Science Institute, University of Iceland, Dunhagi 3, 107, Reykjavik, Iceland.
Department of Inorganic Spectroscopy, Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.
Chemistry. 2021 Dec 1;27(67):16788-16800. doi: 10.1002/chem.202102730. Epub 2021 Oct 15.
The iron-molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E , reduced states of FeMoco are much less well characterized. The E state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E state can, however, relax back the E state via a H side-reaction, implying a hydride intermediate prior to H formation. This E →E pathway is one of the primary mechanisms for H formation under low-electron flux conditions. In this study we present an exploration of the energy surface of the E state. Utilizing both cluster-continuum and QM/MM calculations, we explore various classes of E 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 state. These models feature either i) a bridging hydride between Fe and Fe and an open sulfide-bridge with terminal SH on Fe (E -hyd) or ii) a double belt-sulfide protonated, reduced cofactor without a hydride (E -nonhyd). We suggest both models as contenders for the E redox state and further calculate a mechanism for H evolution. The changes in electronic structure of FeMoco during the proposed redox-state cycle, E →E →E →E , are discussed.
铁-钼辅因子（FeMoco）负责钼氮酶中的二氮还原。与静息态 E 相比，还原态的 FeMoco 特征研究较少。据推测，E 态可能含有一个氢化物，但目前仍缺乏直接的光谱证据。然而，E 态可以通过 H 侧反应弛豫回 E 态，这意味着在 H 形成之前存在一个氢化物中间体。这种 E→E 途径是低电子通量条件下 H 形成的主要机制之一。在这项研究中，我们探索了 E 态的能量表面。利用簇连续体和 QM/MM 计算，我们探索了各种 E 模型的类别：包括末端氢化物、具有闭合或开放硫桥的桥接氢化物，以及没有氢化物的模型。重要的是，我们发现质子化的带硫桥的半配位能力强烈影响氢化物的稳定性。令人惊讶的是，非氢化物模型几乎与氢化物模型同样有利。虽然簇连续体计算表明有多种可能性，但 QM/MM 只建议两种模型作为 E 态的候选者。这些模型的特征是：i）Fe 和 Fe 之间的桥接氢化物和具有末端 SH 的开放硫桥（E-hyd），或 ii）双带硫桥质子化、还原辅因子无氢化物（E-nonhyd）。我们建议这两种模型都可以作为 E 氧化还原态的候选者，并进一步计算 H 演化的机制。讨论了在提议的氧化还原态循环 E→E→E→E 期间 FeMoco 电子结构的变化。
