ESR determination of the reactions between selected phenolic acids and free radicals or transition metals

Five phenolic acids commonly present in wheat grain and fractions were examined and compared for their radical-scavenging properties and chelating capacities. The free radical-scavenging properties were evaluated against 2,2-diphenyl-1-picryhydrazyl radical (DPPH center dot), radical cation (ABTS(center dot+)), peroxide radical anion (O-2(center dot-)) and hydroxyl radical (HO center dot),whereas the chelating properties were evaluated against Fe (II) and Cu (II) using spectrophotometric and electron spin resonance measurements. These phenolic acids differed in their abilities to react with and quench HO center dot,O-2(center dot-), ABTS(center dot+)and DPPH center dot, as well as their capacities to form chelating complexes with transition metals. 4-Hydroxybenzoic acid had neither free radical-scavenging nor chelating activity under the experimental conditions. Strong structure-activity relationships were observed in the present study. Both substituents on the phenyl ring and the conjugated carbon skeleton may influence the antioxidant properties of phenolic acids. The presence of an additional methoxyl group in the ortho position of the hydroxyl group showed a strong influence on the chelating property of phenolic acids and their radical-scavenging capacity against O-2(center dot), ABTS(center dot+) and DPPH center dot, but not on their HO center dot-scavenging activity. (c) 2005 Elsevier Ltd. All rights reserved.