Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions

Nitrite reductase (NiR) catalyzes nitrite (NO₂ ^-) to nitric oxide (NO) transformation in the presence of an acid (H⁺ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO₂ ^-, in the presence of H⁺, generates NO with a H₂O molecule utilizing two H⁺ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H⁺ ions on NO₂ ^- reduction, we report the acid-induced NO₂ ^- reduction chemistry of a nonheme Fe^II-nitrito complex, [(12TMC)Fe^II(NO₂ ^-)]⁺ (Fe^II-NO₂ ^-, 2), with variable amounts of H⁺. Fe^II-NO₂ ^- upon reaction with one-equiv. of acid (H⁺) generates [(12TMC)Fe(NO)]²⁺, {FeNO}⁷ (3) with H₂O₂ rather than H₂O. However, the amount of H₂O₂ decreases with increasing equivalents of H⁺ and entirely disappears when H⁺ reaches ≅ two-equiv. and shows H₂O formation. Furthermore, we have spectroscopically characterized and followed the formation of H₂O₂ (H⁺ = one-equiv.) and H₂O (H⁺ ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H₂O. Mechanistic investigations, using ¹⁵N-labeled-¹⁵NO₂ ^- and ²H-labeled-CF₃SO₃D (D⁺ source), revealed that the N atom in the {Fe^14/15NO}⁷ is derived from the NO₂ ^- ligand and the H atom in H₂O or H₂O₂ is derived from the H⁺ source, respectively.