Interfacial Proton Diffusion

Abstract

Lateral diffusion along the membrane represents the most efficient pathway for proton transport between membrane proteins. The recent observation that protons do not jump between ionizable residues but travel along interfacial water molecules (A. Springer et al., PNAS, 2011) suggested that membrane lipids are not required and that lateral proton diffusion may be observed adjacent to any hydrophobic surface. To test that prediction we measured surface proton diffusion in a minimalistic system at the water-decane interface. Although the interface did not offer any proton binding sites, lateral proton diffusion occurred. Fitting a mathematical model to the proton density in the observation spot suggested that proton diffusion was decoupled from bulk at low buffer capacity. At higher aqueous buffer concentrations and in heavy water, proton bulk diffusion became more important. This proves that the retarded proton surface to bulk release is (i) a fundamental property of the boundary between hydrophobic and hydrophilic phases, and (ii) independent on the presence of specific lipids or amino acids.