Sequential hydration-dehydration studies of lysozyme by the thermally stimulated depolarization currents (TSDC) technique

To investigate the physical state of water in hydrating biological macro-molecules, the dielectric properties of water in hen egg lysozyme pellets with various moisture contents were studied using the thermally stimulated depolarisation currents technique. The water dipoles appeared to be directly involved in the relaxation processes, such that, by increasing the content of water of sorption from ho = 0.075 to ho = 0.29, the current density recorded increased abruptly at moisture content above 0.075. At a fixed starting hydration level, the time evolution of water content was also studied by isothermal sample aging in dynamic vacuum: the TSDC spectra changed in both intensity and position of their main peaks (TM = 245 K, 190 K, 150 K) with moisture content and showed hysteresis. The complex behaviour of the TSDC response can be compared with the results obtained with the same technique on other biological macromolecules and suggests possible models for water configurations and rearrangements.