Assignment of the μ4-O5 atom in catalytic center for water oxidation in photosystem II

The detailed structure of catalytic center of water oxidation, Mn₄Ca-cluster, in photosystem II (PSII) has been reported recently. However, due to the radiation damage induced by X-ray and the complexity of the Mn₄Ca-cluster, the assignment of the μ₄-O5 atom coordinated by three Mn and one Ca²⁺ ions is still lack of essential evidences. In this article, we synthesized one Mn complex containing two μ₄-O atoms. It is found that the lengths of all μ₄-O-Mn bonds in this Mn complex are in the range of 1.89–2.10 Å, which are significantly shorter than 2.40–2.61 Å distance of μ₄-O5-Mn bonds in Mn₄Ca-cluster observed in the crystal structure of PSII. In addition, DFT calculations have been carried out on the Mn₄Ca-cluster. It is found that the O atom of μ₄-O or μ₄-OH always trends to deviate from the center position of four metal ions, resulting in unequal bond lengths of four μ₄-O-M (M=Mn or Ca), which is obviously different with larger and nearly equal distances between μ₄-O and four metal ions observed in the crystal structure. Based on these results, we suggest that the μ₄-atom in Mn₄Ca-cluster of PSII is unlikely to be a μ₄-O, μ₄-OH or μ₄-OH₂, and its assignment is still an open question.