Kinetics and mechanism of peroxyl radical reactions with nitroxides

Cyclic nitroxides (> NO^̇) are stable radicals of diverse size, charge, lipophilicility, and cell permeability, which provide protection against oxidative stress via various mechanisms including SOD-mimic activity, oxidation of reduced transition metals and detoxification of oxygen- and nitrogen-centered radicals. However, there is no agreement regarding the reaction of nitroxides with peroxyl radicals, and many controversies in the literature exist. The question of whether nitroxides can protect by scavenging peroxyl radicals is important because peroxyl radicals are formed in biological systems. To further elucidate the mechanism(s) underlying the antioxidative effects of nitroxides, we studied by pulse radiolysis the reaction kinetics of piperidine, pyrrolidine, and oxazolidine nitroxides with several alkyl peroxyl radicals. It is demonstrated that nitroxides mainly reduce alkyl peroxyl radicals forming the respective oxoammonium cations (>N⁺=O). The most efficient scavenger of peroxyl radicals is 2,2,6,6-tetramethylpiperidine-N- oxyl (TPO), which has the lowest oxidation potential among the nitroxides tested in the present study. The rate constants of peroxyl reduction are in the order CH₂(OH)OO^̇ CH₃OO^̇ > t-BuOO^̇, which correlate with the oxidation potential of these peroxyl radicals. The rate constants for TPO vary between 2.8 × 10 ⁷ and 1.0 × 10⁸ M^-1 s^-1 and for 3-carbamoylproxyl (3-CP) between 8.1 × 10⁵ and 9.0 × 10⁶ M^-1 s^-1. The efficacy of protection of nitroxides against inactivation of glucose oxidase caused by peroxyl radicals was studied. The results demonstrate a clear correlation between the kinetic features of the nitroxides and their ability to inhibit biological damage inflicted by peroxyl radicals.