Assignment of the mu(4)-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 mu(4)-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 mu(4)-O atoms. It is found that the lengths of all mu(4)-O-Mn bonds in this Mn complex are in the range of 1.89-2.10 , which are significantly shorter than 2.40-2.61 distance of mu(4)-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 mu(4)-O or mu(4)-OH always trends to deviate from the center position of four metal ions, resulting in unequal bond lengths of four mu(4)-O-M (M=Mn or Ca), which is obviously different with larger and nearly equal distances between mu(4)-O and four metal ions observed in the crystal structure. Based on these results, we suggest that the mu(4)-atom in Mn4Ca-cluster of PSII is unlikely to be a mu(4)-O, mu(4)-OH or mu(4)-OH2, and its assignment is still an open question.