Secondary-sphere interactions in the activation of dinitrogen and other small molecules by synthetic models of the nitrogenase FeMo cofactor

项目摘要(库珀CITEK)：“氮素活化中的次级球体相互作用 和其他小分子的固氮酶FEMO辅因子的合成模型“ 氮基化肥是养活地球不断扩大的人口的关键组成部分。不仅 化肥能让现代农业养活70多亿人吗？它们能帮助农民生产更多吗？ 用更少的土地获取食物，从而限制过度用水或清理对环境的影响 对多样化生态系统至关重要的栖息地。今天，哈伯-博世工艺提供了大量的 从大气氮素中提取的合成氨。然而，这一过程的总费用相当于 全球能源消耗占天然气的4%，以及由此产生的温室气体的影响。 某些细菌和古细菌能够从大气中的氮素中提取氨，从而有利于 它们本身以及它们所属的生态系统。众所周知，生物固氮有助于 由一种铁硫协同因子在保守的固氮酶内。这些辅因(最广为人知的 是FEMO辅助因子)结合和还原氮，与生物衍生的质子和 电子和三磷酸腺苷的水解。虽然固定氮素对有能力的生物来说是一笔巨大的成本， 这一投入可能会为固氮建立一个能量投入的下限。然而，这名特工 固氮酶FEMO辅因子的作用机制在基础、化学水平上仍有争议。 这一建议旨在探讨二级结构中保守残基的机制意义。 FEMO辅因子的配位球，即被认为促进氢键或 催化过程中的质子穿梭。我们将使用合成模型来模拟假设的氮素结合 与保守的组氨酸相邻的位点，并研究小分子的质子偶联还原途径 分子通过这些模型。含二次球状氢键的分子铁络合物 功能性(组氨酸类似物)可以通过已知的制剂获得，并可能允许稳定 活性物种在促进氮素固定的途径上，或者相反地，用于增强催化氮素 还原。在前一种情况下，我们可能能够表征以前未观察到的 质子与还原的铁氮物种的反应(用~(57)Fe穆斯堡尔，EPR，X射线结晶学， 电化学等)并将它们逐步转化为其他已知的络合物。在后一种情况下，我们可以 探索赋予FEMO辅因子的化学基序，为困难的小分子提供这样的便利 通过考察催化动力学、能量学和质子的最密切的步进顺序--和 电子加成(通过反应速率、电化学、质子相关性等)。人们希望氢- 键合结构甚至可能允许使用更温和的试剂进行均匀的氮素还原。如果分子铁 系统要成为未来的基本技术，必须从根本上理解它们。 并且必须与可持续的能源相兼容。

PROJECT SUMMARY (COOPER CITEK): “Secondary-sphere interactions in the activation of dinitrogen and other small molecules by synthetic models of the nitrogenase FeMo cofactor” Nitrogen-based fertilizers are a crucial component of feeding the Earth’s expanding population. Not only do fertilizers allow modern agricultures sustain more than 7 billion people, they help farmers to produce more food with less land, thereby limiting the environmental impacts of excess water consumption or of clearing habitats critical to diverse ecosystems. Today, the Haber-Bosch process supplies enormous quantities of synthetic ammonia from atmospheric dinitrogen. However the total expense of this process amounts to 2% of global energy consumption, 4% of natural gas, and the associated impact of greenhouse gases produced. Certain bacteria and archaea are able to derive ammonia from atmospheric dinitrogen for the benefit of themselves and the ecologies of which they are members. Biological nitrogen fixation is known to be facilitated by an iron-sulfur cofactors within conserved nitrogenase enzymes. These cofactors (the most well-known being the FeMo cofactor) both bind and reduce dinitrogen, with equivalents of biologically-derived protons and electrons and ATP hydrolysis. While dinitrogen fixation represents a significant cost to competent organisms, this input might establish a lower bound to energetic investment for nitrogen fixation. However the operative mechanism of nitrogenase FeMo cofactor is still debated at the fundamental, chemical level. This proposal aims to probe the mechanistic significance of conserved residues in the secondary coordination sphere of the FeMo cofactor, namely a histidine that is thought to facilitate hydrogen bonding or proton shuttling during catalysis. We will employ synthetic models to mimic a hypothesized dinitrogen binding site that is adjacent to the conserved histidine and to study the pathways of proton-coupled reduction of small molecules by these models. Molecular iron complexes incorporating secondary-sphere hydrogen-bonding functionality (histidine analogs) can be accessed by known preparations and may allow for the stabilization of reactive species on-path toward dinitrogen fixation or, conversely, for the enhancement of catalytic dinitrogen reduction. In the former case, we may be able to characterize previously unobserved intermediates of the reaction of protons with reduced iron-dinitrogen species (by 57Fe Mössbauer, EPR, X-ray crystallography, electrochemistry, etc.) and their stepwise conversion to other known complexes. In the latter case, we may probe the chemical motifs that impart the FeMo cofactor with such facility for a difficult small-molecule transformation by examining catalysis kinetics, energetics, and the intimate step-by-step order of proton- and electron-additions (by reaction rates, electrochemistry, proton dependences, etc.). It is hoped that hydrogen- bonding structures may even permit milder reagents for homogeneous dinitrogen reduction. If molecular iron systems are to become essential technologies for the future, they must be understood at the fundamental level and must be compatible with sustainable sources of energy.