Protonic percolation on hydrated iysozyme powders studied by the method of thermally stimulated depolarisation currents

Abstract

Thermally stimulated depolarisation current measurements on hydrated Iysozyme powder in the temperature range 77-300 K and water content range 0.07-0.29 g water/g dry protein show a broad low-temperature peak in the range 100-60 K and a complex high-temperature band in the range 160-300 K. Four dispersions were found to contribute to the high-temperature band, the alpha 2-dispersion, the alpha -dispersion, the Omega -dispersion and a 'new' dispersion. The alpha -dispersion and the new dispersion occurring in the same temperature range were studied in detail. The new dispersion appears for water contents higher than about 0.15 and it shifts to lower temperatures with increasing water content while its magnitude rapidly increases. Its characteristics help one to identify it with a dispersion observed by Careri and co-workers (1985,6,8) on hydrated Iysozyme powders in the frequency range 10 kHz-10 MHz at 302 K and attributed to percolative proton transfer along threads of hydrogen-bonded water molecules on the surface of single protein macromolecules. The activation energies of the alpha -dispersion and the new dispersion were determined. Both dispersions are characterized by distributions of relaxation times and activation energies.