EFFICIENT PHOTOINDUCED ORTHOGONAL ENERGY AND ELECTRON-TRANSFER REACTIONS VIA PHOSPHOLIPID MEMBRANE-BOUND DONORS AND ACCEPTERS

A three component, liposome-bound photochemical molecular device (PMD) consisting of energy and electron transfer reactions is described. Bi layer membrane surface-associated dyes, akis[4-(trimethylammonio)-phen yl]-21H,23H-porphine tetra-p-tosylate salt and bis[(3-trimethylammonio )propyl]thiadicarbocyanine, tribromide, are the energy donor and accep tor, respectively, in a blue light stimulated energy transfer reaction along the vesicle surface. The electronically excited cyanine is quen ched by electron transfer from the phospholipid membrane bound triphen ylbenzyl berate anion, which is located in the lipid bilayer interior. The PMD exhibits sequential reactions following electronic excitation with the novel feature that the steps proceed with orthogonal orienta tion: energy transfer occurs parallel to the membrane surface, and ele ctron transfer occurs perpendicular to the surface. Photobleaching and fluorescence quenching experiments verify the transfer reactions, and Stern-Volmer analysis was used to estimate the reaction rate constant s. At the highest concentrations examined of energy and electron accep tor ca. 60% of the photoexcited porphyrins were quenched by energy tra nsfer to the cyanine. The use of liposomes of well defined composition and dimensions in conjunction with molecular components that associat e with the bilayer in a predictable manner permit the accurate estimat ion of molecular binding volumes and local concentrations for the anal ysis.