TEAC antioxidant activity of 4-hydroxybenzoates

The influence of pH, intrinsic electron donating capacity, and intrinsic hy drogen atom donating capacity on the antioxidant potential of series of hyd roxy and fluorine substituted 4-hydroxybenzoates was investigated experimen tally and also on the basis of computer calculations. The pH-dependent beha vior of the compounds in the TEAC assay revealed different antioxidant beha vior of the nondissociated monoanionic form and the deprotonated dianionic form of the 4-hydroxybenzoates. Upon deprotonation the radical scavenging a bility of the 4-hydroxybenzoates increases significantly. For mechanistic c omparison a series of fluorobenzoates was synthesized and included in the s tudies. The fluorine substituents were shown to affect the proton and elect ron donating abilities of 4-hydroxybenzoate in the same way as hydroxyl sub stituents. In contrast, the fluorine substituents influenced the TEAC value and the hydrogen atom donating capacity of 4-hydroxybenzoate in a way diff erent from the hydroxyl moieties. Comparison of these experimental data to computer-calculated characteristics indicates that the antioxidant behavior of the monoanionic forms of the 4-hydroxybenzoates is not determined by th e tendency of the molecule to donate an electron, but by its ability to don ate a hydrogen atom. Altogether, the results explain qualitatively and quan titatively how the number and position of OH moieties affect the antioxidan t behavior of 4-hydroxybenzoates (C) 1999 Elsevier Science Inc.