Lateral proton migration along membranes

Transmembrane proton concentration gradients drive the mitochondrial ATP-synthase or facilitate transport across the bacterial translocation channel. These gradients differ from their respective counterparts in the bulk because an energy barrier, G, delays proton surface-to-bulk transfer, thereby enabling proton migration towards the respective molecular machine along the membrane surface. This interfacial proton migration is also likely to be important for a variety of other processes, like proton coupled uptake of nutrients, proton delivery to active centers of proteins, and protein folding. According to conflicting theories, proton surface diffusion occurs (i) totally decoupled from bulk or (ii) under conditions of rapid equilibrium between bulk and surface protons. Here we aim to distinguish between both theories. Therefore, we directly observe surface proton diffusion by releasing the protons either instantaneously or gradually at one spot on planar membranes and by detecting their arrival at a distant spot. By identifying the conditions under which transmembrane diffusion may compete with surface-to-bulk release, we will be able to estimate G in a model-independent manner. We will reveal the relative location of the proton pathway with respect to membrane structure by performing the first systematic investigation of proton’s residence time on the interface as a function of membrane surface and dipole potentials. These experiments will also allow dissection of the contributions of these two components toG. The accompanying analysis of electrostatics will help clarify whether the hydroxyl ion might substitute the proton as the migrating species....