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(center dot-) 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 degrees C). The systematically estimated second-order rate constants (k(s)) of these phenolic compounds span a wide range (from 4.5 x 10 to 1.0 x 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(center dot-) radical scavengers with k(s) values above 3.8 x 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 x 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(center dot-) 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)(center dot)) of the intermediate aroxyl radicals (k(s)-pK(a)(center dot) plots) revealed that the second proton transfer process could potentially be the rate-determining step of the O-2(center dot-) radical scavenging reaction of phenolic compounds. The k(s)-E-p plots provide practical information to predict the O-2(center dot-) radical scavenging activity of plant-derived phenolic compounds based on those molecular structures.