ ]2 mol% squalene; Al ]2 mol% arachidonic acid ; AS ]2 mol% arachidic acid ; LPC ]2 mol% lysophosphatidylcholine ; PCAR ]2 mol% palmitoylcarnitine.(B) More complex lipids. PE ]2 mol% phosphatidylethanolamine ; Ch ]2 mol% cholesterol ; SM ]2 mol% egg sphingomyelin. Faraday Discuss. 1998 111 55»68 62 Fig. 6 Lag times of phospholipase C hydrolysis of egg PC to which small amounts of other lipids have been added. Data are taken from experiments of the kind shown in Fig. 5. (A) and (B) are as in Fig. 5. all cases the burst of enzyme activity required the formation of the same amounts of diacylglycerol (B7»10%) while the three systems have a totally diÜerent phase behaviour PC PE becoming isotropic (probably inverted cubic) with B10% diacylglycerol while PC SM mixtures remain purely lamellar even with 20% diacylglycerol and PC Ch represent an intermediate situation. 28 This is also against an involvement of non-lamellar phase formation in phospholipase C activation.An additional series of experiments have been carried out in which the substrate egg PC has been doped with small amounts (less than 5 mol%) of lipids that are not enzyme substrates. These are SM known to preserve the lamellar structure lysoPC and palmitoylcarnitine (PCAR) that induce a positive curvature in the monolayers thus opposing the formation of inverted phases 63 Faraday Discuss. 1998 111 55»68 cholesterol (Ch) squalene (SQ) and arachidonic acid (Al) that favour in several ways the transition to non-lamellar phases and arachidic acid (AS) that appears to be neutral in this respect. PE a substrate for the enzyme and a negative-curvature inducer has also been included for comparison.All the mixtures under study are perfectly lamellar at 37 °C according to 31P-NMR measurements (not shown). The results of phospholipase C activity on these samples are shown in Fig. 5»7. More speci–cally Fig. 5 shows turbidity curves of the enzyme activity. Phospholipase C phosphohydrolase activity and vesicle suspension turbidity always change in parallel.13,28 The eÜects of single-chain lipids can be seen in Fig. 5A. Squalene and arachidonic acid lipids with a Fig. 7 Rates of phospholipase C hydrolysis of egg PC to which small amounts of other lipids have been added. Data are taken from experiments of the kind shown in Fig. 5. (A) and (B) are as in Fig. 5. Faraday Discuss. 1998 111 55»68 64 tendency to favour formation of non-lamellar phases decrease the lag time and increase the maximum rate of phospholipase C.LysoPC and palmitoylcarnitine stabilizers of the lamellar phase inhibit the enzyme activity and increase the lag times. Arachidic acid that has little or no eÜect on the monolayer curvature is also inactive on the enzyme. The eÜects of more complex lipids SM PE and Ch are shown in Fig. 5B. SM that stabilizes the lamellar phase in lamellar-tonon-lamellar transitions increases notoriously the lag time. Interestingly PE and Ch that favour the formation of inverted non-lamellar phases have little eÜect (PE) or even increase the lag time (Ch). All these eÜects are dose-dependent (Fig. 6 7) with the exception of SM in which case the dose-eÜect relation is more complex for reasons that remain as yet obscure.Also unexpected is the eÜect of cholesterol that increases both the lag times (Fig. 6B) and the enzyme rates (Fig. 7B) of phospholipase C activity. At concentrations above 10 mol% cholesterol decreases the lag time in agreement with its role as an enzyme activator.29 The behaviour at low concentrations described in Fig. 5»7 may be related to the complexities of the PC»Ch phase diagram and requires a more detailed investigation. It should be stressed that all the mixtures in Fig. 5»7 are lamellar at 37 °C also when 10 mol% diacylglycerol is substituted for PC to include the eÜect of phospholipase C activity during the latency period. Again this is not in favour of an association between enzyme activation and nonlamellar phase formation.It could be suggested that the transition from lag to burst would be linked to a particular form of enzyme docking to the membrane through a non-lamellar lipidic stem. This possibility would be difficult to rule out experimentally considering that the proportion of lipid involved in the putative stem could be too low to be detected by 31P-NMR. However because inverted non-lamellar phases are favoured by high temperatures the hypothetical stem would be more easily formed at these higher temperatures and as a consequence less diacylglycerol would have to be synthesized during the lag phase. However phospholipase C assays carried out between 25 and 55 °C show that although the lag times decrease clearly with temperature the proportion of diacylglycerol generated at the end of the latency period remains constant at B10 mol%.13 This observation is clearly against the involvement of non-lamellar phases in the process of interfacial enzyme activation.Non-lamellar structures could also be related to the post-burst enzyme activity causing an increase in enzyme rates. This possibility was explored by recording 31P-NMR spectra of aqueous dispersions of ten lipid compositions namely pure PC PC Ch (88 12) PC PE Ch (1 2 1) PC PE Ch (1 1 1) and PC PE Ch (2 1 1) plus mixtures derived from these –ve by substituting part of the phospholipid for diacylglycerol in the precise proportion that marks the end of the lag period for each mixture. The –ve mixtures not containing diacylglycerol gave oÜ purely lamellar 31P-NMR signals (not shown).The 31P-NMR spectra of the –ve diacylglycerolcontaining samples are shown in Fig. 8 together with the post-burst enzyme rates observed in each case. The mixtures are ordered from top to bottom in order of increasing maximal enzyme activities. The phase behaviour does not follow any pattern that can be accommodated to that of the enzyme rates non-lamellar structures are clearly seen only in a mixture (originally PC PE Ch 1 2 1) that allows an intermediate enzyme rate. Both the higher and lower enzyme activities occur with the substrate lipids in the lamellar phase. The conclusion of these experiments is that the presence or absence of non-lamellar phases and the rates of phospholipase C activity while being both very sensitive to lipid composition are unrelated phenomena.What is then the explanation for the repeatedly observed phenomenon of the activation of phospholipase C by lipids that favour non-lamellar phase formation and conversely its inhibition by lamellar lipids as seen in Fig. 1 5»7 Table 1 and ref. 10»12? Kinnunen31 has recently suggested a hypothesis partly related to previous proposals,32,33 according to which a number of enzymes that interact with membranes as peripheral proteins would be activated by a certain propensity of lipid bilayers to adopt the inverted hexagonal disposition while remaining in the lamellar phase. Such propensity would be given by the presence of non-bilayer lipids in the membrane that would induce a frustrated lamellar state. The results in the present paper can certainly be interpreted in the light of this hypothesis.Thus PLC would join in a large group of enzymes reviewed in ref. 31 whose activities are enhanced by the presence of non-bilayer lipids in essentially lamellar systems. This hypothesis appears to be physiologically relevant since it allows the possibility of enzyme regulation in cell membranes without loss of the bilayer structure or its barrier properties. 65 Faraday Discuss. 1998 111 55»68 Fig. 8 31P-NMR spectra of aqueous lipid dispersions representing bilayer compositions at the start of the phospholipase C activity burst. The corresponding post-burst enzyme rates are also indicated for each composition. Line broadening 80 Hz. 5 Conclusions (a) When phospholipid-based bilayers are treated with phospholipase C enzyme-induced generation of diacylglycerol leads ultimately to a phase transition into one or more inverted nonlamellar phases.This occurs under virtually any experimental conditions at a rate that depends essentially on the enzyme activity. (b) At least when its substrate is in the form of a low-curvature bilayer phospholipase C activity can be modulated even by small (\5 mol%) amounts of lipids with the capacity to modulate monolayer curvature. Lipids that favour negative monolayer curvature will stimulate the enzyme activity and decrease the latency times. The opposite eÜects are caused by lipids that increase the positive curvature of a monolayer. These eÜects are unrelated to the formation of non-lamellar phases by the lipid.(c) When the bilayer lipid composition is such that the appropriate non-lamellar phases may be formed and the rate of generation of diacylglycerol in the bilayer is fast enough to allow the formation of diacylglycerol-rich patches phospholipase C activity leads to vesicle aggregation and fusion the latter event being mediated by a non-lamellar lipid structure. Faraday Discuss. 1998 111 55»68 66 (d) In summary phospholipase C may give rise to non-lamellar lipidic structures that in turn permit liposomal fusion to occur but neither enzyme activity is directly modulated by nonlamellar phase formation nor will whatever kind of enzyme-induced non-lamellar structure give rise to fusion both kinetic and thermodynamic parameters playing their roles in de–ning the –nal outcome of the process.6 Experimental Phospholipase C (EC 3.1.4.3) from Bacillus cereus was supplied by Boehringer»Mannheim. Egg phosphatidylcholine egg lysophosphatidylcholine egg phosphatidylethanolamine and 1,2-diacylglycerol obtained by phospholipase C hydrolysis of egg PC were grade 1 from Lipid Products (South Nut–eld UK). Egg sphingomyelin was purchased from Avanti Polar Lipids Inc. (Alabaster AL). Cholesterol free fatty acids hexadecane squalene and palmitoylcarnitine were from Sigma (St. Louis MO). 8-Aminonaphthalene-1,3,6-sulfonate (ANTS) and p-xylenebis(pyridinium bromide) (DPX) were purchased from Molecular Probes (Eugene OR). Lipid dispersions were prepared by rehydrating lipid –lms dried from organic solvents under high vacuum.Large unilamellar vesicles (LUV) were prepared by the extrusion method34 using Nuclepore –lters of 0.1 lm pore diameter at room temperature. Enzyme assays were carried out in 100 mM NaCl 10 mM CaCl2 10 mM Hepes pH 7.0. Assays were performed at 37 °C and with continuous stirring. Lipid concentration was 0.3 mM and enzyme was used at 0.16 U ml~1 unless otherwise stated. Phospholipase C was assayed by measuring phosphorus contents35 in the aqueous phase of an extraction mixture (chloroform methanol 2 1) after addition of aliquots from the reaction mixture at diÜerent times. Enzyme activity was also monitored through changes in the vesicle suspension turbidity (absorbance at 500 nm) in a Cary 3 Bio Varian UV-vis spectrophotometer. Mixing of aqueous vesicle contents was estimated using the ANTS/DPX —uorescent probe system.25 31P-NMR spectra were recorded in a KM300 Varian spectrometer operating at 121.4 MHz for 31P.Spectral parameters were 45° pulses (10 ls) 3 s pulse interval 16 kHz sweep width and full proton decoupling. Two thousand free induction decays were routinely accumulated from each sample; the spectra were plotted with a line broadening of 80 Hz. Samples (B200 mM in lipid) were equilibrated at 37 °C for 10 min before data acquisition. Phase transitions were also detected through changes of TMA-DPH —uorescence polarization.9 Acknowledgements We are grateful to Ms Sara Loç pez for her help with the NMR experiments. G.B. and M.B.R.A. were predoctoral students supported by the Basque Government.This work was supported by grants from DGICYT (No. PB96-0171) and The Basque Government (No. PI96/46). References 1 L. V. Chernomordik V. A. Frolov E. Leikina P. Bronk and J. Zimmerberg J. Cell Biol. 1998 140 1369. 2 J. L. Nieva F. M. Gon8 i and A. Alonso Biochemistry 1989 28 7364. 3 F. M. Gon8 i J. L. Nieva G. Basaç n8 ez G. D. Fidelio and A. Alonso Biochem. Soc. T rans. 1994 22 839. 4 E. R. S. Roldaç n and R. A. P. Harris Biochem. J. 1989 259 397. 5 B. Spungin I. Margalit and H. Breitbart J. Cell Sci. 1995 108 2525. 6 J. L. Nieva F. M. Gon8 i and A. Alonso Biochemistry 1993 32 1054. 7 J. L. Nieva A. Alonso G. Basaç n8 ez F. M. Gon8 i A. Gulik R. Vargas and V. Luzzati FEBS L ett. 1995 368 143. 8 M. B. Ruiz-Argué ello F. M. Gon8 i and A.Alonso J. Biol. Chem. 1998 273 22977. 9 G. Basaç n8 ez J. L. Nieva E. Rivas A. Alonso and F. M. Gon8 i Biophys. J. 1996 70 2299. 10 G. Basaç n8 ez G. D. Fidelio F. M. Gon8 i B. Maggio and A. Alonso Biochemistry 1996 35 7506. 11 G. Basaç n8 ez F. M. Gon8 i and A. Alonso Biochemistry 1998 37 3901. 12 G. Basaç n8 ez F. M. Gon8 i and A. Alonso FEBS L ett. 1997 411 281. 13 G. Basaç n8 ez J. L. Nieva F. M. Gon8 i and A. Alonso Biochemistry 1996 35 15183. 14 G. M. Carman R. A. Deems and E. A. Dennis J. Biol. Chem. 1995 270 18711. 15 M. M. Kozlov and V. S. Markin Bio–zika 1983 28 255. 16 D. P. Siegel Biophys. J. 1993 65 2124. 67 Faraday Discuss. 1998 111 55»68 17 L. V. Chernomordik Chem. Phys. L ipids 1996 81 203. 18 D. P. Siegel J. Banschbach and P. L. Yeagle Biochemistry 1989 28 5010.19 L. C. M. van Gorkom S. Q. Nie and R. M. Epand Biochemistry 1992 31 671. 20 P. L. Yeagle F. T. Smith J. E. Young and T. D. Flanagan Biochemistry 1994 33 1820. 21 H. Ellens D. P. Siegel D. Alford P. L. Yeagle L. Boni L. J. Lis P. J. Quinn and J. Bentz Biochemistry 1989 28 3692. 22 V. Luzzati Curr. Opin. Struct. Biol. 1997 7 661. 23 D. P. Siegel and R. M. Epand Biophys. J. 1997 73 3089. 24 S. Leikin M. M. Kozlov N. L. Fuller and R. P. Rand Biophys. J. 1996 71 2623. 25 H. Ellens J. Bentz and F. C. Szoka Biochemistry 1986 25 285. 26 H. Ellens J. Bentz and F. C. Szoka Biochemistry 1986 25 4141. 27 G. Basaç n8 ez M. B. Ruiz-Argué ello A. Alonso F. M. Gon8 i G. Larlsson and K. Edwards Biophys. J. 1997 72 2630. 28 A. D. Bangham and R. M. C. Dawson Biochem. J. 1959 72 486. 29 M. B. Ruiz-Argué ello F. M. Gon8 i and A. Alonso Biochemistry 1998 37 11621. 30 S. W. Hui T. P. Steward L. T. Boni and P. L. Yeagle Science 1981 212 921. 31 P. K. J. Kinnunen Chem. Phys. L ipids 1996 81 151. 32 A. Sen T. V. Isac and S. W. Hui Biochemistry 1991 30 4516. 33 H. De Boeck and R. Zidovetzki Biochemistry 1989 28 7439. 34 L. D. Mayer M. H. Hope and P. R. Cullis Biochim. Biophys. Acta 1986 858 161. 35 C. S. F. Boé ttcher C. M. Van Gent and C. Fries Anal. Chim. Acta 1961 1061 297. Paper 8/06352D Faraday Discuss. 1998 111 55»68 68

