P680, the primary electron donor of photosystem II

The primary electron donor of photosystem II is a special form of chlorophyll a known as P680. Its detection and subsequent biophysical characterisation has relied heavily on the technique of flash photolysis of Norrish and Porter [Nature 164 (1949) 658] and on the physical principles which emerged from photochemical studies of isolated chlorophyll a using this technique. When oxidised the P680 radical has a midpoint redox potential estimated to be 1.17 V or more which is needed to drive the oxidising reactions of the water-splitting process. Such a high oxidising potential dictates special properties of P680 which are discussed in terms of robustness and structural organisation of photosystem II. Of particular importance has been the recent finding that P680 is not a 'special pair' of chlorophyll molecules as is the case for the primary electron donors of other types of photosynthetic reaction centres. Instead P680 is composed of a cluster of four weakly coupled monomeric chlorophylls which together with the local protein environment enables this primary donor to generate a redox potential capable of oxidising water.