Evidence for conduction of protons along the interface between water and a polar lipid monolayer

Movements of H+ along the polar heads of phospholipids spread in monolayers were compared to movements of H+ in the aqueous subphase. The probe for detecting H+ movement along the monolayer was a pH-sensitive fluorescein chromophore covalently bound to the head group of phosphatidylethanolamine. The behavior of this probe was not affected by the electrical properties of the lipid/water interface. Lateral diffusion of H+ along the phospholipid/water interface was then studied by acid-jump experiments in which advantage was taken of the large size of the monolayer. H+ was injected a few centimeters away from the probe observation area. The time needed for H+ diffusion to the probe was monitored by the change in the fluorescence signal, fluorescein being nonfluorescent in an acid medium. Diffusion of H+ in the bulk phase was monitored by the fluorescence change of water-soluble fluorescein isothiocyanate. Diffusion along the lipid monolayer was found to be 20 times faster than in the bulk water phase and required a structured monolayer in order to occur, as revealed by variation of the molecular area occupied by the lipid molecules. The molecular basis of rapid H+ transfer along the lipid monolayer may be the existence of a hydrogen-bond network along the polar heads, capable of supporting a rapid "hop and turn" of H+.