Mechanisms Associated with Superoxide Radical Scavenging Reactions Involving Phenolic Compounds Deduced Based on the Correlation between Oxidation Peak Potentials and Second-Order Rate Constants Determined Using Flow-Injection Spin-Trapping EPR Methods.

Flow-injection spin-trapping electron paramagnetic resonance (FI-EPR) methods that involve the use of 5,5-dimethyl-pyrroline- N -oxide (DMPO) as a spin-trapping reagent have been developed for the kinetic study of the O 2 •- radical scavenging reactions occurring in the presence of various plant-derived and synthetic phenolic antioxidants (Aox), such as flavonoid, pyrogallol, catechol, hydroquinone, resorcinol, and phenol derivatives in aqueous media (pH 7.4 at 25 °C). The systematically estimated second-order rate constants ( k s ) of these phenolic compounds span a wide range (from 4.5 × 10 to 1.0 × 10 6 M -1 s -1 ). The semilogarithm plots presenting the relationship between k s values and oxidation peak potential ( E p ) values of phenolic Aox are divided into three groups (A1, A2, and B). The k s - E p plots of phenolic Aox bearing two or three OH moieties, such as pyrogallol, catechol, and hydroquinone derivatives, belonged to Groups A1 and A2. These molecules are potent O 2 •- radical scavengers with k s values above 3.8 × 10 4 (M -1 s -1 ). The k s - E p plots of all phenol and resorcinol derivatives, and a few catechol and hydroquinone derivatives containing carboxyl groups adjacent to the OH groups, were categorized into the group poor scavengers ( k s < 1.6 × 10 3 M -1 s -1 ). The k s values of each group correlated negatively with E p values, supporting the hypothesis that the O 2 •- radical scavenging reaction proceeds via one-electron and two-proton processes. The processes were accompanied by the production of hydrogen peroxide at pH 7.4. Furthermore, the correlation between the plots of k s and the OH proton dissociation constant (p K a • ) of the intermediate aroxyl radicals ( k s -p K a • plots) revealed that the second proton transfer process could potentially be the rate-determining step of the O 2 •- radical scavenging reaction of phenolic compounds. The k s - E p plots provide practical information to predict the O 2 •- radical scavenging activity of plant-derived phenolic compounds based on those molecular structures.