Regulation of Proton Migration along the Membrane Surface
Sprache des Titels:
Proton diffusion along biological membranes is vitally important for cellular energetics. Both migration speed and span weakly depend on the presence of titratable residues, suggesting that interfacial water molecules serve as proton railways. This hypothesis is supported by (i) the observation of proton migration along the water-decane interface and (ii) ab initio simulations that demonstrate proton travel along structured water molecules in the second hydration shell of this interface. To test the importance of interfacial water structure, we have now monitored proton migration along the membrane surface in the presence of DMSO. Therefore, we photoreleased protons from a membrane-adsorbed caged compound by illuminating a small patch of a planar lipid bilayer, and we monitored their arrival at a distant membrane patch by measuring the fluorescence of the membrane-anchored pH dye. The synthesis of these caged protons is outlined. DMSO facilitated proton surface-to-bulk release and decreased the surface proton diffusion constant as has been anticipated from its disturbing effect on the hydration layer of membranes. We conducted similar experiments with osmolytes, which likewise affect membrane hydration. Finally we tested the effect electrostatic attraction of protons may have on their surface migration. The presence of negatively charged lipids decreased both the rate of proton surface-to-bulk release and the surface diffusion constant. We conclude that proton migration between the source and the sink on the membrane surface may be regulated by both membrane electrostatics and compounds that affect interfacial water structure.