Dielectric studies of protein hydration

Abstract

This work deals with dielectric studies of the binding modes of water in hydrated powders of the proteins casein and lysozyme by means of the thermally stimulated depolarization currents (TSDC) method in the temperature range 77–300 K. The water content was varied between 0.01 and 0.61 w/w. The measurements revealed the existence of two complex dispersions, a low-temperature dispersion with maxima in the temperature region 115–165 K and a high-temperature dispersion with maxima in the temperature region 155–280 K. Both dispersions were studied in detail by using several experimental techniques offered by the TSDC method. The low-temperature dispersion is due to the dielectric relaxation of loosely bound and free water molecules. The fractions of tightly bound, loosely bound and free water molecules are obtained from the dependence of the characteristics of this dispersion on water content. Apart from dc conductivity, two main relaxation mechanisms contribute to the high-temperature dispersion : the relaxation of polar side chains and the displacement of protons on the surface of the protein molecules, a mechanism first proposed by Careri and coworkers. Both mechanisms are strongly affected by the presence of water. They are characterized by distributions of relaxation times.