Despite a comprehensive study on the biosynthesis and function of nitric oxide, biological metabolism of nitric oxide, especially when its concentration exceeds the cytotoxic level, remains elusive. Oxidation of nitric oxide by O-2 in aqueous solution has been known to yield NO2. On the other hand, a biomimetic study on the metal-mediated conversion of NO to NO2/NO3- via O-2 reactivity disclosed a conceivable pathway for aerobic metabolism of NO. During the NO-to-NO2-/NO3- conversion, transient formation of metal-bound peroxynitrite and subsequent release of (NO2)-N-center dot via O-O bond cleavage were evidenced by nitration of tyrosine residue or 2,4-di-tert-butylphenol (DTBP). However, the synthetic/catalytic/enzymatic cycle for conversion of nitric oxide into a nitrite pool is not reported. In this study, sequential reaction of the ferrous complex [(PMDTA)Fe(kappa(2) -O,O'- NO2)(kappa(1) -O-NO2)] (3; PMDTA = pentamethyldiethylenetriamine) with NO(g), KC8, and O-2 established a synthetic cycle, complex 3 -> {Fe(NO)(2)}(9) DNIC [(PMDTA)Fe(NO)(2)][NO2] (4) -> {Fe(NO)(2)}(10) DNIC [(PMDTA)Fe(NO)(2)] (1) -> [(PMDTA)(NO)- Fe(kappa(2)-O,N-ONOO)] (2) -> complex 3, for the transformation of nitric oxide into nitrite. In contrast to the reported reactivity of metal-bound peroxynitrite toward nitration of DTBP, peroxynitrite-bound MNIC 2 lacks phenol nitration reactivity toward DTBP. Presumably, the [(PMDTA)Fe] core in {Fe(NO)}(8) MNIC 2 provides a mononuclear template for intramolecular interaction between Fe-bound peroxynitrite and Fe-bound NO , yielding Fe-bound nitrite stabilized in the form of complex 3. This [(PMDTA)Fe]-core-mediated concerted peroxynitrite homolytic O-O bond cleavage and combination of the O atom with Fe- bound NO - reveals a novel and effective pathway for NO-to-NO2- transformation. Regarding the reported assembly of the dinitrosyliron unit (DNIU) [Fe(NO)(2)] in the biological system, this synthetic cycle highlights DNIU as a potential intermediate for nitric oxide monooxygenation activity in a nonheme iron system.