Protons migrate along interfacial water without significant contributions from jumps between ionizable groups on the membrane surface.

Proton diffusion along membrane surfaces is thought to be essential for many cellular processes such as energy transduction. Commonly, it is treated as a succession of jumps between membrane-anchored proton-binding sites. Our experiments provide evidence for an alternative model. We released membrane-bound caged protons by UV flashes and monitored their arrival at distant sites by fluorescence measurements. The kinetics of the arrival is probed as a function of distance for different membranes and for different water isotopes. We found that proton diffusion along the membrane is fast even in the absence of ionizable groups in the membrane, and it decreases strongly in D(2)O as compared to H(2)O. We conclude that the fast proton transport along the membrane is dominated by diffusion via interfacial water, and not via ionizable lipid moieties.