ISSN:1359-6640    

DOI:10.1039/a806352d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

General Discussion Dr Sansom opened the discussion of Prof. Bayerlœs paper Please could you say how much protein (Streptavidin) was added i.e. what fraction of the membrane surface was coated by protein ? Have you been able to perform experiments in which the surface was incompletely covered with protein ? Prof. Bayerl responded For streptavidin we worked under excess protein conditions i.e. we can safely assume that all biotin groups exposed by the black lipid membrane (BLM) surface are coupled to a streptavidin molecule. We did not perform experiments at low streptavidin resulting in incomplete coverage of the BLM. Prof. Holzwarth asked What do you expect as the time resolution of the dynamic light scattering method you applied ? Is there any angle dependence in your light scattering signal ? (0 30 60 90°) ? Prof.Bayerl responded The best time resolution which we can achieve now is 0.1 ls. An improved setup which is presently under construction will allow us to push this limit one order of magnitude. In answer to your second question no angular dependence has been observed yet since we are restricted to measurements around the specular angle. Prof. Neumann asked Are those proteins which increase the transversal shear undulations by themselves conformationally —exible such that they enforce their —uctuations to the membrane? Prof. Bayerl responded It is generally possible that protein internal degrees of motional freedom couple to the BLM motion. However to be detectable by dynamic light scattering (DLS) a coherent internal motion of all proteins would be required which appears rather unlikely.Dr Pawlak asked When you immobilize biomembranes (e.g. as part of a vesicle) on a solid support how is the frequency spectrum of undulations modi–ed upon the surface contact spontaneous contact or receptor mediated contact ? Will the undulations disappear ? If not how far can undulations compete energetically with e.g. low affinity attractive forces of receptor-mediated surface contacts ? Prof. Bayerl responded We did some preliminary experiments with bilayers on a solid support (mostly planar silica) using solid state NMR and infrared techniques. We found that the undulations are rendered undetectable in the mesoscopic range of wavevectors due to the presence of the support.This is understandable considering the fact that the average distance between bilayer and solid surface is in the range of 15 ” (neutron re—ection results). Hence there are just a few (bound) water molecules separating bilayer and support establishing a high surface viscosity. Only for highest frequencies (upper GHz range) we observed by coherent quasi-elastic neutron scattering that low amplitude undulations still persist for fully hydrated multilamellar bilayer stacks on a planar silicon support. Dr Amblard said What the proteins exactly do and how they alter the mechanical properties of the membrane is not clear. Nevertheless it seem that some eÜects could be due to the properties of the contact between individual proteins and the lipid heads ììbelowœœ them while others could involve lateral interactions between the protein molecules.These two situations could be resolved by studying how the mechanical eÜect varies with the surface concentration of adsorbed protein. Did you study that ? 69 Faraday Discuss. 1998 111 69»78 Prof. Bayerl responded We have not yet undertaken a systematic study of this eÜect. However the results we obtained for the so called S-layer proteins (two-dimensionally crystallized protein layers) from bacteria (Bacillus coagulans) seem to indicate that at protein concentrations at the bilayer surface which are insufficient for two-dimensional crystallization protein»protein interaction is not a dominant feature for the change of the undulation pattern. It is conceivable that this might change at higher protein concentrations but we have no experimental proof yet.This problem is currently being studied in our laboratory and we should have the data soon. Dr Jones said You describe the adsorption of —accid vesicles to a glass surface that shows that some spreading occurs and bilayer undulations in the region of contact (Fig. 6). How large were these vesicles and how frequent was this event ? Also is there a lower limit to the vesicles size to observe this phenomenon and can you observe vesicle disruption by your technique? Prof. Bayerl responded Typical vesicle diameters used in this study are in the range of several tens of micrometers. The lower size limit for observation by this technique is given by the lateral resolution of the microscope thus vesicles below 2 lm are hardly accessible since insufficient numbers of fringes will be resolvable.Nevertheless disruption of smaller vesicles at the solid surface is detectable by the interferometric technique since the formation of a supported bilayer at the disruption/fusion site can be clearly observed. Dr P. N. Edwards commented The changes in undulation behaviour on binding proteins to membranes in tight-packed arrays is likely to in part re—ect protein»protein interactions. Pegylation in situ should reduce and might eliminate that in—uence thus allowing protein»lipid eÜects to dominate any residual changes. Prof. Bayerl responded Protein»protein interaction contributes to the measured vertical shear motions only if the coupling between adjacent proteins indirection normal to the BLM is strong like in a two-dimensionally crystallized protein layer.We have not yet encountered such situations for the two proteins studied. Prof. Holzwarth said It is well known that cholesterol clusters in bilayer membranes depend on cholesterol concentration as we demonstrated.1 Can you deduce any information about the size of clusters induced by cholesterol in lipid membranes and do you see any change of lipid mobility in the surrounding of cholesterol clusters which might be re—ected in the elastic modulus of the membrane? 1 A. Genz J. F. Holzwarth and T. Y. Tsong Biophys. J. 1986 50 1043. Prof. Bayerl responded We assume that the DLS method is not sensitive to the clustering of cholesterol. However we know from our in-plane neutron scattering work on DPPC»cholesterol mixtures in the liquid ordered phase (40 mol% cholesterol) that if clusters exist in this phase they must be exceedingly small (less than 1 nm diameter).So I cannot comment on the eÜects the cluster size may have on the elastic modulus of the membrane. Dr Bezrukov asked From your measurements you deduce the values of lateral tension for lipid and lipid»cholesterol membranes; how well do they compare to the values obtained by other methods? Also you employ a variation of the Montal»Mueller technique for lipid bilayer formation but use decane as the lipid solvent. Decane is rather heavy and not particularly volatile. For this reason it is most probably incorporated into bilayer structures in your experiment and in—uences their mechanical properties signi–cantly.Can you comment on that ? Prof. Bayerl responded Note that the tension values deduced from our DLS measurements is the tension due to the vertical shear motion and thus a dynamical tension. It is not useful to compare this dynamical value with the static tension values measured by other (static) techniques. Decane is certainly a major contributor to the virtually zero viscosity value obtained by the DLS data analysis in terms of Kramerœs theory. We did some preliminary experiments where decane was substituted by squalane which is supposed to leave the BLM completely after sufficient Faraday Discuss. 1998 111 69»78 70 equilibration. For this case we observed a signi–cant decrease of the tension i.e.decane seems to pose a hindrance to vertical shear motion. Prof. Roux said Your results indicate that the continuum model of Kramer1 provides a very good description of the dynamical —uctuations of pure membranes and of the in—uence of proteins associated with the membrane on those —uctuations. Similar continuum models are currently used to describe the microscopic details of the membrane in the neighbourhood of a protein (e.g. mattress model hydrophobic mismatch etc.). Could you provide an upper bound in wave number gmax\2p/jmin up to which you trust the validity of a continuum description of membranes? 1 L. Kramer J. Chem. Phys. 1971 55 2097. Prof. Bayerl responded According to the extension of Kramerœs theory by Fan1 we should expect measurable deviations due to bending energy eÜects at wavenumbers beyond 105 cm~1.This is well above the maximum of 3]104 cm~1 which are currently accessible by our DLS technique. 1 C. Fan J. Colloid Interface Sci. 1979 44 363. Dr L. Fisher commented In Faraday Discussion No. 81 Parker Haydon and I reported interferometric measurements of the interaction between a pair of freely suspended glycerol monooleate bilayers. One result of these measurements was the establishment of a highly reproducible critical separation (30^5 nm) below which the aqueous –lm between the bilayers collapsed and the bilayers fused in a manner described in detail in the original paper. Models available at the time suggested that the collapse was induced by mutual reinforcement of independent bilayer —uctuations inducing a catastrophic increase in the amplitude of these —uctuations.Do your dynamic light scattering studies cast any further light on the possible mechanisms of the process ? 1 N. S. Parker D. A. Haydon and L. R. Fisher Faraday Discuss. Chem. Soc. 1986 81 249. Prof. Bayerl responded We have not yet studied the approach between two membranes or a membrane and a solid surface by this technique. We have to establish a device for precise interferometric distance measurements in the DLS setup –rst. This is presently under construction. Therefore I cannot suggest any new aspects of this behaviour at the moment. Dr Amblard opened the discussion of Prof. Laggnerœs paper In your paper your suggest that the quasi-immediate thinning of the interbilayer water space is achieved by a martensitic lattice disclination mechanism.It is clear from Fig 1b that this type of transition involves very large deformations. How is that possible given the viscosity of the environment? Prof. Laggner responded Our suggestion of a martensitic transition is based upon an indirect argument the absence of any signi–cant lattice perturbations immediately after the jump as seen from the sharp Bragg peaks leads to this notion. We have no direct information about the ensuing deformations and movements of the liposome. This could be done and will be done by time-resolved video microscopy. Prof. Holzwarth said You investigated the in—uence of Li` ions on the lifetime observed in your system. Did you ever use Cs` ions which occupy just the space of a water dipole ? I would expect a big diÜerence between the two alkali ions because their ionic radii are diÜerent and more importantly the lifetime of their coordinated water diÜers by six orders of magnitude as Eigen and co-workers demonstrated in the 1960s1 and as we were able to show for electron transfer reactions.2 A temperature jump in aqueous systems is always accompanied by a pressure jump if you donœt work around 4 °C.Can you distinguish between the pressure and the temperature in—uence on the relaxation time observed in your system? 1 H. Diebler M. Eigen G. Ilgenfritz G. Maaê and R. Winkler Pure Appl. Chem. 1969 20 93. 2 H. Bruhn S. Nigam and J. F. Holzwarth Faraday Discuss. Chem. Soc. 1982 74 129.71 Faraday Discuss. 1998 111 69»78 Prof. Laggner replied So far we have only undertaken static diÜraction experiments with CsCl. They show no qualitative diÜerence to what is observed with LiCl but a diÜerent concentration dependence. The L -phase separation does occur also with Cs and I assume that transient La*- phase will also be observable. Our intention with the present study was to prolong the lifetime of the intermediates and therefore the shorter lifetime of Cs»water coordination would work in the other direction I believe. The dynamic experiments with Cs will be done however. a a The sample cell also contains an air volume so that the volume expansion is practically unhindered. I only expect a shock wave which runs through the sample capillary of about 5 cm length in some 10 ls and thus by about two orders of magnitude faster than the time resolution of the diÜraction experiment.Dr Dijkstra asked In the model presented (LiCl-nuclei with lamellar layers with more outer layer undulation freedom and thus larger d-spacing) wouldnœt one expect a distribution of dspacings between the two extremes i.e. outer and inner most layers rather than two discrete peaks? The observed smaller d-spacing of the transient L*-state is a striking reminder of the diÜerent d-spacings observed in fully hydrated lamellar phosphatidylcholines (PCs) depending on whether the PC is hydrated by excess liquid water or by 100% vapour pressure. The hypothesized ìì lentils œœ of water in the L*-state could evaporate in 100% vapour pressure situations resulting in stabilized L a trapping of the and correlating to the lower d-spacings observed in 100% vapour pres- *-state sure vs.liquid water. Do you have any arguments/data that refute or augment this hypothesis ? a Prof. Laggner replied Yes one would indeed expect that but the two discrete and sharp peaks show that reality in this case does not follow our expectations. To answer your second question this similarity to the ìvapour pressure paradoxœ is very much along our lines of thinking. Recent experiments with diÜerent liquid concentrations and on the coagulation of liposomes have shown that this suppresses undulation in those liposomes that form the inner part of the coacervate leading to a smaller d-value and sharper peaks and that the larger d-values of the coexisting L -phase come from the outer unrestricted surface.Faraday Discuss. 1998 111 69»78 a Prof. Peterson asked How certain are you about the structural identi–cation of the transient phase? In particular have you performed any measurements on ordered lipid systems (e.g. cast lipid –lms) to verify the expected angular relationship between the original and transient phase re—ections ? Secondly can you exclude the transient phase being one of the known L -phases ? 72 b Prof. Laggner responded We have not yet done any experiments on cast lipid –lms but this is planned for the near future. Now we can only be positive that the transient phase is lamellar rather than non-lamellar from the observation of up to three integral orders of Bragg peaks and the absence of any additional non-integral orders.Prof. Evans opened the discussion of Dr Templerœs paper Could the low pressure sensed by the short dipyrenyl PC probe re—ect preferential partitioning in regions of positive (splay) curvature and coupling to undulations? Dr Templer responded Indeed we cannot rule out the possibility that the short di-pyrenyl PC probe may partition preferentially towards regions of curvature and this would indeed reduce the value of g (the ratio of the excimer to the monomer —uorescence intensity). We see precisely such a reduction when we make measurements in the inverse hexagonal phase (results not presented in our paper). This is a fundamental problem with regard to probes such as these namely that the bulk of the probe moieties may have a sizeable eÜect on the very conditions one is hoping to measure.Prof. Holzwarth asked What do you understand by a low pyrene ratio and have you tested whether there is any disturbance of the system in the environment of the pyrene molecules? Dr Templer responded We are interpreting a decrease in the ratio of pyrene to excimer —uorescence ratio in terms of a concomitant decrease in the lateral pressure in the region of the probe. We are of course worried about the disturbance the probe may cause on its surroundings; something which can be said of almost all molecular probes used in soft condensed systems. However we have not been able to devise a way of testing if there is a signi–cant disturbance.Prof Roux commented Assuming that the decay rate of the excited monomer and dimer are the same what you really detect is the population fraction in associated dimer vs. the separated monomers. Exactly how this population is correlated with the local pressure at the microscopic level is not so clear. Have you performed similar measurements in bulk solvent in which the hydrostatic pressure can be set externally ? In other words could you comment on the calibration of such measurements? Dr Templer responded Indeed we believe that g is detecting the population of probe molecules that are in an excimeric state vs. the separate state. We have not performed measurements on dipyPCs in bulk solvent to calibrate the measurement of g with respect to applied pressure.Doing such measurements is not directly related to the lateral pressure since a measurement in bulk solvent will tell us about the response to a three-dimensional pressure whereas the lateral pressure at an interface is a two-dimensional pressure under signi–cant geometrical constraints. We believe that it may be possible to make such a calibration on vesicle dispersions. This will tell us how g varies with pressure but we will still not know what the value at zero lateral pressure is. We have made a –rst attempt without success to determine the value at zero lateral pressure by measuring g on a Langmuir –lm as we increase the molecular area. Prof. Almgren commented It is an interesting idea to try monitor the lateral pressure variation in bilayer membranes in this way.However it is far from clear that the excimer to monomer emission intensity ratio is proportional to the pressure as assumed. Modifying eqn. (3) in your paper somewhat (mainly stressing that it is a proportionality and that the intramolecular excimer formation is governed by a –rst order rate constant kDM the mean encounter frequency of the two pyrene moieties) we have gP k kfD kDM fM(kD]kMD) In this equation all the rate constants could be pressure dependent. From studies of the pressure dependence of excimer formation of pyrene molecules in diÜerent solvents1 it was concluded that the main pressure dependence was in the rate constants kDM and kMD . Quenching by oxygen can occur in the present case which would eÜect k and also make this rate constant pressure depen- D dent.A test of the method using varying pressures appears important. This could maybe be achieved perhaps by using liposomes in a simple pressure cell. 1 M. Okamoto and M. Sasaki J. Phys. Chem. 1991 95 6548. Dr Templer responded First may we thank Prof. Almgren for noticing the typographical error in eqn. (3) which has been recti–ed in the –nal version of our paper. It is indeed correct that if oxygen quenching is occurring in our system then our measurement of g will not be directly proportional to the lateral pressure. We have undertaken tests to see if our samples were ììcontaminatedœœ with oxygen. Freeze dried samples of both DOPC and DOPE with 4dipyPC at 0.1 mol% were prepared in a nitrogen glove box. Water was added that had either been bubbled with argon for 15 min or left exposed to air and the excimer to monomer ratio then measured.To within the precision of our measurements we were unable to discern any diÜerence in the samples with g being 1.07^0.03 in DOPC and 0.91^0.03 in DOPE. We agree entirely that a pressure calibration is the next important step to demonstrate whether or not this is really a probe of lateral pressure. Prof. Bohne asked Could the smaller values for the excimer-to-monomer ratio at the position close to the double bond be due to an eÜect of the structural environment on the radiative rate constants for the monomer and/or excimer? Secondly are the spectra for the monomer and 73 Faraday Discuss. 1998 111 69»78 excimer emission the same for the pyrene at diÜerent positions in the lipid ? Finally I was surprised by your comment that the pyrene —uorescence is not sensitive to the presence of oxygen.Could you expand on whether the oxygen eÜect was determined for all probes and if the lack of an eÜect is on the absolute monomer and excimer emission intensities or on the excimer-tomonomer ratios. Dr Templer responded We are aware that the polarizability of the local environment aÜects the relative intensities of the monomer peaks. We have so far been unable to detect such changes in relative intensities as a function of chain length. Similarly we do not observe signi–cant diÜerences in relative monomer peak heights with temperature or composition. Furthermore we do not see variations in the peak position within the 0.5 nm resolution of our measurements of the –rst monomer peak at 376.5 nm.Yes it is perhaps surprising that we are not able to detect any eÜects of oxygen. We only tested for the eÜect on 4dipyPC and it is possible that the eÜect is only to be seen at the longer chain lengths. Our measurements were made with respect to g not the absolute values of the —uorescence intensity. Prof. Laggner said Intuitively it would seem to me that the lateral pressure and its pro–le along the lipid molecules is related to the —exibility/mobility gradients which has been extensively studied. How do your present results –t into this picture ? Cholesterol is perhaps the most widely studied moderator of —exibility/mobility in lipid bilayers have you done experiments with cholesterol and what were the results ? 0 Dr Templer responded I am not clear what a mobility gradient is but certainly the —exibility of the monolayer is related to the –rst moment of the lateral pressure.The –rst moment of the lateral pressure of the monolayer is proportional to iH where i is the bending modulus and H is the 0 0 spontaneous curvature. Our measurements indicate that the total lateral pressure in the chain region appears to be rising as we add DOPE to DOPC. This is entirely consistent with our knowledge of the product iH which is increasing as we add DOPE to DOPC. We have not done experiments with cholesterol yet. Dr P. N. Edwards said Excimer formation involves a reduction in the lipid-accessible surface area particularly p-surface area of pyrene molecules.Thus any diÜerential interaction energy between saturated and ole–nic lipid part-structures will alter the monomer excimer ratio independent of pressure eÜects. The interaction energy will be signi–cantly greater in the region of the cis-double bond and this will reduce excimer formation. Solution partial molar volumes of pyrene (or a more soluble derivative) in cyclohexane and cyclohexene could be used to demonstrate such eÜects. Dr Templer responded This is a subtle idea that I have to admit we had not thought of. As I understand the statement you are making is that the dip we observe in our pro–le of g with respect to chain position may be an artefact of the probeœs interaction with the oleic double bond. The idea would be that pyrene is more readily solvated by carbon double bonds so that there is hindrance to pyrene forming excimers as one approaches regions where the double bond is present.I have to accept that this is indeed a possibility and we will go on to test the idea. Prof. Lee asked Why is the order parameter low at the double bond in dioleoyl phosphatidylcholine ? Isnœt it a purely geometric eÜect due to the orientation of the CxC with respect to the long axis of the fatty acyl chain? Is the excimer ratio higher for pyrene at the 4 position than at the 6 position etc. not due to a diÜerence in pressure in the membrane but rather due to tethering of the fatty acyl chains to the glycerol backbone keeping the tops of the chains closer together than the bottoms of the chain? Dr Templer responded Yes the low order parameter calculated by Fattal and Ben-Shaul1 at the double bond is a geometric eÜect but one might anticipate that the orientation of the bond relative to the chain segments above and below would lead to an enhancement of the lateral pressure below the double bond.Faraday Discuss. 1998 111 69»78 74 I am not sure that I understand the second question correctly. The excimer formation requires that the monomer is –rst excited and that before the monomer decays that it comes into proximity with its (unexcited) companion. At this point it can form the excited dimer. Certainly the number of allowable chain conformations which give rise to an excimer are quite diÜerent between the top and bottom of the chain.It is these diÜerences which give rise to the variation in the lateral pressure as we probe at diÜerent depths in the monolayer. 1 D. R. Fattal and A. Ben-Shaul Biophys. J. 1994 67 983. Dr Sansom asked Can you distinguish between an eÜect of percentage DOPE on the lateral pressure along the bilayer normal as opposed to an eÜect on the degree of conformational freedom of the chains of dipyPC unrelated to lateral pressure gradients but more related to changes in order parameter pro–le which would also be expected to increase the probability of excimer formation? Dr Templer responded Once again I think the answer to this question is that the lateral pressure in a monolayer is a combination of several eÜects. First the pressure is two-dimensional and in the plane of the monolayer because we have an interface.Second colliding CH groups are not disembodied particles since they are constrained by the con–guration of all the CH2 groups above 2 them. This latter statement makes it clear that the lateral pressure is therefore related to the order parameter pro–le. Formally the chain lateral pressure is de–ned as the negative change in the free energy of the chains with respect to changes in the cross sectional area per chain. To calculate the lateral pressure one allows the lateral volume at some interval along a chain to increase whilst the lateral volumes at all other points and for all other chains is held constant and the change in the number of allowed conformations is calculated. Prof. Bohne commented The excimer-to-monomer ratio will depend on the monomer lifetime in the absence of excimer formation.Have you done time-resolved experiments for the monosubstituted lipids ? In addition time-resolved experiments when excimers are formed could be helpful to interpret if the decrease for the excimer-to-monomer ratio for 6dipyPC is due to a change in lateral pressure or to a change in the excimer and/or monomer photophysical behaviour. Dr Templer responded We have not yet done time resolved experiments. They are on our ìto doœ list and I could not agree more with the comments. Prof. Bayerl said In comparing order parameter pro–les obtained by your approach with those of NMR one has to consider the intrinsic timescales of the methods. What is the characteristic timescale of your method? Dr Templer responded The excited pyrene monomer has a lifetime in cyclohexane of 400 ns.We have not of course measured it in our system but nevertheless one would anticipate a lifetime in the hundreds of nanoseconds region. Prof. Svetina asked Is there any reason to expect that the lateral pressure in the middle of the phospholipid membrane approaches zero as the sketch in Fig. 1 in your paper suggests ? Dr Templer responded A sketch is of course only that a sketch. Nevertheless the statistical mechanical calculations made on bilayers by Szleifer and co-workers1 do all show the lateral pressure to be zero at the middle of the bilayer. But a calculation is of course only a calculation. 1 I. Szleifer A. Benshaud and W. M. Gelbart J.Phys. Chem. 1990 94 5081. Prof. Holzwarth opened the discussion of Dr Gon8 iœs paper Did you –nd a correlation between the activity of the enzyme and the induction of membrane fusion ? Both processes should be temperature dependent and they are also dependent on the —uidity of the membrane. It was demonstrated by several groups (Tsong et al.1 and by Holzwarth et al.2,3) that lipid bilayers are especially sensitive to cross membrane processes in the middle of the main phase 75 Faraday Discuss. 1998 111 69»78 transition because there are diÜerent states of order coexisting and lipid vesicles show a facetted surface instead of a spherical surface which is observed above the main phase transition. Our own experiments clearly showed that lipid vesicles can be handled for weeks if they are kept several degrees above the main phase transition temperature but the same preparations become very unstable if kept around the main phase transition temperature.Your system is very complex and I wonder if you might not have some internal phase separation of the diÜerent components which could induce fusion processes. Especially cholesterol is known to cluster in membranes. 1 A. Genz J. F. Holzwarth and T. Y. Tsong Biophys. J. 1986 50 1043. 2 R. Groll A. Boé ttcher J. Jaé ger and J. F. Holzwarth Biophys. Chem. 1996 58 53. 3 J. F. Holzwarth T he Enzyme Catalysis Process ed. A. Cooper J. L. Houben and L. C. Chien Plenum New York 1989 pp. 383»412. Dr Gon8 i responded The relationship between enzyme activity and induction of membrane fusion is described in Fig.3 in our paper. Both processes are seen to be temperature dependent. Under our conditions the lipids (egg PC egg PE and cholesterol) are always in the —uid state i.e. well above the main gel»—uid transition. The transition that is referred to in Fig. 3 occurs from the —uid lamellar to the inverted cubic phase. The complexity of the system is re—ected in the rather complex phase diagram in Fig. 4. The transient formation of microdomains enriched in certain lipids cannot be excluded. In fact the geometry of the fusion intermediate or stalk would favour a certain enrichment in ì conicallyshapedœ lipids such as PE or cholesterol. Prof. Svetina commented The facilitation of vesicle fusion by the activity of phospholipase could also occur owing to the redistribution of diacylglycerol in between the two membrane lea—ets and the consequent release of the membrane elastic energy in the fusion process.1 Do your observations support such a possibility ? 1 S.Svetina A. Iglicó and B. Zã eksó Ann. N.Y . Acad. Sci. 1994 710 179. Dr Gon8 i responded Redistribution of diacylglycerol between the two membrane lea—ets may be very important in the latter stages of the fusion event i.e. transition between stages C and D in your model. However for the initial steps of vesicle aggregation and (putative) stalk formation we propose that asymmetric localized spots of diacylglycerol are essential. Dr P. N. Edwards said The —ip-—op kinetics in your systems are important in the understanding of concentration gradients of diacylglycerol produced in membranes by the action of phospholipase C.What is known about such rate constants ? Dr Gon8 i responded No precise data on the transmembrane or —ip-—op rate of diacylglycerol in our system are available. However Hamilton et al.1 have measured using NMR methods a rate constant of ca. 62 s~1 (t1@2 ca. 11 ms) at 38 °C for DAG transbilayer movement in a phosphatidylcholine bilayer. 1 J. A. Hamilton D. T. Fujito and C. F. Hammer Biochemistry 1991 30 2894. Dr Templer commented Although the evidence you have presented indicates that the activity of phospholipase is not aÜected by the presence of non-lamellar phases doesnœt your data indicate that it is aÜected by the curvature elastic stresses which are built up in the lipid bilayer ? In other words the presence of a non-lamellar phase indicates that at least some of the curvature elastic stress has been reduced and this might reduce the activity of phospholipase relative to a lamellar phase where the curvature elastic stress is still present.Dr Gon8 i responded Elastic stresses may well constitute one perhaps the main factor of what has been called the ì frustrated lamellar state œ (ref. 31 of our paper). It is also true that once the enzyme has been acting for some time its rate decreases. However the obvious eÜect of inhibition Faraday Discuss. 1998 111 69»78 76 by substrate accumulation may under those circumstances be more important than the release of curvature elastic stress that will accompany the lamellar-to-nonlamellar phase transition.Prof. Laggner said A suggestion on the correlation between fusion propensity and phospholipase-C activity. The –rst step in fusion is the drainage of water from the space between opposing monolayers. I think polyethylene glycol could facilitate this local dehydration which is also a prerequisite for phospholipase-C action. Dr Gon8 i responded The correlation between fusion propensity and phospholipase-C activity is discussed at various stages in the paper (see in particular Fig. 1»3 and Table 1). Removing water from the intermembrane space is certainly an important step in fusion. We have studied in detail the process of liposome fusion induced by polyethylene glycol that is partly driven by the ability of the polymer to remove free water from the aqueous suspension.1 Interestingly when polyethylene glycol is covalently bound to the lipid bilayer (ref.12 in the paper) water cannot be removed from the interbilayer interstice and fusion is inhibited. We have not tested as yet the combined actions of polyethylene glycol and phospholipase-C. 1 A. Alonso R. Saez and F. M. Gon8 i FEBS L ett. 1982 30 137. Prof. Almgren said In connection with the discussion of the eÜects of the bilayer lipid structure on the activity of the enzyme it would be of interest to know if the curvature of the bilayer for example a transformation to a reversed cubic membrane aÜects the enzyme activity (and not only the presence of lipids with a tendency to induce such a transformation). I would also like to draw attention to cubosomes1 and other dispersed particle2 of reversed phases as possible model systems in this –eld.1 K. J. Larsson J. Phys. Chem. 1989 93 7304; J. Gustafsson H. Ljusberg-Wahren M. Almgren and K. Larsson L angmuir 1997 13 6964. 2 J. Gustafsson T. Nylander M. Almgren and H. Ljusberg-Wahren J. Colloid Interface Sci. 1999 211 326. Dr Gon8 i responded Data such as those shown in Fig. 8 of this paper argue against a direct role of phase transitions in the regulation of enzyme activity. However a more detailed exploration of this point would be welcome and in that context cubosomes and related systems may be very useful. Prof. Robinson asked Can you provide some further information concerning experimental protocols ? How for example is the enzyme introduced into the system? The data in Table 1 shows that ganglioside additives have a very dramatic eÜect on the hydrolysis and fusion rates.Can you oÜer an explanation for these observations ? Dr Gon8 i responded Further experimental details may be found in ref. 2 6 and 7 of the paper. The enzyme is added to the preformed large unilamellar vesicles in suspension at the start of the reaction. The eÜect of gangliosides on enzyme activity has been explored in detail by Daniele et al.1 Gangliosides do not aÜect the adsorption process of the enzyme. Instead their modulatory eÜect occurs at the level of the interface perhaps modifying the electrostatic –eld con–guration across the interface through their polar head group dipoles. Further to their inhibitory properties on the enzyme hydrolytic activity gangliosides exhibit a direct inhibitory eÜect on fusion itself due to their ability to stabilize the lamellar phase thus opposing the formation of any nonlamellar fusion intermediates. 1 J. J. Daniele B. Maggio I. D. Bianco F. M. Gon8 i A. Alonso and G. D. Fidelio Eur. J. Biochem. 1996 239 105. Miss Bucak communicated What is the role of cholesterol in your membrane? Do you use it to make the membrane more rigid or because you want the membrane to be more biological as you carry out an enzymatic reaction ? 77 Faraday Discuss. 1998 111 69»78 Dr Gon8 i responded Cholesterol is essential for fusion to occur under our conditions (see. ref. 2 in our paper). Our hypothesis is that cholesterol facilitates the formation of inverted nonbilayer structures (hexagonal cubic) thus contributing to the architecture of the nonbilayer fusion intermediate or ì stalk œ (ref. 7 and 15). Faraday Discuss. 1998 111 69»78 78

