The affinity of protons to the hydration layer of membranes
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European Bioenergetics Conference
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Protons have a general propensity for the boundary between aqueous and hydrophobic phases. Their affinity for the membrane/water interface enables lateral diffusion along the membrane surface. This is vitally important for biological energy transduction, where one protein will pump protons to the membrane surface which are subsequently used by another protein for ATP synthesis. Energy transduction would be less efficient if protons would take the alternative route through the aqueous solution, because (i) transport would occur via the more slowly diffusing buffer molecules that carry the protons and (ii) proton dispersion throughout the aqueous compartment would lower the chemical gradient. However, the origin of the proton?s surface affinity has thus far remained enigmatic. By photo-uncaging protons from a membrane patch and by detecting their subsequent arrival at a distant spot via changes in fluorescence intensity of lipid anchored pH dyes, we established that both proton migration speed and span only weakly depend on lipid composition. This observation indicates that protons do not migrate along the surface by jumping between titratable groups (Springer et al., 2011). Similar experiments for the decane/water interface and corresponding ab-initio simulations both suggest that the protons move along interfacial water molecules (Zhang et al., 2012). As we show here, the affinity of protons to the membrane?s hydration layer is mostly entropic in origin. The temperature dependence of the surface to bulk escape rates shows only a minor enthalpic component. Taken together, this work reconciles the delayed proton surface to bulk release with protons weak bonding to surface water molecules.
This work was supported by Grant P25981 from the Austrian Science Fund (FWF) to P.P.