The molecular mechanism of photosynthetic water oxidation in photosystem II was investigated, focusing on the elusive O₂-evolving S₃ → S₀ transition using time-resolved infrared spectroscopy, supported by quantum mechanics/molecular mechanics calculations. It was suggested that the initial ∼ 200 μs phase, which was significantly retarded by Cl^- → NO₃^- substitution but not much by Ca²⁺ → Sr²⁺ substitution, is attributed to proton release from W1, promoted by Y_Z oxidation, via the Cl-1 channel. The resultant W1 = OH^- form is in thermal equilibrium with the W2 = OH^- form. The slow millisecond phase was significantly retarded by both Sr²⁺ and NO₃^- substitutions, maintaining electron transfer to Y_Z^• as the rate-limiting step even in very slow kinetics with simultaneous Sr²⁺/NO₃^- substitution, indicating that the hydrogen-bond network of water molecules between the Cl and Ca sites plays a crucial role in the electron transfer to form the transient S₄ state. It is proposed that electron transfer is coupled with internal proton transfer from O6H^- to W2(OH^-) through this hydrogen-bond network. These results highlight the key role of the hydrogen-bond network in the catalytic site in the molecular mechanism of the O₂-evolving process, the slowest step in photosynthetic water oxidation.

研究了光系统II中光合水氧化的分子机制，重点关注难以捉摸的O₂-释放S₃ → S₀ 转变，并使用时间分辨红外光谱和量子力学/分子力学计算进行了支持。研究表明，在初始约200微秒的阶段，Cl^- → NO₃^- 替换显著延迟了这一过程，而Ca²⁺ → Sr²⁺ 替换影响不大。这归因于W1从质子释放并通过Cl-1通道由Y_Z氧化促进的质子释放。由此产生的W1 = OH^- 形式与W2 = OH^- 形式处于热平衡状态。在毫秒阶段，Sr²⁺ 和NO₃^- 替换都显著延迟了这一过程，并且即使是在同时进行的非常慢的Sr²⁺/NO₃^- 替换过程中，电子转移到Y_Z^• 仍然是限速步骤，这表明水分子之间的氢键网络在形成瞬态S₄ 状态的电子转移中起着关键作用。从Cl和Ca位点之间的位置可以看出，该氢键网络对内部质子从O6H^- 到W2(OH^-) 的传递与电子传输耦合。这些结果强调了在光合作用水氧化过程中最慢步骤的O₂-释放过程分子机制中催化位点氢键网络的关键作用。