On-line strategies for the identification of unknown flavone glycosides in Dracocephalum tanguticum Maxim

In this text we present a series of LC–MS/UV-based strategies for on-line structure identification of unknown flavone glycosides in Dracocephalum tanguticum. The aglycone portion, the glycosylation position, the order of sugars and the identity of sugars of each glycoside were unambiguously identified. The new strategy—identifying the partial structure of a molecule by comparison with a different compound possessing the same assumed partial structure—is quite effective for structure identification of the aglycone of 1–4; the unusual non-radical [Y0–H–CH2]− ion observed for the methoxylated flavonoid glycoside 8 is quite useful for structure elucidation of the aglycone of 5–8; the UV data obtained on-line with post-column addition of UV-shift reagents are powerful for determination of the glycosylation and methoxylation positions for each flavone glycoside. Finally, isolation and subsequent individual analysis by NMR were performed to provide complementary information for the configurations of each sugar and the linkage types of each disaccharide. Thus, the structures of the eight detected flavone glycosides, which all have never been reported in D. tanguticum before, were positively identified.