Flavonol 3-O-Glycosides Series of Vitis vinifera Cv. Petit Verdot Red Wine Grapes

To gain knowledge of the characteristic flavonol 3-O-glycosides of red grape Vitis vinifera varieties, we isolated the flavonol fraction from V. vinifera cv. Petit Verdot grape skins by solid-phase extraction using a combination of reverse-phase and ion-exchanging materials. This procedure allowed us to separate a fraction of anthocyanin-free flavonol 3-O-glycosides that was further split into neutral and acidic subfractions, thus facilitating flavonol identification. By means of semipreparative reverse-phase high-performance liquid chromatography, we isolated several of these flavonol 3-O-glycosides for structural elucidation. The identification of different flavonol 3-O-glycosides was based on liquid chromatography-diode array detection-electrospray ionization-tandem mass spectrometry and NMR data when available. The results suggest that red grape flavonol 3-O-glycosides comprise three different complete series, according to the nature of the sugar moiety linked to the C-3 position. The 3-O-glucosides were the main derivative of the six possible flavonol aglycones (kaempferol, quercetin, isorhamnetin, myricetin, laricitrin, and syringetin), whereas the 3-O-galactoside derivatives were found as minor compounds for all of the flavonol aglycones. The 3-O-glucuronides are the third kind of red grape flavonol derivatives and normally account as minor compounds for all of the flavonol aglycones, with the exception of quercetin 3-O-glucuronide, which was as abundant as quercetin 3-O-glucoside. In addition, the presence of quercetin 3-O-(6 ''-rhamnosyl)-glucoside (rutin) was also detected as a trace compound in the skins of Petit Verdot grapes.