Rational tuning of the thiolate donor in model complexes of superoxide reductase: Direct evidence for a trans influence in FeIII-OOR complexes

Frances Namuswe, Gary D. Kasper, Amy A. Narducci Sarjeant, Takahiro Hayashi, Courtney M. Krest, Michael T. Green, Pierre Moënne-Loccoz, David P. Goldberg

Research output: Contribution to journalArticle

48 Scopus citations

Abstract

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))FeII complexes that react with tert-butyl hydroperoxide (tBuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)FeIII-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the FeII complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the FeIII-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the FeIII-OOR unit comes from resonance Raman (RR) spectroscopy, which shows ν(Fe-O) modes between 600-635 cm-1 and ν(O-O) bands near 800 cm-1. These Fe-O stretching frequencies are significantly lower than those found in other low-spin FeIII-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.

Original languageEnglish (US)
Pages (from-to)14189-14200
Number of pages12
JournalJournal of the American Chemical Society
Volume130
Issue number43
DOIs
StatePublished - Oct 29 2008

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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