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1.
The helix–coil transition of poly-γ-N-carbobenzoxy-L -α,γ-diaminobutyrate (PCLB) and poly-δ-N-carbobenzoxy-L -ornithine (PCLO) in chloroform–dichloroacetic acid mixtures was followed by optical rotatory dispersion. PCLB displays a “normal” temperature-induced transition, but PCLO an “inverse” one. The thermodynamic parameters for helix formation of the two polymers were determined using the Zimm-Bragg theory. The enthalpy for adding an amide residue to a helical region, ΔH, and the initiation factor σ were ΔH = ?180 cal/mole and σ = 9.2 × 10?5 for PCLB and ΔH = +490 cal/mole and σ = 1.9 × 10?5 for PCLO. 相似文献
2.
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 fN 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 fN, 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. 相似文献
3.
The helix–coil transition of poly-N5-(2-hydroxyethyl)L -glutamine (PHEG) in aqueous isopropanol was examined by means of optical rotatory dispersion (ORD) and intrinsic viscosity [η] measurements. The Zimm–Bragg parameters σ and s for the transition were determined from the ORD data as a function of molecular weight. It was found that the transition was characterized by a relatively low cooperativity; the values of \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt \sigma $\end{document} were in the range from 0.039 to 0.066, depending on the solvent composition. These σ values are much larger than those reported for other polypeptide–solvent systems. The transition enthalpy was negative and its magnitude varied with the solvent composition, with a maximum of 620 cal/mol at 40 wt% isopropanol. The curve of [η] versus helical content for a high-molecular-weight sample exhibited a very broad minimum, and this behavior was attributed to the low cooperativity of the transition. 相似文献
4.
The Zimm–Bragg parameters s and σ were determined for poly(γ-benzyl L -glutamate) (PBLG) in m-cresol and in dimethylformamide (DMF) from ORD data as a function of molecular weight. It was found that, within the temperature range between 10 and 55°C and on the average, s = 1.61 ± 0.1 and √σ = 0.04 ± 0.01 in m-cresol and s = 1.65 ± 0.05 and √σ = 0.045 ± 0.015 in DMF. The values of s in m-cresol decreased with increasing temperature, while the values of σ in the same solvent increased. This result for s suggests that PBLG in m-cresol will undergo a thermal helix–coil transition of normal type. The parameters in DMF showed no appreciable trend to vary with temperature. Aside from the difference between the two solvents, our results are consistent with existing data for various conformation-dependent properties such as light-scattering radius, intrinsic viscosity, and dipole moment, each indicating that the polypeptide chain has some flexibility in helicogenic solvents. 相似文献
5.
Conformational aspects of a series of copolymers of L -Leucine and L -leucine [poly-(LysxLeuy)] containing 0 to 0.41 mole fraction L -leucine have been studied by circular dichroism (CD) and potentiometric titration in 0.05M KF solution. CD studies on the α-helical conformation showed a dependence of the magnitude of the CD ellipticity band at 222 nm on copolymer composition; the [θ]222 decreasing with higher leucine contents. This was interpreted as the result of an increase of the hydrophobicity of the environment of the amide group due to the presence of the leucyl residues. Values of the Zimm-Rice parameter, σ, for the copolymers were obtained from the potentiometric titrations and used to fit theoretical curves to the experimental data. Using the variation of σ with polymer composition, a value of σ for the leucyl residue was estimated to be 6.3 × 10?2, assuming independence of σ on the amino acid sequence in the copolymer. The free energy change for the conversion of one mole residue from uncharged helix to uncharged coil, ΔGhc°, was also obtained from the titration data for each copolymer up to a leucine mole fraction of 0.16; a value of 385 cal mole?1 was estimated for ΔGhc° for a leucyl residue. These values for σ and ΔGhc° are compared with other values in the literature for various amino acid residues obtained from titration and melting curve data. 相似文献
6.
The potentiometric titration of random copolymers of L -lysine and L -alanine containing 0–35% alanine was carried out. The standard free-energy change for the transition of coil to helix was calculated from the titration curve, and was treated by taking account of first-neighbor interactions. For uncharged lysine ΔG° = ?140 cal/mole, and for alanine ΔG° = ?50 cal/mole in 0.06M NaBr at 25°C, indicating that the alanine helix is thermodynamically less stable than the lysine helix. The randomness in co-polymerization was confirmed by trypsin treatment. 相似文献
7.
