Photosynthesis

Among many processes occurring in oxygenic photosynthesis, the water oxidation reaction catalyzed by the Mn 4 Ca cluster provides various types of insights into the field of the metal coordination chemistry. The water oxidation reaction in nature is carried out by Photosystem II (PS II), a multi subunit membrane protein complex. This light-driven reaction is made possible by a spatially separated, yet temporally connected series of cofactors along the electron transfer chain of PS II over 40 Å, through the donor—the Mn 4 CaO 5 catalytic center, the reaction center chlorophylls, to the mobile quinone electron acceptors. Such chemical architecture provides an ideal platform to investigate how to control multi-electron/proton chemistry, using the flexibility of metal redox states, in coordination with the protein and the water network. Understanding the insights of nature's design gives inspiration of how to build artificial photosynthetic devices, where the controlled accumulation of charge and high-selectivity of products is currently challenging. The electronic and geometric structure of this catalyst have been extensively investigated, but its step-wise water oxidation mechanism is not yet completely understood. In this chapter, we summarize our current understanding of the water oxidation reaction in nature.