The E2 state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H2 Evolution

The iron-molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E₀, reduced states of FeMoco are much less well characterized. The E₂ state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E₂ state can, however, relax back the E₀ state via a H₂ side-reaction, implying a hydride intermediate prior to H₂ formation. This E₂→E₀ pathway is one of the primary mechanisms for H₂ formation under low-electron flux conditions. In this study we present an exploration of the energy surface of the E₂ state. Utilizing both cluster-continuum and QM/MM calculations, we explore various classes of E₂ models: including terminal hydrides, bridging hydrides with a closed or open sulfide-bridge, as well as models without. Importantly, we find the hemilability of a protonated belt-sulfide to strongly influence the stability of hydrides. Surprisingly, non-hydride models are found to be almost equally favorable as hydride models. While the cluster-continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E₂ state. These models feature either i) a bridging hydride between Fe₂ and Fe₆ and an open sulfide-bridge with terminal SH on Fe₆ ( E₂-hyd ) or ii) a double belt-sulfide protonated, reduced cofactor without a hydride ( E₂-nonhyd ). We suggest both models as contenders for the E₂ redox state and further calculate a mechanism for H₂ evolution. The changes in electronic structure of FeMoco during the proposed redox-state cycle, E₀→E₁→E₂→E₀, are discussed.