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Radical Rebound Hydroxylation Versus H-Atom Transfer in Non-Heme Iron(III)-Hydroxo Complexes: Reactivity and Structural Differentiation

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posted on 10.04.2019, 16:06 by Michael J. Drummond, Courtney L. Ford, Danielle L. Gray, Codrina V. Popescu, Alison R. Fout
The characterization of high-valent iron centers in enzymes has been aided by synthetic model systems that mimic their reactivity or structural and spectral features. For example, the cleavage of dioxygen often produces an iron­(IV)-oxo that has been characterized in a number of enzymatic and synthetic systems. In non-heme 2-oxogluterate dependent (iron-2OG) enzymes, the ferryl species abstracts an H-atom from bound substrate to produce the proposed iron­(III)-hydroxo and caged substrate radical. Most iron-2OG enzymes perform a radical rebound hydroxylation at the site of the H-atom abstraction (HAA); however, recent reports have shown that certain substrates can be desaturated through the loss of a second H atom at a site adjacent to a heteroatom (N or O) for most native desaturase substrates. One proposed mechanism for the removal of the second H-atom  involves a polar-cleavage mechanism (electron transfer-proton transfer) by the iron­(III)-hydroxo, as opposed to a second HAA. Herein we report the synthesis and characterization of a series of iron complexes with hydrogen bonding interactions between bound aquo or hydroxo ligands and the secondary coordination sphere in ferrous and ferric complexes. Interconversion among the iron species is accomplished by stepwise proton or electron addition or subtraction, as well as H-atom transfer (HAT). The calculated bond dissociation free energies (BDFEs) of two ferric hydroxo complexes, differentiated by their noncovalent interactions and reactivity, suggest that neither complex is capable of activating even weak C–H bonds, lending further support to the proposed mechanism for desaturation in iron-2OG desaturase enzymes. Additionally, the ferric hydroxo species are differentiated by their reactivity toward performing a radical rebound hydroxylation of triphenylmethylradical. Our findings should encourage further study of the desaturase systems that may contain unique H-bonding motifs proximal to the active site that help bias substrate desaturation over hydroxylation.