Oxidative stability of (-)-epigallocatechin gallate in the presence of thiols

Polyphenols are attractive ingredients due to their purported health benefits, but their addition to foods is limited by their chemical instability, as they are rapidly oxidized under many conditions. This oxidation not only compromises the potential biological activity of the phenolic compound, but can also affect the chemical stability of the surrounding food matrix. Polyphenols bearing catechol or gallate groups, when oxidized to their benzoquinone forms, are strong electrophiles capable of reacting with nucleophilic thiols via 1,4-Michael addition reactions. These reactions are known to proceed in foods during processing and storage, and can profoundly affect the quality and biological efficacy of polyphenols when they are added as functional food ingredients. The stability of (-)-epigallocatechin gallate (EGCG) in the presence of three thiol-containing species [cysteine (Cys), glutathione (GSH), 3-mercaptohexan-1-ol (3SH)] was followed under both neutral and acidic conditions. Both Cys and GSH increased the rate of EGCG oxidation at pH 4. At pH 7, only Cys was found to increase the rate of EGCG oxidation. On the basis of these results, the reactivity of thiols toward EGCG follows the trend: Cys > GSH > 3SH, which is consistent with observed thiol-quinone adduct formation rates. Contrary to the results observed for Cys and GSH, 3SH was observed to inhibit EGCG oxidation.