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Photosystem II (PS II) sustains the biosphere by enabling oxygen evolution in the S₃-to-S₀ transition (duration?～1–2?ms) of its catalytic cycle. Using time-resolved Mn K_β X-ray emission spectroscopy, we observed spectroscopic changes (which were unimpeded in D₂O buffer) consistent with a modified electronic structure as early as ～50?μs in the transition. These changes are consistent with the reduction of the Mn centers at the ～50–200?μs time window in H₂O and at ～50–500?μs in D₂O, ahead of the reduction of the redox-active Tyr_Z^?+. These results indicate the multi-step nature of O–O bond formation and O₂ release by the oxygen-evolving complex (OEC), where an O–O bond is most likely formed prior to the final electron-transfer step. The D₂O-induced extension of up to ～500?μs in the lifetime of the early transient state of a possible peroxo nature opens it up to further spectroscopic and structural analysis.

中文翻译：

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观察光系统 II 的 Mn4Ca 簇中 O-O 键形成的挑战


光系统 II （PS II） 通过其催化循环的 S₃ 到 S₀ 转变（持续时间 ～1-2 ms）中实现氧气释放来维持生物圈。使用时间分辨 Mn K_β X 射线发射光谱，我们观察到光谱变化（在 D₂O 缓冲液中不受阻碍）与转变中 ～50 μs 的修改电子结构一致。这些变化与 H₂O 中 ～50-200 μs 时间窗口和 D₂O 中 ～50-500 μs 的 Mn 中心减少一致，领先于氧化还原活性 Tyr_Z^?+ 的减少。这些结果表明，析氧复合物 （OEC） 形成 O-O 键和 O₂ 释放是多步骤性质的，其中 O-O 键最有可能在最后的电子转移步骤之前形成。在可能的 peroxo 性质的早期瞬态的生命周期中，D₂O 诱导的延伸高达 ～500 μs，这使其为进一步的光谱和结构分析打开了大门。