Identification of autoxidation oligomers of flavan-3-ols in model solutions by HPLC-MS/MS

Autoxidation of flavan-3-ols was carried out in aqueous/methanol model solutions under mildly acidic conditions (pH 6.0), and these autoxidation products were analyzed by using high performance liquid chromatography (HPLC) coupled with tandem mass spectrometry (MS/MS). The results showed that (+)-catechins and (-)-epicatechins generated autoxidation reaction with each other to form a series of oligomers that had the same [M - H](-) molecular ions (MS(1)) as those of natural procyanidins, but had completely different fragment ions (MS(2)). According to MS/MS analysis, the major fragments of these oligomers were derived not only from the retro-Diels-Alder (RDA) dissociations on the C-rings of the flavan-3-ol units, but also from the quinone-methide (QM) cleavage of the interflavan linkages (IFL), and thus they were identified as B-type dehydrodicatechins, B-type dehydrotricatechins and A-type dehydrotricatechins, respectively. The potential structures of their [M - H](-) molecular ions and partial fragment ions were deduced on the basis of the MS/MS characterization and the oxidation of flavan-3-ols in previous reports. Some specific fragment ions were found to be very useful for identifying the autoxidation oligomers (the B-type dehydrodicatechins at m/z 393, the B-type dehydrotricatechins at m/z 681 and the A-type dehydrotricatechins at m/z 725).