Structural elucidation of alfalfa root saponins by mass spectrometry and nuclear magnetic resonance analysis

Abstract

Structures of saponins may be fully elucidated using a combination of n.m.r. and m.s. instead of the usual degradation techniques. Well resolved proton n.m.r. spectra and chromatographic separation can be obtained from peracetylated mixtures of saponins. Under these conditions, genins are identified mainly by ¹³C n.m.r. spectroscopy; sugars and their points of attachment are determined by examining ¹H chemical shifts and coupling constants following assignments made by 2D COSY and relayed COSY experiments. Sugar chains are sequenced by observing intersugar connectivities through long-range couplings (delayed COSY) or n.o.e. Alternatively, the use of Californium plasma desorption m.s. provides quasimolecular ions and fragments corresponding to the sequential rupture of the chain of sugars. Free carboxylic acids are located on an acetylated saponin by looking at the disappearance, in the presence of Eu complexes, of the carbon situated α to the carbonyl in the ¹³C n.m.r. spectrum. The above methods have been employed to determine the structures of the seven major saponins of alfalfa root, which are allegedly responsible for the antifeedant properties of the vegetable.