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.