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High Resolution Image Download MS PowerPoint Slide Two key characteristics differentiate molybdenum-dependent (MoFe) and iron-only (FeFe) nitrogenases: their efficacy for N 2 fixation and their product distributions for CO 2 reduction, yielding HCO 2 – and CH 4, respectively. Despite their divergent properties, prior research argues that the distinct cofactors share a common mechanism and equivalent structures with the addition of “ n ” electrons and protons at their E n catalytic states. The proposed equivalence between the cofactors was foundational in the assignment of an Fe-hydride at their E 1 states based on interpretation of FeFeco’s photolabile and thermolabile E 1 isomers ( Inorg. Chem . 2022, 61, 5459–5464). However, the E 0 state crystal structures of FeMoco and FeFeco have sharp, previously unaddressed distinctions in their metal–metal distances and proximal residue identities at their respective octahedral M-sites (M = Mo or Fe). Herein, we study QM/MM models of the E 0 and E 1 states of FeMoco and FeFeco to distinguish their geometric and electronic structures. Our analysis shows diminished metal–metal bonding and increased hydrogen bonding at FeFeco’s M-site, supporting the presence of two energetically low-lying E 1 states differentiated by protonation of a μ 3 - or μ 2 -sulfide. The calculated thermodynamic and kinetic properties of FeFeco’s sulfide-protonated states agree with experimental data, without invoking Fe-hydride formation. Unlike FeFeco, FeMoco’s E 1 state strictly favors μ 2 -sulfide protonation. FeFeco’s distinct μ 3 -sulfide-protonated E 1 isomer has a five-coordinate, reduced M-site that could explain its divergent reactivity relative to FeMoco.
Joyce et al. (Tue,) studied this question.