Dynamical structure and function of purple membrane as seen by neutrons

Abstract

In the context of structural investigations concerning the function of the membrane protein bacteriorhodopsin (BR) as part of purple membrane (PM) we have investigated the diffusive motion occurring in the PM. In order to analyse these motions on the molecular level, a study using quasielastic incoherent neutron scattering (QINS) was performed with oriented PM stacks. Experiments with samples, hydrated in D 2O, were carried out under dark conditions (without pump activity) at room temperature. The spectra obtained with two energy resolutions (50 and 16 μeV (FWHM)) were used for fits with a model consisting of five components. The treatment follows the idea of describing one part of the scattering intensity with a certain number of motions by theoretical terms which are already known from other studies, and representing the remaining scattering intensity, corresponding to as yet unknown types of motion, by a general two-site jump model. This procedure allows one to characterize the predominant components, the three-site jump motions of protons in methyl groups and the general two-site ‘jumps’ of protons in polypeptide side groups, by correlation times, jump distances and statistical weights. The jump distances obtained from the two-site model are similar to the spatial extent of the side groups ( d ts = 2–4 Å); the correlation times show, that rotational diffusion of protons in methyl groups ( τ met = 4.3 ps) is a much faster process than reorientation of the larger side groups ( τ ts = 19 – 110 ps).