Energetics of Lateral Membrane Proton Diffusion

Abstract

The diffusion of protons along biological membranes is vitally important for cellular energetics. The weak dependence of both migration speed and span on lipid composition suggests that proton migrate along water (Springer et al., 2011). However, it is unclear how to reconcile the apparently high proton affinity to the phase boundary (Zhang et al., 2012) with the poor proton acceptability of water, i.e. with the low pK(=0) value of water. Here we monitored the diffusion of excess protons along the phosphatidylcholine lipid bilayer/water interface at different temperatures. The kinetics of proton arrival from a distant spot of proton release to lipid-anchored fluorescent pH-sensitive dyes indicated that the in vitro Gibbs activation energy ΔG‡ for proton surface-to-bulk release harbours only a minor enthalpic constituent. We observed that more than 2/3 of ΔG‡ are entropic in origin, which explains the high proton affinity 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.1. Springer, A., Hagen, V., Cherepanov, D.A., Antonenko, Y.N., and Pohl, P. (2011). Protons migrate along interfacial water without significant contributions from jumps between ionizable groups on the membrane surface. Proc. Natl. Acad. Sci. U. S. A. 108, 14461-14466.2. Zhang, C., Knyazev, D.G., Vereshaga, Y.A., Ippoliti, E., Nguyen, T.H., Carloni, P., and Pohl, P. (2012). Water at hydrophobic interfaces delays proton surface-to-bulk transfer and provides a pathway for lateral proton diffusion. Proc. Natl. Acad. Sci. U. S. A. 109, 9744-9749.