Low-Coordinate Synthetic Models for Nitrogenase Activity

描述（由申请人提供）：固氮酶是一种非常重要的酶，可以进行惊人的化学反应，将N2还原为氨。在固氮酶中，铁硫簇催化小分子的多电子还原，这一作用不同于铁硫簇作为电子转移位点的更常见的作用，避免了键形成和键断裂反应。关于固氮酶还原底物的机制知之甚少。我们的指导性假设是固氮酶和其他反应性铁硫簇合酶在铁原子上产生一个短暂的开放位点用于底物结合。铁钼固氮酶最近的光谱工作强烈表明，该机制涉及N2与铁的结合，并涉及铁氢化物物种。然而，对于具有开放位点的铁硫簇合物或具有氢化物的铁硫簇合物，没有化学先例。N2与高自旋铁结合的例子很少见，而且研究不足。需要具有这些特征的合成化合物来评估铁硫簇上所提出的官能团的可行性，以建立这些官能团的光谱特征，并了解它们的反应性是否与酶促产物一致。在拟议的研究中，我们将创建具有这些功能的合成含铁化合物：不饱和铁硫簇，铁硫化物氢化物簇和铁-N2络合物。这些化合物的分离和表征通过使用非常大的支持基团而成为可能。大体积基团还促进结晶，并提高在可在低温下使用的有机溶剂中的溶解度。晶体学，动力学研究，电化学和反应性将被用来阐明小分子结合和还原的基本步骤的原子级细节。将通过ENDOR、红外、拉曼、穆斯堡尔和X-射线吸收光谱对合成的复合物进行评价，以提供新的模型化合物的结构与固氮酶的已知数据之间的联系。我们预计，拟议的工作将导致固氮酶的反应途径的第一个坚实的先例。虽然在生物无机化学中对多电子氧化反应的机理了解较多，但对多电子生物还原反应的认识却相对较少，因此研究固氮酶中硫化铁团簇如何结合和转化生命所必需的小分子具有重要意义.从长远来看，了解生物系统中小分子还原的机制也可能导致用于化学合成的新催化剂。 公共卫生相关性：含有铁和硫的酶产生许多对生命至关重要的分子，但还不清楚。铁硫固氮酶将大气中不活泼的氮转化为可利用的形式，我们所知道的生命就是依赖于这种“固氮”过程。“然而，科学界还不知道固氮酶是如何工作的。该项目旨在展示固氮机制的基本原理，这也可能导致转化有机和无机分子的新催化剂。

DESCRIPTION (provided by applicant): Nitrogenases are vitally important enzymes that perform an amazing chemical reaction, the reduction of N2 to ammonia. In nitrogenases, iron-sulfur clusters catalyze multielectron reductions of small molecules, a role that differs from the more common role of iron-sulfur clusters as electron-transfer sites that avoid bond-making and bond-breaking reactions. Little is known about the mechanism of substrate reduction by nitrogenases. Our guiding hypothesis is that nitrogenases and other reactive iron-sulfur cluster enzymes generate a transient open site on an iron atom for substrate binding. Recent spectroscopic work on iron- molybdenum nitrogenase strongly suggests that the mechanism involves binding of N2 to iron, and involves iron-hydride species. However, there are no chemical precedents for iron-sulfur clusters that have an open site, or for iron-sulfur clusters with a hydride. Examples of N2 binding to high-spin iron are rare and understudied. Synthetic compounds with these features are needed to evaluate the feasibility of the proposed functional groups on iron-sulfur clusters, to establish the spectroscopic signatures of these functional groups, and to learn whether their reactivity is consistent with the enzymatic products. In the proposed research, we will create synthetic iron-containing compounds with each of these functionalities: unsaturated iron-sulfur clusters, iron-sulfide-hydride clusters, and iron-N2 complexes. The isolation and characterization of these compounds is made possible by the use of very bulky supporting groups. The bulky groups also facilitate crystallization, and enhance solubility in organic solvents that can be used at low temperature. Crystallography, kinetic studies, electrochemistry, and reactivity will be used to elucidate the atomic-level detail of the elementary steps of small-molecule binding and reduction. The synthetic complexes will be evaluated by ENDOR, infrared, Raman, M"ssbauer, and X- ray absorption spectroscopies to provide a link between the structures of novel model compounds and the known data for nitrogenases. We anticipate that the proposed work will lead to the first solid precedents for reaction pathways in nitrogenases. Although much is known about the mechanisms of multielectron oxidation reactions in bioinorganic chemistry, the knowledge about multielectron biological reductions is much less. Therefore, there is fundamental importance in learning how the iron-sulfide cluster in nitrogenase binds and transforms small molecules that are essential for life. In the long run, understanding the mechanisms of small-molecule reduction in biological systems may also lead to new catalysts for use in chemical synthesis. PUBLIC HEALTH RELEVANCE: Enzymes that contain iron and sulfur produce many of the molecules that are essential for life, but are not understood well. The iron-sulfur enzyme nitrogenase converts unreactive nitrogen in the atmosphere into forms that can be used, and life as we know it is dependent upon this process of "nitrogen fixation." However, the scientific community does not yet know how nitrogenase works. This project aims to show the principles underlying the mechanism of nitrogen fixation, which may also lead to new catalysts for transforming organic and inorganic molecules.