Effect of counterion concentration on the dielectric behavior of a polypeptidic chain in the helix-coil transition

On the basis of the two-state model of a polyelectrolyte solution, the ion concentration in the polymer domain has been calculated by using the spherical Poisson-Boltzmann equation. The ion accumulation in the neighboring of the polyion influences, on different time scales, various electrical properties of the solution, in particular the low-frequency electrical conductivity and the high-frequency dielectric dispersion. These predictions have been compared with recent dielectric measurements on poly(L-glutamic acid) aqueous solutions during the conformational transition from the alpha-helix to random coil, and a satisfactory agreement has been found. This finding suggests that counterion distribution plays a different role in determining the electrical properties of charged polymer solutions, causing a electrophoretic contribution of the polymer domain to the electrical conductivity and influencing the high-frequency dielectric dispersion.