Kinetic properties and the electric field effect of the helix-coil transition of poly(gamma-benzyl L-glutamate) determined from dielectric relaxation measurements

Dielectric relaxation of poly(γ-benzyl L -glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*_max = 5 × 10^?7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 10¹⁰ sec^?1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.     

Deuteration and solvent composition effects in the helix–coil transition of poly-γ-beiizyl-L-glutamate

The effects of deuteration and of changes in solvent composition on the thermo dynamics of the helix–coil transition have been studied by calorimetric and optical measurements in the poly-γ-benzyl-L -glutamate–dichloroacetic acid–1,2-dichloro-ethanc system. For a given solvent composition, deuteration of the polypeptide and of the acid lowers the transition temperature T_c, while an increase in the volume fraction of acid in the solvent raises T_c. A rise in T_c is accompanied by a decrease in both the van't Hoff and the calorimetric heats of transition, but at different rates. The result is a temperature dependency in the Zimm-Bragg cooperation parameter σ. Possible causes of this result and its implications are discussed.     

Thermodynamics of the helix-coil transition in polypeptides in mixed organic solvents: the influence of inert solvent and side chain

The thermally induced coil–helix transition of poly(γ-benzyl-L -glutamate) (PBLG) and poly(γ-methyl-L -glutamate) (PMLG) in binary solvent mixtures was investigated by calorimetric and optical rotatory dispersion (ORD) measurements. Dichloroacetic acid was the common active solvent, and the inert solvent was one of the chlorinated hydrocarbons, such as chloroform, 1,3-dichloropropane, 1-chlorobutane, or 1-chlorooctane. The thermodynamic parameters characterizing the intramolecular polypeptide and polypeptide–solvent interactions were calculated using the Karasz and Gajnos theoretical model [(1973) J. Phys. Chem. 77 , 1139–1145]. It was found that the enthalpy (ΔH₁) and entropy (ΔS₁) of helix stabilization in the absence of the active solvent depend on the inert solvent, but only in the case of PBLG. This is explained by the additional helix stabilization achieved by the stacking of the benzyl groups. The stacking is more pronounced in less polar chlorinated hydrocarbons with longer aliphatic chains. The results obtained indicate that the maximum helix stability is reached in chlorinated hydrocarbons with 12 C atoms. In the case PMLG, with an aliphatic ester side group, ΔH₁ and ΔS₁ are independent of the inert solvent. The ORD measurements were used to determine the maximum fraction of helicity attained at constant solvent composition and the transition temperature, T_c, at the point where f_H = 0.5. It was found that, for the same solvent composition, T_c was higher than the temperature of the midpoint of the calorimetric peak. This is explained by the fact that the maximum fraction of helicity is less than unity. The finite transition width was taken into account by calculating the phase boundaries for different fractions of helicity using the value of σ estimated from the calorimetric and van't Hoff enthalpies in the usual manner.     

A Study of stepwise conformational changes in poly(β-benzyl-L-aspartate) with the helix–coil transition by means of proton and carbon-13 NMR

The helix–coil transition for poly(β-benzyl-L -aspartate) [poly(Asp[OBzl])] in solvent mixtures of trifluoroacetic acid/deuterated chloroform (F₃AcOH/CDCl₃) was studied by means of proton and carbon-13 nmr. Conformational fixation of the side chain occurs before the coil–helix transition of the backbone, when neighboring phenyl rings face each other. Another type of conformational fixation occurs in the side chain after the coil–helix transition of the backbone. These conformational changes of the side chain are due to the changes of the strength of the interaction between the side-chain ester group and the F₃AcOH molecule. In the absence of F₃AcOH (coil-forming solvent), the polymer has a rather rigid structure in which the side chain may wrap around the backbone. These conformational changes of the polymer are closely related to the changes of the interaction between the polymer and F₃AcOH molecules.     

