Photoinduced electron transfer, energy transfer and polymerization reactions in phospholipid membranes.

Abstract

The differential physical properties found at different depths of a phospholipid membrane permit design of systems for vectorial reactions (which are not possible in isotropic solution). In the system described in Chapter IV, a hydrophobic electron donor (triphenylbenzylborate) binds to the membrane interior while a hydrophilic electron acceptor (a cyanine dye) binds to the surface. Irradiation with red light leads to vectorial electron flow via photoinduced electron transfer (PET), as monitored by fluorescence quenching and photobleaching techniques. The PET reaction efficiency is enhanced over the case where the donor and acceptor are present in water without membranes. In that case, strong dimeric complexes are formed which reduce the efficiency of PET by enhancing nonradiative decay modes for the dye (Chapter III). Addition of a porphyrin to the membrane surface extends the sensitivity of the system to blue light (Chapter V). Excitation of the porphyrin at 417 nm and subsequent energy transfer to the cyanine trigger the same PET chemistry observed with red-light irradiation. Furthermore, the electron- and energy-transfer reactions are enhanced on polymerized, phase-separated membranes (Chapter VI). In these applications, membranes are media for chemical reactions. Membranes also make interesting substrates for chemical reactions, in this case, polymerization. A system consisting of a membrane-bound, amphiphilic cyanine dye and molecular oxygen is described in Chapter VII which effectively initiates the polymerization of vesicles upon irradiation with visible light. Potential utility in drug delivery applications is discussed.