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), radical cation (ABTS+), peroxide radical anion (O2-) and hydroxyl radical (HO), 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, O2-, ABTS+ and DPPH, 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 O2-, ABTS+ and DPPH, but not on their HO-scavenging activity.