A DFT Study of the cis-Dihydroxylation of Nitroaromatic Compounds Catalyzed by Nitrobenzene Dioxygenase

The mechanism of cis-dihydroxylation of nitrobenzene and 2-nitrotoluene catalyzed by nitrobenzene 1,2-dioxygenase (NBDO), a member of the naphthalene family of Rieske non-heme iron dioxygenases, was studied by means of the density functional theory method using four models of the enzyme active site. Different possible reaction pathways for the substrate dioxygenation initiated either by the Fe-III-OOH or HO-Fe-V=O attack on the aromatic ring were considered and the computed activation barriers compared with the Gibbs free energy of activation for the oxygen oxygen cleavage leading to the formation of the iron(V) oxo species from its ferric hydroperoxo precursor. The mechanism of the substrate cis-dihydroxylation leading to the formation of a cis-dihydrodiol was then investigated, and the most feasible mechanism was found to be starting with the attack of the high-valent iron-oxo species on the substrate ring yielding a radical intermediate, which further evolves toward the final product.