ISSN:1359-6640    

DOI:10.1039/a900727j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope Yves F. Dufre� ne,a,b,§ Thomas Boland,a James W. Schneider,a William R. Bargera and Gil U. Lee*a a Chemistry Division Code 6177 Naval Research L aboratory W ashington DC 20375-5352 USA. E-mail glee=stm.2.nrl.navy.mil b Center of Marine Biotechnology University of Maryland Columbus Center 701 E. Pratt Street Baltimore MD 21202 USA Receiøed 1st October 1998 Interaction forces and topography of mixed phospholipid»glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG).The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3»1.0 nm after which the probe makes mechanical contact with the bilayers. At loads \5 nN the bilayer was elastically deformed by the probe while at higher loads plastic deformation of the bilayer was observed. Surprisingly a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) –lm despite the identical head groups of DOPE and DSPE. This provides direct evidence for the in—uence of the structure and mechanical properties of lipid bilayers on their interaction forces an eÜect which may be of major importance in the control of biological processes such as cell adhesion and membrane fusion.The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modi–ed AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.§ Current address Uniteç de Chimie des Interfaces Universiteç Catholique de Louvain Place Croix du Sud 2/18 B-1348 Louvain-la-Neuve Belgium. 79 Faraday Discuss. 1998 111 79»94 Introduction Many cellular phenomena including cell adhesion and membrane fusion involve speci–c molecular interactions and non-speci–c interactions between bilayer membranes. While the function of (glyco)proteins such as integrins,1 selectins2 and lectins,3 in these interactions has long been recognized there is now growing evidence supporting the involvement of lipids. Gangliosides are charged glycosphingolipids found in most animal membranes which are thought to have important functions in cell adhesion molecular recognition oncogenesis and diÜerentiation.4h6 On the other hand phosphatidylcholine phosphatidylethanolamine and galactolipids the most common lipids in animal and plant membranes prevent membranes and vesicles from coming into contact and reacting adhering or fusing due to strong repulsion between their highly hydrated head groups.7h10 Interaction forces between opposing supported lipid bilayers have been directly measured using the surface forces apparatus (SFA).For uncharged galactolipid bilayers in NaCl solutions two types of forces have been identi–ed i.e. long-range attractive van der Waals forces and shortrange repulsive hydration/steric forces.8 A doubling of the range of the repulsive force was found when comparing monogalactosyldiglyceride with digalactosyldiglyceride.For phosphatidylethanolamine and phosphatidylcholine bilayers in the presence of millimolar levels of Ca2` and Mg2` a third force was detected in agreement with theory i.e. long-range electrostatic double-layer.9 A second approach the osmotic stress (OS) method has been widely used to probe short-range hydration forces between lipid bilayers in multilamellar systems.11h14 SFA and OS studies have provided valuable insight into membrane»membrane interactions but they are not suited for probing local properties of complex biosurfaces i.e. surfaces made of multiple component mixtures and exhibiting lateral organization at the nanometer scale. Since its invention in 1986 atomic force microscopy (AFM Fig.1) has been widely used to image the surface of materials down to molecular resolution.15 AFM has been applied to many biological systems due to its high resolution and ability to operate in aqueous environments. Subnanometer scale resolution has been achieved on proteins,16h18 DNA»protein complexes19,20 and bilayers,21h24 but the resolution of cell surfaces has been limited to 10»30 nm18,19,25,26 due to the complex interplay between topographic mechanical and frictional forces. One solution to this problem is to reconstruct more rigid models for lipid bilayer membranes using the Langmuir» Blodgett (LB) deposition technique.27,28 Such an approach has enabled several research groups to image supported lipid membranes at high resolution under physiological conditions.22,24,29 Besides imaging the very high force sensitivity of the AFM has been used to measure forces with nanometer scale lateral resolution.Quantitative measurements have been made of van der Waals and electrostatic forces,30h32 hydration forces,31,33,34 steric repulsive forces,35,36 speci–c Fig. 1 Schematic of an atomic force microscope. A molecular resolution image of a surface is constructed by rastering a microfabricated probe over a surface under constant low loads. The critical elements of the instrument are the microfabricated cantilever-probe (A) piezoelectric electromechanical actuator (B) optical-lever displacement detector (C) and digital feed-back loop (not shown). The dimensions of the cantilever can be modi–ed to select a spring constant and cantilevers have been microfabricated with spring constants of 100» 0.0001 N m~1.Microfabrication gives the force transducer a high temporal resolution due to the minute mass of the cantilever. Faraday Discuss. 1998 111 79»94 80 intermolecular interactions37,38 and molecular mechanics.39 Of particular interest is the mapping of interaction forces with nanometer scale resolution by recording force»distance curve maps in the (x,y) plane.40h42 A key issue that must be addressed when measuring forces with the AFM is the probe surface chemistry. The most commonly used probes for surface imaging are made of silicon oxynitride. These probes have a complex surface chemistry made of ionizable silanol and amine groups but also of contaminants.43 Reproducible measurements of forces attributable to interactions between speci–c functional groups on a surface require rigorous control of the surface chemistry of the probe.Sharp probes bearing single chemical moieties such as alcohol or aliphatic groups can be produced using self-assembly of functionalized alkanethiols onto Au-coated probes. This method has allowed mapping of the spatial arrangement and interaction forces on surfaces patterned with diÜerent chemical groups.44,45 In this paper we describe the nanometer scale topography mechanical properties and surface forces of lipid bilayers that model biological membranes in aqueous solution. To measure unambiguously short-range forces in the absence of electrostatic double-layer contribution we used lipid bilayers with a net zero surface charge and probes modi–ed by self-assembly of alkanethiol monolayers terminating in hydroxy groups.Bilayers made of phosphatidylethanolamines and galactosyldiglycerides were selected for several reasons ( these lipids are the most common in animal and plant membranes respectively ; (ii) controlled deposition of these bilayers on mica using the LB technique has been described previously ;9 (iii) data on surface forces between such bilayers are available from other techniques for comparison.46 These results represent the –rst systematic analysis of short-range interaction forces of lipid bilayers using chemically de–ned AFM probes and set the stage for the characterization of the physical properties of complex supported bilayers and possibly even living cells with molecular resolution.3)49 and then rinsed 16 S 2p Si 2s O 1s Au 4f C 1s 7@2 Cr 2p3@2 \0.5 3 2 1 11 8 57 37 29 52 \0.5 \0.5 Materials and methods Preparation of thiol monolayers Cleaved [0001] muscovite mica and AFM probes were functionalized with self-assembling alkanethiols using conditions optimized for the formation of organized monolayers.47,48 To this end mica surfaces and probes were coated with a 2 nm thick Cr layer followed by 15 nm of Au using electron beam thermal evaporation at high vacuum (CVC Products Inc. Rochester NY USA). The gold coated surfaces were immediately immersed for 18 h in 0.2 mM ethanolic solutions of 16-hydroxyhexadecane-1-thiol (C -OH) and hexadecane-1-thiol (C -CH 16 15 with ethanol to remove excess alkanethiol.The chemical composition of the C -OH coating was 16 determined by X-ray photoelectron spectroscopy (XPS) using Au/Cr-coated silicon as a standard (Table 1). Sulfur was detected in signi–cant concentrations at the C -OH surface and a dramatic 16 increase in the carbon surface concentration was observed correlating with a decrease in the gold surface concentration. These results are consistent with previous XPS studies of alkanethiol monolayers. 47 Table 1 Surface chemical composition [atomic fraction (%)] determined by XPS for Au/ Cr-coated silicon and Au/Cr-coated silicon after self-assembly of C -OHa Sample Au/Cr-silicon Au/Cr-silicon]C -OH 16 a n-Type S100T silicon wafers (Wafernet San Jose CA USA) cleaved into 1]1 cm2 chips were submitted to the same coating treatments as the cantilevers and directly analyzed by XPS (220iXL Fisons East Grinstead UK).The pressure during analysis was D10~7 Pa. Spectra were collected with monochromatic Al-Ka X-rays with a nominal 150]800 lm spot size. The pass energy was 20 eV and the take-oÜ angle was 90°. The atomic concentrations were calculated using the standard sensitivity factors with a program supplied with the instrument. Results are expressed as atomic fractions (%) with respect to the sum of all elements except hydrogen. 81 Faraday Discuss. 1998 111 79»94 Preparation of supported bilayers The lipids used in the study dioleoylphosphatidylethanolamine (DOPE) distearoylphosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG) were obtained from Sigma Chemical Co.(St. Louis MO USA) and Matreya Inc (Pleasant Gap PA USA) and their purity was con–rmed with electrospray mass spectroscopy. For DGDG the hydrocarbon chains were saturated and mostly made of 18 carbons. All lipids were dissolved at 0.5 mM in chloroform»methanol (4 1). Monolayers were spread at 25 °C at the air/water (triply distilled) interface of a KSV5000 Langmuir»Blodgett system (KSV Instruments Helsinki Finland) compressed at a rate of 1 mN m~1 min~1 and transferred at a constant surface pressure of 25 mN m~1. Mixed monolayers (1 1 molar ratio) of DSPE/DOPE and DGDG/DOPE were transferred onto mica for imaging in air or onto DSPE-coated mica substrates for imaging in water as described previously.50 Water was replaced by 0.15 M NaCl solutions before AFM measurements.Pure lipid monolayers were deposited onto silicon wafers with a 250 Aé thick oxide layer (Transition Technology Interactions Sunnyvale CA USA) for ellipsometry. Atomic force microscopy Measurements were made at room temperature (20»25 °C) using an optical lever AFM equipped with a liquid cell (Nanoscope III Digital Instruments Santa Barbara CA USA). Thermal oxide sharpened microfabricated Si3N4 probe-cantilevers (Park Scienti–c Instruments Mountain View CA USA) with a nominal tip radius of 25 nm were used. Topographic images were taken in the constant-de—ection mode with an applied force kept as low as possible i.e. \1 nN and scan rates of D4 Hz.The sensitivity of the AFM detector was estimated using the slope of the retraction force curve in the region where the probe and sample are in contact. Adhesion maps were obtained by reconstructing 64]64 force»distance curves per image and displaying the pull-oÜ force measured for each force curve. Images were then resampled to 512]512 pixels. For quantitative interpretation of the force»distance curves the non-linear response (displacement/applied voltage) of the piezoceramic actuator of the scanner was corrected using a capacitance-based displacement calibrator (Capacitec Boston MA). The cantilever spring constants ranging from 0.01 to 0.6 N m~1 were determined using micromachined reference cantilevers of precisely controlled spring constants following the procedure of Tortonese and Kirk.51 Ellipsometry Ellipsometric measurements of monolayer properties were made in air at 44 wavelengths between 400 and 800 nm at angles of incidence of 65 70 75 and 85° (M-44 spectroscopic ellipsometry J.A. Woollam Co Inc.). The rotating analyzer ellipsometer uses a broad band light source with a monochromator built into the solid state detector. The calcite Glan Taylor polarizer is mounted in a high precision rotation stage with accurate positioning below 0.01° and a maximum beam deviation of 0°1@. The angle of incidence phase (D) and amplitude (W) were known with an uncertainty of less than 0.005 0.02 and 0.01° respectively. The optical properties of the –lms were –t with a three-layer model.52 The four unknown physical parameters are the –lm thickness –lm coverage and the real and imaginary parts of the permittivity of the –lm.For each wavelength the ellipsometer yields the two re—ection parameters D and W thus measurements at two angles of incidence should in principle be sufficient to solve for all unknowns. In our case W is largely independent of the layer thickness and experiments at additional angles were necessary. The optical constants of the initial bare gold or silicon surface were used as substrate references and the optical constants of the organic layer were –t with a two parameter Cauchy model with no absorptive component (k\0). Film thickness and optical constants of the –lm were then evaluated in an iterative procedure to satisfy variation with wavelength and angle of incidence.Results Knowledge of the physical and chemical properties of lipid bilayer membranes at high spatial resolution is a key step towards a better understanding of complex biological surfaces and cellular 82 Faraday Discuss. 1998 111 79»94 16 15 3 surfaces (Fig. 2A). The C -OH functionalized probes were then used to study the nanometer events such as cell adhesion and membrane fusion. Functionalized AFM probes have been used to investigate the nanometer scale surface properties of mixed lipid bilayers under aqueous conditions. To evaluate the relative contributions of short-range surface forces and mechanics in the measured forces we –rst made force measurements between model hydrophilic and hydrophobic surfaces in water i.e.between C -OH functionalized AFM probes and either C -CH or C16- OH 16 scale surface properties of mixed phospholipid/glycolipid bilayers prepared as follows either DSPE or DGDG (saturated hydrocarbon chains) were mixed with DOPE (unsaturated hydrocarbon chains) and the mixed monolayers were transferred as a second layer onto DSPE-coated surface yielding bilayers with hydrophilic phosphatidylethanolamine and galactoside head groups exposed to the aqueous phase (Fig. 2B). Alkanethiol monolayers 16 and C -CH monolayers was D5° and 110^2° respectively which is consistent with 3 The alkanethiol –lms were characterized with ellipsometry and contact angle measurements. The ellipsometric thickness of the C -CH and C -OH –lms was 2.1^0.1 nm (refractive index 15 3 16 n\3) and 2.4^0.1 nm (n\3) respectively.The thickness of the C -CH –lm is in quantitative 15 3 agreement with earlier ellipsometric studies and con–rms the alkanethiol forms a closed packed monolayer tilted 30° to the surface normal.47 The hydroxy terminated C -OH –lm is 0.3 nm 16 thicker than the methyl terminated C -CH monolayer which we attribute to the formation of a 15 3 water monolayer on the C -OH –lm under ambient conditions. The water contact angle on the C -OH 16 15 close packed monolayers. The forces measured between the C -OH and C -CH surfaces and C -OH probe in water 16 15 3 16 are plotted as a function of surface separation in Fig. 3. The form of the force curve was not aÜected by the maximum force applied to the surface for forces up to 20 nN.Only repulsive forces were observed as the C -OH surface was approached to and retracted from the C -OH probe 16 16 Fig. 3A. The lack of signi–cant adhesive force suggests a strong repulsive force counterbalances the attractive interactions. In contrast to the C -OH/C -OH measurements signi–cant hyster- 16 16 esis was observed in the retracting force curve between the C -CH surface and C -OH probe 15 3 16 Fig. 3B. ìPull-oÜœ forces of D7.5 nN magnitude were observed which are indicative of a strong attractive force when the probe is in contact with the surface. and DSPE were constructed using an energy minimization molecular dynamics algorithm (Chem3D 16 16-OH Fig. 2 Schematic diagrams of forces measurements made between (A) a C -OH modi–ed probe and surface and (B) a C -OH modi–ed probe and a homogeneous DSPE/DSPE lipid bilayer.The space –lling models of 16 C -OH CambridgeSoft Cambridge MA USA). The theoretical length (L ) of the C 16 DSPE and DGDG molecules are 2.4 3.3 and 3.7 nm respectively. The tilts (h) of the DSPE and C -OH in this image were set at 25 and 30° respectively to re—ect the measured thickness (T ) of the monolayers. 16 83 Faraday Discuss. 1998 111 79»94 Fig. 3 Force measurements between a C -OH coated AFM probe and (A) C16-OH and (B) C15-CH3 surfaces in water. The force and separation resolution of the two measurements were varied by an order of 16 magnitude in order to capture the hydration force region of the C16-OH surface and the adhesive force region of the C -CH surface. Data in (A) are shown with circular symbols (1 symbol/5 data points) and the theoreti- 15 cal –t is shown with a solid line.3 Supported lipid bilayers Fig. 4 shows the surface pressure vs. area (n-A) isotherms of DSPE DOPE DSPE/DOPE (1 1) DGDG and DGDG/DOPE (1 1) monolayers at the air/water interface. While DSPE and DGDG monolayers show a two-dimensional ì solid-like œ behavior the DOPE monolayers show a 2-D ì liquid-like œ behavior due to the unsaturated hydrocarbon chains of the molecule. The area per molecule surface compression modulus and theoretical space –lling (SF) thickness of pure lipid –lms were determined from the surface pressure diagrams at the deposition pressure of 25 mN m~1 and are summarized in Table 2. Topographic images made of mixed DSPE/DOPE and DGDG/DOPE monolayers on DSPEcoated mica are presented in Fig.5A and 6A respectively with topographic cross-sections. These images are consistent with previous AFM images of saturated lipid/DOPE monolayers50 in that all the bilayers show phase-separation in the form of elevated microscopic domains surrounded by a continuous matrix. In light of the surface fractions covered by the domains in the diÜerent images as well as the molecular lengths calculated from SF models the higher level domains in the images are ascribed to DSPE and DGDG and the lower level domains to DOPE. Two observations can be made from the topographic images. First defects are observed in the continuous DOPE phase in the form of holes associated with the DOPE layer images.50 These holes appar- Fig.4. Surface pressure»area (n»A) isotherms for pure and mixed (1 1) lipid monolayers at the air/water interface at 25 °C (A) DSPE DOPE and DSPE/DOPE mixture; (B) DGDG DOPE and DGDG/DOPE mixture. Faraday Discuss. 1998 111 79»94 84 Table 2 Physical properties of the LB deposited lipid monolayer and bilayer –lms Ellipsometry AFM step height Cauchy parameters Monolayer thickness/ nm SF thickness/ nmb Bilayer/ nme Monolayer/ nmd Lipid A Bc Surface compression modulus/ mN m~1 a Area molecule~1/ nm2 a » » 1.9 0.67 DOPE 1.9^0.1 79^1 3.0 0.41 DSPE 3.3^0.3 1.8^0.2 3.3^0.1 183^1 2.7 0.47 DGDG 4.9^0.4 2.1^0.1 2.9^0.1 193^1 1.865 [0.0866 1.456 [0.00856 1.6045 [0.0416 a At the deposition pressure of 25 mN m~1 of the –lms.b The theoretical space –lling (SF) thicknesses of the monolayers (T ) were determined from the area per molecule (a) of the lipids using a constant speci–c volume (l) i.e. T \l/a. A speci–c volume of 1.03 cm3 g~1 was chosen; the molecular weights of the DGDG DOPE and DSPE lipids are 778 744 and 748 respectively. c The wavelength (j) dependence of the refractive index (n) of the lipid –lms was –tted with a Cauchy form n\A]B/j. d Mixed DSPE/DOPE and DGDG/DOPE monolayers supported on mica were made by raising freshly cleaved mica vertically through the air/water interface and imaged in air at forces of D1 nN. The step heights between DSPE and DGDG domains and the DOPE matrix were measured from cross-sections in the topographic images.Mean value and standard deviation of height diÜerences of three independent experiments. e Step heights measured between the DSPE and DGDG domains and the DOPE matrix for bilayers made of mixed DSPE/ DOPE and DGDG/DOPE monolayers deposited onto DSPE-coated mica and imaged in 0.15 M NaCl solutions. Fig. 5 (A) Topographic (z-range 20 nm) and (B) adhesion images (5 lm]5 lm) of a mixed DSPE/DOPE monolayer onto DSPE-coated mica in a 0.15 M NaCl solution. Lighter levels in the images correspond to higher height and adhesion. The topographic image was obtained at an applied force of \1 nN and the maximum force applied to the sample for adhesion force mapping was D1 nN. A cross-section (C) taken along a line indicated by the arrow is shown beneath the topographic image while the histogram of the magnitude of the adhesion force (D) (4096 events per image) is shown beneath the adhesion map.85 Faraday Discuss. 