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

► The potent synergistic effect of a thiol on antioxidant activity of caffeates was found. ► Maximum effect was observed with two equivalents of the thiol to one of methyl caffeate. ► For methyl dihydrocaffeate maximum effect was observed with three equivalents of the thiol. ► Several thiol adducts were formed in situ in the antioxidant reactions. ► The synergism was accounted for by the potent antioxidant activity of thiol adducts.