One-electron oxidation of acetohydroxamic acid: The intermediacy of nitroxyl and peroxynitrite

The pharmacological effects of hydroxamate derivatives have been attributed not only to metal chelation or enzyme inhibition but also to their ability to serve as nitroxyl (HNO/NO^-) and nitric oxide (NO) donors. However, the mechanism underlying the formation of these reactive nitrogen species is not clear and requires further elucidation. In the present study, one-electron oxidation of acetohydroxamic acid (aceto-HX) by ^•OH, ^•N₃, ^•NO₂, CO ₃^•-, and O₂^•- radicals was investigated using pulse radiolysis. It is demonstrated that only ^•OH, ^•N₃, and CO₃ ^•- radicals attack effectively and selectively the deprotonated form of the hydroxamate moiety, yielding the respective transient nitroxide radical. This nitroxide radical is a weak acid (CH₃C(O)NHO ^•, pK_a = 9.1), which decays via a pH-dependent second-order reaction, 2k(2CH₃C(O)NO^•-) = (5.6 ± 0.4) × 10⁷ M^-1 s^-1 (I = 0.002 M), 2k(CH₃C(O)NO^•- + CH₃C(O)NHO ^•) = (8.3 ± 0.5) × 10⁸ M^-1 s^-1), and 2k(2CH₃C(O)NHO^•) = (8.7 ± 1.3) × 10⁷ M^-1 s^-1. The second-order decomposition of the nitroxide yields transient species, one of which decomposes via a first-order reaction whose rate increases linearly upon increasing [CH₃C(O)NHO^-] or [OH^-]. One-electron oxidation of aceto-HX under anoxia does not give rise to nitrite even after exposure to O₂, indicating that NO is not formed during the decomposition of the nitroxide radical. The presence of oxidants such as Tempol or O₂ during CH₃C(O)NO^•- decomposition had no effect on the reaction kinetics. Nevertheless, in the presence of Temopl, which does not react with NO but does with HNO, the formation of the hydroxylamine Tempol-H was observed. In the presence of O₂, about 60% of CH₃C(O) NO^•- yields ONOO^-, indicating that 30% NO^- is formed in this system. It is concluded that under pulse radiolysis conditions, the transient nitroxide radicals derived from one-electron oxidation of aceto-HX decompose bimoleculary via a complex mechanism forming nitroxyl rather than NO.