Protons have a general propensity for the boundary between aqueous and hydrophobic phases. An
apparent increase in that propensity at the membrane/water interface enables protons to hop along
the membrane surface. This is important for biological energy transduction, where one pro- tein
will pump protons to the membrane surface which are subsequently used by another protein for ATP
synthesis (ATP is the energy ?currency? of the cell.) Energy transduction is less ef?cient if
protons 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 af?nity 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 flu- orescence
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). To clarify the origin of their
unusually high af?nity to surface water molecules, we monitored the diffusion of excess protons
along the lipid bilayer/water interface at different temperatures. We determined that more than 2/3
of the binding free energy are entropic in origin. Our ?nding thus explains the high proton af?nity
to membranes in the absence of a potent proton acceptor.
This work was supported by Grant P25981 from the Austrian Science Fund (FWF) to P.P.
Sprache der Kurzfassung:
Englisch
Vortragstyp:
Hauptvortrag / Eingeladener Vortrag auf einer Tagung
Forschungs-