1998 111 79»94 Fig. 6 (A) Topographic (z-range 20 nm) and (B) adhesion images (5 lm]5 lm) of a mixed DGDG/DOPE monolayer onto DSPE-coated mica in 0.15 M NaCl solution. A cross-section and adhesion map are shown in Fig. (C) and (D) respectively. ently form during or shortly after deposition and appear to be stable for over 24 h periods. Second the step heights measured between the DSPE and DGDG domains and the surrounding DOPE matrix in the topographic images are 3.3^0.3 and 4.9^0.4 nm respectively clearly larger than predicted from the SF thicknesses of the bilayers. The adhesion force maps shown in Fig.5B and 6B exhibit high contrast the DSPE and DGDG domains presenting systematically low levels of adhesion compared to DOPE. Indeed the corresponding adhesion force histograms show two discrete levels of adhesion one centered at about 0.1 nN for DSPE and DGDG the other centered between 0.5 and 2 nN for DOPE (the magnitude of the adhesive forces can not be directly compared between the diÜerent –lms due to the variations in the radius of curvature of the microfabricated probe). The force measured between the DOPE surface and a hydroxy-terminated probe is plotted as a function of surface separation in Fig. 7A. The form of the force curve was not aÜected by the maximum force applied to the surface for forces up to 20 nN. Upon approach the probe jumpsto-contact at 4.5 nm without experiencing repulsive forces.Upon retraction a hysteresis or pulloÜ force is observed of D5 nN magnitude indicating contact and a strong attractive interaction adhesion between the probe and –lm. A set of typical force»distance curves recorded between a modi–ed AFM probe and the DSPE and DGDG surfaces is given in Fig. 7 B»E. The curves clearly diÜer from those obtained on DOPE and depend on the maximum load applied on the surface. At low maximum loads (\1.5 nN) the approach and retraction curves are similar. The lack of signi–cant jump-to-contact suggests that a strong repulsive force balances the attractive interactions between the lipid surface and the probe. Upon retraction no signi–cant adhesion pull-oÜ forces are observed.These two observations suggest that at low loads molecular contact is not established between the probe and surface of the DSPE and DGDG domains. In contrast when the maximum applied force is increased the probe experiences a steep shortrange repulsion upon approach until it jumps into contact when the force is greater than 5 nN. Faraday Discuss. 1998 111 79»94 86 Fig. 7 Typical force»distance curves recorded over the (A) DOPE (B,C) DSPE (D,E) DGDG surfaces in 0.15 M NaCl solutions ; (B,D) Low maximum applied loads (\1.5 nN) (C,E) high maximum applied forces ([1.5 nN). The occurrence and form of the jump-to-contact were found to be probe dependent which is consistent with other measurements of short-range forces with the AFM.53 Upon retraction adhesion forces of D8 and 4 nN magnitude are found for DSPE and DGDG respectively suggesting that the lipid and alkanethiol head groups are in direct contact.The repulsive portion of the approach curves of the DSPE and DGDG can be –tted to an exponential form at low loads with decay lengths of 0.5^0.2 nm and 0.8^0.2 nm respectively. At high loads the data points no longer –t through an exponential function but through a straight line. (1) Discussion Image contrast mechanisms of supported lipid monolayer We have shown that the topographic contrast of mixed fatty acid and lipid monolayers in air is determined by three factors i.e. length of the molecule (L ) tilt of the molecule (h) and the mechanical properties of the –lm.50,54 The step height (*Z) measured between two lipid phases is *ZM\*T ](dM[dM{) where *T is the diÜerence between the thickness of the two –lms hereafter referred to as relative thickness and d is the deformation of –lm under the load of the AFM probe (M and M@ designate the thinner and thicker –lm respectively).The thickness of the pure monolayers has been independently determined using a simple SF model and ellipsometry Table 2. Two trends are evident in the data. First both techniques 87 Faraday Discuss. 1998 111 79»94 produce similar thicknesses second the solid-like DSPE and DGDG –lms are of similar thickness and are signi–cantly thicker that the more loosely packed liquid-like DOPE –lm. Using contact mechanics the elastic deformation produced by the probe under a known load can be determined if the physical properties of the system are known.Given an applied load P and probe radius R the Hertzian deformation is d\(9P2/16RE*2)1@3 (E Table 2) through the monolayer thickness56,57 (2) E*\[(1[v12)/E1](1[v22)/E2] E where 1 v1 and E2 v2 are Youngœs modulus and and Poissonœs ratio of the probe and sample respectively.55 If volume compressibility is neglected the isothermal thickness compression modulus (ET) of a LB monolayer is related to the surface compression modulus a (3) ET\Ea/T Using ellipsometric thicknesses the E of the DOPE DPSE and DGDG monolayers is 4.3 6.1 T and 7.1]107 N m~2 respectively and for soft materials v\0.5. The elastic modulus of the alkanethiol –lm is about 2]1010 N m~2 58 and thus will make a negligible contribution to E*.If one assumes the mechanical properties of the supported monolayer are dominated by the LB –lm the calculated deformation produced by a 50 nm radius probe at 0.5 nN of load in the DOPE DSPE and DGDG –lms is 1.0 0.8 and 0.7 nm respectively. Using the ellipsometric thickness of the –lms and the Hertzian –lm deformations the theoretical AFM step height of the mixed DSPE/DOPE and DGDG/DOPE monolayers in air (*ZM) is 1.6 and 1.3 nm respectively. The DSPE/DOPE monolayer *Z and measured step height (Table M 2) is in reasonable agreement but *Z of the DGDG/DOPE –lm is 0.8 nm smaller than the M measured step height. We attribute the signi–cant diÜerence between the theoretical and the measured DGDG/DOPE step height to a water –lm between the lipid head groups and mica surface.This nanometer scale water –lm is produced by the repulsive hydration»steric interactions between the lipid head groups and surface which make the water –lmœs thickness highly dependent on the form of the lipid head group. Fig. 8A schematically presents how the water –lmœs thickness in—uences the measured step height. The theoretical AFM step height of the monolayers is (4) *ZM\*T ](dM[dM{)](dL{[dL) where d and dL{ is the thickness of the water –lm associated with the thin and thick lipid –lm L respectively. Interpretation of the force»distance curves (see discussion below) indicates that the range of the hydration»steric interaction (dF) of the DGDG DSPE and DOPE phase is B1.2 0.3 and 0 nm respectively. If we assume that the water –lm thickness is equal to the range of the repulsive hydration»steric force *Z for the DSPE/DOPE and DGDG/DOPE monolayers is 1.9 M and 2.5 nm respectively.The theoretical step heights are in good agreement with their respective measured step heights (Table 2). This discussion indicates that the measured step height of mixed lipid monolayers is determined from molecular length molecular tilt mechanical properties and the nature of the lipid head group»surface interaction. The contribution of all these factors to the AFM imaging mechanism makes step height a sensitive probe of the physical state of a bilayer. Image contrast mechanisms of supported lipid bilayers The step heights measured between the DSPE and DGDG domains and the surrounding DOPE for the bilayers in water are much larger than the step heights measured for the corresponding monolayers in air (Table 2).The increase in the bilayer step height is unexpected as no new factors have been introduced into the image contrast mechanism. However comparison of topographic images of the bilayers (Fig. 5A and 6A) and monolayers50 indicates that deposition of the mixed monolayer in water changes its physical state. Speci–cally the formation of holes in the DOPE phase indicates that DOPE desorbs from the mixed monolayer during or shortly after deposition. A mechanism emerges to explain the image contrast of the mixed bilayers from this observation (Fig. 8B). At the low imaging force used in this study (\1 nN) topographic imaging of the DOPE region is conducted with the probe indenting into the –lm.We expect the deformation of Faraday Discuss. 1998 111 79»94 88 Fig. 8 Schematic diagram of the probe position on the lipid monolayer in air (A) and lipid bilayer in water (B). The unsaturated and saturated lipid»probe interaction is presented on the left and right side of the diagrams respectively. d is the deformation of the lipid monolayers d is the thickness of the water layer *T is M L the relative thickness of the –lms d is the distance of the probe from the surface due to repulsive forces and F *Z is the step height measured by the AFM. the DOPE phase by the probe to be signi–cantly larger in water than it was in air as reorganization will produce a signi–cant decrease in elastic modulus of this phase (Fig. 4). Reorganization of the DOPE phase does not appear to change the DSPE and DGDG internal surface pressure as the surface area occupied by these phases does not increase signi–cantly from air to water images.50 Imaging of the DSPE and DGDG regions appears to take place in a short-range repulsive force mode with the probe some distance oÜ the surface of the lipid –lm.Noncontact imaging has also been reported using repulsive long-range double layer interactions.32,42,59 The theoretical step height of the bilayer resulting from this model is (5) *ZB\*T ]dM]dF{ where *T is the relative thickness of the monolayer. Using the ellipsometric thickness of the –lms (d and assuming the probe penetrates through the entire DOPE –lm MB1.9 nm) *ZB for the mixed DSPE/DOPE and DGDG/DOPE bilayers is 3.6 and 4.1 nm respectively.The theoretical and measured step heights are in reasonable agreement considering the assumptions that have been made in estimating d and dF . Accordingly the image contrast mechanism of lipid bilayers M in water is controlled by the same factors that have been identi–ed for monolayers in air but the physical properties of the mixed lipid –lm is signi–cantly changed by deposition onto a DSPE monolayer on the down stroke. Surface force measurements between alkanethiol monolayers 3) Both imaging and interaction force measurements on the mixed lipid bilayers were made with a C -OH functionalized probe. To gain an understanding of surface forces between alkanethiol 16 surfaces in water we made force measurements between C -OH functionalized probes and model 16 hydrophilic (C -OH) and hydrophobic (C -CH surfaces.16 15 Faraday Discuss. 1998 111 79»94 89 Forces measured between amphiphilic surfaces in aqueous solutions typically result from longrange electrostatic double-layer and van der Waals (vdW) interactions and short-range hydration/ steric interactions. The role of electrostatic double-layer interactions was tested by making force measurements on each surface in water and 0.15 M NaCl. Consistent with our expectation that the surfaces were uncharged no diÜerence in the forces could be detected. As a rule vdW forces between surfaces of identical chemistry are attractive and for a sphere on —at geometry (6) FvdW\[AR/6r2 where A is the Hamaker constant R is the radius of curvature of the sphere and r is the distance between surfaces.60 As both probe and sample surfaces are made of hydrocarbon based monolayers which have similar dielectric constants (eD2) we anticipate that the vdW forces will be attractive with Hamaker constants in the range of 4»7]10~21 J.61,62 Hydration/steric forces are short-ranged and have the form (7) o Fhydration\Fo e~(r@j) where F is the preexponential factor r is the separation between the surfaces and j is the hydration decay length.Repulsive hydration/steric interaction clearly dominate the force measured between the C -OH surface and probe Fig. 3A. These forces are believed to be associated with the reorientation 16 of water molecules at surfaces12h14,63 and entropic —uctuation forces arising from the overlap of thermally excited surface modes of molecular vibrations including long-wavelength undulation forces and molecular scale protrusion forces.10,64,65 A reasonable –t of the total force curve (solid line in Fig.3A) is obtained for decay lengths of 0.3 nm a preexponential factor of 2 nN a vdW radius of 100 nm and Hamaker constant of 6]10~21 J if the vdW origin is set 0.6 nm behind the point of zero separation (PZS) of the measured forces (the AFM measures relative displacement and PZS is de–ned as the position of the probe at maximum force). A vdW radius larger than the speci–ed AFM probe was used for the vdW force calculation which is justi–ed due to the pyramidal shape of the probe. Locating the vdW plane behind the PZS suggests that a water layer is bound to the surfaces at the PZS and a distance of 0.6 nm corresponds to two monolayers of water.These results are similar to the SFA and OS measurements between lipid bilayers in which repulsive hydration forces have an exponential form with 0.3 nm decay length.8,9,11h14 Signi–cant hysteresis is observed in the retracting force curve between the C -CH surface 15 3 and C -OH probe Fig. 3B. The strong adhesion between the C -CH surface and C -OH 16 15 3 16 probe appears to result from a decrease in the magnitude of the hydration force between these surfaces and/or an increase in the vdW forces. These results are similar to force measurements made between identical hydrophobic surfaces in that strong adhesion is observed between the surface but diÜer in that the long-range attractive hydrophobic eÜect is not observed.66 Forces between lipid bilayers and alkanethiol-functionalized probes interplay of hydration/steric forces and mechanical eÜects As in the case of alkanethiol monolayers the interaction forces between the C -OH probe and 16 lipid bilayers is expected to result from a balance between repulsive hydration/steric and attractive vdW interactions.At low loads the surface forces on the solid-like DSPE and DGDG –lms appear to be dominated by repulsive hydration/steric interactions Fig. 7B and D. This seems consistent with measurements of surface forces between symmetric lipid –lms with the SFA and OS but on closer examination there are several important diÜerences. First exponential –ts of the repulsive portion of the DSPE and DGDG forces curve result in decay lengths of 0.5^0.2 and 0.8^0.2 nm respectively which are 2»5 times larger than the decay lengths measured with the SFA and OS.8,9,11h14 Second the probe jumps to contact over a distance of 5»8 nm at loads [5 nN Fig.7C and E. If the jump-to-contact were associated with an attractive vdW force this would place the observed repulsive force beyond the range typically associated with hydration/steric forces. The AFM and SFA instruments are based on the same physical principles and use similar geometries i.e. the equivalent of a sphere on a —at thus they should generate similar results. On closer consideration however there is a signi–cant diÜerence in the pressure ranges over which the AFM and SFA typically make measurements.The theoretical Hertzian pressure generated by Faraday Discuss. 1998 111 79»94 90 a D50 nm radius sphere at 1 nN of load is 2]107 Pa. SFA measurements are typically made at pressures less than 105 Pa due to compliance of an epoxy used in the mechanical loop. Direct comparison to OS measurements is complicated by diÜerences in geometry and the isotropic nature of the osmotic force. The high pressures in the AFM suggests that the mechanical properties of the bilayer may contribute to the measured forces. The role of bilayer mechanics is supported by –tting the repulsive portion of the DSPE and DGDG force curve with models that combined hydration/steric forces with mechanical deformation. A reasonable –t of the repulsive portion of the DSPE force curve Fig.9A is achieved with the combined model (8) d\j ln(P/Fo)](9/16RE*2)1@3P2@3 where j\0.35 nm Fo\0.8 nN R\50 nm and E*\1.7]108 N m~2 if mechanical contact (Hertian plane) takes place 0.3 nm after the hydration/steric force can –rst be detected (hydration/ steric plane). The repulsive portion of the DGDG force curves Fig. 9B is also reasonably well –tted with a combined model for j\0.3 nm Fo\0.15 nN R\50 nm and E*\2.1]108 N m~2 if the Hertzian plane is placed 1.2 nm inside the hydration/steric plane. The distance separating the hydration from the vdW origin re—ects the range of the hydration/steric force. The diÜerence in the range over the DSPE and DGDG –lms is directly correlated with diÜerences in the measured step heights thus con–rming the imaging mechanism discussed above.The diÜerent behaviors observed for DSPE and DGDG surfaces may be understood in terms of diÜerences in hydration and steric properties. The fact that the mechanical component of the force is applied to DSPE after only 0.3 nm may be explained by the relatively low degree of hydration of the phosphatidylethanolamine (PE) head group.12,46 This has been attributed to the fact that PEs form a compact lattice that is stabilized by strong hydrogen bonding between the ammonium and phosphate groups.67 In contrast the fact that the mechanical model only becomes relevant at a separation of D1 nm for DGDG is consistent with the mobility of the digalactoside head group and the large number of hydration water molecules associated with head group.67 As the load is increased mechanical deformation of the bilayers by the probe plays an increasingly important role in the measured forces of the bilayer surfaces.Several observations suggest that the jump-to-contact results from a transition in the mechanical behavior of the bilayer from elastic to plastic deformation representing the penetration of the probe through the bilayer to the Fig. 9 Approaching force curve recorded over (A) DSPE and (B) DGDG domains with theoretical –ts + raw data »»»» Hertzian contribution óó hydration/steric forces and »» combined theoretical force curve. The relative separation axis has been set with its origin at the point at which the probe jumps-tocontact with the surface. 91 Faraday Discuss.1998 111 79»94 mica surface. First the distance over which the jump to contact takes place is in reasonable agreement with the bilayer thickness i.e. TDSPE@DSPE\6.6 nm and TDSPE@DGDG\6.2 nm. Second the strong adhesive forces between the probe and surface after jump-to-contact are consistent with the forces measured between the hydrophilic and hydrophobic surfaces (Fig. 3B). Third nanoindentation studies of thick LB –lms suggest the hardness of the –lm to be on the order of 108 N m~2.68 If we assume the yield pressure of the –lms is equivalent to hardness the transition from elastic to plastic deformation should take place at loads [5 nN. A similar conclusion was reached by Ducker and Clarke69 for self-assembled zwitterionic surfactant monolayers in water.Accordingly these results lead us to conclude that on DSPE and DGDG the probe experiences repulsive forces that are a combination of hydration/steric forces and mechanics. Summary Although the surface properties and interaction forces of lipid bilayers have been investigated for many years direct information at high lateral resolution has been inaccessible up to now. This study demonstrates that chemically functionalized AFM probes can be used to characterize the surface properties (surface morphology surface forces and mechanics) of heterogeneous lipid bilayers on the nanometer scale. Short-range repulsive forces are detected between the hydroxy-terminated probe and the 2-D solid-like DSPE and DGDG domains. The ranges and exponential decay lengths of the forces are larger than those found so far between amphiphilic bilayers using the OS and SFA techniques which may be attributed to the fact that mechanical properties of surfaces are sampled at the high loads that can be achieved with the AFM.This highlights the important role that mechanics may play in biomolecular interactions. The direct correlation between the range of the repulsive force over the two solid lipid domains and the step heights in the topographic images validates the quantitative analysis of the measured forces and sheds new light on the fundamental mechanisms of AFM imaging of lipid bilayers in aqueous solutions. Surprisingly no repulsive force is found over the 2-D liquid-like DOPE region which is related to the low packing density of this phase.Taken together these results provide direct evidence for the in—uence of the structure of lipid bilayers on their interaction forces a behavior that may be of prime importance in the control of cellular interactions. The methodology developed here has promising applications for the nanometer scale mapping of the properties (mechanical properties and molecular interactions) of biosurfaces and the study of surface forces in complex asymmetric systems. Cell surface components such as gangliosides integrins and lectins can be incorporated into a patterned bilayer and characterized with AFM probes functionalized with relevant (bio)chemical moieties. This would greatly improve our understanding of the molecular mechanisms of cellular processes such as cell adhesion membrane fusion molecular recognition and intercellular communication.Acknowledgements This research was supported by the Office of Naval Research (ONR) and by a NATO Research Fellowship (Y.F.D.). We also thank M. Fletcher David L. Allara and R.J. Colton and all members of NRL Code 6177 for valuable discussion M.L. Stevens and J.-B. Green for calibration of the AFM M.D. Porter for providing some of the alkane thiols M. Tortonese for supplying micromachined reference cantilevers N.H. Turner for performing XPS analysis K.Lee for programming associated with the adhesion images and J. Callahan and M. Shahgholi for electrospray mass spectroscopy analysis. References 1 R. O. Hynes Cell 1992 69 11. 2 A. Varki Proc. Natl. Acad. Sci. USA 1994 91 7390.3 N. Sharon and H. Lis Science 1989 246 227. 4 W. Curatolo Biochim. Biophys. Acta. 1987 906 137. 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Hochmuth in Current T opics in Membranes and T ransport ed. A. Kleinzeller and F. Bronner Academic Press New York 1978 pp. 1»10. 57 E. A. Evans and P. Kwok Biochemistry 1982 21 4874. 58 K. J. Tupper and D. W. Brenner L angmuir 1994 10 2335. 59 S. Manne J. P. Cleveland H. E. Gaub G. D. Stucky and P. K. Hansma L angmuir 1994 10 4409. 60 S. Nir Prog. Surf. Sci. 1976 8 1. 61 D. Gingell and V. A. Parsegian J. T heor. Biol. 1972 36 41. 93 Faraday Discuss. 1998 111 79»94 62 D. B. Hough and L. R. White Adv. Colloid Interface Sci. 1980 14 3. 63 S. Marcelja and N. Radic Chem. Phys. L ett. 1976 42 129. 64 J. N. Israelachvili and H. Wennerstroé m L angmuir 1990 6 873. 65 J. N. Israelachvili and H. Wennerstroé m J. Phys. Chem. 1992 96 520. 66 J. N. Israelachvili and R. M. Pashley Nature (L ondon) 1982 300 341. 67 J. M. Boggs Biochim. Biophys. Acta 1987 906 353. 68 T. P. Weihs Z. Nawaz S. P. Jarvis and J. B. Pethica Appl. Phys. L ett. 1991 59 3536. 69 W. A. Ducker and D. R. Clarke Colloids Surf. A 1994 93 275. Paper 8/07637G Faraday Discuss. 1998 111 79»94

