Oxidation of caffeic acid in the presence of l-cysteine: isolation of 2-S-cysteinylcaffeic acid and evaluation of its antioxidant properties

Caffeic acid (3,4-dihydroxycinnamic acid) was oxidized in wine-like model solutions (citrate buffer, pH 3.5) containing l-cysteine, by means of sodium periodate that mimics the mechanism of polyphenol oxidase (PPO). The reaction lead to the formation of a l-cysteine/caffeic acid adduct, which was isolated and tentatively identified as 2-S-cysteinylcaffeic acid (2-CCA), on the basis of LC-MS and 1H NMR data. The antioxidant properties of 2-CCA were assessed by employing the DPPH and a ferric-reducing test, and compared with both caffeic acid and l-cysteine, but also with gallic acid, which was used as a reference antioxidant. The results indicated that the adduct exhibits slightly improved antiradical activity in relation with the parent molecule (caffeic acid), but its reducing capacity was dramatically reduced, a fact that was theoretically ascribed to its strong chelating ability.