Structural characterization and identification of flavonoid aglycones in three Glycyrrhiza species by liquid chromatography with photodiode array detection and quadrupole time-of-flight mass spectrometry

Flavonoids, including flavones, isoflavones, flavanones, chalcones, and isoflavans, have long been recognized as the main active ingredients in licorice. A method combining liquid chromatography with photodiode array detection and quadrupole time-of-flight mass spectrometry was developed to characterize components in three Glycyrrhiza species, and to expound the characteristic fragmentation behaviors in the positive ion mode. Based on the fragmentation patterns of reference compounds, a total of 39 compounds, including 37 flavonoid aglycones and two coumestans, were identified or tentatively identified. Besides, some common features, such as H2 O, CO, and CH2 O2 losses, together with retro-Diels-Alder fragmentation, were observed in these compounds. Furthermore, diagnostic fragmentations of C-ring cleavages and UV absorption on the skeleton groups were observed to structurally characterize flavonoid aglycones. In addition, typical losses of different substituent groups were detected: Neutral losses of 56 (C4 H8 ) and 68 Da (C5 H8 ) were yielded from a prenyl chain; neutral losses of 42 (C3 H6 ), 54 (C4 H6 ), and 70 Da (C4 H6 O) were generated by a pyran ring. Particularly, neutral losses of 18 (H2 O), 16 (CH4 ), 112 (C8 H16 ), and 98 Da (C7 H14 ) predicted a hydroxyl, a methoxyl, double prenyl chains, and a prenyl chain with a pyran ring, respectively.