Structural Identification and Conversion Analysis of Malonyl Isoflavonoid Glycosides in Astragali Radix by HPLC Coupled with ESI-Q TOF/MS

In this study, four malonyl isoflavonoid glycosides (MIGs), a type of isoflavonoid with poor structural stability, were efficiently isolated and purified from Astragali Radix by a medium pressure ODS C18 column chromatography. The structures of the four compounds were determined on the basis of NMR and literature analysis. Their major diagnostic fragment ions and fragmentation pathways were proposed in ESI/Q-TOF/MS positive mode. Using a target precursor ions scan, a total of 26 isoflavonoid compounds, including eleven malonyl isoflavonoid glycosides coupled with eight related isoflavonoid glycosides and seven aglycones were characterized from the methanolic extract of Astragali Radix. To clarify the relationship of MIGs and the ratio of transformation in Astragali Radix under different extraction conditions, two MIGs (calycosin-7-O-glycoside-6″-O-malonate and formononetin-7-O-glycoside-6″-O-malonate) coupled with related glycosides (calycosin-7-O-glycoside and formononetin-7-O-glycoside) and aglycones (calycosin and formononetin) were detected by a comprehensive HPLC-UV method. Results showed that MIGs could convert into related glycosides under elevated temperature conditions, which was further confirmed by the conversion experiment of MIGs reference compounds. Moreover, the total contents of MIGs and related glycosides displayed no obvious change during the long-duration extraction. These findings indicated that the quality of Astragali Radix could be evaluated efficiently and accurately by using the total content of MIGs and related glycosides as the quality index.