Nitrite Formation at a Diiron Dinitrosyl Complex

Pathogenic bacteria employ iron-containing enzymes to detoxify nitric oxide (NO center dot) produced by mammals as part of their immune response. Two classes of diiron proteins, flavodiiron nitric oxide reductases (FNORs) and the hemerythrin-like proteins from mycobacteria (HLPs), are upregulated in bacteria in response to an increased local NO center dot concentration. While FNORs reduce NO center dot to nitrous oxide (N2O), the HLPs have been found to either reduce nitrite to NO center dot (YtfE), or oxidize NO center dot to nitrite (Mka-HLP). Various structural and functional models of the diiron site in FNORs have been developed over the years. However, the NO center dot oxidation reactivity of Mka-HLP has yet to be replicated with a synthetic complex. Compared to the FNORs, the coordination environment of the diiron site in Mka-HLP contains one less carboxylate ligand and, therefore, is expected to be more electron-poor. Herein, we synthesized a new diiron complex that models the electron-poor coordination environment of the Mka-HLP diiron site. The diferrous precursor (FeFeII)-Fe-II reacts with NO center dot to form a diiron dinitrosyl species ({FeNO}(7)(2)), which is in equilibrium with a mononitrosyl diiron species (Fe-II{FeNO}(7)) in solution. Both complexes can be isolated and fully characterized. However, only oxidation of {FeNO}(7)(2) produced nitrite in high yield (71%). Our study provides the first model that reproduces the NO center dot oxidase reactivity of Mka-HLP and suggests intermediacy of an {FeNO}(6)/{FeNO}(7) species.