Research School of Chemistry, Australian National University, Canberra, ACT, Australia.
Phys Chem Chem Phys. 2012 Apr 7;14(13):4651-7. doi: 10.1039/c2cp23935c. Epub 2012 Feb 24.
The most recent XRD studies of Photosystem II (PS II) reveal that the His337 residue is sufficiently close to the Mn(4)Ca core of the Water Oxidising Complex (WOC) to engage in H-bonding interactions with the μ(3)-oxo bridge connecting Mn(1), Mn(2) and Mn(3). Such interactions may account for the lengthening of the Mn-Mn distances observed in the most recent and highest resolution (1.9 Å) crystal structure of PS II compared to earlier, lower-resolution (2.9 Å or greater) XRD structures and EXAFS studies on functional PS II. Density functional theory is used to examine the influence on Mn-Mn distances of H-bonding interactions, mediated by the proximate His337 residue, which may lead to either partial or complete protonation of the μ(3)-oxo bridge on models of the WOC. Calculations were performed on a set of minimal-complexity models (in which WOC-ligating amino acid residues are represented as formate and imidazole ligands), and also on extended models in which a 13-peptide sequence (from His332 to Ala344) is treated explicitly. These calculations demonstrate that while the 2.9 Å structure is best described by models in which the μ(3)-oxo bridge is neither protonated nor involved in significant H-bonding, the 1.9 Å XRD structure is better reproduced by models in which the μ(3)-oxo bridge undergoes H-bonding interactions with the His337 residue leading to expansion of the 'close' Mn-Mn distances well known from EXAFS studies at ∼ 2.7 Å. Furthermore, full μ(3)-oxo-bridge protonation remains a distinct possibility during the process of water oxidation, as evidenced by the lengthening of the Mn-Mn vectors observed in EXAFS studies of the higher oxidation states of PS II. In this context, the Mn-Mn distances calculated in the protonated μ(3)-oxo bridge structures, particularly for the peptide extended models, are in close agreement with the EXAFS data.
最近对光合作用系统 II（PS II）的 XRD 研究表明，His337 残基与水氧化复合物（WOC）的 Mn(4)Ca 核心足够接近，能够与连接 Mn(1)、Mn(2)和 Mn(3)的 μ(3)-氧桥形成氢键相互作用。这种相互作用可能解释了与早期分辨率较低（2.9Å 或更高）的 XRD 结构和功能 PS II 的 EXAFS 研究相比，最近（1.9Å）晶体结构中观察到的 Mn-Mn 距离的延长。密度泛函理论用于研究由邻近 His337 残基介导的氢键相互作用对 Mn-Mn 距离的影响，这种相互作用可能导致 WOC 模型中 μ(3)-氧桥的部分或完全质子化。在一组最小复杂度模型（其中 WOC 配位氨基酸残基表示为甲酸盐和咪唑配体）和扩展模型（其中 13 肽序列（从 His332 到 Ala344）被明确处理）上进行了计算。这些计算表明，虽然 2.9Å 结构最好用模型来描述，其中 μ(3)-氧桥既未质子化也未参与显著的氢键相互作用，但 1.9Å XRD 结构更好地用模型来描述，其中 μ(3)-氧桥与 His337 残基形成氢键相互作用，导致众所周知的来自 EXAFS 研究的“紧密”Mn-Mn 距离扩展，约为 2.7Å。此外，在水氧化过程中，μ(3)-氧桥的完全质子化仍然是一种明显的可能性，这可以从 PS II 更高氧化态的 EXAFS 研究中观察到的 Mn-Mn 矢量的延长得到证明。在这种情况下，在质子化 μ(3)-氧桥结构中计算的 Mn-Mn 距离，特别是对于肽扩展模型，与 EXAFS 数据非常吻合。
