2-DIMENSIONAL PROTON DIFFUSION ON PURPLE MEMBRANE

The results are reported of high-precision pulsed-field gradient (PFG) NMR measurements concerning the transport of protons along the surfac e of purple membrane (PM) at room temperature. PM stacks were studied as a function of orientation and hydration. At 100% relative humidity the following results were obtained: (1) Translational diffusion of pr otons on the PM surface is fast, although about five times slower than in bulk water at room temperature; D(s) = 4.4 x 10(-6) cm2/s. (2) Fas t diffusion was observed only parallel, but not perpendicular, to the PM planes, i.e. this diffusion process is clearly two-dimensional on t he timescale of our experiments. (3) The diffusion is much slower at 8 6% relative humidity; this observation may be related to the slowing-d own of biological function of PM (and in particular the proton pumping mechanism of bacteriorhodopsin) with decreasing hydration level. Thes e results are in good agreement with those of a preliminary analysis o f quasielastic incoherent neutron scattering (QINS) data. A new, impro ved analysis of the latter, making use of the NMR value of D(s) and in cluding multiple scattering (MSC) corrections, is also presented. Fina lly the relation of the proton diffusion results with proton conductiv ity is discussed.