A comparative study of purple membranes partially rehydrated with water and deuterium oxide

Abstract

The photoelectric signals of dried oriented purple membrane samples were studied at various hydration degrees (humidities between 0.036–0.13 gH20/gbR) in water and deuterium oxide. A modified photocycle was found both in water and deuterium oxide at very low humidities, as obtained previously in the case of water. The dependence of the lifetime on temperature and hydration degree, for the L↔M and M→bR transitions, was calculated by using an exponential decomposition of the electric signals. The Eyring parameters were calculated from the temperature dependence, in order to obtain comparative information concerning the isotope effect following deuteration. The activation enthalpies and entropies for the L decay showed an abrupt change at a water content of about 0.06 gH20/gbR, but the isotope effect was present only at humidities below this value. In the case of the M decay, an isotope effect was found at all humidities, the values of Eyring parameters being smaller in deuterium oxide. The activation entropies have negative values in the case of strongly dehydrated samples, both in water and deuterium oxide.