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

Nitrite reductase (NiR) catalyzes nitrite (NO(2)(−)) 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 NO(2)(−), in the presence of H(+), generates NO with a H(2)O 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 NO(2)(−) reduction, we report the acid-induced NO(2)(−) reduction chemistry of a nonheme Fe(II)-nitrito complex, [(12TMC)Fe(II)(NO(2)(−))](+) (Fe(II)–NO(2)(−), 2), with variable amounts of H(+). Fe(II)–NO(2)(−) upon reaction with one-equiv. of acid (H(+)) generates [(12TMC)Fe(NO)](2+), {FeNO}(7) (3) with H(2)O(2) rather than H(2)O. However, the amount of H(2)O(2) decreases with increasing equivalents of H(+) and entirely disappears when H(+) reaches ≅ two-equiv. and shows H(2)O formation. Furthermore, we have spectroscopically characterized and followed the formation of H(2)O(2) (H(+) = one-equiv.) and H(2)O (H(+) ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H(2)O. Mechanistic investigations, using (15)N-labeled-(15)NO(2)(−) and (2)H-labeled-CF(3)SO(3)D (D(+) source), revealed that the N atom in the {Fe(14/15)NO}(7) is derived from the NO(2)(−) ligand and the H atom in H(2)O or H(2)O(2) is derived from the H(+) source, respectively.