Synthetic Models of the Oxygen Evolving Complex of Photosystem II

描述(申请人提供)：蓝藻和植物在光系统II(PSII)内产生生物氧气。负责这一转化的活性部位是放氧复合体(OEC)，它由嵌入在大型蛋白质复合体中的Mn4CaOn簇组成。鉴于水裂解制取二氧化氧的广泛基础兴趣和潜在应用，该簇合物的结构和催化机理一直是许多光谱、计算、合成、结晶学和生物化学研究的主题。尽管取得了重大进展，但人们对氧气产生的机制仍然不太清楚。沿着催化循环的确切的锰氧化态和O-O键的形成位置仍然存在争议。大的蛋白质基质对OEC活性部位的直接研究很复杂，而精确的小分子模型的合成受到簇的复杂性的阻碍。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，并进行机理研究，以更深入地了解不同成分(金属，辅助和氧配体，质子化状态)对团簇的化学和物理性质的影响，这些性质与实现高氧化态和影响氧气产生有关。为此，我们将测试这些复杂体系中关于电子、氧原子和质子转移以及配体取代的假设，并对团簇的组装和O-O键的形成产生影响。除了极少数例外，氧配体桥接并形成复杂低聚结构的倾向阻碍了可预测的氧化锰簇的合成。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络合物，这些络合物已经被细化到位置不同的金属-氧簇，包括Mn3CaO4部分。这些结构模型将允许与生物系统相比较的光谱基准(EPR、XES、XAS)。我们在合成络合物方面的工作将补充对蛋白质进行的研究，使系统的结构-反应性研究能够揭示控制这些簇合物的性质和催化机制的化学特征。

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.