Date Published: February 23, 2018
Publisher: Public Library of Science
Author(s): Trung Hai Nguyen, Chao Zhang, Ewald Weichselbaum, Denis G. Knyazev, Peter Pohl, Paolo Carloni, Colin Johnson.
Proton transport at water/membrane interfaces plays a fundamental role for a myriad of bioenergetic processes. Here we have performed ab initio molecular dynamics simulations of proton transfer along two phosphatidylcholine bilayers. As found in previous theoretical studies, the excess proton is preferably located at the water/membrane interface. Further, our simulations indicate that it interacts not only with phosphate head groups, but also with water molecules at the interfaces. Interfacial water molecules turn out to be oriented relative to the lipid bilayers, consistently with experimental evidence. Hence, the specific water-proton interaction may help explain the proton mobility experimentally observed at the membrane interface.
Proton transport between membrane-bound proteins along biological membrane plays a crucial role for bioenergetics of living cells [1–4]. An efficient pathway between protons’ source and sink [3, 5, 6] is represented by proton’s fast and persistent lateral diffusion [7–9]. This process appears to be only weakly dependent of the membrane used [7, 8]: protons move fast along the membrane-water interface prior to being released into the bulk: with the lateral diffusion coefficient in the order of 10−5 cm2s-1 [7–9]. This is similar to the one found in bulk liquid water [7, 10]. The long distance traveled by protons along the membrane [7, 8] is due to a substantial free energy barrier that prevents them from escaping to the bulk [8, 9, 11]. Since its enthalpy component corresponds to the breakage of only a single hydrogen bond, the barrier appears to be mainly entropic .
We have presented an ab initio MD study on an excess proton at DOPC and DPhPC membrane interfaces. Our calculations suggest that the excess proton is preferentially located at the interface, in line with previous MS-EVB  and HYDYN  studies. The excess proton is quite mobile in spite of its strong interactions with the membrane because of the competing attraction of both the negatively charged phosphate groups and the lone electron pairs of interfacial water molecules. The water molecules are oriented differently from those of bulk water. Due to their preferable orientation, the water molecules at the membrane interface play a key role for proton transport. This finding might help to explain why titratable residues are not required for proton migration along membranes .