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 O₂^•– 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 × 10⁶ 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 O₂^•– radical scavengers with k_s values above 3.8 × 10⁴ (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 × 10³ M^–1 s^–1). The k_s values of each group correlated negatively with E_p values, supporting the hypothesis that the O₂^•– 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 (pK_a^•) of the intermediate aroxyl radicals (k_s–pK_a^• plots) revealed that the second proton transfer process could potentially be the rate-determining step of the O₂^•– radical scavenging reaction of phenolic compounds. The k_s–E_p plots provide practical information to predict the O₂^•– radical scavenging activity of plant-derived phenolic compounds based on those molecular structures.