Stabilization of short helices by intramolecular cluster formation

The intramolecular formation of multiple clusters of interacting helices has been characterized in a homopolymer. The configuration partition function permits the formation of clusters in which the number of interacting helices may be as large as the greatest integer in n/2, where n denotes the number of amino acid residues in the chain. The theoretical formulation has its origin in a recent [Mattice, W. L. & Scheraga, H. A. (1984) Biopolymers 23 , 1701–1724], tractable matrix expression for the configuration partition function for intramolecular antiparallel β-sheet formation. Reassignment of the expression for one of the n(n+3)/2 elements in the sparse statistical weight matrix, along with a simple change in notation, converts that treatment into a matrix formulation of the configuration partition function for a chain containing multiple clusters of interacting antiparallel helices. The five statistical weights used are δ, f_l, w, and the Zimm-Bragg σ and s. Each tight bend that connects two interacting helices contributes a factor of δ, f_l is used in the weight for larger loops between interacting helices, and w arises from helix–helix interaction. The influence of the helix–helix interaction is well illustrated by two helix–coil transitions in a chain with n = 156 and σ = 0.001. In the absence of helix–helix interaction, the transition occurs by the nucleation and subsequent elongation of a small number of helices. When helix–helix interaction is attractive, the transition can occur by a different mechanism. Formation of a single pair of interacting helices is followed by addition of new helices to the initial cluster. In the latter process, individual helices experience relatively little growth after they are formed.     

NMR studies of poly-gamma-benzyl-l-glutamate in trifuloroacetic-deuterochloroform solutions: temperature dependence

The helix–random-coil transition process for poly-γ-benzyl-L -glutamate (PBG) in solvent mixtures trifluoroacetic acid/deuterochloroform (TFA/CDCl₃) at different temperatures has been studied by nmr. The chemical shift behavior of the α-CH resonances of the peptide chain and of the TFA carboxylic protons is reported.     

Experimental theromodynamics of the helix-random coil transition. I. Influence of polymer concentration and solvent composition in the PBG-DCA-EDC system

The course of the reversible helix formation of poly(γ-benzyl L -glutamate) (PBG) dissolved in a mixture of dichloroacetic acid (DCA) and 1,2-dichloroethane (EDC) was followed by measuring the heat capacity and the optical rotation of the system through the transition region. The results of these measurements indicate that the transition enthalpy ΔH the transition temperature T_c, and the Zimm-Bragg parameter σ depend considerably on the PBG concentration as well as on the composition of the solvent. For the standard state of infinite dilution, however, a linear extrapolation of the measured ΔH if values results in a standard value ΔH° = 950 cal./mole, independent of the solvent composition. The results of the calorimetric measurements are discussed in relationship to changes in optical rotation. Some peculiarities in the measured thermodynamic and optical properties in solutions with relatively high content of dichloroacetic acid are reported.     

Microcalorimetric studies of solvent-induced conformational change of sodium and cesium salts of poly(L-glutamic acid) in aqueous media

Organic solvent-induced coil → helix conformational change of poly(sodium) L -glutamate (NaPLG) and poly(cesium L -glutamate) (CsPLG) in solution in aqueous mixed solvents have been studied at 25°C. Heats of dilution of NaPLG in the water–dioxane pair have been measured as a function of polymer concentration and solvent composition. The results indicate that the overall chain conformation in the disordered form is not too different from that in the α-helical form. Heat capacity measurements by flow microcalorimetry have also been done. The apparent monomolar heat capacity at constant pressure of the polymer, C_{p, ?}, decreases with dilution similarly to other strong polyelectrolytes in aqueous media. In the water–dioxane pair, C_{p, ?} increases with the dioxane content due to partial desolvation of ionic species resulting from increasing ionic association. In the case of the water-2-chloroethanol (CE) pair, the transition takes place at low CE content and results show a fast decrease in C_{p, ?} when the α-helical conformation predominates. It is believed carboxylate groups and CE molecules associate themselves into a complex formation responsible for the transition. The size of the cation plays a significant role in the thermodynamic properties of these polyelectrolytes in solution since sodium ions are more strongly bound to the chain than cesium ions.     

Synthesis and conformational study of poly(N delta-carbobenzoxy, N delta-benzyl-L-ornithine) and poly(N delta-benzyl-L-ornithine)

