Structural organization of cycloartane-based saponins in the genus Astragalus (Fabaceae)

Cycloartane represents the most common aglycone in saponins of Astragalus genus. These saponins revealed high structural diversity through continuous elucidation of new molecules. Traditionally, saponins' diversity was updated through reviews listing molecules with respect to their pharmacological activities and/or producing plant species. Beyond the long lists of reviewed molecules, this work provides statistical classification of 178 Astragalus saponins published between 1983 and 2014, leading to different molecular clusters grouping structurally close saponins and separating dissimilar ones. Clustering highlighted hierarchical factors governing structural diversification of cycloartane-based saponins in Astragalus, including rare aglycone forms, rare substitution positions and relative substitution levels of the carbon C3. Rare cycloartane forms were due to different epoxydations occurring at different degrees and positions in the lateral part of cycloartane. In common cycloartane forms, rare saponins were due to rare substituted positions consisting of atypical hydroxylations. Within the most common saponins, several classes were highlighted by the variations of C3-substitution levels in both epoxydated and not epoxydated cycloartane. Structural variations within and between saponin classes provided a strong basis for highlighting significant effects of cycloartane forms and C3-substitution levels on molecular ramification, elongation levels and substitution types (glycosylation and acetylation). Finally, these two key structural factors seemed to influence compartmental distribution of saponins in Astragalus genus.