Proton Migration on Top of Charged Membranes

Abstract

Proton relay between interfacial water molecules allows rapid two-dimensional diffusion. An energy barrier, (Formula presented.), opposes proton-surface-to-bulk release. The (Formula presented.) -regulating mechanism thus far has remained unknown. Here, we explored the effect interfacial charges have on (Formula presented.) ’s enthalpic and entropic constituents, (Formula presented.) and (Formula presented.), 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 presented.), while positive net charges decreased (Formula presented.). The accompanying variations in (Formula presented.) compensated for the (Formula presented.) modifications so that (Formula presented.) 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 presented.) 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.