Chemical evidence for the synergistic effect of a cysteinyl thiol on the antioxidant activity of caffeic and dihydrocaffeic esters

Antioxidant activity of methyl caffeate and methyl dihydrocaffeate in the presence of a cysteinyl thiol was measured in an azo-initiator-induced lipid oxidation system. The coexistence of the thiol was observed to display a synergistic effect on the antioxidant activity of both caffeates. The synergism was observed mainly with respect to the elongation of the induction period, rather than the inhibition rate for lipid oxidation. For methyl caffeate, the maximum elongation of the induction period was observed in the presence of more than two equivalents of the thiol, whereas the maximum effect on the activity of methyl dihydrocaffeate was observed in the presence of more than three equivalents of the thiol. These synergistic effects were analysed by high-performance liquid chromatography and liquid chromatography–mass spectrometry analyses of the intermediates produced during the antioxidation period. The analytical results clarified that the mono-thiol adduct of methyl caffeate and the mono- and di-thiol adducts of methyl dihydrocaffeate contributed to the synergism in the antioxidant activity of both caffeates.