摘要:

AbstractWe describe a method for determining chemical kinetic constants and diffusion coefficients by measuring the rates of decay of spontaneous concentration fluctuations. The equilibrium of the system is not disturbed during the measurement. We measure the number of molecules of a specified type in a defined open volume as a function of time and compute the time course of the deviations from the thermodynamic mean concentration. The method is based on the principle that the rates of decay of spontaneous microscopic fluctuations are determined by the same phenomenological rate coefficients as those of macroscopic departures from equilibrium which result from external perturbations. Hence, an analysis of fluctuations yields the same chemical rate constants and diffusion coefficients as are measured by conventional procedures. In practice the number of the specified molecules is measured by a property such as absorbance or fluorescence which is specific and sensitive to chemical change. The sample volume is defined by a light beam which traverses the cell. As the molecules appear in or disappear from the light beam, either due to diffusion or chemical reaction, their concentration fluctuations give rise to corresponding fluctuations of the intensity of absorbed or emitted light. This paper presents the theory needed to derive chemical rate constants and diffusion coefficients from these fluctuations in light intensity. The theory is applied to three examples of general interest: pure diffusion in the absence of chemical reaction; the binding of a small rapidly diffusing ligand to a larger slowly diffusing macromolecule; and a unimolecular isomerization. The method should be especially useful in studying highly cooperative systems, relatively noncooperative systems with intermediate states closely spaced in free energy, small systems, and systems not readily subject to perturbations of state.

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThis paper describes the first experimental application of fluorescence correlation spectroscopy, a new method for determining chemical kinetic constants and diffusion coefficients. These quantities are measured by observing the time behaviour of the tiny concentration fluctuations which occur spontaneously in the reaction system even when it is in equilibrium. The equilibrium of the system is not disturbed during the experiment. The diffusion coefficients and chemical rate constants which determine the average time behaviour of these spontaneous fluctuations are the same as those sought by more conventional methods including temperature‐jump or other perturbation techniques. The experiment consists essentially in measuring the variation with time of the number of molecules of specified reactants in a defined open volume of solution. The concentration of a reactant is measured by its fluorescence; the sample volume is defined by a focused laser beam which excites the fluorescence. The fluorescent emission fluctuates in proportion with the changes in the number of fluorescent molecules as they diffuse into and out of the sample volume and as they are created or eliminated by the chemical reactions. The number of these reactant molecules must be small to permit detection of the concentration fluctuations. Hence the sample volume is small (10−8ml) and the concentration of the solutes is low (∼ 10−9M). We have applied this technique to the study of two prototype systems: the simple example of pure diffusion of a single fluorescent species, rhodamine 6G, and the more interesting but more challenging example of the reaction of macromolecular DNA with the drug ethidium bromide to form a fluorescent complex. The increase of the fluorescence of the ethidium bromide upon formation of the complex permits the observation of the decay of concentration fluctuations via the chemical reaction and consequently the determination of chemical rate co

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThe extended Hückel theory (EHT) was used as a quantum mechanical approach for the determination of the best conformation of cyclic AMP. Both geometrical and quantum mechanical studies were necessary to ascertain the configuration of the sugar ring and phosphate ring. This was due to the fact that no crystallographic data are known for the ribose phosphate part of this nucleotide.The calculations have shown that the sugar ring takes a3T4(C3′‐endo–C4′‐exo) configuration while the phosphate ring exhibits a chair form. Once the geometry of the rigid ribose–phosphate block is resolved, the problem of the determination of the best conformation for cyclic AMP is reduced to the determination of the position of the adenine base with respect to this ribose phosphate part.The study of the rotation of the adenine base around the glycosidic bond C1′–N9 revealed that the two conformationsanti(χ = 30°) andsyn(ξ = 210°) are possible with an energy difference of only 1 kcal × mole−1. This theoretical calculation is in good agreement

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractNonactin is a cyclic actinomycete metabolite which has been implicated as an ion carrier in the passive transport of potassium across certain biological membranes. In order to discover the conformations of the molecule which are involved in its biochemical function, computer calculations were initiated to derive the energetically favored conformations of the nonactin ring. By assuming that all the relevant three‐dimensional conformations of nonactin have the same symmetry property as that suggested by the chemical structure of the molecule (S4) it was possible to generate a representative sample of all sterically allowed conformations of nonactin. The energies of these conformations were calculated by taking into account the nonbonded interactions among the 116 atoms of the molecule and the torsional potential energy of the 20 rotatable backbone bonds of the ring. The initial results reported in this paper indicate that even in the absence of potassium ion the nonactin ring folds into the same compact tennis‐ball seam‐like conformation that was found by an X‐ray crystallographic investigation of the nonactin/KNCS

