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Conversion of solar to chemical energy in natural photosynthesis is the most abundant and sustainable source of energy production. As a central part of the latter process, the oxidation of water to oxygen, which is a four-electron and four-proton transfer process, takes place in the photosystem II (PS II) at the oxygen evolving centers (OEC) of Mn4Ca clusters.[1]

Although there have been substantial developments in revealing the structural and functional motifs of PSII by various spectroscopic techniques and X-ray crystallography, the resolution acquired by these methods were insufficient to recognize crucial features. Recently, Umena et al. solved the crystal structure of PS II at unprecedented resolution (1.9) with CaMn4O5 units as the OEC ligated with the highly conserved amino acids environment,[2] which is closely related to the structure reported by Dau et al. by EXAFS analysis.[3] In the CaMn4 cluster, five …