Synthetic Models of the Oxygen Evolving Complex of Photosystem II

摘要 生物双氧产生发生在蓝细菌和植物的光系统II（PSII）中。的 负责这种转化的活性位点，即氧释放复合物（OEC），由一个 Mn4CaOn簇嵌入在大的蛋白质复合物中。这个金属簇负责 地球上的含氧大气，因此对我们所知道的大多数生命来说。 水裂解的基本兴趣和潜在应用，以使双氧，结构， 该簇和催化机制已经成为许多光谱学的主题， 计算、合成、晶体学和生物化学研究。尽管取得了重大进展， 氧气产生的机制仍然没有得到很好的理解。精确的Mn氧化态沿着 催化循环和O-O键形成的位置仍然存在争议。大蛋白质基质 具有复杂的OEC活性位点的直接研究和精确的小分子的合理合成， 适合结构-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MnxMOn模型（x = 3，4; M = Ca，Mn，其他金属）的合成路线， OEC及其子站点，并进行机制研究，以便更深入地了解 不同成分（金属、辅助和氧代配体、质子化状态）对 与实现高氧化态有关的簇的化学和物理性质， 产生O2。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与O-O键的关系 阵除了少数例外，可预测的锰氧化物簇的合成一直是 受到氧代配体桥接和形成复杂低聚结构的倾向的阻碍。我们 克服这个问题的方法是使用相对较小但刚性的有机框架来支撑 三锰络合物，已被精心制作的网站分化的金属氧簇，包括 生物系统的结构和功能模型。这些合成簇将允许 光谱基准（EPR，XES，XAS）与生物系统的比较。我们的工作 合成复合物将补充对蛋白质进行的研究， 结构-性质研究，以揭示控制反应性的化学特征， 这些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.