Conversion of light energy to proton potential in liposomes by artificial photosynthetic reaction centres

During photosynthesis, photoinduced electron transport across membranes is carried out by pigment molecules organized into reaction centres by membrane-spanning proteins. The resulting transmembrane electrochemical potential is then coupled to the movement of protons across the membrane. Photoinduced electron transport followed by thermal electron transfer, leading to charge separation over distances of 8 nm, has been demonstrated in artificial mimics of the photosynthetic reaction centre comprising covalently linked electron donors and acceptors. Here we report the assembly of an artificial mimic of the photosynthetic apparatus which transports protons across a lipid bilayer when illuminated. Our model reaction centre is a molecular 'triad' consisting of an electron donor and acceptor linked to a photosensitive porphyrin group. This triad is incorporated into the bilayer of a liposome. When excited, it establishes a reduction potential near the outer surface of the bilayer and an oxidation potential near its inner surface. In response to this redox potential gradient, a freely diffusing quinone molecule alternates between its oxidized and reduced forms to ferry protons across the bilayer with an overall quantum yield of 0.004, creating a pH gradient between the inside and outside of the liposome.