Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(iii) porphyrin: Formation of the nitrosylated Mn(ii) porphyrin as an intermediate

József Kalmár, Bernadett Biri, Gábor Lente, István Bányai, Ana Budimir, Mladen Biruš, Ines Batinić-Haberle, István Fábián

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Abstract

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn IIITTEG-2-PyP5+, a synthetic water soluble Mn(iii) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of MnIIITTEG-2-PyP5+, which is followed by a ligand-to-metal electron transfer to get MnIITTEG-2-PyP4+ and the free radical derived from HU (HU). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of MnIITTEG-2-PyP4+, is the reaction of MnIITTEG-2-PyP4+ with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO2- and the initial MnIIITTEG-2-PyP5+. The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Original languageEnglish
Pages (from-to)11875-11884
Number of pages10
JournalDalton Transactions
Volume41
Issue number38
DOIs
Publication statusPublished - Oct 14 2012

ASJC Scopus subject areas

  • Inorganic Chemistry

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