Proton Migration on Top of Charged Membranes

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