Poly(N^δ-carbobenzoxy, N^δ-benzyl-L -ornithine) (PCBLO) was prepared by the standard NCA method. PCBLO was converted into poly(N^δ-benzyl-L -ornithine) (PBLO) through decarbobenzoxylation with hydrogen bromide. The monomer N^δ-benzyl-L -ornithine was synthesized by reacting L -ornithine with benzaldehyde, followed by hydrogenation. The conformation of the two polypeptides was studied by optical rotatory dispersion and circular dichroism. PCBLO forms a right-handed helix in helix-promoting solvents. In mixed solvents of chloroform and dichloroacetic acid (DCA) it undergoes a sharp helix–coil transition at 12% (v/v) DCA at 25°C, as compared with 36% for poly(N^δ-carbobenzoxy-L -ornithine) (PCLO). Like PCLO, the helix–coil transition is “inverse,” that is, high temperature favors the helical form. PBLO is soluble in water at pH below 7 and has a “coiled” conformation. In 88% (v/v) 1-propanol above pH (apparent) 9.6 it is completely helical. In 50% 1-propanol the transition pH (apparent) is about 7.4; this compares with a pH_tr of about 10 for poly-L -ornithine in the same solvent.     

Conformational change of poly-N epsilon-glutaryl-L-lysine and poly-N epsilon-succinyl-L-lysine in aqueous salt solutions

The conformational changes of poly-N^ε-glutaryl-L -lysine (PGL) and poly-N^ε-succinyl-L -lysine (PSL) in various salt solutions were studied by use of ORD and potentiometric titration measurements. The addition of alkali metal salts to the fully ionized PGL or PSL solution caused helix formation. The helical content of the polymers increases in the following sequences: at salt concentration 0–2 M, CsCl < KCl < LiCl < NaCl; and at 2–3 M, LiCl < CsCl < KCl ～ NaCl. The preferential binding of the solvent components with various alkali metal salts of PGL or PSL was measured in LiCl, NaCl, and KCl solutions by means of equilibrium dialysis and differential refractometry. It was found that with increasing salt concentration, the polymers were preferentially hydrated in NaCl and KCl soultions; however the salt was preferentially bound to the polymers in LiCl solution. Such preferential binding was suggested to be closely related to conformational change. The addition of CaCl₂ to polymer solutions led to the stabilization of the helical structure of PGL or PSL.     

Use of sequence-specific tri-block copolymers to determine the helix-forming tendencies of amino acids

In order to examine the helix-coil transition of water-insoluble polypeptides, without requiring interspersion of charged or polar residues within the sequence, a tri-block copolymer strategy has been developed to determine the helix propensity of amino acids in short (15-residue) peptides. The method is also well suited to evaluate specific interactions that contribute to helix formation. In this approach, measurement is made of the helix content of the central block of tri-block copolymers of the type Lys₁₅-X-Lys₁₅, where X is the 15-residue peptide whose helix content is being investigated. The suitability of tri-block copolymers of this type has been verified experimentally by using the water-soluble peptide (Ala₂GlnAla₂)₃ as the central block. The CD spectrum of the central block in the tri-block copolymer and of Ac-(Ala₂GlnAla₂)₃-NH₂ are indistinguishable within experimental error. © 1996 John Wiley & Sons, Inc.     

Cooperative nonenzymic base recognition II. thermodynamics of the helix-coil transition of oligoadenylic + oligouridylic acids

The properties of oligonucleotide helices of adeuylic- and uridylic acid oligomers have been investigated by measurements of hypo-and hyperchromieity. High ionic strengths favor the formation of triple helices. Thus, the double helix-coil transition can be studied (without interference by triple helices) only at low ionic-strength. A “phase diagram” is given representing the T_m-values of the various transitions at different ionic strengths for the system A(pA)₁₇ + U(pU)₁₇. Oligonucleolides of chain lengths <8 always form both double and triple helices at the nucleotide concentrations required for base pairing. For this reason the double helix-coil transition without coupling of the triple helix equilibrium can only be measured for chain lengths higher than 7. Melting curves corresponding to this transition have been determined for chain lengths 8, 9, 10, 11, 14 and 18 at different concentrations. An increase in nucleotide concentration leads to an increase in melting temperature. The shorter the chain length the lower the T_m-value and the broader the helix-coil transition. The experimental transition curves have been analysed according to a staggering zipper model with consideration of the stacking of the adeuylic acid single strands and the electrostatic repulsion of tlip phosphate charges on opposite strands. The temperature dependence of the nucleation parameter has been accounted for by a slacking factor x. The stacking factor expresses the magnitude of the stacking enthalpy. By curve fitting xwas computed to be 0.7, corresponding to a stacking enthalpy of about S kcal/mole. The model described allows the reproduction of the experimental transition curves with relatively high accuracy. In an appendix the thermodynamic parameters of the stacking equilibrium of poly A and of the helix-coil equilibria of poly A + poly U at neutral pH are calculated (ΔH_A = ?7.9 kcal/mole for the poly A stacking and ΔH₁₂ = ?10.9 kcal/mole for the formation of the double helix from the randomly coiled single strands). A formula for the configurational entropy of polymers derived by Flory on the basis of a liquid lattice model is adapted to calculate the stacking entropies of adenylic oligomers.     

