Figure 11.

Schematic representation of the three observed diffusion modes for a proton on a membrane surface. In Mode I, the hydronium is bound to the lipids. Proton transfer is absent and diffusion is determined by the lipid, to which it is bound. In Mode II, the proton is captured within a free-energy well composed of a small lipid-enclosed cluster of water molecules. Within this well, the proton can transfer freely, and diffusion is a superposition of the proton diffusion within the well, and the diffusion of the whole well. In Mode III, the proton desorbs from the membrane. The proton migrates freely over the surface or through the bulk before the proton readsorbs onto the membrane, leading to large-scale surface diffusion. To see this figure in color, go online.