Dielectric dispersion of polypeptide solutions. II. Helix-coil transition of poly(epsilon-carbobenzoxy-L-lysine) in m-cresol.

Abstract

Dielectric dispersion measurements were made on dilute solutions of poly(epsilon-carbobenzoxy-L-lysine) in m-cresol; this system underwent a very sharp thermal helix-coil transition of inverse type around 30 degrees. Mean-square dipole moments "mu-2" and mean rotational relaxation times t were obtained as functions of molecular weight and helical fraction fN. It was found that "mu-2"-one-half varied almost linearly with fN-one-half over a substantial portion of the transition. The relaxation times corrected for solvent viscosity and temperature changed only slightly with fN except in the region of small fN, while, at fixed fN, they displayed molecular weight dependence characteristic of rod-like molecules. These results can be explained if the transition is assumed to proceed almost in all-or-none fashion. This assumption is consistent with the previous finding from statistical thermodynamic analyses that the transition of the system PCBL-m-cresol is highly cooperative, with standard deviation of population one-half being 0.0025 to 0.0027, where standard deviation of population is the cooperativity parameter. Application of Nagai's theory of "mu-2" for interrupted helices yielded the result that standard deviation of population-one-half was in the range 0.001 to 0.005 and the monomeric dipole moment in the helical conformation was 5.4 to 6.2 D.