Modeling oxygen activation at mononuclear nonheme iron(II) alpha-keto acid-dependent dioxygenases

Mononuclear high-spin iron(II) complexes were prepared to examine oxygen activation at nonheme iron active sites. The bidentate chelation of the α-keto carboxylate to an iron(II) center gives rise to metal-to-ligand charge-transfer transitions in both model complexes and enzymatic systems. Analysis of the Raman features of the chelated iron(II)-(α-keto carboxylate) in both systems demonstrates a susceptibility to nucleophilic attack and a sensitivity to the metal coordination number. Substrate binding leads to a coordinatively unsaturated metal center readied for interaction with dioxygen.; For the [FeII(TpPh2)(X)] complexes, both the α-keto carboxylate and carboxylate compounds react with dioxygen resulting in the hydroxylation of a single ortho phenyl position of the TpPh ₂ ligand. The hydroxylation of the ligand phenyl ring occurs with concomitant oxidative decarboxylation of the α-keto acid. Isotope labeling studies show that 18O₂ ends up in the TpPh2* phenolate oxygen and the carboxylate derived from the α-keto acid. The isotope incorporation mirrors the dioxygenase nature of the enzymatic systems. A mechanism is proposed in which an iron(III)-superoxo intermediate is formed upon the binding of dioxygen, which then does a nucleophilic attack at the activated α-keto carbonyl forming a peroxyhemiketal intermediate. Interestingly, the carboxylate complexes can also carry out this aryl hydroxylation chemistry. Isotope labeling studies demonstrate that the oxygen in the hydroxylated ligand is also derived from molecular oxygen. However, the α-keto carboxylate complexes react two orders of magnitude faster than the carboxylate complexes, thus emphasizing the key role that the α-keto function plays in oxygen activation by α-keto acid-dependent iron enzymes. In an attempt to gain more information about possible high-valent iron(IV)oxo intermediates, a study has been initiated to examine the reacitivty of the air stable [Fe II(TpPh2)(OTf)(CH₃CN)] complex with alternative oxidants.; The aryl hydroxylation in the model complexes is similar to the observed aromatic amino acid hydroxylation activity of α-keto acid-dependent enzymes in the absence of substrate. Here evidence is provided for post-translational modification of a tyrosine residue to form dihydroxyphenylalanine in the iron metalloenzyme, (S)-2-hydroxypropylphosphonic acid epoxidase (HppE). Resonance Raman studies of the iron(III)-catecholate chromophore have led to a proposal for the biogenesis of this colored protein.