Structure revision of hupehensis saponin F and G and characterization of new trace triterpenoid saponins from Anemone hupehensis by tandem electrospray ionization mass spectrometry

Electrospray ionization ion-trap tandem mass spectrometry (ESI-MSn) was first employed for reinvestigating the structures of hupehensis saponin F and G previously isolated from Anemone hupehensis in our lab. Hupehensis saponin G was determined to contain one more trisaccharide unit (Rha-(1→4)-Glc-(1→6)-Glc-), not a glucose residue, than saponin F based on their molecular weights deduced from their [M+Na]+ ions in ESI-MS spectra. The 2,4A4α-ion at m/z 551.3 formed by retro-Diels–Alder (RDA) rearrangement in positive mode illustrated that the C-28 sugar chains of the two saponins were composed of trisaccharide repeating moieties with (1→4) linkages rather than (1→3) linkages. The interpretation of 2D-NMR spectra of the two compounds also confirmed the results obtained by ESI-MSn. Moreover, from the water soluble part of A. hupehensis, two novel triterpene saponins were tentatively characterized to contain 4 and 5 (1→4)-linked above trisaccharide repeating moieties at C-28 position according to their ESI-MSn behaviors, respectively.

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► The chemical structures of hupehensis saponin F and G were revised by ESI-MSn.
► The linkages between the repeated trisaccharide units were (1→4), not (1→3).
► Saponin G was deduced to contain 12 sugar residues, rather than 10 ones.
► Two novel saponins were detected and identified by ESI-MSn.