Chapter 4 - Structural Elucidation of Saponins: A Combined Approach Based on High-Resolution Spectroscopic Techniques

Abstract

Abstract Saponins are a group of naturally occurring bioactive compounds often classified in several groups based on their variable structural features. They are constituted by an aglycone or sapogenin (with triterpenoid or steroid skeletons) and one or more sugar chains of different length and branching. Based on their complexity, the structural elucidation of saponins is a very challenging task, requiring the use of several analytical techniques. High-resolution spectroscopic techniques such as nuclear magnetic resonance (NMR) and mass spectrometry (MS) are proved to be valuable tools for the structural characterization of chemicals. In particular, the latest advances in NMR techniques allowed the resolution of highly crowded spectra as those of saponins. These enhancements are especially due to the introduction of new pulse sequences (i.e., heteronuclear single quantum correlation–total correlation spectroscopy, heteronuclear 2-bond correlation, constant time inverse-detected gradient accordion rescaled long-range–heteronuclear multiple bond correlation, etc.) used in combination with more “classical” 1D and 2-D NMR experiments. In addition, the capability of obtaining peculiar fragmentation patterns by ESI-MS (electrospray ionization–MS) and ESI-MSn further allowed the acquisition of complementary structural information of unknown saponins. These spectroscopic techniques are particularly useful tools for the characterization of the oligosaccharide chains without using chemical reactions and/or derivative processes. The combined use of NMR and MS approaches makes it possible to assess the complete and definitive aglycone structure, including stereochemistry, the identification of sugars (ring conformation and size, anomeric configuration), the linkage positions, and sequence of the sugar chain and, in specific cases, the characterization of acyl moieties as well as their linkage positions. In this chapter, the most recent advances in saponin structural elucidation by NMR and MS will be discussed, providing an overview of the potentiality of these techniques for the determination of such complex structures.