Conformational Transition Of Poly Research Articles

Conformational transitions of poly(dA-dC).poly(dG-dT) induced by high salt or in ethanolic solution.

Poly(dA-dC).poly(dG-dT) was studied by circular dichroism in the presence of high CsCl concentrations and in ethanolic solutions. This alternating purine-pyrimidine duplex may undergo two conformational transitions from a B-type to a novel structure and subsequently into an A-form. Cs+ ions or increasing ethanol concentrations induced a change of the B-type CD spectrum and an inversion of the long wavelength CD band. Lowering the temperature below 0 C or addition of small amounts of Ca++ ions were particularly potent in producing a large negative CD band. A modified B-type structure or a conversion into a left-handed Z-form is considered for this conformational transition.

Conformational transitions of poly(dA-dT)poly(dA-dT) in ethanolic solutions.

Examination of circular dichroic and phosphorus nuclear magnetic resonance spectra showed that poly(dA-dT)-poly(dA-dT) exhibited an ethanol-induced transition to the A form in an Na+ containing medium like natural DNAs. A mere replacement of the Na+ by Cs+ counterions meant that the polynucleotide was with a little cooperativity transformed into a novel conformation displaying a deep negative band in the long wavelength part of the CD spectrum. The presence of very low concentration of Cs2+ shifted the midpoint of the transition to a lower content of ethanol.

Conformational transition in poly[4‐(1,1,3,3‐tetramethylbutyl)phenyl methacrylate] and poly(4‐tert‐butylphenyl methacrylate) in dilute solutions

AbstractTemperature dependence of viscosity data and refractive index increment was investigated on two polymers: poly[4‐(1,1,3,3‐tetramethylbutyl)phenyl methacrylate] (1a) and poly(4‐tert‐butylphenyl methacrylate) (1b) in dilute solution. A discontinuity in intrinsic viscosity was observed over a narrow temperature range. From this behaviour it was possible to visualize sharp changes of unperturbed dimensions (Kθ) and thermodynamic parameters (B). This phenomenon can be accounted for by assuming conformational changes of the chain in the chosen solvent in a specific temperature range. These changes can be also observed by discontinuities in refractive index increments dn/dc. In the case of 1b the conformational change disappears on addition of a polar solvent (CHCl3).

Investigation of conformational transition in poly(cyclohexyl methacrylate)

The variations of intrinsic viscosity and refractive index increment with temperature for poly(cyclohexyl methacrylate) in isobutyl methyl ketone have been studied. A conformational transition has been found in the range 10–30°C. The influence of the molar volume of the side-group on the conformational transition temperature has also been discussed with respect to results for related polymers.

Conformational transition in poly(methyl methacrylate)

This paper refers to a systematic and comparative study of the intrinsic viscosities of fractions of atactic and syndiotactic samples of PMMA in various polar solvents as a function of temperature. The data allow determination of important changes in the conformation of the polymer.

Characterization of deformation phenomena in polymers by rapid‐scanning fourier transform IR (FTIR) spectroscopy and mechanical measurements, 2. The effect of stress relaxation on the reversible, stress‐induced conformational transition in poly(butylene terephthalate)

Characterization of deformation phenomena in polymers by rapid‐scanning fourier transform IR (FTIR) spectroscopy and mechanical measurements, 2. The effect of stress relaxation on the reversible, stress‐induced conformational transition in poly(butylene terephthalate)

Conformational changes of poly(L‐histidyl‐L‐alanyl‐α‐L‐glutamic acid) in solution. Transition pathways and conformational intermediates

AbstractThe conformation and conformational transitions of poly(His‐Ala‐Glu) have been investigated by ir, nmr, and CD measurements. The results obtained—as well as the results of our previous investigations by potentiometric titration and hydrodynamic techniques [Goren et al., Biopolymers (1977) 16, 1541–1555]—indicate that when dissolved in water, the co‐polymer assumes a disordered conformation. On changing the pH of the solution, the states of ionization of the side‐chain imidazole and carboxyl groups change in the same manner as in low‐molecular‐weight model compounds. Concomitantly, the overall shape of the macromolecule is altered, while the conformation of the polypeptide backbone changes from one disordered state to another but never assumes a regular form. In water/methanol and water/trifluoroethanol mixtures, transitions from a disordered state to the α‐helix conformation were observed on increasing the alcohol content of the system. The conformational transitions followed pathways which differ from one another according to the experimental conditions employed. Conformational landmarks (intermediates) have been identified along these pathways.

