Kinetics of charge transfer at the lipid bilayer-water interface on the nanosecond time scale

Advances in instrumentation allow electrical measurements across the planar lipid bilayer to be made with nanosecond time resolution. The electron transfer reaction between photoexcited magnesium octaethylporphyrin in the lipid to a variety of ionically charged acceptors in the water is found to be purely dynamic over a wide range of concentrations of acceptors and up to the time constant of the apparatus, 4 ns. The saturation of the amplitude of the photovoltage with increasing concentration of acceptor is caused by the finite lifetime of the excited state, not by formation of a static pigment-acceptor complex. The reactions are an excellent probe of the lipid-water interface over an extended time scale. No appreciable barrier to reaction exists at this interface beyond the 5-ns time. That is, any water or choline group structure may be evanescent on this time scale. Electrostatic interactions indicate that the acceptor molecules penetrate to the level of the phosphocholine groups with differing orientations. It will be possible to extend the time scale into the picosecond range by decreasing the response time and by deconvolutions.