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.

自然光合作用中的光系统II（Photosystem II）利用太阳能驱动放氧复合体（OEC）处的水发生四电子氧化反应，生成氧气。该复合体包含一个Mn₄CaO₅团簇，该团簇会历经Si（i=0–4）共5种氧化态的循环过程。其中关键的S₂→S₃转变是氧气生成前的最后一步，属于多步骤过程，涵盖去质子化、氧化以及水分子结合至Mn₄CaO₅团簇的反应。目前，该过程的反应事件顺序、水分子进入位点以及潜在中间体的本质仍存在较大争议，部分原因在于S₂态存在多种存在形式，且尚不清楚其中哪一种会顺利转变为S₃态。我们提出采用锰K边处的高能量分辨荧光探测X射线吸收谱（HERFD XAS），测量与水分子结合步骤相关的S₂和S₃态的高自旋构型，并将其与密度泛函理论（DFT）模型进行关联分析。