A possible mechanism of the photooxidation of water sensitized by chlorophyll adsorbed at the interface

Abstract A four-electron mechanism is proposed for the photooxidation of water in a system of two immiscible liquids containing chlorophyll a , a water-soluble electron acceptor, a hydrophobic proton acceptor, and a buffer for maintaining optimal proton concentration. A hydrated chlorophyll oligomer, which is adsorbed at the interface and closely packed so that the electron clouds of porphyrin rings overlap, becomes excited upon illumination; an oxidized form of the pigment and a reduced form of the electron acceptor are formed. In the reaction centre, water is coordinatively bound to the magnesium of one of the chlorophyll molecules, by means of hydrogen bonds to a carboxyl group of another chlorophyll molecule and to a proton acceptor (the pentachlorophenyl anion) required in a model system for the protection of chlorophyll against pheophytinization. The oxidized chlorophyll oligomer, consisting of two hydrated dimers or a tetramer, can be responsible for the dark stage: the four-electron oxidation of water to molecular oxygen. Experimental data in favour of this mechanism are presented.