Structural investigations of flavonol glycosides from sea buckthorn (Hippophae rhamnoides) pomace by NMR Spectroscopy and HPLC-ESI-MSn

Four flavonol glycosides were isolated from an extract of sea buckthorn pomace (Hippophae rhamnoides) by Sephadex LH-20 gel chromatography and semipreparative HPLC. Their structures were elucidated by hydrolysis studies, ESI-MSn, UV, and H-1 and C-13 NMR spectroscopy. The occurrence of the major flavonol glycoside kaempferol 3-O-beta-sophoroside-7-O-alpha-rhamnoside in sea buckthorn is described here for the first time. A further 21 flavonol glycosides of Sephadex LH-20 fractions of sea buckthorn pomace were characterized by HPLC-DAD-ESI-MS. The characteristic MS-MS and MS3 fragmentation pattern of flavonol glycosides previously identified in sea buckthorn juice and of flavonol glycosides identified by NMR spectroscopy gave valuable indications for their identification. The results demonstrate that loss of the sugar moiety from C-7 of the aglycon is more favored than fission of the glycosidic linkage at the C-3 position. Thus, most of the compounds identified were 7-rhamnosides of isorhamnetin, kaempferol, and quercetin, which exhibit different substitution patterns at the C-3 position, mainly glucosides, rutinosides, and sophorosides. In addition, numerous flavonol glycosides were detected lacking a sugar moiety at C-7. Finally, eight flavonol derivatives were identified that are acylated by hydroxybenzoic or hydoxycinnamic acids.