10th International Frumkin Symposium on Electrochemistry
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Surface coupled proton exchange between membrane proteins is pivotal for cell bioenergetics. It is under debate since Mitchell published his chemiosmotic theory, because the origin of the energy barrier preventing interfacial and bulk protons from equilibrating remains enigmatic. Proton release from the membrane is opposed by a substantial Gibbs activation energy barrier ?G?. The removal of titratable moieties from the surface of planar lipid bilayers did not abrogate proton surface diffusion indicating that ?G? does not reflect the energy of a chemical bond between protons and interfacial proton acceptors (Springer et al., 2011). We concluded that instead of hopping between tritratable moieties, the proton diffuses along ordered waters of membrane hydration. To test that hypothesis, we removed the lipid bilayer altogether and replaced it by an organic solvent. We microinjected protons close to a decane-water interface and observed their arrival at a distant spot. Both the proton surface diffusion constant and the dwell time of the protons on the surface differed surprisingly little from the respective values measured at the bilayer-water interface (Zhang et al., 2012). Temperature dependent measurements of both parameters disproved the model that assumes quasi-equilibrium between surface proton layer and bulk solution. Instead, a model that assumes irreversible proton release is consistent with the experiment. Within this model, we find that ?G? has only a minor enthalpic contribution, of the order of the energy of two hydrogen bonds. The free energy barrier is mostly entropic, which could arise from hydrogen bond orientation that strongly favors proton movement towards the membrane. This observation reconciles the delayed proton surface to bulk release with protons weak bonding to surface water molecules...
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