NMR structural elucidation of dehydrodimers resulting from oxidation of 5-O-caffeoylquinic acid in an apple juice model solution

Abstract

During apple juice and cider-making processes, phenolic compounds undergo enzymatic oxidation. 5-O-caffeoylquinic acid (CQA) is one of the major hydroxycinnamic acid derivatives and it is the preferential substrate for polyphenol oxidase (PPO) in apple juices. Consequently, CQA dehydrodimers (MW 706 Da) are among the main products resulting from CQA oxidation. CQA dehydrodimers were previously synthesized in a biomimetic apple juice model solution. Following their purification and characterization using UV-Visible spectra and mass spectrometry, the structures of seven CQA dehydrodimers were elucidated using ¹H and ¹³C one- and two-dimensional NMR spectroscopy. Six of them exhibited dihydrobenzofuran, benzodioxane, or dihydronaphtalene skeletons, which are caffeicin-like structures. Interestingly, a new dehydrodicaffeoyldiquinic acid molecule was also characterised for which two novel structures showing a symmetric dicatechol skeleton were also proposed.

Keywords: CQA dehydrodimers; Caffeicin; Chlorogenic acid; PPO; Phenolic compounds.