Comparative LC−MS Study of Colored Compounds Derived from Oxidative Coupling of Caffeic Acid Esters with α- or β-Alanine

Abstract

Oxidative coupling of caffeic acid esters with primary amino compounds may cause the formation of colored compounds, but there is still uncertainty about the mechanisms and structures, especially when derived from α-amino acids. Therefore, methyl caffeate (MCA) or 5-caffeoylquinic acid (5-CQA) was reacted with either α- or β-alanine, and the products were characterized using UHPLC-DAD-ESI-LIT-MS and UHPLC-DAD-ESI-QToF-MS. For both caffeic acid esters, UV–vis data, fragmentation patterns, and exact masses revealed the formation of analogous reaction products, supposedly based on a benzacridine core. However, data derived from model systems based on MCA were less complex due to the absence of acyl migration. As expected, presumed N-unsubstituted benzacridines were found with α- but not β-alanine, whereas N-substituted benzacridines were surprisingly found with both. The findings expand current knowledge of benzacridine formation from different precursors and provide a sound basis for the generation of reference substances and studies in real food systems.