Analysis of oligosaccharide sequences of trace Caulophyllum robustum saponins by direct infusion multiple-stage tandem mass spectrometry.

The saponins in Caulophyllum robustum have not yet been fully characterized. Furthermore these saponins are often present in trace amounts and are structurally complex. Here, a simple direct infusion electrospray ion trap multiple-stage tandem mass spectrometry (DI-ESI-IT-MS(n)) method was described for the characterization of trace C. robustum saponins. Eight reference saponins from the C. robustum hairy root were investigated by DI-ESI-IT-MS(n) in positive ion mode. Some fragmentation approaches were proposed through analysis of the M+Na ions: (1) preferential cleavage of the C-28 ester glycosidic bond to provide complementary Y0α+Na and Bα+Na ions for bidesmosidic saponins; (2) diagnostically neutral loss of CO2 from free carboxyl groups at C-28 for monodesmosidic saponins; and (3) the ion intensity ratio between C2β+Na and B2β+Na, which is sensitive to the structural differences between the two isomeric β-sugar chains (Glc → (2)Ara and Glc → (3)Ara). The DI-ESI-IT-MS(n) method was successfully used for the analysis of trace C. robustum saponins with M+Na ions at m/z 1745.6, 1729.5, 1583.7, 1567.7, 1421.7 and 1405.7. This article highlights the discovery and identification of complex α- and β-oligosaccharide moieties in Caulophyllum saponins by glycosidic product ions along with cross ring cleavage product ions. Five oligosaccharide moieties were unambiguously or tentatively identified as Rha → (4)Glc → (6)Glc → (4)Rha → (4)Glc → (6)Glc, Glc → (4)Glc → (6)Glc → (4)Rha → (4)Glc → (6)Glc, Rha → Glc → Glc (Glc) → (2,3)Ara, Glc → Glc (Glc) → (2,3)Ara and Glc (Glc) → (2,3)Ara. Accuracy of the analytical procedure was demonstrated by structural identification of two saponins isolated using 1D and 2D-NMR spectroscopy. The DI-ESI-IT-MS(n) method facilitates rapid discovery and analysis of trace Caulophyllum saponins and is a powerful and practical tool for structural characterization of complex oligosaccharide moieties in triterpene saponins.