Screening and structural characterization of alpha-glucosidase inhibitors from hawthorn leaf flavonoids extract by ultrafiltration LC-DAD-MS(n) and SORI-CID FTICR MS.

In vitro alpha-glucosidase inhibition assays and ultrafiltration liquid chromatography with photodiode array detection coupled to electrospray ionization tandem mass spectrometry (ultrafiltration LC-DAD-ESI-MS(n)) were combined to screen alpha-glucosidase inhibitors from hawthorn leaf flavonoids extract (HLFE). As a result, four compounds were identified as alpha-glucosidase inhibitors in the HLFE, and their structures were confirmed to be quercetin-3-O-rha- (1-4)-glc-rha and C-glycosylflavones (vitexin-2"-O-glucoside, vitexin-2"-O-rhamnoside and vitexin) by high-resolution sustained off resonance irradiation collision-induced dissociation (SORI-CID) data obtained by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). Several other C-glycosylflavones (vitexin, isovitexin, orientin, isooriention) and their aglycones apigenin and luteolin were evaluated by in vitro assays, and were found to possess strong alpha-glucosidase inhibitory activities as well. Moreover, the substituent groups on the flavones had a great impact on the enzyme inhibition activity. C-3'-OH of the B-ring of flavones in particular increased the alpha-glucosidase inhibition activity, whereas C-glycosylations at C-6 or C-8 of the A ring weakened the inhibition activity.