PROTON DIFFUSION ON PURPLE MEMBRANE STUDIED BY NEUTRON-SCATTERING

Oriented stacks of purple membrane (PM) were studied with high-resolution quasielastic incoherent neutron scattering (QINS). The experiments were carried out under dark conditions at room temperature, for two different orientations, such that the momentum transfer Q was either parallel or perpendicular to the membrane plane at 90 degrees scattering angle. The samples were equilibrated in H2O as well as in D2O, both at 100% relative humidity. The analysis of the spectra shows that the hydration water is performing anisotropic translational diffusion (TD), preferentially parallel to the membrane plane, with a diffusion coefficient about 5 times smaller than that of bulk water at room temperature. From the behaviour at large scattering angles an average jump distance of about 4 Angstrom was estimated. This fast translational diffusion process is accompanied by an even faster local diffusive motion of the water protons. The approximate description of the latter by a rotational diffusion model for H2O molecules yields a quasielastic line width of about 70 mu eV (HWHM). The observed local diffusive motion of protons in PM is an order of magnitude slower (angular average of the corresponding linewidth: HWHM = 9 mu eV).