P680: what is it and where is it?

Electron and X-ray crystallography have provided intermediate structural models for photosystem II (PSII), the membrane located multisubunit complex which uses light energy to split water into its elemental constituents. This reaction is thermodynamically demanding and involves the production of redox potentials in excess of 1 V. Structural analyses have now shown that the primary oxidant, P680, is not a ‘special pair’ of chlorophylls, as in other types of photosynthetic reaction centres, but a tetramer of equally spaced chlorophyll a molecules. Its high redox potential, and the involvement of four weakly coupled isoenergetic monomers rather than a strongly excitonically coupled ‘special pair’, has implications for redox mechanisms which are unique to PSII, and therefore not found in any other photosynthetic system. The importance of these features is discussed.