VECTORIAL PHOTOINDUCED ELECTRON-TRANSFER IN PHOSPHOLIPID-VESICLES ANDLB BILAYERS

Photoinduced electron transfer (PET) was studied in phospholipid vesic les and in Langmuir-Blodgett bilayers in the attempt to produce a mode l for electron transfer processes in biological media. Spatial organiz ation of the reaction centers in lipid membranes needs to be controlle d in order to provide high efficiency of light-to-chemical energy conv ersion. Pie designed a phospholipid system where the donor is localize d in the inner bilayer whereas the acceptor is at the polar groups-wat er interface. We used dipalmitoylphosphatidic acid vesicles containing low molar fractions of dipalmitoylphosphatidylcholine with pyrene (do nor) bound to one of the alkyl chains. Methylviologen (acceptor) was a dded to the external aqueous phase; upon photoexcitation of the donor we observed the electron transfer to take place in a unidirectional ma nner from the inside of the bilayer to the interface. Information abou t the location of the donor was obtained studying the photophysical pr operties of the pyrene chromophore in vesicles and in LB layers. The p hotoinduced electron transfer reaction was evidenced by quenching of p yrene fluorescence in the presence of increasing concentrations of acc eptor, the process was studied both with steady-state and time-resolve d fluorescence emission. Fluorescence intensity was found to decrease with increasing concentration of methylviologen, similar results were obtained for vesicles and LB layers of analog composition immersed in a methylviologen solution. Lifetimes of the excited species were found to be of the same order of magnitude in vesicle and LB-layer systems.