The triple helix ⇌ coil conversion of collagen-like polytripeptides in aqueous and nonaqueous solvents. Comparison of the thermodynamic parameters and the binding of water to (L-Pro-L-Pro-Gly)n and (L-Pro-L-Hyp-Gly)n

The collagen-like peptides (L -Pro-L -Pro-Gly)_n and (L -Pro-L -Hyp-Gly)_n with n = 5 and 10, were examined in terms of their triple helix ? coil transitions in aqueous and nonaqueous solvents. The peptides were soluble in 1,2-propanediol containing 3% acetic acid and they were found to form triple-helical structures in this solvent system. The water content of the solvent system and the amount of water bound to the peptides were assayed by equilibrating the solvent with molecular sieves and carrying out Karl Fischer titrations on the solvent phase. After the solvent was dehydrated, much less than one molecule of water per tripeptide unit was bound to the peptides. Since the peptides remained in a triple-helical conformation, the results indicated that water was not an essential component of the triple-helical structure. Comparison of peptides with the same chain length demonstrated that the presence of hydroxyproline increased the thermal stability of the triple helix even under anhydrous conditions. The results, therefore, did not support recent hypotheses that hydroxyproline stabilizes the triple helix of collagen and collagen-like peptides by a specific interaction with water molecules. Analysis of the thermal transition curves in several solvent systems showed that although the peptides containing hydroxyproline had t_m values which were 18.6° to 32.7°C higher, the effect of hydroxyproline on ΔG was only 0.1 to 0.3 kcal per tripeptide unit at 25°C. The results suggested, therefore, that the influence of hydroxyproline on helical stability may be explained by intrinsic effects such as dipole–dipole interactions or by changes in the solvation of the peptides by alcohol, acetic acid, and water. A direct calorimetric measurement of the transition enthalpy for (L -Pro-L -Pro-Gly)_n in 3% or 10% acetic acid gave a value of ?1.84 kcal per tripeptide unit for the coil-to-helix transition. From the value for enthalpy and from data on the effects of different chain lengths on the thermal transition, it was calculated that the apparent free energy for nucleation was +5 kcal/mol at 25°C (apparent nucleation parameter = 2 × 10^?4 M^?2). The value was dependent on solvent and on chemical modification of end groups.     

Proton and phosphorus NMR studies of d-CpG(pCpG)n duplexes in solution. Helix-coil transition and complex formation with actinomycin-D.

The Watson–Crick imino and amino exchangeable protons, the nonexchangeable base and sugar protons, and the backbone phosphates for d-CpG(pCpG)_n, n = 1 and 2, have been monitored by high-resolution nmr spectroscopy in aqueous solution over the temperature range 0°–90°C. The temperature dependence of the chemical shifts of the tetramer and hexamer resonances is consistent with the formation of stable duplexes at low temperature in solution. Comparison of the spectral characteristics of the tetranucleotide with those of the hexanucleotide with temperature permits the differentiation and assignment of the cytosine proton resonances on base pairs located at the end of the helix from those in an interior position. There is fraying at the terminal base pairs in the tetranucleotide and hexanucleotide duplexes. The Watson–Crick ring imino protons exchange at a faster rate than the Watson–Crick side-chain amino protons, with exchange occurring by transient opening of the double helix. The structure of the d-CpG(pCpG)_n double helices has been probed by proton relaxation time measurements, sugar proton coupling constants, and the proton chemical shift changes associated with the helix–coil transition. The experimental data support a structural model in solution, which incorporates an anti conformation about the glycosyl bonds, C(3) exo sugar ring pucker, and base overlap geometries similar to the B-DNA helix. Rotational correlation times of 1.7 and 0.9 × 10^?9 sec have been computed for the hexanucleotide and tetranucleotide duplexes in 0.1 M salt, D₂O, pH 6.25 at 27°C. The well-resolved ³¹P resonances for the internucleotide phosphates of the tetramer and hexamer sequences at superconducting fields shift upfield by 0.2–0.5 ppm on helix formation. These shifts reflect a conformational change about the ω,ω′ phosphodiester bonds from gauche-gauche in the duplex structure to a distribution of gauche-trans states in the coil structure. Significant differences are observed in the transition width and midpoint of the chemical shift versus temperature profiles plotted in differentiated form for the various base and sugar proton and internucleotide phosphorous resonances monitoring the d-CpG(pCpG)_n helix–coil transition. The twofold symmetry of the d-CpGpCpG duplex is removed on complex formation with the antibiotic actinomycin-D. Two phosphorous resonances are shifted downfield by ～2.6 ppm and ～1.6 ppm on formation of the 1:2 Act-D:d-CpGpCpG complex in solution. Model studies on binding of the antibiotic to dinucleotides of varying sequence indicate that intercalation of the actinomycin-D occurs at the GpC site in the d-CpGpCpG duplex and that the magnitude of the downfield shifts reflects strain at the O-P-O backbone angles and hydrogen bonding between the phenoxazone and the phosphate oxygens. Actinomycin-D is known to bind to nucleic acids that exhibit a B-DNA conformation; this suggests that the d-CpG(pCpG)_n duplexes exhibit a B-DNA conformation in solution.     

