Identification of antioxidants from Taraxacum mongolicum by high-performance liquid chromatography–diode array detection–radical-scavenging detection–electrospray ionization mass spectrometry and nuclear magnetic resonance experiments

Taraxacum mongolicum was a traditional Chinese medicine for the treatment of inflammatory disorders and viral infectious diseases. Furthermore, fresh leaves of T. mongolicum have been used by local people as vegetable food in Northern China. An on-line rapid screening method, high-performance liquid chromatography-diode array detection-radical-scavenging detection-electrospray ionization mass spectrometry (HPLC-DAD-RSD-ESI-MS) system, has been developed for the separation and identification of radical scavengers from the methanolic extract of T. mongolicum. In addition, the detected antioxidants were isolated directly by preparative HPLC (PHPLC) and Sephadex LH-20, then the purified compounds were sampled to off-line nuclear magnetic resonance (NMR) spectrometer to acquire NMR spectra. The structure of the active compounds was elucidated by ultraviolet (UV), ESI-MS and NMR spectral data. Thirty-two radical-scavenging compounds including sixteen flavonoid derivatives, ten phenylpropanoid derivatives and six benzoic acid derivatives were screened, isolated and identified. Among them, seventeen compounds including three new compounds were first isolated from Taraxacum genus by our group. Caffeic acid (6), isoetin-7-O-beta-d-glucopyranosyl-2'-O-alpha-l-arabinopyranoside (9), isoetin-7-O-beta-d-glucopyranosyl-2'-O-alpha-d-glucopyranoside (10) and isoetin-7-O-beta-d-glucopyranosyl-2'-O-beta-d-xyloypyranoside (12) were found to be the major metabolites in T. mongolicum based on their relative peaks in the HPLC chromatogram. Antioxidant activity of three new compounds was assessed for their scavenging capacity on 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical, and all of them showed potent activity.