Metal substitution studies of an extradiol dioxygenase and X-Ray absorption studies of high-valent nonheme iron complexes.

Abstract

An understanding of the mechanisms by which Nature employs mononuclear nonheme iron centers and dioxygen to carry out biological oxidative transformations of substantial biological and environmental relevance is of great fundamental and practical interest. Direct biochemical studies of nonheme iron oxygenases themselves can be complemented by spectroscopic studies of small model complexes to afford significant mechanistic insight, and this work employs a combination of these approaches. Substitution of Fe(II) into the active center of a Mn(II)-dependent extradiol dioxygenase under aerobic expression conditions leads to in vivo formation of a selfhydroxylated form of the enzyme that is not observed for the native Mn(II) form, and we suggest that this reflects a significantly lowered redox potential for the Fe(II) form. We have also found that Co(II) substitution into an Fe(II)-dependent extradiol dioxygenase affords active enzyme, with X-ray absorption spectroscopy (XAS) providing metrical parameters for the resting state of this enzyme and its complexes with substrates. XAS studies were carried out for an assortment of high-valent nonheme iron complexes of relevance to the oxoiron(IV) intermediates proposed in the catalytic cycles of mononuclear nonheme iron oxygenases. Studies of a series of pseudo-octahedral lowspin oxoiron(IV) complexes employing pentadentate ligands with a pendant axial ligand trans to the oxo moiety demonstrated that the properties of the oxoiron unit were not significantly affected by the identity of the trans ligand, in agreement with earlier observations on chemically similar systems. We also characterized the products of oxo transfer to a thiolate-ligated nonheme iron(II) complex, providing structural evidence for conversion of the thiolate to an O-bound sulfinate. We structurally characterized the first example of a high-spin oxoiron(IV) complex in a trigonal bipyramidal (TBP) geometry, and also examined the XAS properties of a series of TBP iron complexes with different axial ligands as part of efforts to provide a basis for interpretation of the spectroscopic properties of TBP iron centers. Finally, in studies of a diiron(IV) complex of relevance to intermediate Q in methane monooxygenase, we demonstrated that the μ-oxo bridge of its diiron(III) precursor is retained upon oxidation.