First Evidence for a Stable, Aqueous Nitroxyl on a Non-Heme Iron Platform.

MONTENEGRO, ANDREA; BARI, SARA E.; RONCAROLI, FEDERICO; MURGIDA, DANIEL; OLABE, JOSÉ

Montevideo, Uruguay

Congreso; Free Radicals in Montevideo; 2007

First Evidence for a Stable, Aqueous Nitroxyl on a Non-Heme Iron Platform.             The coordination chemistry of nitroxyl (HNO/NO-) in iron complexes is relevant to several processes regarding nitrogen metabolism. Although the coordination to different metals (Re, Ru, Os, Ir) has been described, 1 the iron complexes remain elusive. Only the myoglobin complex (MbHNO) has been characterized showing a surprising stability, traced to the interaction of the HNO with proximal aminoacids in the heme cavity.2             We focused in the characterization of bound HNO, by reducing nitroprusside ([Fe(CN)5NO]2-, NP) with sodium dithionite, in the presence of excess cyanide (pH 10, anaerobiosis). There is a precedent on the two-electron electrochemical reduction of NP, which leads to an ill defined product.3 Redox titration led to [Fe(CN)5NO•]3– and to a new complex, P445 (lmax, 445 nm; ε, 3300 M–1 cm–1), respectively. The reduction process was reversible upon sequential addition of hexacyanoferrate(III), suggesting the following equilibria:   [Fe(CN)5NO]2– ↔ [Fe(CN)5NO•]3– ↔ [Fe(CN)5HNO]3–               Accordingly, a quantitative comproportionation reaction was observed upon addition of one equivalent of NP to P445, yielding two equivalents of [Fe(CN)5NO•]3-. A pKa = 11.0 has been estimated spectrofotometrically for P445. The absorption of P445 completely decayed at pH 12, and was recovered upon acidification, probably due to [Fe(CN)5HNO]3–↔[Fe(CN)5(NO-)]3–.             The assignment of P445 to [Fe(CN)5HNO]3– is also supported by the proposed MLCT character of the intense absorption band.. Resonance Raman spectra (irradiation near 445 nm) displayed typical bands for the cyano-ligands (2050-2100 cm–1), and for the N=O (1300-1400 cm–1). The latter shifted downwards upon labeling with 15N. DFT calculations also support the FeHNO complex.4             The proposed [Fe(CN)5HNO]3– ion is robust toward dissociation at pH 10 (hours time scale), but decays at lower pH or in the presence of oxygen. Kinetic reactivity studies of bound HNO and NO- are underway.   1. Farmer, P. J.; Sulc, F. J. Inorg. Biochem. 2005, 99, 166. 2. Sulc F.; Immoos C.; Pervitsky D.; Farmer P. J. J Am Chem Soc. 2004, 126, 1096. 3. Masek, J.; Maslova, E. Coll. Czech. Chem. Commun. 1974, 39, 2141. 4. González Lebrero, M.; Scherlis, D.; Estiú, G.; Olabe, J. A.; Estrín, D. Inorg. Chem. 2001, 40, 4127