Conformation of poly(L‐arginine). I. Effects of anions

AbstractThe conformational transition of poly(L‐agrignine) by binding with various mono‐, di‐, and polyvalent anions, especially with SO, was studied by CD measurements. The intramolecular random coil‐to‐α‐helix conformational transition and the subsequent transition to the β‐turn‐like structure was caused by binding with SO. The binding data obtained from equilibrium dialysis experiments showed that the α‐helical conformation of poly(L‐arginine) is stabilized at a 1:3 stoichiometric ratio of bound SO to arginine residue; at higher free SO concentrations, the α‐helix converts to the β‐turn‐like structure accompanied by a decrease in amount of bound SO. The same conformaitonal transition of poly(L‐arginine) also occurred in the solutions of other divalent anions (SO, CO, and HPO) and polyvalent anions (P2O, P3O). Among the monovalent anions examined, CIO and dodecyl sulfate were effective in including α‐helical conformation, while the other monovalent anions (OH−, Cl−, F−, H2PO, HCO and CIO) failed to induce poly(L‐arginine) to assume the α‐helical conformation. Thus, we noticed that, except for dodecyl sufate, the terahedral structure is common to the α‐helix‐forming anions. A well‐defined model to the α‐helical poly(L‐arginine)/anion complex was proposed, in which both the binding stoichiometry of anions to the arginine residue and the tetrahedral structure of anions were taken into consideration. Based on these results, it was concluded that the tetrahedral‐type anions stabilize the α‐helical conformation of poly(L‐arginine) by crosslinking between two guanidinium groups of nearby side chains on the same α‐helix through the ringed structures stabilized by hydrogen bonds as well as by electrostatic interaction. Throughout the study it was noticed that the structural behavior of poly(L‐arginine) toward anions is distinct from that of poly(L‐lysine).

Investigation of conformational transition in poly(methylmethacrylate)—III Aromatic esters as solvents

Light scattering and viscosity measurements on fractionated samples of atactic PMMA in methyl and ethyl benzoates show the existence of a conformational transition in the temperature ranges 45–50° and 40–45° respectively. The study of unperturbed dimensions and polymer-solvent interaction parameter shows that the phenomenon affect both inter- and intramolecular interaction. This effect modifies the flexibility of the macromolecular chain and other properties in solution, e.g. intrinsic viscosity.

The two beta forms of poly(L-glutamic acid).

Abstractβ‐Helical poly(L‐glutamic acid) in a gel state was found to be easily converted to the antiparallel β form by heating. Two β forms were obtained, depending on the temperature of heating. Temperatures between 40° and 85°C produced a β form with a spacing between pleated sheets (d001) of 9.03 Å, termed β1. If the heating was carried out at temperatures higher than 85°C, the β1 form underwent another conformational transition reducing the d001 value from 9.03 to 7.83 Å (termed β2) without any prominent change in the fiber repeat distance (i.e., the polypeptide backbone conformation). The time course of these two transitions was followed by measuring the infrared spectra of the samples, and it was concluded that the α → β1 transition in its initial stage obeys a pseudo‐first order rate process with activation enthalpy and entropy of 54 kcal/mol and 92 eu, respectively. On the other hand, the typical sigmoidal conversion curves observed for the transition between the two types of β forms (β1 → β2) indicate that this transition proceeds via a socalled “nucleation and growth” process. The kinetic theory of phase transitions developed by Avrami can be applied with success to explain this transition. The infrared spectra, in the region from 1800 to 200 cm−1, were measured for these two β forms and the results showed that the conformation of the side chains and the mode of the hydrogen bonding between the side‐chain carboxyl groups undergo appreciable change during the transition.The heat‐induced conformational transition of poly(L‐Glu78 L‐Val22) was also studied. The copolymer was transformed from the α‐helical conformation directly to the β2 form. The reason for this was thought to be due to the fact that the L‐valine residues and the L‐glutamyl residues near the L‐valine residues have a strong tendency to take the more compact β2 form.

The conformational transition in poly(methacrylic acid) and butyl vinyl ether/maleic anhydride copolymers studied by 1H NMR linewidth measurements

AbstractLinewidth measurements on the 1H NMR spectra of D2O solutions of poly(methacrylic acid), (PMA), and the hydrolyzed form of butyl vinyl ether/maleic anhydride copolymer show a sharp decrease in the linewidths of side chain resonances at a degree of neutralization α ≈ 0,2. This is the region in which potentiometric titrations previously have shown a conformational transition to occur. Such a change does not occur in poly(acrylic acid) on the one hand and methyl vinyl ether/maleic anhydride copolymer on the other. The nature of the transition is discussed in terms of the freedom of rotation of the side chains about the main chain.

ポリ(L-ホモアルギニン)の陰イオンによる分子形態転移

ポリ(L-ホモアルギニン)の陰イオンによる分子形態転移

A DSC study of the conformational transition of poly- 7-benzyl-L-glutamate

The thermally induced random coil-to-helix conformational transition of poly- 7-benzyl-L-glutamate (PBG) in dichloroacetic acid (DCA)—chloroform, DCA-1,2-dichloroethane (DCE), and DCA-l,1,2,2-tetrachloroethane (TCE) mixtures has been studied as a function of solvent composition. The experimental data were obtained using the Perkin—Elmer DSC-2 differential scanning calorimeter with a modified sample container. Results of a systematic investigation of the transition temperature and the heat of transition in the above solvent systems are presented. The data are interpreted in terms of current theory.