ISSN:1359-6640    

DOI:10.1039/a807637e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Modelling and simulation of light-activated membrane proteins Dynamical transitions in bacteriorhodopsin Christian Simon,a Malika Aalouacha and Jeremy C. Smith*ab a L aboratoire de Simulation Moleculaire Section de Biophysique des Proteç ines et des Membranes DBCM CEA-Saclay 91191 Gif-sur-Y vette CEDEX France b L ehrstuhl f ué r Biocomputing IW R Universitaé t Heidelberg Im Neuenheimer Feld 368 D-69120 Heidelberg Germany Receiøed 2nd September 1998 Many of the functions of membranes are carried out by proteins associated with them. A knowledge of atomic-detail membrane protein structures and dynamics is required for a full understanding of these functions. We brie—y discuss recent progress in this –eld using modelling and simulation. One of the best characterised membrane proteins bacteriorhodopsin undergoes dynamical transitions with temperature.Here we present preliminary results of molecular dynamics simulation of this protein as a function of temperature indicating the presence of dynamical transitions at approximately the temperatures seen experimentally. Introduction Many of the functions of biological membranes are performed by proteins bound to them. Among the roles of these proteins are the reception/transmission of messages and/or the transport of materials. However due to difficulty in their crystallization only a small number of atomic-detail three-dimensional structures exist for membrane-spanning proteins. Among the few known structures are those of the light-transducing proteins the photosynthetic reaction centre and bacteriorhodopsin.1h4 In the present paper we brie—y review some recent progress in the modelling and simulation of light-activated membrane proteins before presenting new results on dynamical transitions in bacteriorhodopsin as a function of temperature. The paucity of crystallographic structures has led to a bottleneck in structural membrane protein research and adds impetus to the development of computer modelling techniques for determining their structures. One of these techniques is homology modelling it can be possible to determine an unknown protein structure by using a known X-ray structure as a three-dimensional template if there is sequence homology between the two. The higher the sequence homology the higher the probability of obtaining a reliable model structure.An example of homology modelling at high sequence identity is the recent work on the photosynthetic reaction centre protein from the bacterium Rhodobacter capsulatus.5 This protein has been the subject of a considerable amount of molecular biological and spectroscopic work aimed at improving our understanding of the primary steps of photosynthesis. A structural model was derived by combining information from the experimental structure of the highly homologous (54% sequence identity) reaction centre from Rhodopseudomonas viridis1 with molecular mechanics 95 Faraday Discuss. 1998 111 95»102 and simulated annealing calculations. In the Rb. capsulatus model the orientations of the bacteriochlorophyll monomer and bacteriopheophytin cofactors on the pathway inactive in electron transfer diÜer signi–cantly from those in the reaction centre of Rps.viridis. The orientational diÜerence was found to be in agreement with linear dichroism measurements.6 Moreover the pattern of cofactor hydrogen-bonding to the protein was found to be in agreement with optical spectroscopic experiment.7 The Rb. capsulatus model was used to provide an explanation as to why a partially symmetrized mutant Rb. capsulatus which has been of particular interest for experiments on primary excited states in photosynthesis lacks an electron acceptor bacteriopheophytin (BPhL).8h10 Conformational energy calculations on the partially symmetrised mutant and several BPh -binding revertants also provided an explanation for the relative BPh -binding properties of L L the proteins in terms of interactions involving two residues in the binding pocket these being a tryptophan and a methionine.10 Modelling at lower sequence homology although less reliable can be useful for suggesting experiments as part of an iterative procedure to obtain structural information on a membrane protein of particular interest.An example of this is the recent modelling of the photosystem II reaction centre core in plants for which a model was constructed by exploiting homology existing with the bacterial reaction centre proteins.11 In the rare cases where high-resolution experimental structures do exist modelling and simulation can be undertaken so as to re–ne structural detail and to understand physically how structure leads to function.A good example of such a system is bacteriorhodopsin (bR) a membrane protein that functions as a light-driven proton pump in the purple membrane of the bacterium Halobacterium halobium.12 The light-absorbing chromophore in bR is a retinal molecule that is covalently bonded via its SchiÜ base to the e-amino group of Lys 216.13 The characteristic purple colour of bacteriorhodopsin is due to absorption by the chromophore. The absorption is redshifted with respect to that of related model compounds in solution an eÜect that has been proposed to originate from interactions between the retinal and its polar environment in the protein.14 The retinal interactions may include hydrogen bonds with the SchiÜ base.Structures for bR at high resolution have been obtained.2,3 These revealed a channel through the protein that includes the SchiÜ base. Site-directed mutagenesis experiments suggest that the channel contains the pathway for proton transfer through bR.15h18 A considerable amount of data exist that suggest that the proton transfer channel is at least partially hydrated. Low resolution neutron diÜraction using contrast variation has indicated that about four water molecules are present in the neighborhood of the SchiÜ base although their positions in the direction perpendicular to the membrane plane could not be accurately determined.19 There is however considerable other evidence that water molecules are directly associated with the SchiÜ base. A resonance Raman study suggests that a negatively charged counterion located near the SchiÜ base group is stabilized by water molecules.20 Solid state 13C and 15N NMR experiments led to a model being proposed in which a water molecule is directly hydrogen-bonded to the SchiÜ base.21 Other solid state 1H and 15N NMR experiments suggest that there is a direct exchange of the SchiÜ base NH hydrogen with bulk water.22 A recent resonance Raman study of the SchiÜ base hydrogen»deuterium exchange also led to the conclusion that a water molecule is directly hydrogen bonded to the SchiÜ base NH proton.23 Finally the recent crystallographic structure of Pebay-Peyroula et al.has directly identi- –ed some water molecules associated with the SchiÜ base.3 Clearly a detailed understanding of SchiÜ base hydrogen bonding in the various stages of the photocycle will be required for a complete description of bR function.Computational chemistry has an important role to play in resolving such questions by identifying and quantifying hydrogen-bonding geometries and energies of pertinent model systems. For example quantum chemistry and molecular mechanics techniques have been combined to determine the geometries and energetics of retinal»water interactions.24,25 Ab initio molecular orbital calculations were used to determine potential surfaces for water»SchiÜ base hydrogen bonding and to characterize the energetics of rotation of the CwC single bond distal and adjacent to the SchiÜ base NH group. The ab initio results were combined with semiempirical quantum chemistry calculations to produce a data set used for the parameterization of a molecular mechanics energy function for retinal.Using the resulting molecular mechanics force –eld the hydrated retinal and associated bR protein environment were energy minimized and the resulting geometries examined. Two distinct Faraday Discuss. 1998 111 95»102 96 sites were found in which water molecules can make hydrogen-bonding interactions one near the NH group of the SchiÜ base in a polar hydrophilic region directed towards the extracellular side and the other near a retinal CH group in a relatively hydrophobic region directed towards the cytoplasmic side. To enable further investigations of internal hydration in bR and other systems a statistical mechanical formulation was derived that can be employed using molecular dynamics (MD) simulation to calculate the free energy of transfer of a small molecule from one environment to a speci–c site in another using molecular dynamics simulation.26 The method was used to calculate the free energy of transfer of water molecules from the bulk to individual sites in the proton transfer channel of bR.The channel contains a region lined primarily by nonpolar side-chains. The results obtained indicate that the transfer of water molecules from bulk water to this apparently hydrophobic region is thermodynamically favorable. The presence of two water molecules in direct hydrogen-bonding association with the SchiÜ base was also found to be thermodynamically allowed. Once a complete structural model of bR is obtained theoretical investigations into the photocycles of this protein can be envisaged.One interesting aspect of this in bR is the phenomenon of dark-adaptation in which retinal is found to exist in both all-trans and (13,15) syn conformations in approximately equal proportions. A theoretical investigation into dark-adaptation has been initiated. Initial free energy molecular dynamics calculations on a model of the isolated retinal suggested that the all-trans form is strongly favoured in vacuo.27 Calculations of factors in—uencing the conformational free energy diÜerence in the protein are now in progress. To fully understand bR function structural and thermodynamic examination must necessarily be complemented by dynamical investigations.Several analyses using molecular dynamics have been reported.28h33 In this respect it is of considerable interest that dynamical transitions have been found in bR as a function of temperature and have been correlated with function.34,35 In what follows we present preliminary results on the transitions investigated with molecular dynamics. The atomic position mean-square displacements are computed from a number of simulations at temperatures between 20 K and 300 K. Dynamical transitions are observed in the simulations at D150 K and D240 K. Methods Molecular dynamics The model system consists of 3544 atoms of bR of which 1806 are hydrogens. Four internal water molecules were included placed according to crystallographic data A 3 and each within 1 é of the positions derived in ref.27. The model system was subjected to molecular dynamics simulation using version 25 of the CHARMM program36 with the potential function described in ref. 37 26 and 27. The function includes bonded interactions (bond stretches bond angle bendings and dihedral and improper torsions) and nonbonded pairwise interactions represented by 12»6 Lennard- Jones and Coulombic electrostatic potentials which were cut-oÜ at 12 Aé . The Coulomb term was smoothed by multiplying by a switching cubic function between 8 and 12 Aé . The bR model molecule was simulated without an explicit environment. To approximately mimic the eÜect of the environment the relative permittivity was set to 1 and the a carbon atoms of the residues most surface exposed were harmonically restrained :28 the force constant used for this has a small value of 0.2 kcal mol~1 Aé ~2.This value was chosen so as to prevent gross deviation from the experimental structure while allowing internal —exibility. The system was energy minimized using 500 steps of Steepest Descent minimization followed by 2500 steps of Adopted Basis Newton»Raphson.36 The –nal RMS gradient was 0.15 kcal mol~1 Aé ~1. The energy-minimized structure was used as a starting point for the MD simulations. The equations of motion were solved using the Verlet algorithm. The SHAKE algorithm was applied to –x the lengths of the bonds involving hydrogen atoms. A 2 fs time step was used for integration of the equations of motion. MD simulations were performed at thirty temperatures from 10 to 300 K at 10 K intervals as follows.The minimized structure was heated to 10 K over 1 ps equilibrated over 5 ps then 10 ps of production was performed in microcanonical ensemble. The –nal frame of the production was then used for 1 ps heating to 20 K and the procedure 97 Faraday Discuss. 1998 111 95»102 Fig. 1 Temperature»time series calculated from the MD simulations. repeated. Subsequently the production runs were each extended by 100 ps yielding thirty 110 ps-long production trajectories at each temperature. The con–gurations were dumped to disk every 100 fs (50 steps) i.e. 1100 conformations per trajectory. Experimental connection Dynamical transitions of bR have been observed by incoherent neutron scattering experiments.The measurable quantity in incoherent neutron scattering the dynamical structure factor is the time Fourier transform of the intermediate scattering function I(q t) where q is the scattering wavevector and t is the time. I(q t) is the sum of the I(q t) of the individual atoms.38 In the case of scattering by a single atom in a harmonic potential I(q t) is Gaussian in q:39,40 (1) I(q t)\expS[q2c(t)V where and Sd2(t)T is the mean-square displacement de–ned as c(t)\16Sd2(t)T (2) Sd2(t)T\S[R(t)[R(0)]2T where R(t) is the atomic position vector and S… … …T indicates an ensemble average. The in–nite time limit of Sd2(t)T is (3) Su2T\ limSd2(t)T t?= Assuming limSR(t)R(0)T\0 then t?= (4) Su2T\2SdR2T where dR(t)\R(t)[SRT the displacement from the mean at instant t.The quantities Sd2(t)T and SdR2(t)T can be extracted from molecular dynamics trajectories by replacing the ensemble average with a time average. However the in–nite time Su2T can be obtained only from simulations long enough to sample all the motions involved. Correspondingly the experimental Su2T can be obtained only when the instrumental energy resolution is sufficiently good to resolve all the contributing motions. The neutron cross-section of a protein is dominated by the hydrogen atoms. Faraday Discuss. 1998 111 95»102 98 Results Stability of the simulation Fig. 1 shows time series of the temperature for each simulation. No signi–cant drift is seen at any temperature. The total energy was also found to be stable.The root mean square positional deviation (RMSD) between each bR conformation of the trajectory at each temperature and the initial energy-minimized structure is plotted in Fig. 2. The RMSD increases with temperature but remains approximately constant with time for most temperatures. Existence of dynamical transitions In Fig. 3 the simulation-derived hydrogen-atom Su2T is plotted against temperature. The increase in Su2T between 20 and 300 K is approximately 0.6 Aé 2 in accord with the neutron results obtained for dry purple membrane (PM).41 An in—ection is seen at D150 K a temperature at which a dynamical transition in bR has been experimentally reported.35,41 Fig. 2 Time series of the RMSD from the initial energy minimized structure at each temperature averaged over all the bR atoms.Fig. 3 Simulation-derived mean square displacement averaged over the hydrogen atoms vs. temperature. 99 Faraday Discuss. 1998 111 95»102 Fig. 4 Temperature dependance of the normalized variational contribution of the individual residues to the mean square displacement. The residues displacements were averaged over the hydrogen atoms. We introduce the variation with temperature of Sui2T the mean-square displacement of residue i (5) *Sui2T(T )\Sui2T(T ]dT )[Sui2T(T ) where dT is 10 K in this case. The normalized variational contribution of residue i is (6) **Sui2T(T )\ *Sui2T(T ) *Su2T(T ) where *Su2T\;i *Sui2T. **Sui2T is plotted against T and the residue number i in Fig. 4. Below 150 K all residues have **Su approximately equal i2T close to zero.At D140 K a dynamical transition is revealed by **Su variation of i2T for some residues. A second transition occurs at D240 K and above involving **Su larger variation of i2T than the D150 K transition and concerning a larger number of residues. A dynamical transition at D240 K has also been reported in neutron scattering work and has been correlated to the activation of bR function.34 Conclusions The modelling and simulation of membrane protein structures and dynamics is still in its infancy but will be of growing importance as more and more sequences of membrane proteins are determined. As structural research progresses the investigation of associated dynamical properties can also be expected to gain importance. The presence of a dynamical transition with temperature in water-soluble proteins has been recognized for some while.The recent neutron results on bacteriorhodopsin have demonstrated the presence of transitions also in a membrane protein.35,41 The present preliminary results indicate that transitions may also be apparent in molecular dynamics simulation. More research will be required to test further the present –ndings. In particular simulations of bR with an explicit membrane environment i.e. with the protein in trimer form with lipid and water surroundings rather than with harmonic constraints can be envisaged although they will be computationally demanding. Signi–cant variation in the dynamical transition properties with environmental changes has been documented.Tests of the temperature hysteresis would also be interesting to make. The question also arises as to how long a simulation would have to be performed at any given temperature to obtain converged mean-square displacements. Finally an Faraday Discuss. 1998 111 95»102 100 examination using MD of the eÜect of the application of the Gaussian approximation [Eqn. (1)] to the intermediate scattering function would be of interest. Two transitions are seen in Fig. 4 at D150 and D240 K. These are close to the temperatures at which transitions were seen experimentally. The higher-temperature transition is at about the water-melting temperature. But the present simulation was performed in the absence of water indicating that water is not required for it. However the D240 K transition is not apparent in the Su2T data in Fig.3 and may not therefore correspond to that seen experimentally. Work on this and other related questions is in progress. Acknowledgements We thank E. Pebay-Peroula for providing the crystallographic structure and D. Mihailescu J. Baudry and B. Costescu for useful discussions and preliminary calculations. References 1 J. Deisenhofer O. Epp K. Miki R. Huber and H. Michel Nature (L ondon) 1985 318 618. 2 R. Henderson J. M. Baldwin T. A. Ceska. F. Zemlin E. Beckmann and K. H. Downing J. Mol. Biol. 1990 213 899. 3 E. Pebay-Peyrouola G. Rummel J. P. Rosenbusch and E. M. Landau Science 1997 277 1676. 4 Y. Kimura D. G. Vassylyev A. Miyazawa A. Kidera M. Matsushima K. Mitsuoka K. Murata T. Hirai and Y.Fujiyoshi Nature (L ondon) 1997 389 206. 5 N. Foloppe M. Ferrand J. Breton and J. C. Smith Proteins Structure Function Genetics 1995 22(3) 226. 6 J. Breton E. J. Bylina and D. C. Youvan Biochemistry 1989 28 6423. 7 T. Mattioli personal communication. 8 M. H. Vos F. Rappaport J. C. Lambry J. Breton and J. L. Martin Nature (L ondon) 1993 363 320. 9 S. J. Robles J. Breton and D. C. Youvan Science 1990 248 1402. 10 N. Foloppe M. Ferrand and J. C. Smith Chem. Phys. L ett. 1995 242 238. 11 B. Svensson C. Etchebest P. TuÜery P. van Kan J. C. Smith and S. Styring Biochemistry 1996 35 14486. 12 D. Oesterhelt and W. Stoeckenius Nature (L ondon) New Biol. 1971 233 149. 13 K. J. Rothschild P. V. Argade T. N. Earnest K-S. Huang E. London M-J. Liao H. Bayley H.G. Khorana and J. Herzfeld J. Biol. Chem. 1982 257 8592. 14 R. A. Mathies S. W. Lin J. B. Ames and W. T. Pollard Annu. Rev. Biophys. Biophys. Chem. 1991 20 491. 15 T. Mogi L. J. Stern N. R. Hackett and H. G. Khorana Proc. Natl. Acad. Sci. USA 1987 85 5595. 16 T. Mogi L. J. Stern T. Marti B. H. Chao and H. G. Khorana Proc. Natl. Acad. Sci. USA 1988 84 5595. 17 L. J. Stern and H. G. Khorana J. Biol. Chem. 1989 264 14202. 18 T. Marti H. Otto T. Mogi S. J. Roé sselet M. P. Heyn and H. G. Khorana J. Biol. Chem. 1991 266 6919. 19 G. Papadopoulos N. Dencher G. Zaccaïé and G. Bué ldt J. Mol. Biol. 1990 214 15. 20 P. Hildebrandt and M. Stockburger Biochemistry 1984 23 5539. 21 H. J. M. De Groot S. O. Smith J. Courtin E. van der Berg C. Winkel J. Lugtenburg R.G. Griffin and J. Herzfeld Biochemistry 1990 29 6873. 22 G. S. Harbison J. E. Roberts J. Herzfeld and R. G. Griffin J. Am. Chem. Soc. 1988 110 7221. 23 H. Deng L. Huang R. Callender and T. Ebrey Biophys. J. 1994 66 1129. 24 M. Nina B. Roux and J. C. Smith in Structures and Functions of Retinal Proteins ed. J. L. Rigaud Colloque INSERM/John Libbey Eurotext Ltd. 1992 vol. 221 pp. 17»20. 25 M. Nina J. C. Smith and B. Roux J. Mol. Struc. (T HEOCHEM) 1993 286 231. 26 B. Roux M. Nina R. Pomes and J. C. Smith Biophys. J. 1996 670. 27 J. Baudry S. Crouzy B. Roux and J. C. Smith J. Chem. Inf. Comp. Sci. 1997 37(6) 1018. 28 M. Ferrand G. Zaccaïé M. Nina J. C. Smith C. Etchebest and B. Roux FEBS L ett. 1993 327 256. 29 W. Humphrey I. Logunov K. Schulten and M.Sheves Biochemistry 1994 33 3668. 30 W. Humphrey E. Bamberg and K. Schulten Biophys. J. 1997 72(3) 1347. 31 I. Logunov and K. Schulten J. Am. Chem. Soc. 1996 118(40) 9727. 32 D. Xu C. Martin and K. Schulten Biophys. J. 1996 70(1) 453. 33 D. Xu M. Sheves and K. Schulten Biophys. J. 1995 69(6) 2745 34 M. Ferrand A. J. Dianoux W. Petry and G. Zaccai Proc. Natl. Acad. Sci. USA 1993 90 9668. 35 V. Reç at H. Patzelt M. Ferrand C. P–ster D. Oesterhelt and G. Zaccai Proc. Natl. Acad. Sci. USA 1998 95 4970. 36 B. R. Brooks R. E. Bruccoleri B. D. Olafson D. J. States S. Swaminathan and M. Karplus. J. Comput. Chem. 1983 4 187. 37 A. D. MacKerell Jr. D. Bashford M. Bellot R. L. Dunbrack Jr. J. D. Evanseck M. J. Field S. Fischer J. Gao H. Guo S. Ha D. Joseph-McCarthy L. Kuchnir K. Kuczera F. T. K. Lau C. Mattos S. 101 Faraday Discuss. 1998 111 95»102 Michnick T. Ngo D. T. Nguyen B. Prodhom W. E. Reiher III B. Roux M. Schlenkrich J. C. Smith R. Stote J. Straub M. Watanabe J. Wioç rkiewicz-Kuczera D. Yin and M. Karplus J. Phys. Chem. B 1998 102 3586. 38 M. Beç e Quasi Elastic Neutron Scattering Principles and Applications in Solid State Chemistry Biology and Materials Science Adam Hilger Bristol 1998 UK. 39 A. Rahman K. Singwi and A. Sjoé lander Phys. Rev. 1962 126(3) 986. 40 A. Rahman Phys. Rev. 1963 140(4) 1334. 41 V. Reç at G. Zaccai M. Ferrand and C. P–ster in Biological Macromolecular Dynamics. Proceedings of a W orkshop on Inelastic and Quasielastic Neutron Scattering in Biology ed. S. Cusack H Bué ttner M. Ferrand P. Langan and P. Timmins Adenine Pres New York 1996 p. 117. Paper 8/06840B Faraday Discuss. 1998 111 95»102 102

