Dielectric studies of aqueous solutions of poly(L‐glutamic acid)

Abstract

Abstract The dielectric features of poly( L ‐glutamic acid) are studied by the Fourier synthesized pseudorandom noise method in a time domain combined with a four‐electrode cell. Polymer concentration dependence, the effect of the solvent viscosity, salt effects, and pH dependence are studied concomitantly with measurements of CD. A helix‐to‐coil transition occurs near pH 5.6 for a salt‐free solution; at higher pH values, the polymer has an ionized random‐coil conformation, and at lower pH, it has a deionized α‐helical conformation. When it is in the ionized random‐coil conformation, with the usual features of an electrolytic polymer, the solution shows a relaxation spectrum with a large dielectric increment at low frequencies. In the deionized α‐helical state, no distinct relaxation curves are obtained, which does not deny the existence of a permanent peptide dipole. The pH dependence of the dielectric increment does not mainly correspond to the conformational change from helix to coil, but rather corresponds to the change of chain expansion on account of a charge–charge interaction under low ionic strength, which is conceived of by a viscosity measurement.