A strategy for characterization of triterpene saponins in Caulophyllum robustum hairy roots by liquid chromatography with electrospray ionization quadrupole time-of-flight mass spectrometry

Abstract

Triterpene saponins are important bioactive constituents widely distributed in many plants. Saponins present in Caulophyllum (Berberidaceae) have not been fully characterized. In this study, we studied triterpene saponins from Caulophyllum robustum using liquid chromatography combined with electrospray ionization quadrupole time-of-flight mass spectrometry (LC-qTOF-MS). Rapid identification of Caulophyllum saponins was facilitated using low and high MS cone voltages to induce controlled fragmentation in positive mode. The full scan spectra at low cone voltage of 40V provided considerable structural information relating to aglycone skeletons, sugar types, and linked sequences for Caulophyllum saponins. Seven Caulophyllum aglycones were differentiated and identified by their diagnostic fragment ions combined with accurate mass measurements and characteristic fragmentation pathways. Peak intensity ratio of [aglycone+H−2H2O]+ to [aglycone+H−H2O]+ in full scan spectra acquired with low cone voltage is correlated with structural features of hederagenin and echinocystic acid and is useful for the discrimination of these positional isomers. However, at a high voltage of 200V, the saponin [M+H]+ ion and its fragmentation ions were not present; and the single saponin [M+Na]+ generated [Bα+Na]+ and [Y0α+Na]+ by in-source fragmentation, which provided structural information on the α- and β-sugar chains in the saponins. This approach enabled simultaneous acquisition of structural information on both aglycones and sugar chains from full scan spectra in one injection. Based on the developed strategy, 51 triterpene saponins of seven different classes were fully characterized or tentatively identified, of which 32 constituents were the first to be reported in genus Caulophyllum and 18 compounds were characterized as potentially new compounds.