Structural organization of saponins in Caryophyllaceae

Saponins in Caryophyllaceae were subjected to many structural elucidations for several decades leading to big datasets of molecules essentially based on four aglycones: quillaic acid (QA), gypsogenin (Gyp), gypsogenic acid (GA) and 16-OH-gypsogenic acid (16-OH-GA). Statistical analyses of structural variability of 231 saponins (published between 1971 and 2016) highlighted several diversification trends associating the four aglycones to different glycosylation (Gly) patterns defined by specific, dominant or relatively well-regulated sugars at the carbons C3, C23 and/or C28. The C28 was frankly more glycosylated than C3 in GA and 16-OH-GA versus comparable Gly levels in Gyp and QA. Moreover, GA and 16-OH-GA showed higher regulations of glucose than Gyp and QA. Further structural diversification of GA- and 16-OH-GA-based saponins was provided by specific occurrence, absence or high regulations of other hexoses (Gal, Man), GlcA and/or pentoses (Xyl, Ara) in presence of Glc (dominant sugar). Under Gyp and QA, Gly patterns showed systematic occurrence of 3-Gal with diversification due (i) to presence-absence of Fuc, Rha, Qui (6-deoxyhexoses), Xyl at C3 or C28 or (ii) to high regulations of 3-Glc, 3-GlcA, 3-Ara, 28-Gal and/or 28-Glc. The highlighted structural organization of saponins showed significant associations with several plant genera. At lower biological scale, roots, seeds and aerial parts were revealed to be more or less favorite compartments for the different saponin classes. Finally, a synthesis of published pharmacological activities was made taking into account affiliations of saponins to different aglycones and plant genera. Preliminary average aglycone-glycosylation patterns were established for the most cited activities (cytotoxic, antiproliferative, immunomodulatory).