Fig. 1. (a) Structure of the saponin natural product immunoadjuvant QS-21 (≈2 : 1 mixture of 1a/1b). (b) Structures of a simplified semisynthetic lead compound 2 and corresponding glycosidic linkage variants 3–9 designed to probe the effects of linker length, stereochemistry, and conformational flexibility upon adjuvant activity. Four-number SQS (synthetic QS) codes designate structural variants in each of the four corresponding structural domains of QS-21, left to right, with 0 assigned to the natural product structure.
