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-methoxyphenyl)-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.