Unraveling the reactive species of a functional non-heme iron monooxygenase model using stopped-flow UV-Vis spectroscopy

Low-temperature stopped-flow electronic spectroscopy was utilized to resolve the intermediates formed in the reaction of a diiron(II) compound, Fe-2(H(2)Hbamb)(2)(N-Melm)(2) (H(4)HBamb = 2,3-bis(2-hydroxybenzamido)dimethylbutane), 1, with the oxygen atom donors 2,6-dimethyliodosylbenzene and p-cyanodimethylaniline N-oxide and the mechanistic probe hydroperoxide 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH). Previous studies showed that 1 is capable of catalytically oxidizing cyclohexane to cyclohexanol (300 turnovers) via a pathway involving the heterolytic cleavage of the O-O bond of MPPH (> 98% peroxide utilization). We now report intimate details of the formation of the reactive intermediate and its subsequent decay in the absence of substrates. The reaction, which is independent of the nature of the oxidant, proceeds in three consecutive steps assigned as (i) oxygen-atom transfer to one of the iron centers of 1 to form an Fe-IV=O species, 2, (ii) ligand rearrangement to 3, and (iii) internal collapse of the terminal oxo group to generate a diferric, mu-oxo species, 4. Assignment of the second step as a ligand rearrangement was corroborated by stopped-flow spectroscopic studies of the one-electron oxidation of the starting diferrous 1, which is also known to undergo ligand rearrangement upon the formation of the [Fe-II, Fe-III] mixed-valent complex. Observation of the reaction rates over a temperature range allowed for the determination of activation parameters for each of the three steps. The role of the ligand reorganization in the energetic profile for the formation of the catalytically competent intermediate is discussed, along with the potential biological significance of the internal conversion of the active oxidant to the inert, mu-oxo diiron(III) dimer, 4.