Synthesis, identification, and structure elucidation of adducts formed by reactions of hydroxycinnamic acids with glutathione or cysteinylglycine
Résumé
Grape polyphenols, especially hydroxycinnamic
acids such as caftaric and ca
ff
eic acid, are prone to enzymatic
oxidation reactions during the winemaking process, forming
o
-quinones and leading to color darkening. Glutathione is
capable of trapping these
o
-quinones and thus limiting juice
browning. In this study, the addition of glutathione or cys-
teinylglycine onto caftaric or ca
ff
eic acid
o
-quinones formed by
polyphenoloxidase-catalyzed reactions was investigated by
UPLC-DAD-ESIMS and NMR data analyses. Complete iden-
ti
fi
cation of adducts has been achieved via NMR data. The
results con
fi
rmed that the favored reaction is the substitution
of the sulfanyl group of cysteine at C-2 of the aromatic ring.
Several minor isomers, namely, the
cis
-isomer of the 2-
S
adduct
and
trans
-isomers of the 5-
S
and 6-
S
adducts, and the 2,5-di-
S
-glutathionyl adducts were also identi
fi
ed and quanti
fi
ed by qNMR.
With the exception of 2-(
S
-glutathionyl)- and 2,5-di(
S
-glutathionyl)-
trans
-caftaric acid, these products had never been formally
identi
fi
ed. In particular, the 5-
S
and 6-
S
derivatives are reported here for the
fi
rst time. The
fi
rst formal identi
fi
cation of 2-
S cis
-
derivatives is also provided. Moreover, NMR and UPLC-DAD-ESIMS analysis showed that signature UV and MS spectra can
serve as markers of the conformation and substitution position in the aromatic ring for each of the isomers