Journal
JOURNAL OF INORGANIC BIOCHEMISTRY
Volume 102, Issue 1, Pages 87-100Publisher
ELSEVIER SCIENCE INC
DOI: 10.1016/j.jinorgbio.2007.07.025
Keywords
iron-sulfur proteins; density functional theory; electron transfer; reorganization energy; reduction potential
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This work reports density functional computations of metal-substituted models of biological [Fe3S4] clusters in oxidation states [MFe2S4](+/0/-1) (M = Mn, Fe, Co, Ni, Cu, Zn, and Mo). Geometry optimization with a dielectric screening model is shown to provide a substantial improvement in structure, compared to earlier used standard procedures. The error for average Fe-S bonds decreased from 0.038 angstrom to 0.016 angstrom with this procedure. Four density functionals were compared, B3LYP, BP86, TPSS, and TPSSh. B3LYP and to a lesser extent TPSSh energies were inconsistent with experiment for the oxidized [Fe3S4](+) cluster. BP86 (and to a slightly lesser extent TPSS) was within expected theoretical and experimental uncertainties for all oxidation states, the only qualitative error being 5 kJ/ mol in favor of the M-s = 3/2 configuration for the [Fe3S4](+) cluster, so BP86 was used for quantitative results. Computed reorganization energies and reduction potentials point directly towards the [Fe3S4] cluster as the superior choice of electron carrier, with the [ZnFe2S4] cluster a close second. In addition, partially and fully Mo-substituted clusters were investigated and found to have very low reorganization energies but too negative reduction potentials. The results provide a direct rationale why any substitution weakens the cluster as an electron carrier, and thus why the [Fe3S4] composition is optimal in the biological clusters. (c) 2007 Elsevier Inc. All rights reserved.
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