ISSN:1359-6640    

DOI:10.1039/a806840b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Interactions between poly(2-ethylacrylic acid) and lipid bilayer membranes E� ects of cholesterol and grafted poly(ethylene glycol) David Needham,* JeÜ Mills and Gary Eichenbaum Department of Mechanical Engineering and Materials Science Duke University Durham NC 27708-0300 USA Received 9th November 1998 The exchange of the protonatable polymer poly(2-ethylacrylic acid) (PEAA) has been studied with vesicle membranes containing cholesterol from 0 to 60 mol% or PEG2000-lipid (5 mol%). The release of an entrapped dye from 100 nm extruded liposomes was used as an assay for membrane perturbation by the polymer as a function of pH. The inclusion of cholesterol was found to reduce the pH at which the polymer caused release of the dye from the lipid vesicles and the degree of polymer protonation (i.e.degree of hydrophobicity) correlated well with the increase in elastic expansion modulus of the vesicle bilayer. The results are discussed in terms of a balance between polymer solubility and membrane expansion. With respect to the PEG barrier the presence of 5 mol% PEG2000 which represents full surface coverage did not prevent PEAA from inducing contents release demonstrating that highly hydrated polymeric layers are not eÜective barriers for other water soluble polymers and may point to some association between the two polymers. In pure lipid bilayer systems the aqueous solubility for membrane components is so low (of the order of 10~10 M) that bilayer mass can essentially be considered constant at least during the time of an experiment.In contrast in the presence of other membrane compatible materials like lysolecithin and bile salts which can have a signi–cant solubility in the aqueous phase as monomers and micelles the composition and properties of the bilayer can be changed in seconds and at concentrations in excess of the surfactant critical micelle concentration (c.m.c.) can lyse the membranes causing them to fail.1h4 Another important class of molecules that shows partitioning into bilayers are pH-sensitive polymers such as poly(ethylacrylic acid) (PEAA).5 Here protonation of the carboxyl groups of the polymer reduces the solubility of the molecule in aqueous media making it more hydrophobic. Consequently this promotes solution of the polymer in the hydrophobic interior of lipid bilayers.5 This class of molecule shows a well de–ned cooperative conformational transition from an expanded coil to a collapsed coil that is dependent on pH.The existence of this transition (when compared to the continuous condensation of simple polyelectrolytes such as polyacrylic acid) is a direct result of additional attractive interactions between hydrophobic moieties such as the ethyl group in PEAA. For polymers dissolved in aqueous solution the midpoint of this transition is at pH 6.2.6 Interestingly however in the presence of Ladipalmitoylphosphatidylcholine (DPPC) vesicles the midpoint of the transition is shifted up to pH 6.5.6 This shift indicates some adhesive interaction between the polymer and the membrane at pHs above its natural critical pH for the transition and a role of the bilayer in inducing this transition.103 Faraday Discuss. 1998 111 103»110 Up to now most of the work on this polymer has been carried out with DPPC bilayers. The work presented here was motivated by a desire to examine the way in which the properties of the bilayer and its interface may in—uence the polymer»bilayer interaction. We therefore chose to study bilayers containing cholesterol and bilayers containing a grafted layer of a diÜerent water soluble polymer poly(ethylene glycol) (PEG). The inclusion of cholesterol in lipid bilayer membranes causes a condensation of the interface and a dramatic decrease in elastic area compressibility of the bilayer as well as its permeability to water.7h9 The presence of PEG grafted to the bilayer interface via the incorporation of PEG»lipids into the bilayer can inhibit the close approach of globular macromolecules and micelles.10h12 It is of interest then to determine if these two diÜerent kinds of barriers can hinder the approach and interaction of a water-soluble polymer that can be triggered to undergo a conformational transition due to a lowering of solution pH making it less soluble in aqueous media and more soluble in lipid bilayers which it can then permeabilize.In the present paper then we report the measurement of the release of a —uorescent dye entrapped within lipid vesicles that is induced by the exposure of the vesicles to PEAA solutions as a function of solution pH. These measurements are carried out for lipid vesicle bilayers containing either cholesterol (from 0 to 60 mol%) or PEG-lipid (5 mol%).Based on previous mechanical property data,8 the expectation was that cholesterol-rich membranes would possibly inhibit the polymer»bilayer interaction. For the PEG-lipid membranes interest centered on whether the presence of a PEG covered surface (at the mushroom-brush transition) would inhibit approach and intercalation of the PEAA into the membrane and therefore prevent release of internal contents. Experimental Materials Stearoyloleoylphosphatidylcholine (SOPC) and cholesterol (from Avanti polar lipids) in chloroform (10 mg mL~1) were mixed to give cholesterol at 0 20 30 40 50 and 60 mol%. SOPC and distearoylphosphatidylcholine (DSPE)-PEG2000 (10 mg mL~1) were mixed to give DSPEPEG2000 at 5 mol%.20 mM Phosphate buÜered saline (PBS) [0.8% w/w saline (0.137 M)] was prepared using sodium phosphate monobasic sodium phosphate dibasic and sodium chloride from Mallinckrodt. Carboxy—uoroscence (6-CF) was obtained from Aldrich Inc. Liposome preparation The prepared SOPC and cholesterol solutions in chloroform were dried using a Rotavapor onto a 100 mL round-bottomed —ask.13 Lipid vesicles were prepared by hydrating the dried lipid –lm with a solution of 50 mM 6-carboxy—uoroscene (6-CF) (high enough concentration for —uorescence to be self-quenched) in 20 mM PBS.13 The hydrated lipid mixture was extruded through two 100 nm Poretics polycarbonate membrane –lters using a Lipex extruder to give a homogeneous sample of unilamellar lipid vesicles of 100 nm diameter,14 containing the entrapped carboxy —uoroscene dye in a quenched form.Lipid vesicles were then –ltered using a Sephadex G-50 column (Sigma) to remove unencapsulated 6-CF.13 Contents release measurements 20 mM PBS was prepared at pHs ranging from 5.5 to 7.5. PEAA was dissolved in 20 mM PBS at 33.3 mM (1 mg mL~1) over the same pH range. Both sets of solutions were osmotically matched using glucose to the 6-CF solution used to hydrate and form the liposomes to ensure there were no osmotic pressure gradients across the lipid membranes. 10 lL of liposome solutions were suspended in 1 mL of buÜer containing PEAA (time zero). Fluorescence (jex\436 nm jem\520 nm) intensity was measured every 5 s for 15 min at 30 min and 1 h.(Aminco-Bowman Series 2 Luminescence Spectrometer or a Shimadzu RF-1501 Spectro—uorophotometer were used to measure —uorescence.) As 6-CF leaked out of the liposomes and was diluted by the suspending buÜer self-quenching was eliminated resulting in an increased —uorescence signal. After 1 h 20 lL of Triton X-100 was added to completely solubilize the membranes and a –nal —uorescence reading was made. This –nal reading was used as a maximum or 100% release reference and all Faraday Discuss. 1998 111 103»110 104 other readings were divided by this value to obtain a measure of the ììpercent releasedœœ. This procedure was similar to that used by Gon8 i et al. to study the eÜects of surfactants on release from liposomes.15 In separate experiments the —uorescence of 6-CF in Triton X-100 was found to be unaÜected by the presence of surfactant but was dependent on pH.The pH-dependent intensity was controlled by making control measurements at the same pH. Results andiscussion Cholesterol-containing membranes The eÜect of cholesterol on the pH dependence of PEAA-induced release of contents from liposomes was investigated by measuring release of 6-CF (quenched inside the liposome) as indicated above for liposomes of various cholesterol compositions. First consider the release from liposomes composed of pure SOPC. Fig. 1 shows a plot of percent release of 6-CF vs. time in min for 100 nm liposomes composed of pure SOPC in a solution of 1 mg mL~1 PEAA. Each trace is the cumulated release of carboxy- —uoroscene —uorescence as it leaked out of the liposome sample over a period of 1 h.Each curve represents the pH of the bulk media in the range of 5.5»7.5. As shown on the plot the curve at pH 6.7 was the lowest pH at which the liposomes were stable in the polymer suspension. In polymer solutions more basic than or equal to pH 6.7 baseline release was observed rising from 10% to only 30% release over the time course measured. Similar values are noted for control solutions which did not contain PEAA. In contrast polymer solutions more acidic than pH 6.7 exhibited a burst release reaching 100% of entrapped 6-CF within a few minutes. By comparison Fig. 2 shows a plot of percent release of 6-CF as a function of time for 100 nm SOPC/cholesterol liposomes containing 40 mol% cholesterol.Here release was not observed at pH 6.7 but in solutions more acidic than pH 6.4. Thus the pH at which onset of release occurred was shifted from near pH 6.7 (for 0 mol% cholesterol) to near pH 6.4 (for 40 mol% cholesterol). Then by taking the steady state release values (60 min) at each pH for SOPC systems with cholesterol composition 0 20 30 40 50 and 60 mol% as shown in Fig. 3 a plot of steady state release as a function of pH for each composition was constructed. This –gure shows that all compositions achieved 100% release if the pH was acidic enough. Importantly it also shows that the pH at onset of release was monotonically shifted to more acidic pH values with increasing mol% cholesterol from pH 6.7 for a pure lipid system to pH 6.0 for a cholesterol-saturated (i.e.D60 mol%) system. Thus the uptake of protonated polymer by the bilayer membrane is increasingly inhibited as the bilayer is condensed with cholesterol. What these results then suggest is that there is indeed a Fig. 1 Plot of percent release of 6-CF vs. time in min for a system of pure SOPC 100 nm liposomes suspended in solutions of 1 mg ml~1 PEAA at pHs varying from 5.5 to 7.5. Error bars on each data point represent the standard deviation from the mean for 2 measurements. 105 Faraday Discuss. 1998 111 103»110 Fig. 2 Plot of percent release of 6-CF vs. time in min for a system of 100 nm SOPC liposomes with 40 mol% cholesterol incorporated in the membrane suspended in solutions of 1 mg ml~1 PEAA at pHs varying from 5.5 to 7.5.Error bars on each data point represent the standard deviation from the mean for 2 measurements. The intensity drop noted in some of the curves during the –rst 15 min can be attributed to photobleaching of the 6-CF but this artifact did not aÜect the steady state measurements. correlation between the compressibility of the lipid bilayer (determined by the presence of cholesterol) and the interaction of the polymer with the bilayer which is controlled by pH i.e. the degree of protonation of the polymer which in turn determines its degree of hydrophobicity. In order to develop this correlation further what is needed is a measure of the percent protonation of the polymer as a function of pH. Tirrell and colleagues have characterized the pHdependent solution behavior of the pure polymer.5,6 Titration of the polymer indicates that it has a pK of approximately 5.4.16 Graphically then as shown in Fig.4 as the pH is lowered in the a range 7.4»3.4 the percent of polymer that becomes protonated (and therefore hydrophobic) increases from close to zero to 100%. Critical pHs for some important features of polymer behavior and polymer»bilayer interaction from the previous literature and from the experiments reported here are summarized by this plot. At pH 6.2 as observed by Borden et al. using pyrene —uorescence the free polymer in aqueous solution undergoes a sharp conformational transition.6 Thus D15% of the polymer needs to be Fig. 3 Plot of steady state release of 6-CF (60 min after liposome addition to the polymer solution) vs.pH for 100 nm liposomes composed of SOPC and containing 0 20 30 40 50 and 60 mol% cholesterol. Error bars on each data point represent the standard deviation from the mean for 2 measurements. Faraday Discuss. 1998 111 103»110 106 a Fig. 4 Plot of percent protonation of the polymer vs. pH calculated from the Henderson»Hasselbach equation using the data from Tirrell et al.,16 which measured the pK as 5.4. Shown are critical pHs for several features of polymer and polymer»lipid behavior. protonated in order for the polymer to undergo the collapse transition. At pH 7.4 the polymer was found by Seki and Tirrell,17 to cause a broadening of the main acyl chain gel to liquid crystalline transition temperature for DPPC vesicles. Thus at relatively high pH where the polymer is only marginally (1%) protonated it is intimately associated with the bilayer.This association is sufficient to cause perturbation of the gel/liquid crystalline transition but not enough to cause the polymer collapse transition indicating that at this degree of protonation the interaction with the bilayer is insufficient to ììcatalyzeœœ the collapse. Also it seems that this expanded coil polymer cannot traverse to the inner layer of the bilayer in order to cause bilayer disruption. For DPPC vesicles Tirrell and colleagues showed that at pH 6.5 three events were correlated PEAA induced a sharp vesicle to micelle transition signifying massive reorganization of the lipid ;18 pyrene —uorescence increased sharply signifying the premature induction of the expanded coil to collapsed coil transition due to binding of polymer to the bilayer ;6 and the DPPC lipid acyl chain transition showed maximum broadening.6 Taken together these results implicate the collapse transition as a key event that leads to membrane disruption.In our experiments at pH 6.7 the polymer is D5% protonated when it causes release of dye from the SOPC vesicles (a slightly higher pH than that measured for reorganization of DPPC vesicles,5 and by inference and comparison with the DPPC data of Thomas and Tirrell,5 this signi–es the premature induction of the collapse transition by SOPC bilayers. At pH 6.1 where the polymer is 20% protonated it causes release of dye from cholesterol-saturated vesicles. At this pH the polymer is actually collapsed in solution which represents the maximum potential for the polymer to disrupt the bilayer.A Using KA the data from Fig. 4 and previous measurements of the elastic area expansion modulus for SOPC/cholesterol bilayers,8 Fig. 5 shows that the fractional increase in K is in fact found to be directly proportional to the increase in percent protonation required to cause release of dye from the vesicles. In evaluating this data in terms of energetic considerations there is an energy state for the polymer in aqueous solution that is pH dependent and there is an energy state for the polymer in the bilayer which may be changed by the presence of cholesterol and which determines the release 107 Faraday Discuss. 1998 111 103»110 Fig.5 ììFractional change plotœœ showing the fractional increase in percent protonation of the polymer (over and above that which occurs at pH 6.7) at the pH where dye is released vs. the fractional change in the elastic area expansion modulus (over and above that for the pure SOPC bilayer) for bilayers containing increasing amounts of cholesterol (in parentheses next to each data point). of CF. This latter energy determines the ability of the polymer to create defects and perturb the structure of the bilayer. Assuming that cholesterol does not decrease the pH at the bilayer interface relative to the bulk solution its role is therefore limited to its ability to increase the cohesiveness of the membrane. Then a greater amount of work is required to perturb the structure of the membrane and induce leakage of contents which has to be compensated by increasing the protonation of the polymer.The assay used here was simply the release of the marker dye (i.e. the end point of the interaction) and so does not provide any information on the amount of polymer Fig. 6 Plot of steady state release of 6-CF vs. pH for 100 nm liposomes composed of pure SOPC with 5 mol% PEG2000 as well as 100 nm liposomes composed of only SOPC. Faraday Discuss. 1998 111 103»110 108 bound. Thus at this stage in the work it is not possible to distinguish between the eÜect of cholesterol on polymer partitioning and the formation of defects that result in CF release. Critical information that needs to be obtained includes the concentration dependence of the contents release and a lipid to polymer ratio for the failure event.Grafted PEG Data for CF release from the PEG-grafted liposomes was also obtained as described above. A compilation of the results is shown in Fig. 6 as steady state release vs. pH. The curve for the PEG-grafted liposomes was nearly identical to the plot for the pure SOPC non-PEGylated vesicle case with release occurring near pH 6.8 and reaching 100% by pH 6.6. Thus despite the presence of a complete layer of PEG on the bilayer surface [enough to signi–- cantly retard the approach of globular molecules (avidin) and micelles (MOPC)] PEAA can still approach close enough to the bilayer interface to bind and cause membrane disruption. This indicates an ability of the PEAA polymer to reptate through the PEG polymer layer.As mentioned in the Introduction this same PEG layer cannot prevent MOPC monomer from adsorbing into or desorbing from such PEG-grafted bilayers suggesting that monomer can in fact diÜuse through the PEG layer.10 With a cross-sectional area of 40 ”2 the polymer has a similar size to MOPC of 34 ”2. A binding interaction between PEG and PEAA is also not ruled out. Concluding remarks The results of this study then suggest that membrane compressibility dominates the interaction between lipid bilayers and PEAA such that highly expandable membranes are more likely to take up hydrophobic polymer molecules than less expandable membranes with more condensed interfaces. We plan to obtain additional data on the binding and uptake of polymer by lipid bilayers including the use of the micropipet manipulation technique to measure any area change that may accompany polymer insertion into the membrane at pHs slightly higher than those required to initiate membrane disruption to observe CF release directly for a single vesicle and to measure any changes in the membrane compressibility and tensile strength for polymer-adsorbed but stable bilayers.For the PEG-lipid containing membranes no additional barrier eÜect was observed leading to the conclusion that either these aqueous-soluble polymers can in fact entangle such that the PEAA can come into intimate contact with the lipid interface or an attractive interaction occurs between PEG and PEAA especially for the protonated form of PEAA. This work was supported by grant GM 40162 from the NIH and by the National Science Foundation Engineering Research Center Grant CDR-8622201.The authors would also like to acknowledge useful discussions with Drs. Evan Evans Tom McIntosh Sid Simon and Doncho Zhelev and Mr. Patrick Kiser. References 1 E. Evans W. Rawicz and A. F. HoÜman Bile Acids in Gasteroenterology Basic and Clinical Advances ed. A. F. HoÜman G. Paumgartner and A. Stiehl Kluwer Academic Publishers Dordrecht Boston London 1994 pp. 59»68. 2 D. Needham and D. V. Zhelev Ann. Biomed. Egr. 1995 23 287. 3 D. V. Zhelev Biophys. J. 1996 71 257. 4 D. V. Zhelev Biophys. J. 1998 75 321. 5 J. L. Thomas and D. A. Tirrell Acc. Chem. Res. 1992 25 336. 6 K. A. Borden K. M. Eum K. H. Langley and D.A. Tirrell Macromolecules 1987 20 454. 7 M. Bloom E. Evans and O. G. Mouritsen Q. Rev. Biophys. 1991 24 293. 8 D. Needham and R. S. Nunn Biophys. J. 1990 58 997. 9 D. Needham and D. V. Zhelev V esicles ed. RosoÜ Marcel Dekker New York and Basel 1996. 10 D. Needham T. J. McIntosh and D. V. Zhelev L iposomes Rational Design. ed. A. JanoÜ Marcel Dekker New York 1998 in press. 11 D. Needham N. Stoiceva and D. V. Zhelev Biophysical 1997 73 2615. 12 D. Noppl-Simson and D. Needham Biophys. J. 1996 70 1391. 13 R. R. C. New L iposomes A Practical Approach. ed. R. R. C. New Oxford University Press New York 1990 ch. 2. 109 Faraday Discuss. 1998 111 103»110 14 M. J. Hope R. Nayay L. D. Mayer and C. P. R. Tilcock L iposome T echnology L iposome Preparation and Related T echniques. ed. G. Gregoriadis CRC Press Boca Raton Fl. 1993 ch. 8. 15 F. M. Gon8 i M. A. Urbaneja and A. Alonso L iposome T echnology Entrapment of Drugs and Other Materials ed. G. Gregoriadis CRC Press Boca Raton Fl. 1993 ch. 15. 16 K. H. Langley U. K. O. Shroé der and D. A. Tirrell personal communication. 17 K. Seki and D. A. Tirrell Macromolecules 1984 17 1692. 18 K. A. Borden K. M. Eum K. H. Langley J. S. Tan D. A. Tirrell and C. L. Voycheck Macromolecules 1988 21 2649. Paper 8/08717B Faraday Discuss. 1998 111 103»110 110

