The hydroperoxyl radical scavenging activity of natural hydroxybenzoic acids in oil and aqueous environments: Insights into the mechanism and kinetics.

Foods that contain hydroxybenzoic acid derivatives (HBA) include red fruits, black radish, onion, and potato peel. HBA are widely known for their anti-inflammatory, anti-cancer, and especially antioxidant capabilities; however, a comprehensive study of the mechanism and kinetics of the antiradical action of these compounds has not been performed. Here, we report a study on the mechanisms and kinetics of hydroperoxyl radical scavenging activity of HBA by density functional theory (DFT) calculations. According to the results, HBA exert low HOO antiradical activity in the nonpolar environment with overall rate constants in the range of k = 5.90 × 10 - 4.10 × 10 M s. However, most HBA exhibit significant HOO antiradical activity (k = 10 - 10 M s) by the single electron transfer (SET) reaction of the phenoxide anions in water at physiological pH. The overall rate constant increases with increasing pH values in the majority of the substances studied. At pH ≤ 4, gentisic acid had the best HOO antiradical activity (log(k) = 3.7-4.8), however at pH > 4, the largest HOO radical scavenging activity (log(k) = 4.8-9.8) was almost exclusively found for gallic and syringic acids. Salicylic and 5-sulphosalicylic acids have the lowest antiradical activity across most of the pH range. The activities of the majority of the acids in this study are faster than the reference compound Trolox. Thus, in the aqueous physiological environment, these HBA are good natural antioxidants.