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Metal substitution in the active site of nitrogenase MFe7S9 (M = Mo(4+), V(3+), Fe(3+))

Academic Article
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Overview

authors

  • Lovell, T.
  • Torres, R. A.
  • Han, W. G.
  • Liu, T. Q.
  • Case, David A.
  • Noodleman, Louis

publication date

  • November 2002

journal

  • Inorganic Chemistry  Journal

abstract

  • 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-ion 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 (4Fe(2+)4Fe(3+) or 6Fe(2+)2Fe(3+)) for the iron-only cofactor (FeFeco) best represents the resting state. For the 6Fe(2+)2Fe(3+) 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 S(a) = (7)/(2) antiferromagnetically coupled to a 4Fe' cluster with spin S(b) = (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 4Fe(2+)4Fe(3+) (derived from experimental Mössbauer isomer shifts), our isomer shift calculations for the 4Fe(2+)4Fe(3+) oxidation state are in poorer agreement with experiment. Using the Mo(4+)6Fe(2+)Fe(3+) oxidation level of the cofactor as a starting point, the structural consequences of replacement of molybdenum (Mo(4+)) with vanadium (V(3+)) or iron (Fe(3+)) 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.

subject areas

  • Absorptiometry, Photon
  • Azotobacter vinelandii
  • Cobalt
  • Crystallography, X-Ray
  • Iron
  • Isomerism
  • Metalloproteins
  • Molybdenum
  • Nitrogenase
  • Oxidation-Reduction
  • Proteins
  • Vanadium
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Identity

International Standard Serial Number (ISSN)

  • 0020-1669

Digital Object Identifier (DOI)

  • 10.1021/ic020474u

PubMed ID

  • 12401079
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Additional Document Info

start page

  • 5744

end page

  • 5753

volume

  • 41

issue

  • 22

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