LC-ESI-MS/MS characterization of strophanthin-K

A liquid chromatography-mass spectrometry (LC-MS) method was developed for the characterization of strophanthin-K, a mixture of cardiac glycosides extracted from the seeds of Strophanthus kombè. The method is based on the separation of the cardenolides using high performance liquid chromatography (HPLC) followed by detection with electrospray ionization mass-spectrometry (ESI-MS). Chromatographic separation of the analytes was achieved on a RP C-18 column using water: 1% formic acid in water (v/v):acetonitrile gradient mobile phase. Strophanthin-K glycosides studied in ESI-MS in negative ion mode formed abundant adduct ions [M + HCOO]^- while the pseudomolecular ions [M - H]^- are obtained in ESI-MS/MS experiments. Six different cardiac glycosides were identified and characterized: k-strophanthoside, k-strophanthin-β, helveticoside (erysimin), erysimoside, cymarin and neoglucoerysimoside. Forced degradation investigations done with strophanthin-K showed that k-strophanthidin (the aglycone of strophanthin-K glycosides) was the main product of degradation in acidic conditions; however, in basic conditions, the hydrolysis of the unsaturated 17β-lactones to the corresponding γ-hydroxy acids was the predominant degradation pathway.