Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis. The highly oxidizing, charge-separated state generated within its reaction center (RC) drives water oxidation. Spectroscopic studies on PS II RCs are difficult to interpret due to large spectral congestion, necessitating modeling to elucidate key spectral features. Herein, we present results from time-dependent density functional theory (TDDFT) calculations on the largest PS II RC model reported to date. This model explicitly includes six RC chromophores and both the chlorin phytol chains and the amino acid residues <6 Å from the pigments’ porphyrin ring centers. Comparing our wild-type model results with calculations on mutant D1-His-198-Ala and D2-His-197-Ala RCs, our simulated absorption-difference spectra reproduce experimentally observed shifts in known chlorophyll absorption bands, demonstrating the predictive capabilities of this model. We find that inclusion of both nearby residues and phytol chains is necessary to reproduce this behavior. Our calculations provide a unique opportunity to observe the molecular orbitals that contribute to the excited states that are precursors to CS. Strikingly, we observe two high oscillator strength, low-lying states, in which molecular orbitals are delocalized over Chl_D1 and Phe_D1 as well as one weaker oscillator strength state with molecular orbitals delocalized over the P chlorophylls. Both these configurations are a match for previously identified exciton–charge transfer states (Chl_D1⁺Phe_D1⁻)* and (P_D2⁺P_D1⁻)*. Our results demonstrate the power of TDDFT as a tool, for studies of natural photosynthesis, or indeed future studies of artificial photosynthetic complexes.

光系统 II (PS II) 在光合作用期间捕获太阳能并引导电荷分离 (CS) 穿过类囊体膜。其反应中心 (RC) 内产生的高度氧化、电荷分离状态驱动水氧化。由于光谱拥塞较大，PS II RC 的光谱研究很难解释，因此需要建模来阐明关键光谱特征。在此，我们展示了迄今为止报道的最大 PS II RC 模型的瞬态密度泛函理论 (TDDFT) 计算结果。该模型明确包括六个 RC 发色团以及二氢叶绿醇链和来自颜料卟啉环中心的氨基酸残基 <6 Å。将我们的野生型模型结果与突变体 D1-His-198-Ala 和 D2-His-197-Ala RC 的计算结果进行比较，我们的模拟吸收差光谱再现了实验观察到的已知叶绿素吸收带的变化，证明了该模型的预测能力模型。我们发现包含附近的残基和植醇链对于重现这种行为是必要的。我们的计算提供了一个独特的机会来观察分子轨道，这些轨道有助于激发态，而激发态是CS的前体。引人注目的是，我们观察到两种高振荡强度的低位态，其中分子轨道在 Chl _D1和 Phe _D1上离域，以及一种较弱的振荡强度态，其中分子轨道在 P 叶绿素上离域。这两种配置都与先前确定的激子电荷转移态 (Chl _D1 ⁺ Phe _D1 ⁻ )* 和 ( _PD2 ⁺ P _D1 ⁻ )* 相匹配。 我们的结果证明了 TDDFT 作为自然光合作用研究或人工光合作用复合物的未来研究工具的强大功能。