The thermally induced conformational changes of poly-N5-(3-hydroxypropyl)-L -glutamine in water and in methanol–water (3:7 v/v) have been analyzed in terms of the Lifson-Roig theory. The transitions in both solvents can be described by using v = 0.017. The thermodynamic parameters for the random coil-to-helix transition of one amino acid residue at room temperature were found to be: in water, ΔH = ? 130 cal/mole and ΔS = ? 0.45 e.u.; in methanol–water (3:7 v/v), ΔH = ? 170 cal mole and ΔS = ? 0.45 e.u. The size distribution of helical segments is broad, and the results of numerical calculations are presented for three degrees of polymerization (DP = 100, 300, and 750). 相似文献
8.
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 Tc, 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. 相似文献
9.
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. 相似文献
10.
The helix–coil transitions of poly(Nε-methyl, Nε-carbobenzoxy-L -lysine), poly(Nδ-methyl, Nδ-carbobenzoxy-L -ornithine), and poly(Nδ-ethyl, Nδ-carbobenzoxy-L -ornithine) in chloroform–dichloroacetic acid and their corresponding decarbobenzoxylated polypeptides in alkaline solutions were followed by optical rotation measurements. The introduction of a methyl or an ethyl group to the side chains of the carbobenzoxy derivatives of poly(L -lysine) and poly(L -ornithine) appeared to weaken the helical conformation at 25°C. The thermodynamic quantities of the three water-soluble polypeptides were calculated from the data on potentiometric titrations at several temperatures. For uncharged coil-to-helix transition, ΔH = ?370 cal/mol and ΔS = ?1.1 eu/mol for poly(Nε-methyl-L -lysine), and ΔH = ?540 cal/mol and ΔS = ?1.6 eu/mol for poly(Nδ-ethyl-L -ornithine) (all on molar residue basis). The absolute values of ΔH and ΔS dropped in the region of pH-induced transition and eventually both quantities became positive. The initiation factor σ was about 2 × 10?3, which was essentially independent of temperature. For poly(Nδ-methyl-L -ornithine) the coil-to-helix transition was not complete even when the polymer was uncharged at high pH. 相似文献
11.
Thermodynamic parameters of helix-coil transition in polypeptide chains. II. Poly-L-lysine 总被引:2,自引:0,他引:2
The helix–coil transitions for poly-L -lysine (PL) were investigated by the methods of spectropolarimetry, viscometry and potentiometric titration in 0.2M NaCl at different temperatures as well as in 0.2MNaBr, 1MKCl, and in mixtures of 0.2MNaCl or NaBr with methanol at room temperature. The enthalpy and entropy differences between the helical and coillike states of uncharged PL molecules in 0.2.M NaCl were determined from the potentiometric titration curves. The cooperativity parameters σ for PL in different solvents were determined by two methods (from the sharpness of the transition and from the dependence of the intrinsic viscosity on the helical content in the transition region). In 0.2MNaCl σ has a value of (2.3 ± 0.5) × 10?4 and does not depend on temperature, i.e., the cooperativity of the helix-coil transition, as for PGA, is mainly of an entropy origin (the initiating of the helical region is accompanied by the entropy decrease ΔSi = ?12 eu/mole of helical regions). A comparison of the obtained results for PGA and PL with the molecular theories of the helix-coil transitions shows that the role of dipole-dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment. 相似文献
12.
High-molecular-weight poly(0,0′-dicarbobenzoxy-L -β-3,4-dihydroxyphenyl-α-alanine) was prepared by the N-carboxyanhydride method. From the results obtained by a study of the optical rotation, nuclear magnetic resonance, and solution infrared absorption, the conformation of poly(0,0′-dicarbobenzoxy-L -β-3,4-dihydroxyphenyl-α-alanine) depended greatly on the solvent taking a right-handed helix with [θ]225 = ?13,600 ~ ?18,900 in alkyl halides, a left-handed helix with [θ]228 = 22,100 ~ 24,800 in cyclic ethers or trimethylphosphate, and a random coil structure in dichloroacetic acid, trifluoroacetic acid, or hexafluoroacetone sesquihydrate. The polypeptide underwent a right-handed helix-coil transition in chloroform/dichloroacetic acid (or trifluoroacetic acid) mixed solvents and a left-handed helix-coil transition in dioxane/dichloroacetic acid (or trifluoroacetic acid) mixed solvents. The results were compared with those of poly(0-carbobenzoxy-L -tyrosine). 相似文献
13.
