Epicatechin carbonyl-trapping reactions in aqueous Maillard systems: Identification and structural elucidation

Recently, our group reported via labeling experiments that epicatechin in Maillard reaction aqueous glucose-glycine model systems formed adduct reaction products with C₂, C₃, and C₄ sugar fragments. In the current study, we investigated the identity of the sugar fragment precursors responsible for adduct generation by directly comparing the liquid chromatography-mass spectrometry properties of these reported epicatechin (EC) - sugar fragments adducts with those generated from reactions consisting of only EC and well-known Maillard-generated glucose fragments (i.e., glyoxal, glycolaldehyde, methylglyoxal, glyceraldehyde, etc.). The structural properties of an EC-methylglyoxal adduct reaction product were also analyzed by NMR. The most likely precursors for the C₂, C₃, and C₄ sugar moiety of the EC-sugar fragment adducts were identified as glyoxal, hydroxyacetone, and erythrose, respectively. ¹H NMR analysis of the EC-methylglyoxal indicated that the analyte underwent rapid conformational/ constitutional exchange. Using cold temperature (-25°C) two-dimensional NMR analyses (heteronuclear multiple bond coherence, heteronuclear multiple quantum coherence, and ¹H-¹H correlation spectroscopy), the structure of one of the isomers was reported to consist of a covalent linkage between the C1 position of the methylglyoxal and either the C6 or the C8 position of the EC A ring, presumably generated by hydroxyalkylation and aromatic substitution reactions.