Synthetic Models of the Oxygen Evolving Complex of Photosystem II

描述（由申请人提供）：生物双氧生成发生在蓝藻和植物的光系统II （PSII）内。负责这种转化的活性位点，氧进化复合体（OEC），由嵌入在一个大蛋白质复合体中的Mn4CaOn簇组成。鉴于水裂解制备二氧的广泛基础兴趣和潜在应用，该簇的结构和催化机制已成为许多光谱、计算、合成、晶体学和生化研究的主题。尽管取得了重大进展，但氧气产生的机制仍然没有得到很好的理解。Mn在催化循环中的氧化态和O-O键形成的位置仍在争论中。大的蛋白质基质使OEC活性位点的直接研究变得复杂，而精确的小分子模型的合成也受到了集群复杂性的阻碍。我们的目标包括开发OEC的MnxMOn模型（x= 3,4； M=Ca, Mn，其他金属）的合成路线，并进行机制研究，从而更深入地了解不同成分（金属，辅助和氧配体，质子化态）对团簇的化学和物理性质的影响，从而实现高氧化态和影响O2的产生。为此，我们将测试有关这些复杂系统中电子、氧原子和质子转移以及配体取代的假设，并对簇的组装和O-O键的形成产生影响。除了少数例外，可预测的氧化锰簇的合成受到氧配体桥接和形成复杂的低聚体结构的倾向的挫折。我们克服这一问题的方法是使用相对较小但刚性的有机框架来支持三锰配合物，这些配合物已经被精心设计成定位分化的金属-氧簇，包括Mn3CaO4部分。这些结构模型将允许光谱基准（EPR， XES， XAS）与生物系统进行比较。我们对合成复合物的研究将通过系统的结构-反应性研究来揭示控制这些簇的性质和催化机制的化学特征，从而补充对蛋白质的研究。

DESCRIPTION (provided by applicant): 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. 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 synthesis of accurate small-molecule models 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 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 the assembly of the clusters and 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 a Mn3CaO4 moiety. These structural models 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- reactivity studies to uncover the chemical features that control the properties of these clusters and the mechanism of catalysis.