Nitric oxide reduction forming hyponitrite triggered by metal-containing complexes

Nitric oxide reduction yielding N2O is known as a route to detoxify nitric oxide (NO) to relieve nitrosative stress in pathogenic bacteria and fungi. Nitric oxide enzymes are classified into Cu/Fe-heme NO reductases (NORs) and non-heme flavindiiron NO reductases (FNORs). In biological system, the mechanism of NO reduction generating N2O was proposed to involve NO coordination to metal centers prior to producing cis/trans-hyponitrite-bound intermediate, and the subsequent protonation of hyponitrite-bound-Fe/Cu intermediates releases N2O. In this review article, we compile the recently published biomimetic model studies of NO-to-[N2O2](2-) transformation triggered by the designed transition-metal complexes. In biomimetic model study, the ON-NO bond coupling of metal-nitrosyl complexes yielding [N2O2](2-)-bound species may occur via either the inter/intramolecular radical-[NO](-)-radical-[NO](-) coupling or metal-[NO](2-) radical coupling with exogenous NO. The H-bonding interaction between hyponitrite and protic solvents promoting/stabilizing the formation of hyponitrite complexes was also demonstrated. In addition, the electronic structure of the designed transition-metal-nitrosyl complexes triggering the formation of [N2O2](2-)-bound species and the detailed NO-to-[N2O2](2-) formation pathways were delineated.