Taichi Fujii Ken Terao Maiko Tsuda Shinichi Kitamura Takashi Norisuye 《Biopolymers》2009,91(9):729-736
The z‐average mean‐square radius of gyration 〈S2〉z, the particle scattering function P(k), the second virial coefficient, and the intrinsic viscosity [η] have been determined for amylose tris(phenylcarbamate) (ATPC) in methyl acetate (MEA) at 25°C, in ethyl acetate (EA) at 33°C, and in 4‐methyl‐2‐pentanone (MIBK) at 25°C by light and small‐angle X‐ray scattering and viscometry as functions of the weight‐average molecular weight in a range from 2 × 104 to 3 × 106. The first two solvents attain the theta state, whereas the last one is a good solvent for the amylose derivative. Analysis of the 〈S2〉z, P(k), and [η] data based on the wormlike chain yields h (the contour length or helix pitch per repeating unit) = 0.37 ± 0.02 and λ?1 (the Kuhn segment length) = 15 ± 2 nm in MEA, h = 0.39 ± 0.02 and λ?1 = 17 ± 2 nm in EA, and h = 0.42 ± 0.02 nm and λ?1 = 24 ± 2 nm in MIBK. These h values, comparable with the helix pitches (0.37–0.40 nm) per residue of amylose triesters in the crystalline state, are somewhat larger than the previously determined h of 0.33 ± 0.02 nm for ATPC in 1,4‐dioxane and 2‐ethoxyethanol, in which intramolecular hydrogen bonds are formed between the C?O and NH groups of the neighbor repeating units. The slightly extended helices of ATPC in the ketone and ester solvents are most likely due to the replacement of those hydrogen bonds by intermolecular hydrogen bonds between the NH groups of the polymer and the carbonyl groups of the solvent. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 729–736, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com 相似文献
14.
The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔHh° in poly(L -ornithine) in 0.1 N KCl has been determnined calorimetrically to be ?1530 ± 210 and ?1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH? ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D ,L -ornithine). Since it was discovered that poly(L -ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix–coil transition curve for poly(L -ornithine) at pH 11.68 in 0.1 N KCl in the 0°–40°C region, the transition temperature Ttr and the quantity (?θh/?T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be ?33 cal/mol and ?5.3 cal/mol deg, respectively. The value of the Zimm–Bragg helix–coil stability constant σ has been calculated to be 1.4 × 10?2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C. 相似文献
15.
Intrinsic viscosities of cyclic and linear lamda DNA 总被引:3,自引:0,他引:3
The ratio of the intrinsic viscosities of the linear and circular forms of λ DNA, [η]L /[η]c, has been measured as a function of ionic strength in the range [Na+] = 0.6. M–0.03MCorrections were made for the presence of uncyclizable linear contaminant in circular preparations. By combining data in the literature on the ionic strength dependence of linear DNA of various molecular weights with that obtained here, it was possible to determine the expansion parameter εL as a function of [Na+]. εL is defined by the relation 〈L2〉 = b2N1+εL, where 〈L1〉 is the mean-square end-to-end distance of a chain of N segments of length b. The empirical relation εL = 0.05 ? 0.11 log [Na+] for native NaDNA at 25°C is found. When εL = 0, [η]L /[η]c extrapolates to 1.6, in good agreement with the theoretical prediction of 1.55. As εL increases, [η]L /[η]c increases, in agreement with a theory of Bloomfield and Zimm. 相似文献
16.
The thermal triple helix–coil transition of covalently bridged collagenlike peptides with repeating sequences of (Ala-Gly-Pro)n, n = 5–15, was studied optically. The peptides were soluble in water/acetic acid (99:1) and were found to form triple-helical structures in this solvent system beginning with n = 8. The thermodynamic analysis of the transition equilibrium curves for n = 9–13 yielded the parameters ΔH°s = ?7.0 kJ per tripeptide unit, ΔS°s = ?23.1 J deg?1 mol?1 per tripeptide unit for the coil-to-helix transition, and the apparent nucleation parameter σ ? 5 × 10?2. It was suggested through double-jump temperature experiments that the rate-limiting step during refolding is not only influenced by the difficulties of nucleation, but also by cis–trans isomerization of the Gly-Pro peptide bond. 相似文献
17.
