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

Abstract

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 M s for the reductive reaction TEMPO + HOO· → TEMPOH + O and 5.0 × 10 M s for the oxidative reaction TEMPOH + HOO· → TEMPO + HO. 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 M s). 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.

Hydrogen Atom Transfer from HOO. to ortho‐Quinones Explains the Antioxidant Activity of Polydopamine

“…[(Scheme 3, Eqs. (8) and (9)] [20–23] . As expected, [17–19] 1 and 2 had no effect on the peroxidation of styrene alone (Figure 1 A).…”

Kinetics of oxidation of benzyl alcohols with molecular oxygen catalyzed by N‐hydroxyphthalimide: Role of hydroperoxyl radicals

“…Participation of HOO • in the chains termination explains the inhibition by aromatic nitroxide of the oxidation reaction . Extremely effective reaction HOO • with TEMPO • (2,2,6,6‐ tetramethylpiperidine‐1‐oxyl) breaks down the chain of oxidation, and measured rate constants for reactions HOO • radicals in the inhibitory cycle are very high: …”

“…31 Participation of HOO • in the chains termination explains the inhibition by aromatic nitroxide of the oxidation reaction. 32 Extremely effective reaction HOO • with TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl) breaks down the chain of oxidation, and measured rate constants for reactions HOO • radicals in the inhibitory cycle are very high 33 : It should be noted that by the value of ΔH it is possible to predict a greater reactivity of PINO • than the ТЕМРО • in the reaction of chain breaks with HOO • which can significantly decrease the oxidation rates.…”

Hydrogen Atom Transfer from HOO. to ortho‐Quinones Explains the Antioxidant Activity of Polydopamine

“…and hydroperoxyl (HOO . ) radicals[ 20 , 21 , 22 , 23 ] to understand the reactivity and the mechanisms of antioxidant action of polydopamine ( PDA , Scheme 1 A ) and of two model ortho ( 1 ) and para ( 2 ) quinones (Scheme 2 ).…”