Conformational aspect of polypeptide structure. XLIV. Conformational transitions of poly(N‐methyl‐alanines) induced by trifluoroacetic acid

AbstractThe synthesis of poly(N‐methyl‐L‐alanine) and poly (N‐methyl‐DL‐alanine) are described. The polymers were examined by 220 MHz high‐resolution nuclear magnetic resonance (nmr) and circular dichroism (CD). The results demonstrate that poly(N‐methyl‐L‐alanine) exists as an ordered helical structure with all the amide bonds in the trans configuration in appropriate solvents. As trifluoroacetic acid (TFA) is added to the solutions of the polymer in helix‐supporting solvents, resonances corresponding to both trans and cis amide conformations of N‐methyl, C‐methyl, and α‐CH are observed. The presence of both the trans and the cis peptide bonds in a polymer chain disrupts the ordered structures. Our conclusions from CD data are in agreement with the nmr results. Ultracentrifugation shows that degradation of the polymer chain does not occur during the TFA treatment.

Effect of chain length and concentration of anionic surfactants on the conformational transitions of poly(L-ornithine) and poly(L-lysine) in aqueous solution

The conformation of poly(L-ornithine) (PLO) and poly(L-lysine) (PLL) in solutions of sodium alkyl sulfates, CH 3(CH 2) nSO 4Na with n = 7, 9, 11, 13 and 15 was studied by circular dichroism. PLO adopts a helical conformation in all 5 homologs and PLL a β-form in only 4 of the homologs. With octyl sulfate PLL has a helical conformation instead. These conformations were observed in solution of surfactants both below and above the critical micelle concentration.

A study of the interaction of Ni(II) and Cu(II) with poly(acrylic acid)

AbstractPotentiometric titrations have been carried out on a sample of poly(acrylic acid) of very high molecular weight (Mw = 1.92 × 106). At polymer concentrations greater than about 1 × 10−4 equiv./liter, this material behaves as a weaker acid, under given conditions, than when it is heat degraded, and also compared with a lower molecular weight sample of poly(acrylic acid) (Mw = 3.52 × 105). Only at very high dilutions are there indications that the three materials have the same pKa values. The titration curves reveal a conformational transition in poly (acrylic acid) in the range of α's of 0.2–0.5. Formation curves for the complexes between the high molecular weight poly‐(acrylic acid) and Cu2+ and Ni2+ have been calculated from the titration curves. At a polymer concentration of 2.4 × 10−3 equiv./liter, these metals form complexes in which two carboxylate groups are involved, whereas at a polymer concentration of 1.5 × 10−4 equiv./liter, the formation curves suggest the existence of complexes involving more than two carboxylate groups. This is thought to be due to enhanced intermolecular interaction which results in a reversible physically crosslinked system.

Potentiometric study of the conformational transition in poly(acrylic acid)

AbstractDetailed potentiometric titration curves have been obtained for poly(acrylic acid) (Mn = 2.6 × 106) in solution at four different ionic strengths at 25°C. When plotted as pH + log [(1 − α)/α] versus α (where α is the degree of dissociation), they reveal the presence of two different molecular conformations, denoted PAA(a) and PAA(b), with a transition between them. The proportion of the material present in the two conformations has been calculated for different degrees of dissociation and is found to be independent of ionic strength. PAA(a) is intact up to α = 0.22 and is completely converted into PAA(b) by α = 0.75. The free energy of transition from PAA(a) to PAA(b) when both forms are uncharged decreases numerically from −275 to −157 cal./mole monomer residues as the ionic strength increases from 0.02 to 0.20. The increase of the pH of solutions of the PAA(a) from over those of the PAA(b) form at a given value of α, is largely independent of α. This makes it possible to calculate the free energy of transition from PAA(a) to PAA(b) when both forms are charged to the same extent. This free energy is proportional to α and decreases numerically from −944α to −561α (cal.) as ionic strength increases from 0.02 to 0.20. The charge‐independent part of the free energy of transition is a constant fraction of the total, at a given value of α, independent of the ionic strength. The PAA(a) form is the more compact, less contractile, and more ordered structure. The possibility is suggested that this conformation may be partly helical in nature.

The conformational transition in partially esterified poly‐(methacrylic acid)

AbstractThe conformational transition of poly‐(methacrylic acid), partially esterified with dimethyl sulfate, has been studied using potentiometric titration at constant temperature.It is found that the stability of the form existing at low values of dissociation (α) is not decreased by reducing the number of groups on the chain capable of establishing intramolecular hydrogen bonds. The transition is also shown to depend on the average charge per monomeric unit. This confirms the hypothesis that the stable form at low α‐values is a compact structure stabilized probably by VAN DER WAALS attractions between —CH3 groups.