What is the role of the high spin forms of the S2 and S3 states in water binding to the Mn4Ca cluster of Photosystem II?

Photosystem II of natural photosynthesis utilises the energy of sunlight to drive the four-electron oxidation of water to dioxygen at the oxygen-evolving complex (OEC). This harbours a Mn4CaO5 cluster that cycles through five oxidation states Si (i = 0–4). The critical S2→S3 transition, the last step before O2 formation, is a multistep transition that combines deprotonation, oxidation, and water binding to the Mn4CaO5 cluster. The sequence of events, the water access site, and the nature of the possible intermediates remain highly controversial, partly because multiple forms exist in the S2 state, and it is unclear which one progresses to the S3. We propose to measure the high spin configurations of the S2 and S3 states, which are relevant to the water binding step, using high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) at the Mn K-edge and correlate them with DFT models.