Extremely Fast Hydrogen Atom Transfer between Nitroxides and HOO· Radicals and Implication for Catalytic Coantioxidant Systems
2018
Baschieri, Andrea | Valgimigli, Luca | Gabbanini, Simone | DiLabio, Gino A. | Romero-Montalvo, Eduardo | Amorati, Riccardo
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 × 10⁹ M–¹ s–¹ for the reductive reaction TEMPO + HOO· → TEMPOH + O₂ and 5.0 × 10⁷ M–¹ s–¹ for the oxidative reaction TEMPOH + HOO· → TEMPO + H₂O₂. 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 × 10⁶ 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.
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