An additional water is introduced into the manganese cluster during the formation of the S state of photosystem II

During photosynthetic water oxidation, the Mn4Ca cluster in Photosystem II progresses through five intermediate S-i (i = 0-4) states. X-ray crystallography studies have reported the insertion of one new O ligand during the formation of the S-3 state, but recent studies question the presence of this additional ligand based on cryo-EM and earlier room-temperature crystallography data. There is also controversy about whether the O-O bond interaction already occurs in the S-3 state or in the subsequent S-3 to S-0 transition. Here we report conventional high-resolution data for the S-1, S-2, and S-3 states to a resolution of similar to 1.9 & Aring;, and anomalous diffraction data at two energies (9.5 keV and 7 keV), that was used to model the Mn positions, followed by determination of oxygen positions using the high-resolution maps. We show that the new oxygen atom, O-X (or O6), in the S-3 state is observable as a distinct peak without any restraints, confirming its ligation to Mn1 and Ca. The O-X-O5 distance is similar to 2.1 & Aring;, supporting no strong interaction between them in the S-3 state, suggesting that if this is the O-O bond formation site, it is formed during the S-3 to S-0 transition initiated by the final oxidation of the cluster.