Understanding the water oxidation mechanism in Photosystem II (PSII) stimulates the design of biomimetic artificial systems that can convert solar energy into hydrogen fuel efficiently. The Sr^2+-substituted PSII is active but slower than with the native Ca²⁺ containing PSII as an oxygen evolving catalyst. Here, we use Density Functional Theory (DFT) to compare the energetics of the S₂ to S₃ transition in the Mn₄O₅Ca²⁺ and Mn₄O₅Sr²⁺ clusters. The calculations show that deprotonation of the water bound to Ca²⁺ (W3), required for the S₂ to S₃ transition, is energetically more favorable in Mn₄O₅Ca²⁺ than Mn₄O₅Sr²⁺. In addition, we have calculated the pK_a of the water that bridges Mn4 and the Ca²⁺/Sr²⁺ in the S₂ state using continuum electrostatics. The calculations show that the pK_a is higher by 4 pH units in the Mn₄O₅Sr²⁺cluster.

了解光系统 II (PSII) 中的水氧化机制会刺激仿生人工系统的设计，该系统可以有效地将太阳能转化为氢燃料。Sr ^{2+ -}取代的PSII 是有活性的，但比作为放氧催化剂的含有PSII的天然Ca ²⁺慢。在这里，我们使用密度泛函理论 (DFT) 来比较Mn ₄ O ₅ Ca ²⁺和 Mn ₄ O ₅ Sr ²⁺簇中S ₂到 S ₃跃迁的能量。计算表明，与 Ca ²⁺ (W3)结合的水的去质子化是S在Mn ₄ O ₅ Ca ^{2+ 中}，₂到S _{3 的}转变在能量上比Mn ₄ O ₅ Sr ²⁺更有利。此外，我们已经使用连续静电学计算了在 S ₂状态下桥接 Mn4 和 Ca ²⁺ /Sr ²⁺的水的 pK _a。计算表明，在 Mn ₄ O ₅ Sr ²⁺簇中，pK _a高出 4 个 pH 单位。