Evidence for C–H cleavage by an iron–superoxide complex in the glycol cleavage reaction catalyzed by myo -inositol oxygenase

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Abstract
myo -Inositol oxygenase (MIOX) activates O 2 at a mixed-valent nonheme diiron(II/III) cluster to effect oxidation of its cyclohexan-(1,2,3,4,5,6-hexa)-ol substrate [ myo -inositol (MI)] by four electrons to d -glucuronate. Abstraction of hydrogen from C 1 by a formally (superoxo)diiron(III/III) intermediate was previously proposed. Use of deuterium-labeled substrate, 1,2,3,4,5,6-[ 2 H] 6 -MI (D 6 -MI), has now permitted initial characterization of the C–H-cleaving intermediate. The MIOX·1,2,3,4,5,6-[ 2 H] 6 -MI complex reacts rapidly and reversibly with O 2 to form an intermediate, G, with a g = (2.05, 1.98, 1.90) EPR signal. The rhombic g-tensor and observed hyperfine coupling to 57 Fe are rationalized in terms of a (superoxo)diiron(III/III) structure with coordination of the superoxide to a single iron. G decays to H, the intermediate previously detected in the reaction with unlabeled substrate. This step is associated with a kinetic isotope effect of ≥5, showing that the superoxide-level complex does indeed cleave a C–H(D) bond of MI.