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（N2-ase）是一种介导生物固氮的金属酶，对生命至关重要。他们的 研究相应地引起了生物和化学界的强烈兴趣。不过 固氮酶促进环境下氮的生物还原的机制 情况仍然是个谜激励我们NIH支持的研究计划的广泛问题是， 一个单一的铁位点能介导与催化N2相关的关键键形成和断裂步骤吗？ 固定在合成模型系统，并通过扩展，在生物学？如果是，关键的中间体是什么， 可到达的路径？二次金属中心和/或二次球体相互作用如何影响 这种反应性？这一更新申请建立在上一个赠款期取得的广泛进展的基础上， 通过21篇主要文献发表。我们建议继续设计和研究Fe-NxHy模型配合物 解决上述问题。我们的方法强调功能性，而不是结构性，忠实 铁钼辅因子（FeMoco）模型。低分子量Fe-NxHy络合物将是 开发用于探索低坐标几何结构中的铁位点，其容纳N2和更还原的NxHy。 生物固氮的两个限制性单点机制已经被强调：第一个是一个 交替机制，其中连续的H-原子转移（通过H+/e-步骤）发生在远端和近端N- Fe-N-N亚基的原子以交替的方式（例如，Fe-N=NH → Fe-NH=NH → Fe-NH-NH2→ Fe-NH2- NH 2 → Fe-NH 2 + NH3）;第二种是远端机制，其中三个H原子在远端N原子处转移到 释放NH3当量（例如，Fe-N2 + 3 e- + 3 H+ → Fe <$N + NH3）先于H原子向邻近的N- 原子。我们还探索了一种混合交叉机制，交织这两条路径。我们的合成模型研究 用于测试这些途径中的每一个的可行性，并了解局部几何和电子 Fe-NxHy物种的结构控制它们的反应模式，目的是开发越来越有效的 Fe介导的N2至NH3转化催化剂。无论氮还原的精确机制如何， 酶促中间体的归属依赖于定义明确的光谱参数的可用性， Fe-NxHy模型。我们将继续收集这些数据，并与专门从事以下研究的研究人员合作： 光谱研究，包括固氮酶，使有用的比较。总之， 本文提出的功能性Fe-NxHy模型化学将继续发挥关键作用， 生物化学，光谱和理论模型研究，旨在解开化学机制/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.