Structure–biological activity relationships in triterpenic saponins: the relative activity of protobassic acid and its derivatives against plant pathogenic fungi

Abstract BACKGROUND: Triterpenic saponins from Sapindus mukorossi Gaertn. and Diploknema butyracea JF Gmelin were evaluated for in vitro antifungal activity against four phytopathogenic fungi. The study of the structure–antifungal activity relationships of protobassic acid saponins was widened by including semi‐synthetic derivatives. RESULTS: Diploknema butyracea saponins exhibited significant antifungal activity against three fungi (ED 50 230–455 µg mL −1 ), whereas S. mukorossi saponin was effective against two fungi (ED 50 181–407 µg mL −1 ). The n ‐butanol extract after preparative HPLC separation provided two saponins from D. butyracea saponin mixture: 3‐O‐[β‐ D ‐glucopyarnosyl‐β‐ D ‐glucopyranosyl]‐16‐α‐hydroxyprotobassic acid‐28‐O‐[arabinopyranosyl‐glucopyranosyl‐xylopyranosyl]‐arabinopyranoside (MI‐I), and 3‐O‐β‐ D ‐glucopyranosyl‐glucopyranosyl‐glucopyranosyl‐16‐α‐hydroxyprotobassic acid‐28‐O‐[arabinopyranosyl‐xylopyranosyl‐arabinopyranosyl]‐apiofuranoside (MI‐III). The single saponin extracted from S. mukorossi saponin mixture was identified as 3‐O‐[O‐acetyl‐β‐ D ‐xylopyranosyl‐β‐ D ‐arabinopyranosyl‐β‐ D ‐rhamnopyranosyl] hederagenin‐28‐O[β‐ D ‐glucopyranosyl‐β‐ D ‐glucopyranosyl‐β‐ D ‐rhamnopyranosyl] ester (SM‐I). Monodesmosides resulting from the partial degradation of hederagenin and hydroxyprotobassic acid bisdesmosides exhibited significant reduction in antifungal effect. Further removal of sugar moiety yielded complete loss in activity. The antifungal activity of the triterpenic saponins was associated with their aglycone moieties, and esterification of the hydroxyl group led to change in antifungal activity. CONCLUSION: Sapindus mukorossi saponin, which is effective against Rhizoctonia bataticola (Taub.) Briton Jones and Sclerotium rolfsii Sacc., can be exploited for the development of a natural fungicide. A sugar moiety is a prerequisite for the antifungal activity of triterpenic saponin. Copyright © 2010 Society of Chemical Industry