Saponins of Glycine max Merill.: Isolation and further characterization by NMR and ESI-MS/MS

Abstract

Legumes play a pivotal role in the traditional diets of many regions throughout the world. Soybeans are unique among the legumes because they are a concentrated source of soysaponins. Soysaponins are basically triterpene glycosides and comprise a hydrophobic aglycone linked to one or more hydrophilic mono- or oligosaccharide moieties [1]. Chemical and health effects of soysaponins are numerous and suggest a number of biological properties [2]. As a continuation of our work on the chemical elucidation of bioactive markers from plants, ten soysaponins were isolated from Glycine max. All saponins were isolated from saponin concentrate of said plant by using CHCl3-MeOH-H2O lower layer in an almost isocratic way through a gravity column. All isolated compounds were characterized by NMR. A number of constituents and different possibilities of sugar chain composition and attachment cause greater diversity of soysaponin structures. In order to determine the structures of isolated compounds, ten soysaponins were analyzed under positive and negative mode respectively, by electrospray ionization multi-stage tandem mass spectrometry (ESI-MSn) coupled with an ion-trap analyzer. In positive mode, the sodium adduct molecular ions [M+Na]+ and [M+2Na-H]+ were easily detected. However, β-D-glucopyranosiduronic acid required negative mode to detect [M-H]-, [M-H-sugar(s)]- and [aglycone]-. The proposed fragmentation pathway is useful for structural characterization of soysaponins.