Abstract
Carbon monoxide (CO) is a well-known inhibitor of nitrogenase activity. Under turnover conditions, CO binds to FeMoco, the active site of Mo nitrogenase. Time-resolved IR measurements suggest an initial terminal CO at 1904 cm–1 that converts to a bridging CO at 1715 cm–1, and an X-ray structure shows that CO can displace one of the bridging belt sulfides of FeMoco. However, the CO-binding redox state(s) of FeMoco (En) and the role of the protein environment in stabilizing specific CO-bound intermediates remain elusive. In this work, we carry out an in-depth analysis of the CO–FeMoco interaction based on quantum chemical calculations addressing different aspects of the electronic structure. (1) The local electronic structure of the Fe–CO bond is studied through diamagnetically substituted FeMoco. (2) A cluster model of FeMoco within a polarizable continuum illustrates how CO binding may affect the spin-coupling between the metal centers. (3) A QM/MM model incorporates the explicit influence of the amino acid residues surrounding FeMoco in the MoFe protein. The QM/MM model predicts both a terminal and a bridging CO in the E1 redox state. The scaled calculated CO frequencies (1922 and 1716 cm–1, respectively) are in good agreement with the experimentally observed IR bands supporting CO binding to the E1 state. Alternatively, an E2 state QM/MM model, which has the same atomic structure as the CO-bound X-ray structure, features a semi-bridging CO with a scaled calculated frequency (1718 cm–1) similar to the bridging CO in the E1 model.
Highlights
Nitrogenases are a group of enzymes that can reduce chemically inert N2 to bioavailable ammonia
FeS clusters usually maintain a local high spin in the weak ligand field of the S2− ligands. It is not a priori evident how the interaction between Carbon monoxide (CO) and the Fe centers in FeMoco relates to these two extremes, and it is instructive to discuss the influence of a CO ligand on the local electronic structure of an Fe center in the chemical environment of FeMoco
Diamagnetic substitution generates Fe−CO fragments that are more relevant for a CO−FeMoco adduct than, for example, model complexes with multiple CO ligands coordinated to the same Fe center
Summary
Nitrogenases are a group of enzymes that can reduce chemically inert N2 to bioavailable ammonia. In the substrate-free FeMoco, the members of the BS7 class (BS7235, BS7-247, and BS7-346) have been shown to be the lowest in energy,[18,77−79] which has been rationalized through the high number of antiferromagnetically aligned Fe pairs.[76,95] the study of the diamagnetically substituted cofactor showed that a terminally bound CO reduces the coupling constant between Fe centers by inducing local spin-pairing. CO binding to FeMoco in the wild-type MoFe protein generally requires turnover conditions Consistent with this observation, the calculated CO frequencies in model (1966 and 1957 cm−1 for Fe6 an Fe2, the E0 QM/MM respectively) are about 50 cm−1 higher than the initial, experimental band at. This once more emphasizes the importance of applying multiple and ideally orthogonal experimental techniques when characterizing a given nitrogenase species
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