Mechanistic insights into thein vitrometal-promoted oxidation of (di)azine hydroxamic acids: evidence of HNO release andN,O-di(di)azinoyl hydroxylamine intermediate

The oxidant-dependent ability of hydroxamic acids to release nitroxyl (HNO), a small inorganic molecule endowed with various biological properties, is addressed from a mechanistic standpoint. Indeed, the exact mechanism of the hydroxamic acid oxidation in physiological conditions and the direct or indirect characterization of the intermediates remain elusive. In this work, intermolecular oxidation of isonicotino-, nicotino- and pyrazino-hydroxamic acids with K-3[Fe-III(CN)(6)] at physiological pH (7.4), was monitored by(1)H NMR, MS, EPR and UV-vis techniques. While nitrosocarbonyl (di)azine intermediates, (di)Az-C(O)-NO, could bea priorienvisaged, it was in fact the correspondingN,O-di(di)azinoylhydroxylamines (AzC(O)NHOC(O)Az) and HNO that were identified, the first by(1)H NMR and the second on the basis of EPR and UV-vis experiments using the [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] (cPTIO) spin trap. The decomposition of the unstableN,O-di(di)azinoylhydroxylamine intermediates in aqueous buffer media was shown to generate the corresponding carboxylic acids as final organic products, envisaged as possiblein vivometabolites. The same oxidation experiments performed in the presence of methylamine led to the correspondingN-methyl amides suggesting that, unlike hydroxamic acids,N,O-di(di)azinoylhydroxylamines act as acylating agents in physiological pH conditions.