Thermal motions and function of bacteriorhodopsin in purple membranes: effects of temperature and hydration studied by neutron scattering.

The internal dynamics of bacteriorhodopsin, the light-driven proton pump in the purple membrane of Halobacterium halobium, has been studied by inelastic neutron scattering for various conditions of temperature and hydration. Light activation can take place when the membrane is vibrating harmonically. The ability of the protein to functionally relax and complete the photocycle initiated by the absorption of a photon, however, is strongly correlated with the onset of low-frequency, large-amplitude anharmonic atomic motions in the membrane. For a normally hydrated sample, this occurs at about 230 K, where a dynamical transition from a low-temperature harmonic regime is observed. In moderately dry samples, on the other hand, in which the photocycle is slowed down by several orders of magnitude, no transition is observed and protein motions remain approximately harmonic up to room temperature. These results support the hypothesis, made from previous neutron diffraction studies, that the "softness" of the membrane modulates the function of bacteriorhodopsin by allowing or not allowing large-amplitude motions in the protein.