ISSN:1359-6640    

DOI:10.1039/a808717b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Membrane electroporation and electromechanical deformation of vesicles and cells Eberhard Neumann Sergej Kakorin and Katja Toensing Physical and Biophysical Chemistry Faculty of Chemistry University of Bielefeld P.O. Box 100 131 D-33501 Bielefeld Germany Received 17th August 1998 Analysis of the reduced turbidity (*T ~/T and absorbance *A~/A relaxations of 0) ( 0) unilamellar lipid vesicles doped with the diphenylhexatrienyl[phosphatidylcholine (b- DPH pPC) lipids in high-voltage rectangular electrical –eld pulses demonstrates that the major part of the turbidity and absorbance dichroism is caused by vesicle elongation under electric Maxwell stress. The kinetics of this electrochemomechanical shape deformation (time constants 0.1Oq/lsO3) is determined both by the entrance of water and ions into the bulk membrane phase to form local electropores and by the faster processes of membrane stretching and smoothing of thermal undulations.Moreover the absorbance dichroism indicates local displacements of the chromophore relative to the membrane normal in the –eld. The slightly slower relaxations of the chemical turbidity (*T `/T and absorbance *A`/A modes are both associated with the entrance of 0) ( 0) solvent into the interface membrane/medium caused by the alignment of the dipolar lipid head groups in one of the lea—ets at the pole caps of the vesicle bilayer. In addition (*T `/T indicates changes in vesicle shape and volume. The results for lipid vesicles 0) provide guidelines for the analysis of electroporative deformations of biological cells.1 Introduction The method of membrane electroporation is widely applied in cell biology and medicine to introduce eÜector substances and genes into biological cells and tissue.1 The mechanisms of electric pore formation in lipid bilayer membranes and the resulting transport facilitation for macromolecules are not yet well understood. Basic elements of the electric –eld eÜects have already been derived from electro-optic and conductometric data for model systems such as unilamellar lipid vesicles doped with optical membrane probes.2,3 Here the lipid probe 2-[3-(diphenylhexatrienyl propanoyl]-1-hexadecanoyl-sn-glycero-3-phosphocholine (b-DPH pPC) has been used where one of the hydrocarbon chains of the PC is replaced by the DPH residue.DPH[doped bilayer membranes exhibit characteristic absorbance changes in polarized light when subjected to electric –elds.4 Previously the negative absorbance dichroism of DPH-doped lipid vesicles was analysed in terms of structural rearrangements of lipids and of DPH molecules in the wall of electropores assuming the DPH site to be the origin of pore formation.5 On the other hand the electro-optic data with b-DPH pPC can only be rationalized quantitatively if the major part of the absorbance dichroism is attributed to global vesicle deformation under the electric Maxwell stress.2,6 Here we develop the theoretical and analytical framework to diÜerentiate the contributions of the –eld-induced membrane structural changes and of the vesicle shape deformation to the tur- 111 Faraday Discuss.1998 111 111»125 bidity and absorbance dichroisms. It appears that the two characteristic turbidity *T ~ and absorbance *A~ modes associated with orientational processes are rate-limited by the electric pore formation whereas the chemical turbidity *T ` and absorbance *A` modes reveal further details such as the entrance of water and ions into the interface regions of the lipid head groups.7,8 Additionally *A~ re—ects local chromophore displacements in the membrane. where *Ip\Ip(E)[Ip is the light *C (1) (2) (3) 2 Materials and methods 2.1 Vesicle suspensions Large unilamellar phospholipid vesicles and vesicles doped with the optical lipid probe b-DPH M pPC r\782 were prepared by the extrusion method as described by Toensing et al.9 The vesicle mean diameters determined by dynamic light scattering measurements (data not presented) are U\100^36 nm after extrusion through the 100 nm –lter ; in line with the size distribution of extruded vesicles determined by Mayer et al.10 During vesicle preparation and the following electro-optic relaxation measurements care was taken to protect the DPH samples from photolysis.The –nal total lipid concentration used for the optical and electro-optical measurements was app [LT]\1 mM corresponding to a vesicle density of ca. 7.4]1015 L~1 for vesicles of radius a\50 nm. Under these conditions the average distance between the anionic surfaces of single vesicles is about ca. 0.53 lm qualifying the suspension as dilute with practically no vesicle»vesicle contacts.Actually at the maximum –eld E\8 MV m~1 and at the eÜective permittivity of vesicle et eff e BeL\2.5 where L is the permittivity of the lipid membrane the minimum characteristic time of approach of vesicles due to induced dipole forces taking vesicle»vesicle hydrodynamic appB610 t interactions into account is ls.11,12 Therefore at the pulse duration of tE\10 ls and at –eld strengths EO8 MV m~1 we may safely neglect vesicle»vesicle interactions. 2.2 Electro-optical relaxation spectrometry Rectangular pulses of –eld strength up to 8 MV m~1 and of duration up to tE\10 ls are applied by cable discharge to the sample cell equipped with parallel planar graphite electrodes thermostatted at T \293.0^0.1 K (20 °C).The –eld-induced changes in the transmittance of planepolarized light were measured at the wavelength j\365 nm (Hg-line ; highest accuracy). The light intensity change *Ip caused by the electric pulse and measured at the polarization angle p relative to the direction of the applied external –eld vector E is related to the optical *Cp\Cp(E)[C density change by 0 p \[log(1]*Ip/Ip) intensity change from Ip (at E\0) to Ip(E) in the presence of E Cp(E) and C are the optical 0 p densities at E and at E\0 respectively. Generally C\A]lT comprising both absorbance (A) and turbidity (T ) along the light path length l. The absorbance Ap of the reporter lipids b-DPH pPC in the bilayer of the vesicles is given by the diÜerence *Cp between Cp(V,D) of the doped vesicles and Cp(V) of the vesicles without the reporter lipid but at the same total lipid concentration and vesicle size Ap\Cp(V,D)[Cp(V).The –eld-induced optical change may be decomposed into a deformational/orientational part OR p and a structural/chemical part *CCH p according to *Cp\*COR p ]*CCH p The –eld-induced changes *CA and *CM at the two light polarization modes p\0° (p parallel to the external –eld vector E) and p\90° (o perpendicular to E) are given by *CA\CA[C and 0 *CM\CM[C0 respectively. As outlined for the absorbance dichroism7 both the consumptive dichroism (*A) and the conservative scattering dichroism (*T ) are originally de–ned for optical density changes of purely deformational/orientational origin.13,14 In the notation used here the optical density dichroism is classically de–ned by *C\*COR A [*COR M On the other hand the diÜerence *C~ either *A~ or *T ~ of the actually measured changes *CA and *CM is given by *C~\*CA[*CM\*C](*CCH A [*CCH M ) Faraday Discuss.1998 111 111»125 112 *CCH A B*CCH M we may approximate the diÜerence In the case of small chemical contributions or if mode *C~ by the dichroism *C i.e. :7 *C~\*C (4) The equations *A`\*ACH and *T `\*T CH are straightforward ; analogous to the expression 7,8 *A for *CCH\(*CA]2*CM)/3 . Therefore *CCH generally refers to changes in we obtain CH the scattering cross-section or the immediate environment of the absorbing chromophore due to entrance of water and small ions. Note that if the scattering contribution is negligibly small we have *C\*A and outside the absorbance bands we use *C\l *T .The absorbance dichroism is a quantitative measure of rotational displacements comprising both global shape deformations as well as local chromophore shifts in the lipid bilayer. The turbidity term *TCH/T0 re—ects changes in the refractive index of the membrane due to entrance of water and ions as well as changes in vesicle shape and volume. 3 Results The b-DPH pPC membrane probes greatly increase the absorbance of light in the doped vesicles in the light wavelength range 300\j/nm\400 (Fig. 1). The optical density and the absorbance both increase linearly with the concentration of b-DPH pPC suggesting that at zero external –eld there are no optically detectable interactions between the ììreporter lipids œœ in the vesicle membrane (Fig.2). In the electric –eld pulse the very rapid (ca. 1 ls) increases in the *A`/A *A~/A and *T `/T absorbance terms *T ~/T and (Fig. 3) and the turbidity modes 0 0 0 0 (Fig. 4) are followed by similarly rapid –eld-oÜ relaxations. Outside the absorption band the electro-optic relaxations of labelled and non-labelled vesicles are practically identical (Fig. 5). These data show that the membrane probes incorporated in the lipid molecules change neither membrane electroporatability nor mechanical properties such as bending rigidity or membrane spontaneous curvature. At larger b-DPH pPC concentration there is a weak dependence of the chemical and dichroitic absorbance modes on the probe content in the membrane especially at higher –eld strengths (Fig.6). However no changes are observed in the turbidity terms *T ~/T0 and *T `/T outside the absorbance band with increasing concentration of b-DPH pPC in the 0 vesicle membrane (Fig. 7). At the –eld strengths 2OE/MV m~1O8 the chemical turbidity and Fig. 1 Optical density of a suspension of unilamellar vesicles composed of L-a-phosphatidyl-L-serine (PS) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in the molar ratio PS POPC of 1 2 (» » ») and the M same suspension doped with the optical probe b-DPH pPC r\782 at the concentrations [b-DPH pPC]/ lM\2.5 3.3 5 10 20 (from bottom to top) as a function of the wavelength j. Vesicle radius a\50 nm vesicle density ov\7.4]1015 L~1; total lipid concentration [LT]\1.0 mM; 0.66 mM HEPES-Na (pH 7.4) 0.13 mM CaCl2 T \293 K (20 °C).113 Faraday Discuss. 1998 111 111»125 Fig. 2 Optical density C365 (D V) at j\365 nm (Ö) of a doped PS POPC (1 2) vesicle suspension as a function of the total concentration of b-DPH pPC. The absorbance A365 (>) of the ììreporter lipid œœ b-DPH pPC at j\365 nm is given by the diÜerence of the doped (D V) vesicle system and the non-doped (V) one A365\*C365\C365 (D V)[C365(V) ; experimental conditions as in Fig. 1. Both C365 and A365 are linear in [b-DPH pPC] hence there is no optically visible interaction between the reporter molecules. Fig. 3 Field-on and –eld-oÜ relaxations of the absorbance terms *A`/A and *A~/A0 at the two extreme 0 –eld strengths E\2 (thin line) and E\8 MV m~1 (thick line) [b-DPH pPC]\5 lM.One rectangular electric pulse of –eld strength E and pulse duration tE\10 ls at T \293 K. Note the change in the timescale for tP20 ls. Experimental conditions as in Fig. 1. Faraday Discuss. 1998 111 111»125 114 *T ~/T and Fig. 4 Field-on and –eld-oÜ relaxations of the turbidity terms *T `/T (j\365 nm) for non- 0 0 doped vesicles at the two extreme –eld strengths E\2 (thin line) and E\8 MV m~1 (thick line). Wavelength j\365 nm. Experimental conditions as in Fig. 3. Visual inspection shows that there are at least two kinetic modes I and II in the presence of E. *T `/T0 *A`/A0 are 10-fold smaller than the dichroitic modes (*T ~/T0 justifying the approximations *T /T absorbance terms *A~/A 0\*T ~/T0 and *A/A0\*A~/A0 respectively. 0) 4 Theory and data analysis 4.1 Vesicle orientation If the vesicles shortly after extrusion were slightly elongated,15 they can be oriented in an electric –eld.However the very rapid (ca. 1 ls) after-–eld relaxations of the major part of the absorbance *A~/A (Fig. 3) and the turbidity *T ~/T (Fig. 4) modes exclude the possibility that the optical 0 0 signals are due to orientation of non-spherical vesicles in an external –eld. After-–eld disorientation of slightly elongated vesicles may be described by rotational diÜusion of a sphere of radius a\50 nm with the rotational relaxation time given by16 qrot\4nga3/(3kT ) where g is the viscosity of the solvent here water k the Boltzmann constant and T is the absolute temperature. With g\10.02]10~4 kg m~1 s~1 at T \293 (20°) qrot\130 ls.If the vesicles were modelled by ellipsoids q should be even slightly larger. In any case the time constants of the after-–eld dichroisms (ca. 1 ls) are appreciably smaller than qrot . Since the after-–eld rotational relaxation is rot independent of the –eld strength comparison of the time constants suggests that the major parts of the *T ~/T and *A~/A relaxations are caused by a mechanism diÜerent from the vesicle 0 0 orientation namely electric pore formation as well as membrane stretching and smoothing of thermal undulations under Maxwell stress. 4.2 Hydrophilic pore model If the reduced absorbance mode *A~/A is directly due to the electropores pore formation can be 0 115 Faraday Discuss. 1998 111 111»125 0 *C~/C of a doped vesicle suspension [b-DPH pPC]\5 lM; at wavelengths j\281 (» » ») and 436 Fig.5 Comparison of the reduced dichroitic (turbidity) modes (a) *T ~/T of an unlabelled vesicle suspension with (b) 0 (»») nm outside the b-DPH-residue absorption band (see Fig. 1). The extent and the rate of the turbidity dichroisms are independent of both the wavelength [*T ~/T (281 nm)B*T ~/T (436 nm)B*C~/C (281 0 0 0 nm)B*C~/C (436 nm)] and the presence or absence of the reporter lipid b-DPH pPC (b). One rectangular 0 electric pulse of –eld strength E\5 MV m~1 and pulse duration t Experimental conditions as in Fig. 1. E\10 ls was applied at T \293 K. described in terms of local lipid phase transitions involving cooperative clusters L of n lipids in n the pore edge. During this transition the membrane probes together with lipid molecules are locally rotating (Fig.8). The entrance of the highly polarizable aqueous solvent (water and ions) modi–es the local environment of the chromophores and is described by the term *A`/A0 . Based *A`(t Fig. 6 Dependence of the absorbance terms E)/A0 (open symbols) and *A~(tE)/A0 (–lled symbols) at tE\10 ls and j\365 nm on the b-DPH pPC concentration at –eld strengths E/MV m~1\2.0 (» @); 2.8 4.0 ( ) K ( C) ; 5.0 L ( 7) ; 6.5 +) ; 8.0 (« OP). One rectangular electric pulse –eld strength E and (| >); pulse duration tE\10 ls at T \293 K. Experimental conditions as in Fig. 1. Faraday Discuss. 1998 111 111»125 116 tE 6 ) ) ; 4.0 (K ( C) ; 5.0 L ( 7) ; 6.5 +) ; 8.0 (« OP). Experimental Fig. 7 Dependence of the turbidity terms *T ~(tE)/T0 (open symbols) and *T `(tE)/T0 (–lled symbols) at the end of the pulse at j\436 nm i.e.outside absorbance band on the b-DPH pPC concentration at –eld strengths E/MV m~1\2.0 (» @) ; 2.8 (| conditions as in Fig. 6. on the original concept of HO and HI pores,17 a speci–c scheme for electropore formation has been proposed:5 The state transitions from the closed membrane state (C) to hydrophobic (HO) and hydrophilic (HI) pore states are associated with the rotation of the lipids and chromophores in the HI-pore Fig. 8 HI pore model and vesicle geometry relative to the –eld direction E. Scheme for the molecular rearrangements of the lipids in the pore edges of the lipid vesicle membrane. C denotes the closed bilayer state. The induced membrane –eld leads to entrance of water into the membrane to produce pores (P) cylindrical HO pores or inverted HI pores.In the pore edge of the HI pore states the lipid molecules are rotated into the pore region to minimize the hydrophobic contact with water. In the pole caps the rotation of DPH lipids leads to a negative absorbance dichroism ([) and in the equatorial region to a positive contribution to the dichroism (]). 117 Faraday Discuss. 1998 111 111»125 wall edge. The speci–c structural organization of the lipids and DPH in the HI pore states is modelled by a 90° rotation of the optical transition moment l of the chromophore with respect to the membrane normal (Fig. 8). For the chromophores the scattering component of the molar absorption coefficients is negligibly small compared with the absorbance component.Therefore a change in the scattering of the rotated chromophores is negligible. Perpendicular to the optical transition moment of chromophore the absorbance is zero.18 Hence the absorbance is associated predominantly with the vector l. In the C and HO states l predominantly shows positions parallel to the membrane normal. The orientational distribution of the chromophores around the membrane normal in the C and HO states and around the orthogonal direction in the HI state decreases the amplitude of the dichroitic signal. Because of axial symmetry about the –eld direction and free orientation of the chromophores around the HI-pore axis the Lambert»Beer formalism –nally yields an expression for the absorbance mode (*A~/A (6) 4 *A~ \ 3 A Pn Ab]3 HI[bfHOB(1[3 cos2 h) sin h dh 0) f 2 0 and for the chemical mode (7) 0 (*ACH/A0\*A`/A0) accordingly *A` \ b 2 HI]fHO) sin h dh Pn ( f h h HO(h)\[HO]h/[C0] and fHI(h)\[HI]h/[C0] respectively where and [HI] are the DPH probe concentrations of HO and HI pores respectively at h and f is the total probe concentration.Mass conservation suggests HI(h)]fHO(h)]fC(h)\1 A0 0 where h is the positional angle of the chromophore (Fig. 8) relative to the –eld direction. The relative molar absorption coefficient diÜerence is de–ned by the molar absorption factor b\(e*[eC)/eC. For simplicity we assume that in state C the microscopic molar absorption coefficients of the chromophore in the presence (eE C) and in the absence (eC) of the –eld are equal (eEC\eC).In contrast to C the membrane probes in HO and HI pore edges are partially exposed to water and ions ; hence eHI and eHODeC. We assume equal molar extinction coefficients of membrane probes in HO and HI pore edges respectively eHI\eHO4e*. Similar to the orientation distribution function for chromophores in a solution the angular distribution functions representing the probabilities of –nding chromophores in HO or HI membrane states at an angle between h and h]dh are de–ned by f [HO] [C0] where fC(h)\[C]h/[C0]. Here it is recalled that o*A~/A0 oAo*A`/A0 o (Fig. 6 and Fig. 3) hence the molar absorption factor b is practically zero and the approximation *A~\*A is valid.Substitution of b\0 in eqn. (6) yields the absorbance dichroism (due to the local rotation of the membrane probes in HI pore edges) (8) 8 *A \ 9 A Pn f 0 0 The distribution function is speci–ed as (9) fHI(h)\ 0 f * h*\h\n[h* n[h*OhOn 7HI f ( * h)(1[3 cos2 h)sin h dh 0OhOh* where f * is the fraction of chromophores in HI pore edges in the vesicle pole caps. The h f -average fraction 6 of chromophores in HI pores is then given by (10) f6 \ 1 n fHI(h)sin h dh\f *(1[cos h*) 2 P0 Substitution of eqn. (9) in eqn. (8) and integration yields (11) 4 f * cos h*(cos2 h*[1) *A \ 9 A0 *A/A Analysis of eqn. (11) shows that at a given value of f * has a minimum at h*\ 0 Actually the rotation of chromophores in HI pores at arccos(J1/3)\54.7°.Faraday Discuss. 