Disproportionation of hydroxylamine by water-soluble iron(III) porphyrinate compounds

• Published in: Journal of inorganic biochemistry Vol. 104; no. 1; pp. 30 - 36
• Main Authors: Bari, Sara E., Amorebieta, Valentín T., Gutiérrez, María M., Olabe, José A., Doctorovich, Fabio
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 2010
• Subjects: Catalysis; Hydroxylamine; Hydroxylamine - chemistry; Iron(III) microperoxidase 11; Iron(III) porphyrinates; Metalloporphyrins - chemistry; Nitric Oxide - chemistry; Nitrogen metabolism; Peroxidases - chemistry; Pyridines - chemistry; Solubility; Water - chemistry; Iron(III) porphyrinates; Hydroxylamine; Nitrogen metabolism; Catalysis; Iron(III) microperoxidase 11
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/j.jinorgbio.2009.09.024

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-( N-ethylpyridinium-2yl)-porphyrinate ([Fe III(TEPyP)] 5+), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([Fe III(TPPS)] 3−), and microperoxidase 11 ([Fe III(MP11)]) were studied for different [Fe III(Porph)]/[HA] ratios, under anaerobic conditions at neutral pH. Efficient catalytic processes leading to the disproportionation of HA by these iron(III) porphyrinates were evidenced for the first time. As a common feature, only N 2 and N 2O were found as gaseous, nitrogen-containing oxidation products, while NH 3 was the unique reduced species detected. Different N 2/N 2O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [Fe III(TEPyP)] 5+ and [Fe III(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [Fe III(TPPS)] 3− led to the well characterized soluble intermediate, [Fe II(TPPS)NO] 4−. Free-radical formation was only evidenced for [Fe III(TEPyP)] 5+, as a consequence of a metal centered reduction. We discuss the catalytic pathways of HA disproportionation on the basis of the distribution of gaseous products, free radicals formation, the nature of porphyrinic intermediates, the Fe II/Fe III redox potential, the coordinating capabilities of each complex, and the kinetic analysis. The absence of NO 2 - revealed either that no HAO-like activity was operative under our reaction conditions, or that NO 2 - , if formed, was consumed in the reaction milieu.