By comparisons of N₂ and isoelectronic substrate CO bound FeMo-cofactors (FeMo-cos) in nitrogenases, we have used a classical bond valence method to calculate the oxidation states of the iron and molybdenum atoms in FeMo-cos. For single N₂- and CO-bound FeMo-cos that have the same charge in iron counterparts, Fe1/7 can be unambiguously assigned to Fe(III), while Fe3/4/5 show different degrees of mixed valences related to the individual iron atoms in FeMo-cos. The only difference is the valence of binding sites Fe2 and Fe6, where N₂-bound FeMo-co in 6UG0 shows +3 and +2, respectively, while the opposite is observed in the CO-bound FeMo-cos in 4TKV. For double CO- and N₂-bound FeMo-cos with different coordination modes, the N₂-bound species in 6UG0 performs a similar oxidation state and more electron-localized than those of CO-bound species in 7JRF. Calculations also show an unprecedented Mo(II) in N₂-bound FeMo-cos with monodentate homocitrate, while they show Mo(III) in CO-bound and other high resolution nitrogenases. Bond comparisons imply that monodentate homocitrate and unreasonable long Mo–O distances are attributed to the low valence of Mo(II) in N₂-bound FeMo-cos.

中文翻译：

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FeMo辅因子的CO和N2结合簇的键合价和距离的比较

通过比较固氮酶中 N ₂和等电子底物 CO 结合的 FeMo-辅因子 (FeMo-cos)，我们使用经典的键合价方法计算了 FeMo-cos 中铁和钼原子的氧化态。对于在铁对应物中具有相同电荷的单个 N ₂ - 和 CO 结合的 FeMo-cos，Fe1/7 可以明确地分配给 Fe( III )，而 Fe3/4/5 显示出与个体相关的不同程度的混合价FeMo-cos中的铁原子。唯一的区别是结合位点 Fe2 和 Fe6 的化合价，其中 6UG0 中的 N ₂结合的 FeMo-co分别显示 +3 和 +2，而在 4TKV 中的 CO 结合的 FeMo-cos 中观察到相反的情况。对于双 CO- 和 N ₂结合具有不同配位模式的FeMo-cos，6UG0中的N ₂结合物质表现出相似的氧化态并且比7JRF中的CO结合物质更电子局域化。计算还显示在 N ₂结合的 FeMo-cos 与单齿高柠檬酸盐中前所未有的 Mo( II )，而在 CO 结合的和其他高分辨率固氮酶中显示出 Mo( III )。键比较意味着单齿高柠檬酸盐和不合理的长 Mo-O 距离归因于 N ₂结合的 FeMo-cos中 Mo( II ) 的低价态。