TEAC antioxidant activity of 4-hydroxybenzoates.
The influence of pH, intrinsic electron donating capacity, and intrinsic hydrogen atom donating capacity on the antioxidant potential of series of hydroxy and fluorine substituted 4-hydroxybenzoates was investigated experimentally and also on the basis of computer calculations. The pH-dependent behavior of the compounds in the TEAC assay revealed different antioxidant behavior of the nondissociated monoanionic form and the deprotonated dianionic form of the 4-hydroxybenzoates. Upon deprotonation the radical scavenging ability of the 4-hydroxybenzoates increases significantly. For mechanistic comparison a series of fluorobenzoates was synthesized and included in the studies. The fluorine substituents were shown to affect the proton and electron donating abilities of 4-hydroxybenzoate in the same way as hydroxyl substituents. In contrast, the fluorine substituents influenced the TEAC value and the hydrogen atom donating capacity of 4-hydroxybenzoate in a way different 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 the tendency of the molecule to donate an electron, but by its ability to donate a hydrogen atom. Altogether, the results explain qualitatively and quantitatively how the number and position of OH moieties affect the antioxidant behavior of 4-hydroxybenzoates.
Tyrakowska
B
B
Faculty of Commodity Science, Poznań University of Economics, Poland. btyrak@novci1.ae.poznan.pl
Soffers
A E
AE
Szymusiak
H
H
Boeren
S
S
Boersma
M G
MG
Lemańska
K
K
Vervoort
J
J
Rietjens
I M
IM
D000975:Antioxidants / Q000737:chemistry*; D052160:Benzothiazoles; D002839:Chromans; D004583:Electrons; D005461:Fluorine / Q000737:chemistry; D016166:Free Radical Scavengers; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006900:Hydroxylation; D007202:Indicators and Reagents; D010226:Parabens / Q000737:chemistry*; D013451:Sulfonic Acids / Q000737:chemistry
D000975:Antioxidants; D052160:Benzothiazoles; D002839:Chromans; D016166:Free Radical Scavengers; D007202:Indicators and Reagents; D010226:Parabens; D013451:Sulfonic Acids; D005461:Fluorine; C002502:2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid; D006859:Hydrogen; C038193:4-hydroxybenzoic acid; C010643:6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid