Artificial photosynthesis: Understanding water splitting in nature

In the context of a global artificial photosynthesis (GAP) project,we reviewour current work on nature’s water splitting catalyst. In a recent report (Cox et al. 2014 Science 345, 804–808 (doi:10.1126/science.1254910)), we showed that the catalyst—a Mn₄O₅Ca cofactor—converts into an ‘activated’ form immediately prior to the O–O bond formation step. This activated state, which represents an all Mn^IV complex, is similar to the structure observed by X-ray crystallography but requires the coordination of an additional water molecule. Such a structure locates two oxygens, both derived from water, in close proximity, which probably come together to formthe product O₂ molecule.We speculate that formation of the activated catalyst state requires inherent structural flexibility. These features represent new design criteria for the development of biomimetic and bioinspired model systems for water splitting catalysts using first-row transition metals with the aim of delivering globally deployable artificial photosynthesis technologies.