Synthetic Models of the Oxygen Evolving Complex of Photosystem II

摘要 蓝藻和植物中的生物氧气产生发生在光系统II(PSII)内。这个 负责这一转化的活性部位，放氧复合体(OEC)，由一个 Mn4CaOn簇嵌入一个大的蛋白质复合体中。这个金属星团负责 地球上含氧的大气层，因此对于我们所知的大多数生命来说。考虑到广泛的 水裂解制取氧气的基本兴趣和潜在应用 这种团簇和催化机理一直是许多光谱研究的主题， 计算、合成、结晶学和生化研究。尽管取得了重大进展，但 氧气产生的机制目前还不清楚。准确的锰的氧化状态沿 催化循环和O-O键的形成位置仍在争论中。大的蛋白质基质 对OEC活性部位进行了复杂的直接研究，并合理合成了准确的小分子- 适合于结构-功能研究的分子模型一直受到复杂性的阻碍 集群。 我们的目标包括开发合成路线，以获得MnxMon模型(x=3，4；M=Ca，Mn，其他金属) OEC及其子站点，并进行机械研究，以便更深入地了解 不同成分(金属、辅助和氧配体、质子化状态)对蛋白质的影响 与获得高氧化态相关的团簇的化学和物理性质 影响产氧量。为此，我们将测试有关电子、氧原子和 这些复杂体系中的质子转移和配体取代及其对O-O键的影响 队形。除了少数例外，可预测的氧化锰团簇的合成一直是 对氧代配体桥接并形成复杂低聚结构的倾向感到沮丧。我们的 解决这一问题的方法是使用相对较小但严格的有机框架来支持 已被阐述为位置差异化的金属氧簇的三锰络合物，包括 封闭的生物系统的结构和功能模型。这些合成集群将允许 光谱基准(EPR、XES、XAS)与生物系统的比较。我们的工作是 人工合成的复合体将补充对蛋白质进行的研究，使系统 结构-性质研究，以揭示控制反应性和 这些团簇的光谱和催化机理。

Abstract Biological dioxygen generation occurs within Photosystem II (PSII) in cyanobacteria and plants. The active site responsible for this transformation, the Oxygen Evolving Complex (OEC), consists of a Mn4CaOn cluster embedded in a large protein complex. This metal cluster is responsible for the oxygenic atmosphere on Earth, and consequently for most life as we know it. Given the broad fundamental interest and potential applications of water splitting to make dioxygen, the structure of this cluster and the mechanism of catalysis have been the subject of many spectroscopic, computational, synthetic, crystallographic and biochemical studies. Despite significant advances, the mechanism of oxygen production is still not well understood. The exact Mn oxidation states along the catalytic cycle and the site of O-O bond formation continue to be debated. The large protein matrix has complicated direct studies of the OEC active site and the rational synthesis of accurate small- molecule models suitable for structure-function studies has been hampered by the complexity of the cluster. Our goals include developing synthetic routes to MnxMOn models (x=3, 4; M=Ca, Mn, other metals) of the OEC and its subsites and undertaking mechanistic studies that will allow a deeper understanding of the effects different constituents (metals, ancillary and oxo ligands, protonation state) have on the chemical and physical properties of the cluster relevant to achieving high oxidation states and effecting O2 production. To that end, we will test hypotheses regarding electron, oxygen-atom and proton transfers and ligand substitution in these complex systems with implications for O-O bond formation. With few exceptions, the synthesis of predictable manganese oxide clusters has been frustrated by the propensity of oxo ligands to bridge and form complicated oligomeric structures. Our approach to overcoming this problem is to use relatively small, but rigid organic frameworks to support trimanganese complexes that have been elaborated to site-differentiated metal-oxo clusters, including close structural and functional models of the biological system. These synthetic clusters will allow for spectroscopic benchmarking (EPR, XES, XAS) in comparison with the biological system. Our work on synthetic complexes will complement the studies performed on the protein by allowing systematic structure-property studies to uncover the chemical features that control the reactivity and spectroscopy of these clusters and the mechanism of catalysis.