Abstract
Grape polyphenols, especially hydroxycinnamicacids such as caftaric and caffeic acid, are prone to enzymaticoxidation reactions during the winemaking process, formingo-quinones and leading to color darkening. Glutathione iscapable of trapping theseo-quinones and thus limiting juicebrowning. In this study, the addition of glutathione or cys-teinylglycine onto caftaric or caffeic acido-quinones formed bypolyphenoloxidase-catalyzed reactions was investigated byUPLC-DAD-ESIMS and NMR data analyses. Complete iden-tification of adducts has been achieved via NMR data. Theresults confirmed that the favored reaction is the substitutionof the sulfanyl group of cysteine at C-2 of the aromatic ring.Several minor isomers, namely, thecis-isomer of the 2-Sadductandtrans-isomers of the 5-Sand 6-Sadducts, and the 2,5-di-S-glutathionyl adducts were also identified and quantified by qNMR.With the exception of 2-(S-glutathionyl)- and 2,5-di(S-glutathionyl)-trans-caftaric acid, these products had never been formallyidentified. In particular, the 5-Sand 6-Sderivatives are reported here for thefirst time. Thefirst formal identification of 2-S cis-derivatives is also provided. Moreover, NMR and UPLC-DAD-ESIMS analysis showed that signature UV and MS spectra canserve as markers of the conformation and substitution position in the aromatic ring for each of the isomers