The photosynthetic water-splitting reaction catalysed by the oxygen-evolving center (OEC) in the photosystem Ⅱ of plants releases protons,electrons and dioxygen,which is one of the most important processes for energy and material conversions on Earth.OEC is a Mn4Ca-duster and cycles through five different states (Sn,n =0-4),wherein So is the most reduced state,S1 is the dark stable state and S2,S3 and S4 are intermediate states.OEC rapidly releases O2 in the S4 state,then retus to the So state.The detailed structure of the OEC in the S 1 state was reported by Umena et al.in 2011 [1]and improved by Suga et al.in 2015 [2].These reports from Shens group have revealed that OEC is composed of an asymmetric Mn4CaO5-cluster coordinated by four water molecules,one imidazole and six carboxylate groups from protein side chains (Fig.1).Determination of the accurate structure of the OEC in the S1 state [1,2] is a breakthrough in the field of photosynthesis.However,because of the complexities of the large protein environment and the dynamic changes of the OEC during the water-splitting reaction,it is a great challenge to reveal its detailed reaction mechanism.