Antioxidant properties of ferulic acid dimers

Dimers of ferulic acid were chemically synthesized and the antioxidant properties assessed using (a) inhibition of ascorbate/iron-induced peroxidation of phosphatidylcholine liposomes and (b) scavenging of the radical cation of 2, 2'-azinobis (3-ethyl-benzothiazoline-6-sulphonate (ABTS) relative to the water-soluble vitamin E analogue, Trolox C (expressed as Trolox C equivalent antioxidant capacity, TEAC). The dimers examined were (E, E)-4, 4'- dihydroxy-5, 5'-dimethoxy-3, 3'-bicinnamic acid (5-5' diFA), trans-5-[(E)-2-carboxyvinyl]-2-(4-hydroxy-3-methoxy-phenyl)-7-methoxy-2, 3-dihydrobenzofuran-3-carboxylic acid (5, 8'-BenDiFA) and the methyl ester of 5, 8'-BenDiFA, dimethyl-5, 8'-BenDiFA. In both assays, the order of antioxidant efficacy was: 5, 5'-diFA > 5, 8'-BenDiFA > dimethyl-5, 8'-BenDiFA. From these results, methyl esterification decreases the antioxidant action. Comparison of the TEAC values shows that 1 mol of each of the ferulic acid dimers tested is less effective than 2 mol of free ferulic acid, and so dimerization decreases antioxidant action of these hydroxycinnamates.