The conformational stability of ribosomal proteins L7 and L12 from E. coli. I. Circular dichroism study of the changes induced by ionic strength and solvents

Ribosomal proteins L₇ and L₁₂ are the only acidic proteins found on the 50S ribosomal subunit of Escherichia coli. The effect of ionic strength, helix-promoting solvents and denaturating agents on the conformation of these proteins has been studied. It has been established that the helicity of L₇ and L₁₂ proteins (approx. 45–50% α helix) can be increased to 60–70% when they are exposed to helix-promoting solvents such as methanol or ethanol in the presence of 0.1M salt. High ionic strength by itself was without any effect on the conformation of the proteins. However, the solvent, 2,2,2-trifluoroethanol increased the content of α helices up to 80% even in the absence of salt. Denaturating agents like urea (6M) or guanidine HCl (6M), decreased the content of the ordered structure below 20%. All conformational changes induced by salt or solvents were completely reversible and characterized by a broad transition showing a low degree of cooperativity. This might indicate the presence of discrete segments with variations in amino acid sequences and ordered structures with different stabilities.     

15N-NMR spectroscopy. IV. Comparison of poly(L-lysine) and isopoly(L-lysine)

The natural abundance ¹⁵N-nmr spectroscopy has been used to characterize the isomeric polymers (L -Lys)_n and iso (L -Lys)_n in aqueous solution. Although the peptide nitrogens of the two polymers have nearly equivalent shifts at pH < 10, the amino nitrogens differ by 5–6 ppm at pH < 7 and provide an easy means of identification. Furthermore, the polymers are distinguishable by the pK_a of the amino group and the basicity of the peptide nitrogen. At pH 10.3 and 25°C, (Lys)_n exhibits line broadening and an upfield chemical shift of the peptide nitrogen, indicative of the coil → helix transition. The formation of 100% helix may produce a shift as large as 5 ppm, which probably makes ¹⁵N-nmr spectroscopy more suitable for studies of this transition.     

Étude thermodynamique de la transition hélice–pelote statistique en solvants mixtes de différents esters de l'acide poly(L-glutamique)

Helix–coil transition of poly(γ-methyl-L -glutamate), poly(γ-ethyl-L -glutamate), and poly(γ-benzyl-L -glutamate) has been studied in mixed solvents by calorimetry, polarimetry, and viscometry. The experimental data have allowed the evaluation of solvation enthalpy Δh_b, equilibrium constant K for hydrogen bond formation between the active solvent component and CO and NH groups, and the cooperativity parameter σ. The conformational transition of polypeptides in solution in a mixed solvent containing enough active solvent to maintain the coiled conformation has been produced by dilution with the helix-supporting solvent for the measurements of enthalpy of transition Δh_s. The average value for Δh_s is 3550 ± 300 J/mol and is practically independent of the nature of the side chain for the dichloroacetic acid-ethylene dichloride solvent pair at 25°C. A noticeable concentration effect exists in the case of poly(γ-benzyl-L -glutamate). The helical conformation is less stable for poly(γ-ethyl-L -glutamate), and this is explained by a steric effect hindering the access of dichloroacetic acid to side chains. Constant K has been calculated using polarimetric data and also from values of Δh_s obtained at different temperatures using the Bixon and Lifson theory on the one hand and that of Sayama and coworkers on the other hand. Values of σ for poly(γ-ethyl-L -glutamate) have been calculated according to both theories mentioned, and the results show that the two sets of values are quite similar. The constant σ depends on the nature of the active solvent, on temperature, and on the binary-solvent composition. These conclusions are confirmed by viscometric results. Values of Δh_b calculated from constant K are 5230 J/mol when Bixon and Lifson theory is used and 5569 J/mol when the theory at Sayama and coworkers is used. In both cases the value for Δh_b is much lower than that of an intramolecular hydrogen bond. Experimental results suggest that the solvation mechanism would proceed in a manner so that mechanisms described in both theories are involved.     

