The unifying view that molybdenum is the essential component in nitrogenase has changed over the past few years with the discovery of a vanadium-containing nitrogenase and an iron-only nitrogenase. The principal question that has arisen for the alternative nitrogenases concerns the structures of their corresponding cofactors and their metal valence assignments and whether there are significant differences with that of the more widely known molybdenum-iron cofactor (FeMoco). Spin-polarized broken-symmetry (BS) density functional theory (DFT) calculations are used to assess which of the two possible metal-ion valence assignments (4Fe2+4Fe3+ or 6Fe2+-2Fe3+) for the iron-only cofactor (FeFeco) best represents the resting state. For the 6Fe2+2Fe3+ oxidation state, the spin coupling pattern for several spin state alignments compatible with S = 0 were generated and assessed by energy criteria. The most likely BS spin state is composed of a 4Fe cluster with spin Sa = 7/2 antiferromagnetically coupled to a 4Fe′ cluster with spin Sb = 7/2. This state has the lowest DFT energy for the isolated FeFeco cluster and displays calculated Mössbauer isomer shifts consistent with experiment. Although the S = 0 resting state of FeFeco has recently been proposed to have metal-ion valencies of 4Fe2+4Fe3+ (derived from experimental Mössbauer isomer shifts), our isomer shift calculations for the 4Fe2+4Fe3+ oxidation state are in poorer agreement with experiment. Using the Mo4+6Fe2+Fe3+ oxidation level of the cofactor as a starting point, the structural consequences of replacement of molybdenum (Mo4+) with vanadium (V3+) or iron (Fe3+) in the cofactor have been investigated. The size of the cofactor cluster shows a dependency on the nature of the heterometal and increases in the order FeMoco < FeVco < FeFeco.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry