Revisiting the Mössbauer Isomer Shifts of the FeMoco Cluster of Nitrogenase and the Cofactor Charge

• Published in: Inorganic Chemistry
• Main Authors: Bjornsson, Ragnar, Neese, Frank, DeBeer, Serena
• Other Authors: Raunvísindastofnun (HÍ), Science Institute (UI), Raunvísindadeild (HÍ), Faculty of Physical Sciences (UI), Verkfræði- og náttúruvísindasvið (HÍ), School of Engineering and Natural Sciences (UI), Háskóli Íslands, University of Iceland
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Chemical Society (ACS) 2017
• Subjects: Inorganic Chemistry; FeMo; Ólífræn efnafræði; Efnasambönd; Iceland
• Online Access: https://hdl.handle.net/20.500.11815/521; https://doi.org/10.1021/acs.inorgchem.6b02540

Despite decades of research, the structure–activity relationship of nitrogenase is still not understood. Only recently was the full molecular structure of the FeMo cofactor (FeMoco) revealed, but the charge and metal oxidation states of FeMoco have been controversial. With the recent identification of the interstitial atom as a carbide and the more recent revised oxidation-state assignment of the molybdenum atom as Mo(III), here we revisit the Mössbauer properties of FeMoco. By a detailed error analysis of density functional theory-computed isomer shifts and computing isomer shifts relative to the P-cluster, we find that only the charge of [MoFe7S9C]1– fits the experimental data. In view of the recent Mo(III) identification, the charge of [MoFe7S9C]1– corresponds to a formal oxidation-state assignment of Mo(III)3Fe(II)4Fe(III), although due to spin delocalization, the physical oxidation state distribution might also be interpreted as Mo(III)1Fe(II)4Fe(2.5)2Fe(III), according to a localized orbital analysis of the MS = 3/2 broken symmetry solution. These results can be reconciled with the recent spatially resolved anomalous dispersion study by Einsle et al. that suggests the Mo(III)3Fe(II)4Fe(III) distribution, if some spin localization (either through interactions with the protein environment or through vibronic coupling) were to take place. We thank E. Bill for valuable discussions and comments on the manuscript. S.D. and F.N. acknowledge the Max Planck Society for funding. This work was supported by the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP/2007-2013) ERC Grant Agreement No. 615414 (S.D.). R.B. acknowledges support from the Icelandic Research Fund, Grant Nos. 141218051 and 162880051 and the Univ. of Iceland Research Fund. Peer Reviewed