Resonance Raman studies of the iron(II)-α-keto acid chromophore in model and enzyme complexes

The bidentate coordination of an α-keto acid to an iron(II) center via the keto group and the carboxylate gives rise to metal-to-ligand charge-transfer transitions between 400 and 600 nm in model complexes and 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 resonance Raman features at 460 and 1686 cm^-1, both of which are sensitive to ¹⁸O labeling. Corresponding studies of model complexes, the six-coordinate [Fe(II)(6-Me₃-TPA)(α-keto acid)]⁺ and the five-coordinate [Fe(II)(Tp^Ph2)(α-keto acid)] (6-Me₃-TPA = tris[(6-methyl-2-pyridyl)methyl]amine, Tp^Ph2 = 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 the v(C=O) of the keto carbonyl group, respectively. Furthermore, the chelate mode is sensitive to the coordination number of the metal center; binding of a sixth ligand to the five-coordinate [Fe(II)(Tp^Ph2)(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 metal center to a five-coordinate center, as observed for the iron center of clavaminate synthase from X-ray crystallography (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 the oxygen activation mechanism of α-ketoglutarate-dependent dioxygenases.