Helix-coil dynamics of a Z-helix hairpin

The helix–coil transition of a Z-helix hairpin formed from d(C-G)₅T₄(C-G)₅ has been characterized by equilibrium melting and temperature jump experiments in 5M NaClO₄ and 10 mM Na₂HPO₄, pH 7.0. The melting curve can be represented by a simple all-or-none transition with a midpoint at 81.6 ± 0.4°C and an enthalpy change of 287 ± 15 kJ/mole. The temperature jump relaxation can be described by single exponentials at a reasonable accuracy. Amplitudes measured as a function of temperature provide equilibrium parameters consistent with those derived from equilibrium melting curves. The rate constants of Z-helix formation are found in the range from 1800 s^?1 at 70°C to 800 s^?1 at 90°C and are associated with an activation enthalpy of ?(50 ± 10) kJ/mole, whereas the rate constants of helix dissociation are found in the range from 200 s^?1 at 70°C to 4500 s^?1 at 90°C with an activation enthalpy +235 kJ/mole. These parameters are consistent with a requirement of 3–4 base pairs for helix nucleation. Apparently nucleation occurs in the Z-helix conformation, because a separate slow step corresponding to a B to Z transition has not been observed. In summary, the dynamics of the Z-helix–coil transition is very similar to that of previously investigated right-handed double helices.     

Solution properties of synthetic polypeptides. VI. Helix-coil transition of poly-N5-(3-hydroxypropyl)-L-glutamine

A new procedure for evaluating u and σ characterizing σ-helix-forming polypeptides in solution was derived from Nagai's theory for the helix–coil transition of such polymers. Here u is the activity for helix formation from random coil, and σ is the helix initiation parameter. The necessary data are the helical content f_N at fixed solvent and temperature as a function of N, where N is the degree of polymerization of the polypeptide sample. Such data were obtained from ORD measurements on a number of fractionated samples of poly-N⁵-(3-hydroxypropyl)-L -glutamine (PHPG) in mixtures of water and methanol covering the complete range of composition and at various termperatures (5–40°C). When analyzed in terms of the proposed procedure, they yielded values of σ which were in the range (3.2 ± 0.6) × 10^?4, substantially independent of solvent composition and temperature. These values were much larger than those obtained recently for σ of poly(β-benzyl-L -aspartate) in m-cresol and in a mixture of chloroform and DCA. The data for [η] and s₀ (limiting sedimentation coefficient) as functions of molecular weight indicated that the molecular shape of PHPG in pure methanol is essentially rodlike, whereas that in pure water is not entirely randomly coiled but rather may be regarded as an interrupted helix. These indications were consistent with the results from ORD measurements. When plotted against the corresponding values of f_N, the values of [η] and [s₀] for PHPG in mixtures of water and methanol of various compositions and temperatures formed smooth composite curves, and we attributed these phenomena to the fact that σ of PHPG was nearly constant under these solvent conditions. Here [s₀] stands for a reduced limiting sedimentation coefficient which is equal to the inverse friction factor of the solute molecule.     

The determination of the thermodynamic parameters of the helix-coil transition of polypeptides by means of dipole moment measurements

The theoretical change of the mean-square dipole moment of a polypeptide during the helix-coil transition is compared with the change in helix content. It is shown that, according to the theory, the determination of the helix initiation parameter σ and the enthalpy of helix formation ΔH can be determined. Experimental data on poly-benzyl-L -gluatamate in two different solvent mixtures are given.