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Issue 16, 2012
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Ramifications of C-centering rather than N-centering of the active site FeMo-co of the enzyme nitrogenase

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Abstract

Nitrogenase catalyses the hydrogenation of N2 to NH3, and of CO to hydrocarbons. The active site is FeMo-co, an Fe7MoS9 cluster with an atom (Xc) at the centre of the inner trigonal prism of six Fe atoms. Calculations extending over almost a decade yielded consensus that this atom was nitrogen. The first strong experimental data, reported very recently, indicate that the central atom is carbon. This paper evaluates differences between C-centered and N-centered FeMo-co, and addresses the questions: (a) does the finding of Cc diminish the validity of the many previous theoretical simulations (with Nc) of the reactivity and reactions of FeMo-co? and (b) does the published 21-step mechanism for N2 + 6H → 2NH3 need major revision? Accordingly, this paper first reports comparative (Cc/Nc) calculations of the electronic structure of FeMo-co, describing the ground and low-lying electronic states, and the distribution of electron spin density and of partial charge. The differences are clear, but minor. Then, reaction profiles and structures of intermediates and transition states are reported for the C-centered and N-centered versions of the four key types of reaction step involved in the overall mechanism: (i) binding of N2, (ii) conformational preparation of H on FeMo-co, (iii) H transfers to N, and (iv) N–N breaking. Again the differences are small, and the calculated activation energies for the previous complete mechanism appear to be essentially transferable to C-centered FeMo-co.

Graphical abstract: Ramifications of C-centering rather than N-centering of the active site FeMo-co of the enzyme nitrogenase

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Publication details

The article was received on 08 Jan 2012, accepted on 31 Jan 2012 and first published on 03 Feb 2012


Article type: Paper
DOI: 10.1039/C2DT00049K
Dalton Trans., 2012,41, 4859-4865

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    Ramifications of C-centering rather than N-centering of the active site FeMo-co of the enzyme nitrogenase

    I. Dance, Dalton Trans., 2012, 41, 4859
    DOI: 10.1039/C2DT00049K

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