Coordination chemistry of mononuclear non-heme iron oxygenase enzymes: probing differential or carboxylate and phenolate ligation through functional synthetic model systems
Tarves, Paul C
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Mononuclear non-heme iron oxygenase (MNO) enzymes utilize ferrous iron and dioxygen to perform a variety of thermodynamically challenging reactions at standard temperatures and pressures. The potent oxidizing power of these enzymatic systems has led to increased interest from the bioinorganic and synthetic organic communities. Presented herein is the preparation and characterization of an α-keto acid dependent synthetic system that closely models the active site electronic and dioxygen reactivity properties of the FeII/α-ketoglutarate dependent class of MNH iron oxygenase enzymes. The ferrous complex utilized possesses a facially coordinating N,N,O-donor ligand reminiscent of a common active site motif observed for MNO iron enzymes. The labile coordination sites opposite the ligand framework allow for the ligation of exogenous α-keto acid cofactor as well as the binding and activation of dioxygen. The coordination of exogenous α-keto acid cofactor has been shown to greatly enhance the rate of dioxygen reactivity of the ferrous complex and lead to the catalytic decarboxylation of the cofactor. The enhancement in rate is attributed to the coupling of the dioxygen reduction step to the oxidative decarboxylation of the bound cofactor, which is a thermodynamically favorable process. The oxidative decarboxylation pathway suggests the formation of a high valent iron-oxo intermediate, which has been further supported by the concentration dependence of solvent oxidation during catalysis. The mechanism of dioxygen reactivity was further probed by Hammett analysis using substituted aromatic α-keto acid cofactors. The data presented suggest that the model system prepared proceeds via a biomimetic mechanism capable of catalytic dioxygen activation and substrate oxidation under ambient conditions. Investigation of differential carboxylate and phenolate ligation as it pertains to MNO iron enzymes is also reported. The synthesis and characterization of both ferrous and ferric compounds containing ligands with similar ethylene diamine backbones and either one or two phenolate moities: 2-(((2-(dimethylarnino)ethyl)(methyl)amino)-methyl)phenol (N2O1-Ph) and 2,2'-((ethane-1,2-diylbis(methylazanediyl))bis-(methylene))diphenol (N2O2-Ph). The replacement of carboxylate moiety with a phenolate led to a significant decrease in reduction potential and subsequent enhancement in dioxygen sensitivity. This observation may provide insight into the reactivity of other iron containing enzymes with coordinated tyrosine residues, such as intradiol catechol dioxygenases.
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