VECTORIAL PHOTOINDUCED ELECTRON-TRANSFER BETWEEN PHOSPHOLIPID MEMBRANE-BOUND DONORS AND ACCEPTORS

Photoinduced electron transfer (PET) from the hydrophobic triphenylbenzylborate anion to the water-soluble tricationic N,N'-bis[3-(trimethylammonio)propyl]thiadicarbocyanine was investigated in water and in phosphatidylcholine membrane systems. The borate (donor) and dye (acceptor) form a strong complex in water, leading to inefficient PET. In the presence of liposomes, the hydrophobic borate anion is bound deep within a bilayer. The resulting negative surface charge of the liposomes leads to electrostatic binding of the cationic dye within the head group region of the membrane. This faciliates PET to the photoexcited dye, as observed in photobleaching and fluorescence quenching experiments. The important feature of the system is the simultaneous binding of both donor and acceptor at relatively well-defined but nonoverlapping binding sites within the membrane, leading to vectorial PET from the hydrophobic interior of the membrane to the hydrophilic surface upon red light irradiation. Moreover, the PET efficiency is dependent on the physical state of the bilayer, which indicates that the borate moves closer to the dye at temperatures below the lipid phase transition temperature (T(m)), a probable consequence of the tighter packing of the lipids in the solid-analogous phase.