Advances in Structural Determination of Saponins and Terpenoid Glycosides
Saponins and triterpenoid glycosides are polar constituents of plants many of which exhibit a broad spectrum of biological activities. They often occur as intricate mixtures difficult to separate by conventional procedures. Improvements in liquid chromatographic techniques and availability of new matrices have improved the possibility of purifying even the isomeric saponins. The application of newer isolation and purification processes are illustrated in the present review with examples both from the literature and from the reviewers laboratories. Advances in mass spectrometry, for example tandem mass spectrometry, offer considerable advantages in structure determination of complex saponins, and their use has been illustrated. The major structural information however comes from NMR spectra. Due to the complex structures, interpretation of NMR spectra recorded even with high field instruments is not easy. The present review summarizes the available 2D NMR techniques particularly HMBC, NOESY and HOHAHA for simplifying spectral interpretation with suitable examples. In special cases, use of molecular mechanics and molecular dynamics studies of structure leads to valuable information on conformation of molecules. Their application was found to be of considerable advantage for saponins possessing polysubstituted monosaccharides.