Manchester Institute of Biotechnology, The University of Manchester, 131 Princess Street, Manchester M1 7DN, U.K.
Department of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
Inorg Chem. 2023 Sep 11;62(36):14715-14726. doi: 10.1021/acs.inorgchem.3c02089. Epub 2023 Aug 31.
Nitrogenase is a fascinating enzyme in biology that reduces dinitrogen from air to ammonia through stepwise reduction and protonation. Despite it being studied in detail by experimental and computational groups, there are still many unknown factors in the catalytic cycle of nitrogenase, especially related to the addition of protons and electrons and their order. A recent biomimetic study characterized a potential dinitrogen-bridged diiron cluster as a synthetic model of nitrogenase. Using strong acid and reductants, the dinitrogen was converted into ammonia molecules, but details of the mechanism remains unknown. In particular, it was unclear from the experimental studies whether the proton and electron transfer steps are sequential or alternating. Moreover, the work failed to establish what the function of the diiron core is and whether it split into mononuclear iron fragments during the reaction. To understand the structure and reactivity of the biomimetic dinitrogen-bridged diiron complex [(PFeH)(μ-N)] with triphenylphosphine ligands, we performed a density functional theory study. Our computational methods were validated against experimental crystal structure coordinates, Mössbauer parameters, and vibrational frequencies and show excellent agreement. Subsequently, we investigated the alternating and consecutive addition of electrons and protons to the system. The calculations identify a number of possible reaction channels, namely, same-site protonation, alternating protonation, and complex dissociation into mononuclear iron centers. The calculations show that the overall mechanism is not a pure sequential set of electron and proton transfers but a mixture of alternating and consecutive steps. In particular, the first reaction steps will start with double proton transfer followed by an electron transfer, while thereafter, there is another proton transfer and a second electron transfer to give a complex whereby ammonia can split off with a low energetic barrier. The second channel starts with alternating protonation of the two nitrogen atoms, whereafter the initial double proton transfer, electrons and protons are added sequentially to form a hydrazine-bound complex. The latter split off ammonia spontaneously after further protonation. The various reaction channels are analyzed with valence bond and orbital diagrams. We anticipate the nitrogenase enzyme to operate with mixed alternating and consecutive protonation and electron transfer steps.
固氮酶是生物学中一种引人入胜的酶，它通过逐步还原和质子化将空气中的二氮还原为氨。尽管实验和计算小组对其进行了详细研究，但固氮酶的催化循环中仍有许多未知因素，特别是与质子和电子的添加及其顺序有关。最近的仿生研究将一种潜在的二氮桥联二铁簇描述为固氮酶的合成模型。使用强酸和还原剂，将二氮转化为氨分子，但反应机制的细节仍不清楚。特别是，从实验研究中不清楚质子和电子转移步骤是顺序进行还是交替进行。此外，该工作未能确定二铁核的功能以及在反应过程中是否分裂成单核铁片段。为了了解具有三苯基膦配体的仿生二氮桥联二铁配合物[(PFeH)(μ-N)]的结构和反应性，我们进行了密度泛函理论研究。我们的计算方法通过与实验晶体结构坐标、穆斯堡尔参数和振动频率进行验证，结果表明它们具有极好的一致性。随后，我们研究了电子和质子对该体系的交替和连续添加。计算确定了一些可能的反应途径，即同位质子化、交替质子化和配合物解离成单核铁中心。计算表明，整个机制不是一系列纯的电子和质子转移，而是交替和连续步骤的混合物。特别是，第一步反应将从双质子转移开始，然后是电子转移，此后，再进行另一次质子转移和第二次电子转移，形成氨可以以低能量势垒分裂的配合物。第二个通道从两个氮原子的交替质子化开始，然后是初始双质子转移，电子和质子依次添加形成一个肼键合配合物。后者在进一步质子化后自发分裂出氨。用价键和轨道图分析了各种反应途径。我们预计固氮酶将以混合交替和连续的质子化和电子转移步骤进行操作。
