Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO·Radicals and Implication for Catalytic Coantioxidant Systems

摘要

We report a novel coantioxidant system based on TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) that, in biologically relevant model systems, rapidly converts chain-carrying alkylperoxyl radicals to HOO· . Extremely efficient quenching of HOO· by TEMPO blocks the oxidative chain. Rate constants in chlorobenzene were measured to be 1.1 × 10~(9) M~(–1) s~(–1) for the reductive reaction TEMPO + HOO· → TEMPOH + O_(2) and 5.0 × 10~(7) M~(–1) s~(–1) for the oxidative reaction TEMPOH + HOO· → TEMPO + H_(2)O_(2). These rate constants are significantly higher than that associated with the reaction of HOO· with α-tocopherol, Nature’s best lipid soluble antioxidant (k = 1.6 × 10~(6) M~(–1) s~(–1)). These data show that in the presence of ROO· -to-HOO· chain-transfer agents, which are common in lipophilic environments, the TEMPO/TEMPOH couple protects organic molecules from oxidation by establishing an efficient reductive catalytic cycle. This catalytic cycle provides a new understanding of the efficacy of the antioxidant capability of TEMPO in nonaqueous systems and its potential to act as a chemoprotective against radical damage.