CHARGE-TRANSFER ACROSS A SINGLE LIPID-WATER INTERFACE CAUSES ION PUMPING ACROSS THE BILAYER

The photoformation of magnesium-porphyrin cations (P+) at a single lip id bilayer-water interface can pump lipophilic berate anions completel y across the lipid bilayer and causes an actual reversal of the photov oltage. The system consists of a lipid bilayer containing magnesium oc taethylporphyrin, an aqueous or interfacial electron acceptor on one s ide, and chloro- or fluoro-substituted tetraphenylborate in both aqueo us electrolyte solutions. With 1-mu s pulsed illumination, an immediat e positive photovoltage is observed, which decreases on the microsecon d and millisecond time scales. On the time scale of seconds, as the pf cation concentration decays in reverse electron transfer, the voltage swings negative to a value almost equal to its initial value and fina lly decays with a half-time (similar to 20 s) longer than the time con stant of the system (similar to 5 s). Thus, an ion gradient across the membrane is formed, trapped by the nonlinear relation between ion mob ility and ion concentration. Continuous light illumination confirms th at negative charge moves in the direction opposite that of the initial photoinduced electron transfer. Steady-state measurements indicate an ion pumping efficiency of similar to 30%. This simple mechanism may b e a progenitor of photobiological ion pumps.