Anomalous Surface Diffusion of Protons on Lipid Membranes

Abstract

Abstract: The cellular energy machinery depends crucially on the presence and properties of protons at or in the vicinity of lipid membranes. To asses the energetics and mobility of a proton near a membrane, we simulated an excess proton near a sol- vated DMPC bilayer at 323 K, using a recently developed method to include the Grotthuss proton shuttling mechanism in classical molecular dynamics simulations. We obtained a proton surface a nity of 13 : 0 0 : 5 kJ mol 1 . The proton interacted strongly with both lipid head group and linker carbonyl oxygens. Furthermore, the surface di usion of the proton was anomalous, with a subdi usive regime over the rst few nanoseconds, followed by a superdi usive regime. The time- and distance dependency of the proton surface di usion coe cient within these regimes may also resolve many discrepancies between previously reported di usion coe cients. Our simulations show that the proton anomalous surface di usion originates from re- stricted di usion in two di erent surface-bound states, interrupted by the occasional bulk-mediated long-range surface di usion. Although only a DMPC membrane was considered in this work, we speculate that the restrictive character of the on-surface di usion is highly sensitive to the speci c membrane conditions, which can alter the relative contributions of the surface and bulk pathways to the overall di usion process.