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 membrane (PM) of Halobacterium salinarum. Its function as a light-driven proton pump is associated with a cycle of photointermediates which is strongly hydration-dependent. Using energy-resolved neutron scattering, we analyzed the thermal motions (in the nanosecond-to-picosecond time range) in PM at different hydration levels. Two main populations of motions 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.