Solvent and pH Effects on the Antioxidant Activity of Caffeic and Other Phenolic Acids

• Published in: Journal of agricultural and food chemistry Vol. 54; no. 8; pp. 2932 - 2937
• Main Authors: Amorati, Riccardo, Pedulli, Gian Franco, Cabrini, Luciana, Zambonin, Laura, Landi, Laura
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 19-04-2006
• Subjects: antioxidant activity; antioxidants; Antioxidants - pharmacology; Biological and medical sciences; caffeic acid; Caffeic Acids - pharmacology; dissociation; enthalpy; Food industries; free radical scavengers; free radicals; Fundamental and applied biological sciences. Psychology; Hot Temperature; Hydrogen-Ion Concentration; Hydroxybenzoates - pharmacology; liposomes (artificial); Liposomes - chemistry; metabolites; Oxidation-Reduction; peroxy radicals; phenolic acids; phosphatidylcholines; Solutions; Solvents; Thermodynamics; Solvent effect; Antioxidants; catechol; Phenolic acid; liposomes; phenolic acids; Liposome; pH; pH effect; peroxyl radicals; Antioxidant; Caffeic acid
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf053159+

The antioxidant activity of several phenolic acids and esters has been investigated both in organic solutions and in large unilamellar phosphatidylcholine vesicles. In solution these compounds behaved as good antioxidants, with the exception of protocatechuic acid, due to the presence of the catechol moiety. Because their antioxidant activity followed an inverse dependence on the magnitude of their O−H bond dissociation enthalpies (BDE), the key mechanism of the chain-breaking action was attributed to hydrogen atom transfer (HAT) from the phenolic OH to peroxyl radicals. In unilamellar vesicles the antioxidant activity was strongly dependent on the pH of the buffer solution. In acid media (pH 4) all of the examined phenolic acids or esters behaved as weak inhibitors of peroxidation, whereas, with increasing pH, their antioxidant activity increased substantially, becoming comparable to or even better than that of Trolox. At pH 8 they also gave rise to lag phases 2−3 times longer than that of Trolox. The increased activity being observed in proximity of the pK a value corresponding to the ionization of one of the catecholic hydroxyl groups, this effect has been attributed to the high antioxidant activity of the phenolate anion. Keywords: Antioxidants; catechol; liposomes; peroxyl radicals; pH effect; phenolic acids