The Role of Nonheme Transition Metal-Oxo, -Peroxo, and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes

Abstract

Transition metals are common co-factors in enzymes and enable catalysis to take place via reaction barriers that are accessible at room temperature. Oxygen activating metalloenzymes are versatile species in Nature involved in vital processes ranging from biodegradation to biosynthesis. Since oxygen activating intermediates are not readily amenable to experimental study, research has started to focus on biomimetic model systems that have the active site coordination sphere and structural features, but react in solution. In our research group, we have been involved in computational modeling of heme and nonheme iron dioxygenases as well as biomimetic models of these complexes. In this contribution an overview is given on recent results of the characterization and reactivity patterns of metaloxo, metal-peroxo and metal-superoxo complexes. In particular, in recent studies attempts were made to trap and characterize the short-lived oxygen bound intermediate in the catalytic cycle of cysteine dioxygenase. Many suggested structures could be ruled out by theoretical considerations, yet these also provided suggestions of possible candidates for the experimentally observed spectra. In addition, we review recent studies on the nonheme iron(III)-hydroperoxo species and how its reactivity patterns with arenes are dramatically different from those found for heme iron(III)-hydroperoxo species. The final two projects show a series of computational studies on manganese(V)-oxo and side-on manganese(III)-peroxo moieties that identify a unique spin state reactivity pattern with a surprising product distribution.3

Tuning Electron-Transfer Reactivity of a Chromium(III)–Superoxo Complex Enabled by Calcium Ion and Other Redox-Inactive Metal Ions

“…Redox-inactive metal ions that function as Lewis acids are essential cofactors in modulating the reactivity of metal–oxygen complexes and metalloenzymes, such as the oxygen-evolving complex (OEC) in Photosystem II, where Ca 2+ ion in the Mn 4 CaO 5 cluster is indispensable for water oxidation. − In the catalytic water oxidation in the OEC as well as in the reverse reaction, which is the four-electron reduction of O 2 in respiration, metal–oxygen complexes, such as metal–oxo, metal–peroxo, and metal–superoxo species, are involved as reactive intermediates. − The effects of redox-inactive metal ions, including Ca 2+ ion, on the redox reactivity of high-valent metal–oxo species, such as Fe­(IV)–oxo and Mn­(IV)–oxo complexes, have been studied extensively. − Similarly, calcium ion and other redox-inactive metal ions are shown to modulate the redox reactivity of metal–peroxo complexes, such as Fe­(III)–peroxo complexes. − …”