Theory of the cooperative transition between two ordered conformations of poly(L-proline). III. Molecular theory in the presence of solvent

Abstract

Phenomenological theories of the form I in equilibrium to form II interconversion in poly(L-proline) have been presented by Schwarz (using the parameters s, sigma, beta', and beta'' in a 2 X 2 matrix formulation) and by the present authors (using the parameters s, sigma, betaC, and betaN in a 4 X 4 matrix formulation). In addition, a molecular theory was developed to compute s, sigma, beta', and beta'' under vacuum. In this paper, we take into account the effect of solvent on the parameters s, sigma, beta', and beta'' of the isothermal poly(L-proline) form I in equilibrium to form II interconversion. The growth parameter is sensitive to the binding of solvent molecules to the peptide CO groups, but the nucleation parameters sigma, beta', and beta'' are not affected by this type of solvent effect. The calculated values of s and sigma under vacuum are in good agreement with the corresponding values derived from experimental data. By combining the theoretical values of s, sigma, beta', and beta'' under vacuum with experimentally determined equilibrium constants for the binding of alcohols to the peptide CO groups (which differ in magnitude for form I and form II), it was possible to reproduce the experimental tranistion curves satisfactorily. Alternatively, the binding constants for alcohols, obtained by combining our theoretically computed parameters under vacuum with experimental equilibrium transition curves, are in a satisfactory agreement with those evaluated independently by infrared spectral measurements of the binding of alcohols to the peptide CO groups. It is pointed out that significant errors may arise in analyzing experimental data if short chains are included with long chains in the determination of s, sigma, beta', and beta'' from the equilibrium transition curves. The transition of poly(L-proline) from form II to form I when n-butyl alcohol is added to a solution of the polymer in benzyl alcohol is brought about by the slight difference in the binding free energies of both alcohols to the carbonyl groups of form II. The different binding affinities of two alcohols, ROH, to form II may arise from (a) the different hydrogen-bond strength between the alcohol and the proline carbonyl group, and (b) possible differences in nonbonded and electrostatic interactions between the R group and the binding-site environment of the proline carbonyl group. The greater binding affinity of form II (compared to form I) for given alcohol is attributed to the more open and extended conformation of form II.