Structural characterization of pregnane glycosides from Cynanchum auriculatum by liquid chromatography on a hybrid ion trap time-of-flight mass spectrometer.

A method coupling high-performance liquid chromatography with hybrid ion trap time-of-flight mass spectrometry (TOFMS) using an electrospray ionization source was firstly used to characterize ten major pregnane glycosides including one novel compound auriculoside IV from the roots of Cynanchum auriculatum Royle ex Wight. In the MS/MS spectra, fragmentation reactions of the [M+Na]+ were recorded to provide abundant structural information on the aglycone and glycosyl moieties. Experiments using TOFMS allowed us to obtain precise elemental compositions of molecular ions and subsequent product ions with errors less than 6 ppm. The pregnane glycosides in C. auriculatum were classified into two major core groups: one is caudatin characterized by the neutral loss of one ikemamic acid molecule (128 Da) from the precursor ion, and the other is kidjoranin characterized by the neutral loss of cinnamic acid (148 Da) from the precursor ion. Meanwhile, a series of sugar-chain fragment ions provided valuable information about the compositions of the sugar residues and the sequences of the sugar chain. Logical fragmentation pathways for pregnane glycosides have been proposed and are useful for the identification of these compounds in natural products especially when there are no reference compounds available.