HELIX-COIL TRANSITION OF POLY(GAMMA-BENZYL L-GLUTAMATE) IN DIOXAN-DICHLORACETIC ACID MIXTURES

Measurements have been made on solutions of poly(γ-benzyl-l-glutamate) in solvents comprising dichloracetic acid (volume fraction φ1) and dioxan over the whole range of φ1. The transition point at φ1 = 0.91 found from intrinsic viscosity is close to that obtained by others via dielectric measurements. However, the specific refractive index increments at constant composition and constant chemical potential as well as the selective adsorption coefficient of dichloracetic acid all exhibit sharp changes at about φ1 = 0.75, and the partial specific volume of the polymer increases when φ1 >0.75. An unusual minimum in the dependence of the refractive index increment on solvent composition is attributed to a small change in the refractive index of the polymer. This corresponds to a decrease of about 2% in the molecular polarizability of the polypeptide during the helix → coil transition. © 1979.