A series of copolymers of L -lysine and L -valine [poly(L -lysinef L -valine100-f)] containing 0–13% L -valine have been studied, in 0.10M KF solution, using potentiometric titration and circular dichroism spectroscopy. Incorporation of increasing amounts of valine into the copolymers favors β-sheet formation over α-helix formation at high pH and room temperature. The titrations were analyzed using the method of Zimm and Rice and the partial free energy (ΔG0cβ) for the coil-to-β-sheet transition for valine is estimated at 900 cal/mole at 25°C. From the temperature dependence of the free energy, the partial enthalpy, ΔH0cβ, and entropy, ΔS0cβ, of the transition for valine is estimated to be 854 cal/mole and 6.0 e.u., respectively. The corresponding partial thermodynamic parameters for L -lysine are in agreement with published results. The fraction of β-sheet versus pH has been calculated for poly(L -lysine86.8 L -valine13.2) at 25.0°C using the titration data; data obtained from circular dichroism spectroscopy for the same copolymer are in good accord. It is concluded from these results that L -valine is a very strong β-sheet forming amino acid. Furthermore, these results indicate that the Zimm–Rice method is applicable to transitions between the coil and β-sheet states for a polypeptide containing two different residues. 相似文献
18.
We have measured the thermodynamic parameters of the slow-fast tail-fiber reorientation transition on T2L bateriophage. Proportions of the virus in each form were determined from peak-height measurements in sedimention-velocity runs and from average diffusion coefficients obtained by quasielastic laser light scattering. Computer simulation of sedimentation confirmed that there were no undetected intermediates in the transition, which was analyzed as a two-state process. Van't Hoff-type plots of the apparent equilibrium constant and of the pH midpoint of the transition as function of reciprocal temperature led to the following estimates of the thermodynamic parameters for the transition at pH 6.0 and 20°C: ΔH° = ?139 ± 18Kcal mol?1, ΔS° = ?247 ± 46 cal K?1 mol?1, and ΔG° = ?66 ± 22 kcal mol?1. Per mole of protons taken up in the transition, the analogous quantities were ?15.9 ± 1.7 kcal mol?1, ?26.3 ± 2.2 cal K?1 mol?1, and ?8.22 ± 1.8 kcal mol?1. The net number of protons taken up was about 8.5 ± 1.5. The large values of the thermodynamic functions are consistent with a highly cooperative reaction and with multiple interactions between the fibres and the remainder of the phage. The negative entropy of the transition is probably due to immobilization of the fibres. 相似文献
19.
Thermal and charge induced random coil to α-helix transitions of poly-L -glutamic acid (PGA) were measured by optical rotatory dispersion in various solvents. The data of PGA in 0.1M Nacl were analyzed by the Zimm-Rice theory. Enthalpy and entropy changes for the coil-to-helix transition in the unionized state were obtained: ΔH° = ?1020 ± 100 cal/residue mole; ΔH° = ?3.0 ± 0.4 e.u./residue mole. The initiation parameter, σ, of the Zimm-Rice theory was given by a value of 5 ± 1 × 10-3. Random copolymers of L -glutamic acid and L -alanine containing 10, 30, and 40 molar percents of alanyl residue were synthesized. Stabilities of α-helix of the copolymers were compared to that of PGA. In water and water-ethanol solutions, stabilities of the polymers were almost equal after the simple correction about the ionized charge density of the polymers. In 0.1 M NaCl solution these copolymers showed some deviations from the transition curve of PGA, which would suggest the hydrophobic contribution of the alanyl residues. 相似文献
20.
By using optical rotatory dispersion measurements, the helix content of poly Glu50Lys50 has been investigated and compared with that of poly Glu20Lys20Ala60 in aqueous solutions. Measurements were made at pH 3 and at pH 8 in various concentrations of lithium bromide. Various factors affecting helix stabilization are considered and their perturbation by lithium bromide is related to the shape of the observed transition curves. A residual helix content of 12% in 8M LiBr, based upon a b0 of +100 for a fully random conformation, was observed for poly Glu50Lys50 at pH 3 and 8. The loss of helix content of poly Glu50Lys50 as a function of temperature is also reported. ΔH is approximately ?6.9 kcal./mole for the overall transition, compared to ?6.5 kcal./mole for poly Glu20Lys20Ala60. The midpoint of the broad transition is near 40°C. at pH 3, but much lower, at ?10 to 0°C., at pH 8. These results are discussed in terms of the stabilizing factors for the partial helix content of the polypeptides. 相似文献