Ion trap multiple-stage tandem mass spectrometry as a pre-NMR tool in the structure elucidation of saponins

Abstract

Ion trap MS has been utilized in the structure elucidation of a saponin from Quillaja saponaria. With respect to the carbohydrate moieties, this technique allowed the elucidation of the sequence and branching, of six out of seven glycosidic linkages, the precise location of an appended O-acetyl group, and the ring sizes of three out of four nonterminal monosaccharide residues. The mass spectra could be recorded and interpreted within a day using approximately 1 µg of sample. The identities and absolute configurations of the constituent monosaccharides were determined using trimethylsilylated methyl glycosides and trimethylsilylated (+)-2-butyl glycosides, respectively. Methylation analysis confirmed the MS data and yielded both the missing glycosidic linkage and the ring sizes. The identity of the aglycone-quillaic acid—and the anomeric forms of the monosaccharide residues were established by NMR spectroscopy, while several structural features were confirmed. Ion trap MS proved to be attractive as a pre-NMR tool because it rapidly provided an overall idea of the structure, thereby considerably reducing the time required for interpretation of the NMR data. As a particular advantage, the structure as deduced from MS enabled facile NMR assignments in case of ambiguity due to overlapping signals. Copyright © 2000 John Wiley & Sons, Ltd.