Proton Migration on Top of Charged Membranes

Proton relay between interfacial water molecules allows rapid two-dimensional diffusion. An energy barrier, [Formula: see text] , opposes proton-surface-to-bulk release. The [Formula: see text]-regulating mechanism thus far has remained unknown. Here, we explored the effect interfacial charges have on [Formula: see text] ’s enthalpic and entropic constituents, [Formula: see text] and [Formula: see text] , respectively. A light flash illuminating a micrometer-sized membrane patch of a free-standing planar lipid bilayer released protons from an adsorbed hydrophobic caged compound. A lipid-anchored pH-sensitive dye reported protons’ arrival at a distant membrane patch. Introducing net-negative charges to the bilayer doubled [Formula: see text] , while positive net charges decreased [Formula: see text]. The accompanying variations in [Formula: see text] compensated for the [Formula: see text] modifications so that [Formula: see text] was nearly constant. The increase in the entropic component of the barrier is most likely due to the lower number and strength of hydrogen bonds known to be formed by positively charged residues as compared to negatively charged moieties. The resulting high [Formula: see text] ensured interfacial proton diffusion for all measured membranes. The observation indicates that the variation in membrane surface charge alone is a poor regulator of proton traffic along the membrane surface.