Structural analysis of platycosides in Platycodi Radix by liquid chromatography/electrospray ionization-tandem mass spectrometry

Platycosides extracted from Platycodi Radix were analyzed by HPLC coupled with electrospray ionization multistage tandem mass spectrometry (HPLC/ESI-MSn). Predominant [M + Na]+ ions in positive mode and [M − H]− ions in negative mode in the direct ESI-MS spectra of extract provided information on molecular weights, but minor components and isomers could not be discriminated. However, combining HPLC and ESI-MSn, allowed eleven platycosides, including four acetylated platycodin isomers and two prosapogenines to be analyzed. During MS2 analysis conducted to elucidate the structures of platycosides, fragment ions provided information on sugar moieties attached at C-28 of triterpene structure of the platycosides. Glycosidic bond cleavages at C-3 were revealed by fragment ions in MS3 spectra. Some characteristic fragment ions not related to sugar bond cleavage revealed that an esterified triterpene is linked to sugars at C-28. The only sugar ring-cross cleavage corresponding to 90 Da in the negative MS2 spectrum took place at an arabinosyl sugar moiety. By using HPLC/ESI-MSn, three acetylated platycosides in Platycodi Radix extract were newly identified.