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThe molecular conformations and dynamics of poly(L‐prolyl), poly(hydroxyl‐L‐prolyl), poly(L‐prolyl‐glycyl), poly(hydroxyl‐L‐prolyl), and poly(glycyl‐glycyl‐L‐prolyl‐glycyl), in aqueous solution, have been studied using13C pulse Fourier transform nmr spectroscopy. From a measurement of the intensities of major and minor resonances in the spectra of the copolypeptides, it was determined that 15–20% of the glycyl‐prolyl and glycyl‐hydroxyprolyl peptide bonds arecis.Effective rotational correlation times (τeff), obtained from measurements of spin‐lattice relaxation times (T1) of individual backbone and side‐chain carbons, demonstrated that backbone reorientation is approximately isotropic for the five polypeptides and is characterized by correlation times of ca. 0.3–0.6 nanoseconds as a result of rapid segmental motion. In a given polypeptide glycyl and pyrrolidine residues were found to have the same backbone correlation times, but backbone carbon τeffvalues did decrease as the glycyl content of the peptides increased. A semi‐quantitative analysis of Cβ, Cγ, and Cδcorrelation times suggests that rapid ring motion in both prolyl and hydroxyprolyl involves primarily Cγand Cβ, with the prolyl ring bei

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThe relaxation of the birefringence of native DNA in solution was investigated in a pulsed sine‐wave electric field. Relaxation times were calculated from the degree of damping of the birefringence signal and were studied as a function of the strength and frequency of the applied field, the molecular weight of the DNA, and the viscosity and ionic strength of the solvent. Relaxation times decrease with increasing field strength. For high‐molecular weight DNA (>106daltons), the relaxation times decreased with frequency and increased less than linearly with viscosity. For low‐molecular‐weight DNA (<6 × 105daltons), the relaxation times were independent of frequency, increased linearly with viscosity, and varied with the 1.65 ± 0.1 power of the molecular weight. The average birefringence of high‐molecular‐weight DNA decreased with frequency in 0.001MNa2EDTA plus NaOH, pH 7.0, but is much less frequency‐dependent if the EDTA concentration is reduced tenfold, while the average birefringence of sonicated DNA increases in both solvents with incre

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractTheDandLcopolymerizations ofN‐carboxy γ‐benzyl glutamate anhydride (NCA) were carried out in a homogeneous solution with variousD/Lratios, initiated by eithern‐butylamine or sodium methoxide, and were followed directly by circular dichroism (CD) to observe the behavior of the secondary structure of growing polymer molecules. In then‐butylamine system, the difference of the helical content between the righthanded and the lefthanded (Δα‐helix) gradually increased as the polymerization proceeded, while in the sodium methoxide system, the Δα‐helix had a tendency to decrease during the later stages of the polymerization. These results suggest a difference of the power of stereo‐selection of monomer antipodes by the growing chain end between these systems, the stereoselectivity by the growing chain end in the sodium methoxide system being higher than that in then

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractCircular dichroism spectroscopy has been used to study the interactions of hyaluronic acid, heparitin sulfate, and keratan sulfate with cationic polypeptides. The results indicate that the presence of these mucopolysaccharides has an effect in the conformation of poly(L‐lysine) and poly(L‐arginine), such that the former adopts the “random” form and the latter takes up the α‐helical conformation, rather than the “charged coil” form expected at neutral pH. The relative strengths of the interactions can be judged from the melting temperatures above which they are disrupted. Both the stoichiometry and the strength of the interactions depend on the position, number, and type of anionic groups attached to the polysaccharide backbone. Such considerations place the six common mucopolysaccharides in order of increasing strength of interaction: hyaluronic acid

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThe synthesis of the oligomeric peptides (Glu)n–Phe(NO2)–Phe (up ton= 4) by the solid‐phase method is reported. Fractionation by ion‐exchange column chromatography of the crude materials cleaved from the resin and subsequent amino acid analysis revealed that the desired peptides were obtained, although contaminated by several by‐products whose number depends on the length of peptide chain and on the experimental c

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY

摘要:

AbstractThe circular dichroism of Ac–Ala–NHMe,cyclo(–Ala–Ala–), Ac–Ala–OMe, Ac–Ala–Ala–OMe, and Ac–Ala–Ala–Ala–OMe has been measured in water and in aqueous salt solutions as a function of temperature. Onlycyclo(–Ala–Ala–) exhibits circular dichroism which is independent of temperature. Each of the linear derivatives ofL‐alanine exhibits a positive circular dichroism in the range 208–218 nm at 15°C in water. Heating reduces the intensity of the positive circular dichroism, and only Ac–Ala–OMe retains positive circular dichroism at 75°C in water. Isothermal addition of salts produces changes in the circular dichroism of linear derivatives ofL‐alanine which resemble those seen on heating. The relative effectiveness of the salts tested, at a concentration of 4M, is LiCl ⩽ KCl = NaCl

ISSN:0006-3525    

DOI:10.1002/bip.1974.360130111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1974

数据来源: WILEY