Rybak-Akimova Research Group

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Tufts U.

Kinetic and mechanistic studies:

High-valent iron oxo species are supported by porphyrin ligands and function as reactive intermediates in oxidations catalyzed by heme complexes. The structure and reactivity of multiple iron-based intermediates involved in non-heme systems must be better understood. Mononuclear and dinuclear iron model complexes proved to be very useful in determining the structures of different intermediates formed in enzyme-catalyzed substrate oxidation. One might expect that mechanistic studies of oxygen and peroxide activation at the model complexes would be equally revealing. How are peroxo intermediates formed in both mononuclear and dinuclear model complexes? Is their reactivity in these two types of complexes similar or different? How does this reactivity depend on the protonation state and the coordination mode of the peroxo ligand? How are the high-valent diamond core species formed in dinuclear systems? Are they indeed reactive intermediates, or just the spectroscopically observable, relatively stable species? How can O₂ molecule be activated by the model complexes, producing reactive intermediates? Is there any relationship between the substrate C-H bond strength and the rate of hydrogen atom abstraction by reactive intermediates formed by model complexes? In order to obtain the answers relevant to rapid catalytic reactions, the individual reaction steps (formation of the intermediate from the iron complex and the oxidant, and the interaction between the intermediate and the substrate) should also be rapid. Thus, multi-mixing stopped-flow technique is applicable to the systems of interest, and low temperatures may help uncovering the details of formation and reactivities of short-lived, unstable intermediates.

Detailed low-temperature single- and multi-mixing stopped-flow kinetic studies provide an information on the rates of formation of reactive intermediates, and the rates of their interactions with the substrates. These demanding and challenging studies (low temperatures, fast reaction rates, anaerobic conditions) are currently underway. Mechanism of a reaction between a dinuclear iron complex with an aminopyridine ligand TPA and hydrogen peroxide has been studied, revealing transient formation of a novel η¹ peroxo intermediate followed by a relatively more stable Fe(III)Fe(IV) intermediate.The reactivity of these intermediates with substrates proved to be different, a dinuclear end-on peroxo complex is far more reactive that a high-valent Fe(III)Fe(IV) species. In contrast, mononuclear peroxo complexes of Fe(III) appear to be less reactive with the substrates.

Mechanism of oxygenation of the first functional model of ribonucleotide reductase was investigated in collaboration with L.Que, Jr. (University of Minnesota). The rate limiting step of this process is the first Fe - O₂ bond formation. This is an entropically controlled, low-barrier process. Some decrease in the steric bulk about Fe(II) coordination sites is expected to allow for facile oxygenation of dinuclear Fe(II) model complexes. The oxygenation kinetics of a series of dinuclear iron(II) complexes with sterically hindered carboxylates is being studied in collaboration with L. Que and W. Tolman research groups (University of Minnesota).