THERMAL MOTIONS IN BACTERIORHODOPSIN AT DIFFERENT HYDRATION LEVELS STUDIED BY NEUTRON-SCATTERING - CORRELATION WITH KINETICS AND LIGHT-INDUCED CONFORMATIONAL-CHANGES

Bacteriorhodopsin (BR) is a transmembrane protein in the purple membra ne (PM) of Halobacterium salinarum. Its function as a light-driven pro ton pump is associated with a cycle of photointermediates which is str ongly hydration-dependent. Using energy-resolved neutron scattering, w e analyzed the thermal motions (in the nanosecond-to-picosecond time r ange) in PM at different hydration levels. Two main populations of mot ions were found that responded differently to water binding. Striking correlations appeared between these ''fast'' motions and the ''slower' ' kinetic constants (in the millisecond time range) of relaxations and conformational changes occurring during the photocycle.