Synthesis, Conformation, and Pro-Inflammatory Activity of the Branched Glucorhamnans Relevant to the Polysaccharide of Ruminococcus gnavus.

Glucorhamnan produced by the gut bacterium Ruminococcus gnavus is closely associated with Crohn's disease and exhibits potent pro-inflammatory activity. Synthetic glucorhamnan fragments could serve as critical tools for dissecting the molecular mechanisms underlying the inflammatory responses elicited by the bacterial polysaccharides, and could be further developed into carbohydrate-based vaccines. However, chemical synthesis of these glucorhamnan motifs remains challenging, given the requisite construction of 1,2-trans-glycosidic linkages within the linear rhamnan backbone and 1,2-cis-glycosidic linkages of the 1,6-isomaltose sidechain. In this study, we report the efficient synthesis of such branched glucorhamnans, including pentamer, decamer, and eicosamer. Conformational analysis, employing nuclear magnetic resonance experiments and molecular dynamics simulations, reveals that the glucorhamnans adopt an extended left-handed helicoidal conformation that regularly and continuously exposes the isomaltose sidechains. Biological assays demonstrate that the synthetic glucorhamnans effectively induce the release of tumor necrosis factor-α (TNF-α) and the secretion of interleukin-6 (IL-6) and interleukin-12 (IL-12) in bone marrow-derived dendritic cells (BMDCs).