Low-Coordinate Synthetic Models for Nitrogenase Activity

在固氮酶中，铁-硫团簇(FeS)超越了它们通常作为电子转移中心的角色，通过执行 氮的多电子还原为氨的困难。固氮酶因此展示了惊人的催化能力 生物系统中的铁硫团簇。固氮酶中氮气还原的位置是一个称为 铁钼辅因(FeMoco)，这是生物化学中唯一的金属碳化物的例子。这 碳化物可以以-CH3、-CH2和-CH中间体的形式插入，这些中间体在 FeS团簇化学。这种酶目前被认为是使用化学家未知的物种： 有机金属FeS团簇、含碳化物的FeS团簇、含氢化物的FeS团簇和结合到 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。 我们的指导性假设是碳化物在FeMoco中的作用是保持和释放瞬时低坐标 铁中心，可在催化机理中形成桥连Fe-N_2和Fe-H中间体。这将是 使用在生物无机化学中很有先例的合成类比策略进行测试。合成的 簇上含有N_2、H和C基团的FeS簇可以证明所提出的官能团的可行性 在铁-硫团簇上，建立特定官能团的光谱特征，并显示 它们的反应与FeMoco机理模型一致。类似地，与集群绑定的CH2、CH和C 这些小组将有助于确定FeMoco生物合成中潜在步骤的可行性。 在拟议的研究中，我们将合成和研究合成的FeS化合物，其中每一种都有以下几种 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，以及(3)具有CH2/CH/C桥的铁簇合物。我们将利用庞大的支持小组来稳定局势 活性物种，以促进结晶，并使系统研究其反应。结晶学， 动力学研究、电化学和反应性将被用来理解氮气的结合和还原。 其他固氮酶底物。这进而显示了可以合理预期的 费莫科。结构定义的合成配合物还将通过磁共振、红外、 拉曼、穆斯堡尔和X射线吸收光谱，这将有助于将已知的 将固氮酶的光谱数据转化为合理的结构结论。 固氮酶是最奇怪的金属酶之一，因为它具有很强的多电子还原能力。 还原，与碳化物的辅因子结构，以及与通常惰性的氮气相互作用的能力。理解 因此，它的机制需要对FeS团簇的基本化学有新的发现。这个项目 旨在提供使固氮酶机制站稳脚跟所需的化学先例。

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.