4 - Structure determination

Summary

There are several basic problems to be solved in the structure elucidation of saponins:

1. – the structure of the genuine aglycone

2. – the composition and sequence of the component monosaccharides in the carbohydrate moiety

3. – how the monosaccharide units are linked to one another

4. – the anomeric configuration of each glycosidically linked monosaccharide unit

5. – the location of the carbohydrate moiety on the aglycone.

The necessary approach is to apply a combination of methods in order to arrive at a final conclusion for the structure. A simplified scheme for the structure elucidation of saponins, with a selection of the available techniques, is shown in Fig. 4.1. This is a step wise process, in which the saponin is gradually broken down into smaller fragments which themselves are analysed spectroscopically. By a judicious handling of the data from the fragments, an idea of the composition of the saponin can gradually be built up.

With advances in the modern techniques available, preparative separations of increasing numbers of saponins are becoming conceivable. At the same time, however, the quantities of pure saponins isolated are often small (sometimes only several milligrams) and there is always a need for highly sensitive, high-resolution and, if possible, non-degradative methods in order to aid the structure determination of a saponin. Recourse to innovations in NMR spectroscopy and mass spectrometry (MS) has been essential for further advances in the investigations of complex saponins. Thus, FAB-MS gives information about the molecular weight and, in many cases, the sugar sequence, while 1-D and 2-D NMR techniques permit the localization of sugar linkages and contribute to the structure elucidation of the aglycone.