Lateral proton conduction at lipid–water interfaces and its implications for the chemiosmotic-coupling hypothesis

Abstract

The driving force in energy-transducing membranes is now recognized to be linked to a flux of protons between membrane-bound donors and acceptors. The question of whether the proton pathway is delocalized within the two bulk aqueous phases on each side of the membrane (delocalized chemiosmotic theory¹), or is localized at its surface (semi-localized hypothesis²) still remains open to discussion^3,4. Using an original fluorescence monolayer technique, we have recently been able to provide direct experimental evidence that a phospholipid–water interface can act as an efficient proton conductor^5,6. This observation strongly supports the semi-localized hypothesis². In the present study, comparisons between surface potential and fluorescence measurements in monolayers provide additional evidence of a facilitated proton conduction along phospholipid–water interfaces. They suggest the existence of an induced steep surface pH gradient from a more acidic surface towards the bulk. They also show that lateral proton transfer along the surface of a biological membrane would alter the surface potential of that membrane.