Low-Coordinate Synthetic Models for Nitrogenase Activity

在固氮酶中，铁硫（FeS）簇超越了它们通常作为电子转移位点的作用， N2到NH3的困难的多电子还原。固氮酶因此证明了惊人的催化能力， 生物系统中的铁硫簇。固氮酶中N2还原的位点是一个FeS簇，称为 铁钼辅因子（FeMoco），这是生物化学中金属碳化物的唯一例子。这 碳化物可以通过簇-CH 3、-CH 2和-CH中间体的方式插入，这也是前所未有的。 FeS团簇化学这种酶目前被假定使用化学家未知的物种： 有机金属FeS簇、具有碳化物的FeS簇、具有碳化物的FeS簇和结合到 氮气由于缺乏先例，迫切需要建立评价的实验基础 文献提出的FeMoco生物合成和活性的机制。 我们的指导假设是，碳化物在FeMoco中的作用是保持和释放瞬态低配位 铁中心，其在催化机理期间可形成桥接Fe-N2和Fe-H中间体。这将是 使用合成类似物策略进行测试，这在生物无机化学中是有先例的。合成 在簇上具有N2、H和C基团的FeS簇可以显示所提出的官能团的可行性 在铁硫簇，建立特定官能团的光谱特征，并显示是否 它们的反应符合FeMoco机理模型。类似地，簇结合的CH 2、CH和C 这将有助于确定FeMoco生物合成中潜在步骤的可行性。 在拟议的研究中，我们将合成和研究具有以下各项的合成FeS化合物 新的功能：（1）结合固氮酶底物的不饱和铁硫簇，（2）铁硫化物， 氢化物簇合物;（3）具有CH_2/CH/C桥的铁簇合物。我们将使用庞大的支持团体来稳定 反应性物种，以促进结晶，并能够系统地研究它们的反应。晶体学， 动力学研究、电化学和反应性将用于了解N2的结合和还原， 其他固氮酶底物。这反过来又表明了什么类型的反应是合理的预期与 FeMoco。结构确定的合成复合物也将通过磁共振，红外， 拉曼，穆斯堡尔和X射线吸收光谱，这将有助于“翻译”已知的 固氮酶的光谱数据转化为合理的结构结论。 固氮酶是最奇特的金属酶之一，它具有强还原性多电子 还原，具有碳化物的辅因子结构，以及与通常惰性的N2相互作用的能力。理解 因此，其机制需要对FeS团簇的基本化学进行新的发现。这个项目 旨在提供化学先例，需要把固氮酶机制的坚实基础。

In nitrogenases, iron-sulfur (FeS) clusters transcend their usual role as electron transfer sites, by performing the difficult multielectron reduction of N2 to NH3. Nitrogenase thus demonstrates the amazing catalytic ability of iron-sulfur clusters in biological systems. The site of N2 reduction in nitrogenase is an FeS cluster called the iron-molybdenum cofactor (FeMoco), which is the only example of a metal-carbide in biological chemistry. This carbide may be inserted by way of cluster-CH3, -CH2, and -CH intermediates, which are also unprecedented in FeS cluster chemistry. This enzyme is currently postulated to use species unknown to chemists: organometallic FeS clusters, FeS clusters with carbides, FeS clusters with hydrides, and FeS clusters bound to N2. Because of the lack of precedents, there is an urgent need to build up the experimental basis for evaluating the literature-proposed mechanisms for FeMoco biosynthesis and activity. Our guiding hypothesis is that the role of carbide in FeMoco is to hold and release transient low-coordinate iron sites, which can form bridging Fe-N2 and Fe-H intermediates during the catalytic mechanism. This will be tested using the synthetic analogue strategy, which is well-precedented in bioinorganic chemistry. Synthetic FeS clusters with N2, H, and C groups on the cluster can show the feasibility of the proposed functional groups on iron-sulfur clusters, establish the spectroscopic signatures of specific functional groups, and show whether their reactions are consistent with the models for FeMoco mechanism. Similarly, cluster-bound CH2, CH, and C groups would help to determine the feasibility of potential steps in FeMoco biosynthesis. In the proposed research, we will synthesize and study synthetic FeS compounds with each of the following novel functionalities: (1) unsaturated iron-sulfur clusters that bind nitrogenase substrates, (2) iron-sulfide- hydride clusters, and (3) iron clusters with CH2/CH/C bridges. We will use bulky supporting groups to stabilize reactive species, to facilitate crystallization, and to enable systematic study of their reactions. Crystallography, kinetic studies, electrochemistry, and reactivity will be used to understand the binding and reduction of N2 and other nitrogenase substrates. This in turn shows what types of reactions are reasonable to expect with the FeMoco. The structurally-defined synthetic complexes will also be evaluated by magnetic resonance, infrared, Raman, Mössbauer, and X-ray absorption spectroscopies, which will serve to "translate" the known spectroscopic data for nitrogenase into reasonable structural conclusions. Nitrogenase is one of the strangest metalloenzymes, because of its strongly reducing multielectron reduction, the cofactor structure with a carbide, and the ability to interact with usually-inert N2. Understanding its mechanism thus requires new discoveries about the fundamental chemistry of FeS clusters. This project aims to provide the chemical precedents that are needed to put nitrogenase mechanism on a firm footing.