Functional LnFe-NxHy Models of Biological N2 Fixation

项目总结-生物固氮的功能性LnFe-NxHy模型 固氮酶(N_2-ase)金属酶介导生物固氮，因此是生命所必需的。他们的 相应地，这项研究也引起了生物界和化学界的强烈兴趣。尽管如此， 环境条件下固氮酶促进生物还原氮素的机制 情况仍然是个谜。激励我们NIH支持的研究计划的广泛问题包括 如下：单一的铁中心是否能调节与催化氮气相关的关键的成键和断裂步骤？ 在合成模型系统中固定，进而在生物学中固定？如果是，关键中间体是什么？ 可以进入的小路？次级金属中心和/或次级球体的相互作用如何影响 这样的反应？这次续签申请是在上一次授权期取得广泛进展的基础上提出的，据报道 通过21种主要文学出版物。我们建议继续设计和研究Fe-NxHy模型络合物 来解决上面强调的问题。我们的方法强调功能上的，而不是结构上的忠诚 铁-钼辅助因子(FeMoco)的模型。低相对分子质量的Fe-NxHy络合物 开发的目的是在低坐标几何图形中探索铁位置，以适应氮气和更多还原的NxHy。 强调了生物固氮的两种限制性单点机制：第一种是 交替机制，其中连续的H原子转移(通过H+/e步骤)发生在N-的远端和近端。 以交替方式的Fe-N≡N亚基的原子(例如，Fe-N=NH→Fe-NH=NH→Fe-NH-NH-→Fe-NH-NH- NH_2→Fe-NH_2+NH_3)；第二种是末端机理，其中三个H原子在N原子的末端转移到 在氢原子转移到近端N-之前释放NH_3当量(例如，Fe-N_2+3e-+3H+→Fe≡N+NH_3) 原子。我们还探索了一种将两条路径交织在一起的混合交叉机制。我们的合成模型研究 用来测试每一条路径的可行性，并理解局部几何和电子如何 Fe-NxHy物种的结构控制着它们的反应模式，目标是开发出越来越有效的 铁介导型氮气转氨催化剂。不管减氮的确切机制/S， 酶中间体的指定依赖于确定的光谱参数的可用性 Fe-NxHy模型。我们将继续收集这些数据，并与专门研究 光谱研究，包括在固氮酶内，使有用的比较成为可能。总而言之， 本文提出的功能Fe-NxHy模型化学将继续与电流一起发挥关键作用 旨在解开化学机制的生化、光谱和理论模型研究/S 生物固氮。

Project Summary - Functional LnFe-NxHy Models of Biological N2 Fixation Nitrogenase (N2-ase) metalloenzymes mediate biological nitrogen fixation and as such are essential to life. Their study correspondingly attracts intense interest from the biology and chemistry communities. Nonetheless, the mechanism by which nitrogenase enzymes promote the biological reduction of nitrogen under ambient conditions remains enigmatic. The broad questions that motivate our NIH-supported research program are as follows: Can a single iron site mediate the critical bond-making and breaking steps relevant to catalytic N2 fixation in a synthetic model system and, by extension, in biology? If so, what are the key intermediates and pathways that are accessible? How can a secondary metal center and/or secondary sphere interactions impact such reactivity? This renewal application builds on extensive progress made in the last grant period, reported via 21 primary literature publications. We propose to continue to design and study Fe-NxHy model complexes to address the questions highlighted above. Our approach stresses functionally, rather than structurally, faithful models of the iron-molybdenum cofactor (FeMoco). Low molecular weight Fe-NxHy complexes will be developed to explore iron sites in low coordinate geometries that accommodate N2 and more reduced NxHy. Two limiting single-site mechanisms for biological nitrogen fixation have been emphasized: the first is an alternating mechanism, where successive H-atom transfers (via H+/e- steps) occur at the distal and proximal N- atoms of an Fe-N≡N subunit in an alternating fashion (e.g., Fe-N=NH → Fe-NH=NH → Fe-NH-NH2→ Fe-NH2- NH2 → Fe-NH2 + NH3); the second is a distal mechanism, where three H-atom transfers at the distal N-atom to liberate an NH3 equivalent (e.g., Fe-N2 + 3 e- + 3 H+ → Fe≡N + NH3) precede H-atom transfers to the proximal N- atom. We also explore a hybrid cross-over mechanism that interweaves both paths. Our synthetic model studies are used to test the viability of each of these pathways, and to understand how the local geometry and electronic structure of Fe-NxHy species controls their reactivity patterns, with the goal of developing increasingly efficient Fe-mediated N2-to-NH3 conversion catalysts. Regardless of the precise mechanism/s of nitrogen reduction, the assignment of enzymatic intermediates relies upon the availability of well-defined spectroscopic parameters for Fe-NxHy models. We will continue to collect such data, and to collaborate with researchers that specialize in spectroscopic studies, including within nitrogenase enzymes, to enable useful comparisons to be made. In sum, the functional Fe-NxHy model chemistry proposed herein will continue to play a critical role alongside current biochemical, spectroscopic, and theoretical model studies aimed at unraveling the chemical mechanism/s of biological nitrogen fixation.