Photosystem II. Photosystem II (PSII) is a multi-subunit protein complex embedded in the thylakoid membranes of higher plants, algae and cyanobacteria. It uses solar energy to catalyze a series of electron transfer reactions resulting in the splitting of water into molecular oxygen, protons and electrons. These reactions dissipate power on a scale of ~100 TW, being responsible for the production of atmospheric oxygen and indirectly for almost all the biomass on the planet [4]. Despite its importance, the catalytic properties of PSII have never been reproduced in any artificial system. Understanding its unique chemistry is not only important in its own right, but could have implications for the development of systems that allow us to obtain a clean source of chemical energy, reproducing the natural reaction. The photosystem II complex is composed of more than fifteen polypeptides and at least nine different redox components that have been shown to undergo light-induced electron transfer [5]. However, only five of these redox components are known to be involved in transferring electrons from H2O to the plastoquinone pool, and one of them is the water oxidizing tetra-manganese cluster from the oxygen evolving complex (OEC). Photosystem II is the only known protein complex that can oxidize water, resulting in the release of molecular oxygen into the atmosphere. Despite years of research, the molecular events that lead to water oxidation are still a matter of current investigations. Energetically, water is a poor electron donor. The oxidation-reduction midpoint potential of water is +0.82 V (pH 7). In photosystem II this reaction is driven by the oxidized chlorophyll a in the reaction center, P680+, which has a midpoint potential estimated to be +1.2 V at pH 7). Water oxidation requires two molecules of water and involves four sequential turnovers of the reaction centre. This was shown by an experiment demonstrating that oxygen release by photosystem II occurs with a four flash dependence [6, 7]. Each photochemical reaction creates an oxidant that removes one electron from the system, resulting in a net reaction that involves the release of one oxygen molecule, the deposition of four protons into the inner water phase, and the transfer of four electrons to the QB-site, producing two reduced plastoquinone molecules [8-10].
