Protons drive (i) ATP synthesis and (ii) protein translocation across the bacterial plasma membrane.
These processes may persist, even if the bulk pH difference between the solutions on both sides
of the membrane is too small to serve as an energy source. Lateral proton diffusion from the
proton pump towards the ATP synthase or the translocon is believed to provide an explanation.
Proton migration along the membrane is commonly regarded as a succession of jumps between
membrane-anchored proton binding sites. Our experiments provide evidence for an alternative
model. We released protons at the interface and monitored their arrival at distant sites by fluorescence
measurements. The kinetics of the arrival was probed as a function of distance (i) for
membranes of various compositions  and (ii) for the decane/water interface . We found
that long-range proton diffusion along the interface required neither the presence of ionizable
groups  nor polar groups . It persisted even after the complete removal of lipids . Salt
removal altered the diffusion constant of 5x10-6 cm2 s-1, but did not inhibit long-range lateral
proton migration. Surface-to-bulk transfer was delayed by an energy barrier, which according
to measurements at various temperatures amounted to at least 8.7 kT . The observation of
a large isotope effect supported the conclusion that interfacial water provided the pathway for
rapid lateral proton migration.
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