Application of directly coupled HPLC-NMR-MS to the identification and confirmation of quercetin glycosides and phloretin glycosides in apple peel

Directly coupled HPLC-NMR-MS was used to identify and confirm the presence of quercetin O-glycosides and phloretin O-glycosides in an extract of apple peel. From the MS and MS/MS data, the molecular weights of the intact molecules as well as those of quercetin and phloretin and their sugar moieties were deduced. The NMR data provided information on the identity of the compounds as well as the alpha and beta conformations and the position of the glycosides on quercetin and phloretin. The following O-glycosides of quercetin could be identified: quercetin-3-alpha-L-rhamnosyl-(1 --> 6)-beta-D-glucoside (rutin), quercetin-3-beta-D-galactoside (hyperin), quercetin-3-beta-D-glucoside (isoquercitrin), quercetin-3-beta-D-xyloside (reynoutrin), quercetin-3-alpha-L-arabinofuranoside (avicularin), and quercetin-3-alpha-L-rhamnoside (quercitrin), Phloretin was present as phloretin-2'-beta-D-glucoside (phloridzin) and the 2'-beta-D-xylosyl-(1 --> 6)-beta-D-glucoside. Concentrations were between 0.2 and 5 mg/g of apple peel.