Based on characterization by X-ray absorption spectroscopy, it has been proposed that the Mn4CaO5 cluster in the crystal structure of the water-oxidizing enzyme, photosystem II (PSII), may represent an over-reduced form arising from reduction by the X-ray beam. Using a quantum mechanical/molecular mechanical approach, and assuming that all of the μ-oxo bridges are deprotonated in S1, we analyzed the reduction process of the Mn4CaO5 cluster. In the crystal structure, the O atom (O5), which is linked with three Mn atoms and one Ca atom, has no H-bond. When reduced to S-2, unexpectedly, a water molecule at Ca2+ (W3) reoriented itself, formed a H-bond with O5, and released a proton to O5, resulting in formation of OH- at both W3 and O5. Once generated, the OH- group at O5 was stable, because the W3…O5 H-bond had already disappeared. A weak binding of H2O at Ca2+ led W3 to reorient and serve as a proton donor to O5 upon over-reduction.

中文翻译：

---

光系统II中过度还原的Mn4CaO5团簇的质子化机理。

根据X射线吸收光谱的表征，有人提出水氧化酶光系统II（PSII）的晶体结构中的Mn4CaO5团簇可能代表由于X射线还原而产生的过度还原形式光束。使用量子力学/分子力学方法，并假设在S1中所有的μ-氧代桥都去质子化，我们分析了Mn4CaO5团簇的还原过程。在晶体结构中，与三个Mn原子和一个Ca原子相连的O原子（O5）没有氢键。当还原为S-2时，出乎意料的是，Ca2 +（W3）处的水分子自身重新定向，与O5形成H键，并向O5释放质子，从而在W3和O5处均形成OH-。一旦生成，由于W3…O5的氢键已经消失，因此O5的OH-基团是稳定的。