Low-Coordinate Synthetic Models for Nitrogenase Activity

说明(申请人提供)：固氮酶是一种极其重要的酶，它执行一种惊人的化学反应，将氮气还原为氨。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中心的更常见角色，后者避免了成键和断键反应。对固氮酶还原底物的机理知之甚少。我们的指导假设是，固氮酶和其他活性铁硫簇酶在铁原子上产生一个瞬时开放位置，用于底物结合。最近对铁钼固氮酶的光谱研究有力地表明，其机理涉及氮与铁的结合，并涉及铁-氢化物物种。然而，没有具有开放中心的铁硫簇合物的化学先例，也没有具有氢化物的铁硫簇的化学先例。氮与高自旋铁结合的例子很少见，研究也很少。具有这些特征的合成化合物需要评估所提出的铁-硫簇上官能团的可行性，建立这些官能团的光谱特征，并了解它们的反应活性是否与酶产物一致。在拟议的研究中，我们将创建具有以下每一种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-氮络合物。通过使用非常大的支撑基，这些化合物的分离和表征成为可能。体积大的基团还有利于结晶，并提高了在可在低温下使用的有机溶剂中的溶解度。结晶学、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水平的细节。合成的络合物将通过Endor、红外、拉曼、穆斯堡尔和X射线吸收光谱进行评估，以提供新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反应途径的第一个坚实的先例。尽管生物无机化学中的多电子氧化反应机理已为人们所熟知，但对多电子生物还原反应的了解却很少。因此，了解固氮酶中的铁-硫化物簇如何结合和转化生命所必需的小分子是至关重要的。从长远来看，了解生物体系中小分子还原的机理也可能导致新的催化剂用于化学合成。 与公共健康相关：含有铁和硫的酶会产生许多生命所必需的分子，但人们对这些分子的了解还不够深入。铁-硫酶固氮酶将大气中的非活性氮转化为可利用的形式，而我们所知的生命依赖于这种“固氮”过程。然而，科学界还不知道固氮酶是如何起作用的。这个项目旨在展示固氮机理的基本原理，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.