Abstract
| - The bidentate coordination of an α-keto acid to an iron(II) center via the keto group and thecarboxylate gives rise to metal-to-ligand charge-transfer transitions between 400 and 600 nm in model complexesand in α-ketoglutarate-dependent dioxygenases. Excitation into these absorption bands of the Fe(II)TauD(α-KG) complex (TauD = taurine/α-ketoglutarate dioxygenase, α-KG = α-ketoglutarate) elicits two resonanceRaman features at 460 and 1686 cm-1, both of which are sensitive to 18O labeling. Corresponding studies ofmodel complexes, the six-coordinate [Fe(II)(6-Me3-TPA)(α-keto acid)]+ and the five-coordinate [Fe(II)(TpPh2)(α-keto acid)] (6-Me3-TPA = tris[(6-methyl-2-pyridyl)methyl]amine, TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate), lead to the assignment of these two features to the Fe(II)(α-keto acid) chelate mode and theν(CO) of the keto carbonyl group, respectively. Furthermore, the chelate mode is sensitive to the coordinationnumber of the metal center; binding of a sixth ligand to the five-coordinate [Fe(II)(TpPh2)(benzoylformate)]elicits a 9−20 cm-1 downshift. Thus, the 10 cm-1 upshift of the chelate mode observed for Fe(II)TauD(α-KG) upon the addition of the substrate, taurine, is associated with the conversion of the six-coordinate metalcenter to a five-coordinate center, as observed for the iron center of clavaminate synthase from X-raycrystallography (Zhang, Z.; et al. Nat. Struct. Biol. 2000, 7, 127−133) and MCD studies (Zhou, J.; et al. J.Am. Chem. Soc.1998, 120, 13539−13540). These studies provide useful insights into the initial steps of theoxygen activation mechanism of α-ketoglutarate-dependent dioxygenases.
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