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Title: Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N 2 reduction

Recent spectroscopic, kinetic, photophysical, and thermodynamic measurements show activation of nitrogenase for N2→ 2NH3reduction involves the reductive elimination (re) of H2from two [Fe–H–Fe] bridging hydrides bound to the catalytic [7Fe–9S–Mo–C–homocitrate] FeMo-cofactor (FeMo-co). These studies rationalize the Lowe–Thorneley kinetic scheme’s proposal of mechanistically obligatory formation of one H2for each N2reduced. They also provide an overall framework for understanding the mechanism of nitrogen fixation by nitrogenase. However, they directly pose fundamental questions addressed computationally here. We here report an extensive computational investigation of the structure and energetics of possible nitrogenase intermediates using structural models for the active site with a broad range in complexity, while evaluating a diverse set of density functional theory flavors. (i) This shows that to prevent spurious disruption of FeMo-co having accumulated 4[e/H+] it is necessary to include: all residues (and water molecules) interacting directly with FeMo-co via specific H-bond interactions; nonspecific local electrostatic interactions; and steric confinement. (ii) These calculations indicate an important role of sulfide hemilability in the overall conversion ofE0to a diazene-level intermediate. (iii) Perhaps most importantly, they explain (iiia) how the enzyme mechanistically couples exothermic H2formation to endothermic cleavage of the N≡N triple bond in a nearly thermoneutralre/oxidative-addition equilibrium, (iiib) while preventing the “futile” more » generation of two H2without N2reduction: hydrideregenerates an H2complex, but H2is only lost when displaced by N2, to form an end-on N2complex that proceeds to a diazene-level intermediate.

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Publication Date:
Journal Name:
Proceedings of the National Academy of Sciences
Page Range or eLocation-ID:
p. E10521-E10530
Proceedings of the National Academy of Sciences
Sponsoring Org:
National Science Foundation
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