PROTON DYNAMICS IN A 2D-SINGLE-CRYSTAL OF BACTERIORHODOPSIN

The dynamical aspects of the proton translocation mechanism in bacteriorhodopsin (BR) were under investigation using quasielastic incoherent neutron scattering (QINS). We performed experiments on BR-molecules in well-ordered two-dimensional lattices of purple membranes (PM), using a multi-chopper time-of-flight spectrometer. Spectra were obtained under dark conditions at 112 different scattering angles with momentum transfer Q from 0.1 to 1.8 angstrom-1. The sample, consisting of oriented PM-stacks, exhibits QINS intensities and energy widths varying with sample orientation, temperature and hydration. The results from an analysis of the hydration dependence are consistent with the presence of fast proton diffusion at 100% relative humidity (r.h.). At 86% r.h. no translational diffusion is observed on the time scale of the experiment, although the (local diffusive) motion of water protons appears to be occurring preferentially parallel to the membrane plane.