The E<sub>2</sub> state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H<sub>2</sub> Evolution
Albert Th. Thorhallsson, Ragnar Björnsson
Abstract
Abstract The iron‐molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E 0 , reduced states of FeMoco are much less well characterized. The E 2 state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E 2 state can, however, relax back the E 0 state via a H 2 side‐reaction, implying a hydride intermediate prior to H 2 formation. This E 2 →E 0 pathway is one of the primary mechanisms for H 2 formation under low‐electron flux conditions. In this study we present an exploration of the energy surface of the E 2 state. Utilizing both cluster‐continuum and QM/MM calculations, we explore various classes of E 2 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 E 2 state. These models feature either i) a bridging hydride between Fe 2 and Fe 6 and an open sulfide‐bridge with terminal SH on Fe 6 ( E 2 ‐hyd ) or ii) a double belt‐sulfide protonated, reduced cofactor without a hydride ( E 2 ‐nonhyd ). We suggest both models as contenders for the E 2 redox state and further calculate a mechanism for H 2 evolution. The changes in electronic structure of FeMoco during the proposed redox‐state cycle, E 0 →E 1 →E 2 →E 0 , are discussed.