Session: Bioinorganic Chemistry:

Metalloenzymes known as dioxygenases catalyze the oxidation of a wide variety of substrates, including substituted aromatics (ring-cleaving dioxygenases) and thiols (cysteine dioxygenase, CDO). The majority of dioxygenases employ a mononuclear non-heme iron center, but enzymes with other first-row transition metal ions (Mn, Co, Ni) are sometimes active. The proposed O₂ activation mechanisms involve formation of a metal-superoxo adduct, followed by a metal-peroxo species generated via reaction of the superoxo ligand with coordinated substrate. The study of such intermediates has proven difficult due to their short lifetimes in both biological and synthetic environments. Here, we describe the formation and characterization of cobalt-dioxygen adducts of relevance to dioxygenases with catechol, aminophenol, and thiol substrates. The precursor Co(II) complexes consist of hydrotris(pyrazolyl)borate scorpionates and redox-active bidentate ligands featuring -O, -S, and -NH₂ donors. Reaction with O₂ yields cobalt(III)-superoxo and -peroxo species that were characterized with low-temperature electronic absorption, nuclear magnetic and electron paramagnetic resonance, and resonance Raman spectroscopies. The thermodynamic parameters of the O₂ binding reactions were measured at multiple temperatures, and our findings indicate that ligand identity has substantial impact on the O₂binding affinity of cobalt complexes.

Sergey Lindeman

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