1998 111 111»125 118 54.7°\h\(180°[54.7°) contributes to the dichroism positively reducing the total amplitude of the negative absorbance dichroism. Substitution of h*\54.7° in eqn. (11) and in eqn. (10) yields (12) f *\[1.155 *A A0 and (13) f6 \[0.487 *A A0 respectively. 4.3 HI pore model and absorbance dichroism At the maximum –eld strength E\8 MV m~1 the amplitude value of the absorbance dichroism 0.225Oo*A/*A increases in the interval 0 oO0.28 with increasing concentrations of the membrane probe 2.5O[b-DPH pPC]/lMO20.0 (Fig. 6). Hence eqn. (12) and eqn. (13) yield 0.25Of *O0.32 and 0.11Of6 O0.135 respectively. Note the fractions of membrane probes in HI f *\[HI] /[C pores are de–ned as f6 \[HI]/[C 0] and 0] where [HI] is the concentration of p p b-DPH pPC in HI pores in vesicle pole caps (0OhOh*; n[h*OhOn) and [HI] is the haverage concentration of membrane probes in HI pores.Clearly eqn. (12) and eqn. (13) imply that *AP[HI] A0P[C0]\[b-DPH pPC]. Therefore at constant fraction of membrane p [HI] and area covered by HI pores f f * and 6 should not depend on [b-DPH pPC]. However the data show that there is a 28% increase in f f * and 6 accompanying an eight-fold increase in [b-DPH pPC] at E\8 MV m~1. The reporter lipids in the membrane might decrease the bending rigidity of the membrane and thereby increase the degree of vesicle deformation. However the analogous turbidity terms *T ~/T and *T `/T 0 0 measured outside the absorbance band are independent of [b- DPH pPC] (Fig.7) suggesting that the degree of the vesicle deformation is constant. In any case the fractions f f * and 6 are too large to be identi–ed with the fraction of porated membrane area. Actually 0.11Of6 O 0.135 implies that 11»13.5% of all membrane probes are in the HI pore edges. At random distribution of the chromophores in the membrane it would mean that 11»13.5% of the membrane surface area is covered by HI pores. However the upper limit for the fraction of membrane surface area covered by conductive pores has been estimated to be only ca. 0.002 which does not correlate with 0.11Of6 O0.135.19 Alternatively the b-DPH pPC molecules may locally change the ability of the membrane to be electroporated such that the pore formation is facilitated at the site of the probe.This assumption is in line with comparative —uorescence anisotropy and scanning calorimetry data suggesting that DPH pPC ììdisruptsœœ bilayer order in its near vicinity.20 In this case the electropores are concentrated at DPH sites. However at the maximum concentration [b-DPH pPC]\20 lM and at a lipid concentration of 1.0 mM there is on average one b-DPH pPC molecule per 50 lipid molecules in the two membrane monolayers. The surface area per lipid molecule in the liquid crystalline state of the membrane is about 0.6 nm2 .21 If we assume that the minimum HI pore radius is *S/S should lead to membrane rupture and to vesicle elongation. 0 rpBd/2\2.5 nm the minimum value of the relative increase in the membrane surface area is 0\0.11p]2.52/(0.6]50/2)\0.14 (or 14%).This fraction is again unreasonably large and Thus the absorbance dichroism does not directly re—ect HI pores. Rather the *A~/A mode must be predominantly due to global rotations of membrane probes caused by electroporative shape elongation of the vesicles under the electrical Maxwell stress. 4.4 Electroporative deformation model Vesicles can be elongated if either the membrane area is increased or the intravesicular volume is decreased. However for the short duration of the electric pulse (10 ls) solvent transport through the membrane electropores is usually very small.2 If the optical transition moments of the membrane probes are predominantly aligned along the membrane normal the elongation of the vesicles leads to a global orientation of the membrane chromophores apart from the external –eld 119 Faraday Discuss.1998 111 111»125 direction leading to negative absorbance dichroism (Fig. 9). If chromophores are distributed at –nite angles around the membrane normal (Fig. 10) the amplitude of the dichroism is smaller than that for normal-parallel positions. At small deformations the shape of the elongated vesicles may be approximated by a spheroid.22 In view of iono-electrochromic eÜects or formation of electropores the molar absorption coefficient of the chromophores in the membrane may change.2 Formally we can describe the modi–ed molar absorption coefficients in terms of HO pores with the parameters fHO and eHO. If the water content of the membrane in the –eld is increased without pores because of –eld-induced orientation of the lipid head groups and isotropic penetration of water and ions into the membrane/solvent interface (Fig.10) the radius of the HO pores is theoretically zero. Because of axial symmetry about the –eld direction the reduced absorbance dichroism is given by *A~ \[ 8 3 M3 cos2 a[1N Pn [1]bfHO]M1[3 cos2[arctan(p2 tan h)]N sin h dh (14) is also dependent on h. The chemical absorbance term is given by eqn. (7). A0 0 where p\c/b is the axis ratio of the spheroid (c[b Fig. 9) and a is the average angle between the membrane normal and the optical transition moment of DPH pPC (Fig. 10). It is recalled that fHOApplying eqn. (14) for a\0 to the data in Fig.6 we obtain the axis ratio in the range at E\8 MV m~1). For small vesicle elongations 1.10OpO1.13 (for 0.225Oo*A~/A0 oO0.28 (pO1.13) the relative increase in the membrane surface area at constant vesicle volume is described by (15) 45 (p[1)2 *S B 8 S0 where S0\4na2 is the vesicle surface area. Substituting 1.10OpO1.13 in eqn. (15) we obtain 0.0018O(*S/S0)O0.0029 at 2.5O[b-DPH pPC]/lMO20.0 and E\8 MV m~1. Alternatively without electropore formation the membrane surface area could also increase due to membrane straining by the electrical Maxwell stress.23 Fig. 9 Rotational displacement of the optical transition moment l of the DPH chromophore in the membrane caused by vesicle shape deformation in the electric –eld E. If l is oriented predominantly perpendicular to the membrane surface the vesicle elongation changes the direction of l out of the parallel –eld direction leading to negative absorbance dichroism (*A).The elongated vesicle is modelled by a spheroid with the principal semiaxes c and b. Faraday Discuss. 1998 111 111»125 120 Fig. 10 Positions of the optical transition moments l of b-DPH pPC relative to the membrane normal N (perpendicular to the membrane surface S) characterized by the average angle a are diÜerent without electric –eld a (a) and in a –eld a(tE) (b). In the electric –eld the head-group dipole moments are partially oriented in the –eld direction. Entrance of water and ions in the head-group regions of the pole caps of the lipid mem- 0 brane is enhanced by the electric –eld (b) compared with zero –eld (a).In order to check the mechanism of vesicle elongation the reduced absorbance dichroism *A~/A is compared for vesicles of diÜerent radii a but at a constant transmembrane potential *r0 drop *r m . At constant m the free energy change associated with electroporation should be constant and the extent of membrane electroporation should not change with vesicle radius.5 Moreover vesicle elongation without pores should decrease with increasing curvature H\1/a. Axis ratio and curvature are related by:6,23 (16) p\1]G3 e0 ew(Ea)2 H 64 i H 1 where e e is the vacuum permittivity the relative permittivity constant of the solvent and i is the 0 w membrane bending rigidity. Note that the product (Ea) in eqn. (16) is a constant.Insertion of eqn. o*A~/A (16) in eqn. (14) shows that the absorbance term should decrease with increasing H. If 0 o vesicle deformation occurs without electropore formation controlled solely by the increase in surface area due to stretching of the membrane or reduction of membrane thermal undulations or the smoothing of a membrane super structure the decrease in p with increasing H should be even steeper because of the amplitude of the thermal undulations or superstructure decreases with decreasing vesicle radius.2,24 In any case increasing H should lead to a decrease in the vesicle elongation due to such elastic membrane stretching by the electrical Maxwell stress and thus to a decrease in o*A~/A0o in contrast to the experimental data (Fig. 11). On the other hand the increase in membrane surface area by electroporation is facilitated by the increase in the packing density diÜerence in the two membrane lea—ets due to increasing membrane curvature H.The enhanced formation of conical pores with increasing H is quanti–ed by the concept of the area diÜerence elasticity (ADE) energy25 and has been applied previously by Toensing et al.9 Clearly the experimental data suggest that vesicle elongation is rate limited by surface area increase due to pore formation. The increase in surface area (*S/S0) by the electropores can be readily transformed into the vesicle elongation under Maxwell stress. Moreover the largest increase in the membrane volume is in the pole caps of the vesicle facilitating the vesicle elongation. 121 Faraday Discuss.1998 111 111»125 Fig. 11 Amplitude of the absorbance term *A~/A as a function of membrane curvature H\1/a at constant *rm\[1.5aE\[1.5]25 nm]8 MV m~1\[1.5]50 nm ]4 MV transmembrane voltage drop 0 m~1\[1.5]100 nm ]2 MV m~1\[1.5]200 nm]1 MV m~1\[0.3 V. The circles joined by a solid line are the experimental data. The dotted line corresponds to the vesicle elongation due to membrane stretching or smoothing of membrane undulations under the electric Maxwell stress. The increase in o*A~/A0 o with H suggests that in this case the electroporative vesicle deformation is predominant. Experimental conditions as in Fig. 1 and Fig. 3. The concept of electroporative vesicle elongation is also con–rmed by the increase in the conductivity of a suspension of salt-–lled vesicles exposed to an electric –eld pulse.2,4 Clearly the conductivity increase after the –eld pulse shows convincingly that electroporation causes permeabilization of the vesicle membrane i.e.there are electropores. Interestingly the reduced absorbance dichroism suggests a continuous increase in the electroporated surface area with increasing –eld strength whereas massive ion transport through the membrane only occurs above a threshold value. Therefore turbidity and absorbance electro-optics are suitable for an investigation of the very initial stages of membrane electroporation and global shape deformations. The relaxation time of vesicle deformation due to smoothing undulations can be estimated according to (17) qB[ 5 16 g i a3 lnA1[ 64 3 e ( 0 p e [ wE 1) 2a i 3B Inserting g\10.02]10~4 kg m~1 s~1 the vesicle radius a\50 nm a typical value for the membrane bending rigidity i\2.5]10~20 J and p\1.1 in eqn.(17) we calculate 0.8Pq/ lsP0.009 in the –eld strength range 1OE/MV m~1O8. The time constant of stretching is also small (2.5]10~10 s) ;26 the apparatus time constant is 20 ls and the –eld builds up rapidly (6]10~8 s). Since all the time constants are much smaller than those of electropore formation (3Pqp/lsP0.2) the slow mode (Fig. 4) of vesicle deformation is rate limited by and rapidly coupled to the primary electroporation process. Note that p\1.1 corresponds to a maximum change of 6.6% in the membrane curvature ; therefore the relative motion between monolayers may be neglected.Only a rapid ca. 1000-fold increase in curvature could lead to a retardation of the shape deformation due to viscous impedance arising from relative monolayer motion.27 It is recalled that the absorbance dichroism contains the ratio p and the angle a; see eqn. (14). For electroporative deformation the surface area increase *S(II) of the slow mode is proportional to the concentration [P] of electropores. In our chemical model for membrane electroporation [P]\[P] (CHP) the pore kinetics is described by =(1[exp[[t/qp]) where [P]= is the amplitude and q the time constant of the poration-resealing process. Both relaxation quantities are p dependent on the –eld strength and on the positional angle h (Fig. 8). Hence all measured parameters re—ect h-averages.Application of a Mie type numerical code for confocal coated spheroids28 to the –eld-on time course of the turbidity terms *T ~(t)/T and *T `(t)/T (see e.g. Fig. 4) yields 0 0 the dependence of the total increase *S on the –eld strength (Fig. 12). The increase in membrane = area in Fig. 12 is certainly large compared with 0.002 fraction of conductive pores reported by Hibino et al.19 However not all of *S= is due to electropores. Membrane stretching and smooth- Faraday Discuss. 1998 111 111»125 122 *S=/S0 of the relative change in the membrane surface area of a vesicle as a func- = *S Fig. 12 Amplitude value tion of the external –eld strength E. S is the total surface area of the vesicle membrane. *S is calculated from *T ~(t)/T and *T `(t)/T0 the –eld-on turbidity relaxations 0 assuming constant intravesicular volume.Note =\*S=(I)]*S=(II) is the total amplitude value of the electrochemomechanical surface area increase that 0 = *S\*S (I)(1[exp[[t/q(I)])]*S (II)(1[exp[[t/q(II)]). Experimental conditions as in Fig. = according to 4. *A~(t)/A using eqn. (14) with b\0 and a –nite value of a (Fig. 13) ing undulations in the electric –eld also contribute to *S=. Fortunately the turbidity terms *T ~/T and *T `/T are independent of the presence of the chromophores. Comparison of the p 0 0 values at diÜerent times calculated from the turbidity terms *T ~(t)/T and *T `(t)/T and from 0 0 the absorbance dichroism 0 *A`(t *A`/A0(B0) and *A~/A at the concentration of the membrane probes 0 E)/A0 *A~(tE)/A0 and *T ~(tE)/T0 *T `(tE)/T0 at the end of the Fig.13 Axis ratio p\c/b of the elongated vesicle as a function of the pulse time at the two extreme –eld strengths E/MV m~1\2 (Ö) and 8 (L). The circles are the values of p calculated from the turbidity terms *T ~/T and *T `/T at diÜerent times. The corresponding solid curves are the theoretical simulations with 0 0 *S\*S(I)]*S(II) for the kinetics of the increase in the membrane surface area *S. The dotted curves are calculated from the absorbance [b-DPH pPC]\5 lM. The values t pulse E\10 ls were used to calculate the average angle a(tE)\39.7° between the optical transition moment of b-DPH pPC and the membrane normal. Experimental conditions as in Fig. 3. 123 Faraday Discuss. 1998 111 111»125 Fig.14 Dependence of the angle a on b-DPH pPC concentration (Ö) the angle a extrapolated to the 0 beginning of the pulse (t\0) and (L) a(t t at the pulse end E) E\10 ls. Note for neither a0 nor a(tE) is there any dependence on the –eld strength in the range 2OE/MV m~1O8. Experimental conditions as in Fig. 3. indicates that the angle a also changes in the –eld strength range EP2 MV m~1. It is readily seen that unlike the turbidity dichroism the absorbance dichroism also contains information on the position of the lipid side chains here the DPH residue relative to the membrane normal. The –eld-induced increase in a and the dependence of a on the b-DPH pPC concentration (Fig. 14) cannot be rationalized unequivocally. One possible explanation is the rotational displacement of the chromophore residue caused by the alignment of the polar head group in the –eld direction (Fig.10). 5 Conclusion It appears that the concept of electrochemomechanical vesicle deformation can consistently rationalize the electro-optic data. However the local structural changes associated with the formation of HI pore edges cannot be directly identi–ed with electro-optics. The electropores are optically enhanced by vesicle deformation because the slow mode *S(II) of shape elongation under the electric Maxwell stress is caused by and rapidly coupled to the electroporative increase in the membrane area. This fundamental result of electroporative shape deformation also applies to biological cells and tissue. However the lipid-protein plasma membrane of cells is a part of a larger envelope structure comprising extracellular matrix components and intracellular cytoskeletal elements.29 Therefore the extent and rate of cell deformation will be determined by the elastic properties of this network.30 Acknowledgements We thank the Deutsche Forschungsgemeinschaft for the grant Ne227/9-2 to E.Neumann. References 1 E. Neumann and S. Kakorin Radiol. Oncol. 1998 32 7. 2 E. Neumann and S. Kakorin Curr. Opin. Colloid Interface Sci. 1996 1 790. 3 S. Kakorin E. Redeker and E. Neumann Eur. Biophys. J. 1998 27 43. 4 E. Neumann E. Werner A. Sprafke and K. Krueger in Colloid and Molecular Electro-Optics ed. B. R. Jennings and S. P. Stoylov Institute of Physics Bristol 1992 p. 197. 5 S. Kakorin S. P. Stoylov and E.Neumann Biophys. Chem. 1996 58 109. 6 S. Kakorin and E. Neumann Ber. Bunsen-Ges. Phys. Chem. 1998 102 670. Faraday Discuss. 1998 111 111»125 124 7 S. Kakorin and E. Neumann Ber. Bunsen-Ges. Phys. Chem. 1996 100 721. 8 A. Revzin and E. Neumann Biophys. Chem. 1974 2 144. 9 K. Toensing S. Kakorin E. Neumann S. Liemann and R. Huber Eur. Biophys. J. 1997 26 307. 10 L. D. Mayer M. J. Hope and P. R. Cullis Biochim. Biophys. Acta 1986 885 161. 11 K. R. Foster and A. E. Sowers Biophys. J. 1995 69 777. 12 G. K. Batchelor J. Fluid. Mech. 1976 74 1. 13 M. Planck Berlin Ber. 1904 740. 14 S. Stoylov and E. Neumann Bulgarian Chem. Commun. 1992 25 445. 15 B. Mui P. Cullis E. Evans and T. Madden Biophys. J. 1993 64 443. 16 T. Chang and H. Yu. Comments Mol.Cell. Biophys. 1990 7 27. 17 I. G. Abidor V. B. Arakelyan L. V. Chernomordik Y. A. Chizmadzhev V. P. Pastuchenko and M. R. Tarasevich Bioelectrochem. Bioenerg. 1979 6 37. 18 E. Fredericq and C. Houssier Electric Dichroism and Electric Birefringence. Clarendon Press Oxford 1973 p. 219. 19 M. Hibino H. Itoh and K. Kinosita Biophys. J. 1993 64 1789. 20 B. Lentz Chem. Phys. L ipids 1993 64 99. 21 J. J. Lopez Cascales M. L. Huertas and J. Garcia de la Torre Biophys. Chem. 1997 69 1. 22 B. Zeks and S. Svetina in Springer Proceedings in Physics ed. R. Lipowsky D. Richter and K. Kremer Springer-Verlag Berlin 1992 vol. 66 p. 174. 23 M. Kummrow and W. Helfrich Phys. Rev. A 1991 44 8356. 24 B. Kloesgen and W. Helfrich Eur. Biophys. J. 1993 22 329. 25 U. Seifert and R. Lipowsky in Structure and Dynamics of Membranes ed. R. Lipowsky and E. Sackmann Elsevier North-Holland Amsterdam 1995 vol. 1A p. 519. 26 S. Komura in V esicles ed. M. RosoÜ Marcel Dekker New York 1996 p. 197. 27 E. Evans and A. Yeung Chem. Phys. L ipids 1994 73 39. 28 V. G. Farafornov N. V. Voshcinnikov and V. V. Somsikov Appl. Opt. 1996 35 5412. 29 R. Lipowsky Encyclopedia Appl. Phys. 1998 23 199. 30 F. G. Schmidt F. Ziemann and E. Sackmann Eur. Biophys. J. 1996 24 348. Paper 8/06461J 125 Faraday Discuss. 1998 111 111»125

ISSN:1359-6640    

DOI:10.1039/a806461j

出版商:RSC    

年代:1999

数据来源: RSC