Synthesis and molecular dynamic simulations of a Vi antigen sulfated analogue

Vi capsular polysaccharide (Vi antigen) was first identified as the virulence factor of Salmonella enterica serovar typhi, the causative agent of typhoid fever in humans. It is a linear homopolymer made up of alpha-(1→4)-linked N-acetyl galactosaminuronic acid with a 60-70% O-acetylation at C-3. This antigen is highly immunogenic and has been used for the formulation of one of the currently available vaccines against typhoid.1 In the context of vaccine development, the search for carbohydrate analogues is stimulated by the intrinsic instability of several polysaccharide vaccines from natural sources or by the need to understand/control the immunological events under analysis. We are interested in the preparation of oligomeric sulfated analogues of Vi antigen, i.e. analogues in which the 6-carboxylic group has been substituted with a sulfate, to study the influence of the sulfo pH-independent ionizable group on the biological activity. The modification from galacturonic to 6-O-sulfogalactose does not actually generate an isosteric analogue, but in principle should not significantly interfere with the immunostimulating activity of Vi antigen. Indeed, it has been demonstrated that the immunogenicity of Vi is strongly related to the content of O-acetyl groups and not to the presence of the carboxyl group.2 In fact, the acetate groups confer the hydrophobic properties to the molecular surface and dominate the surface of the polysaccharide, probably shielding the carboxyls from interaction with other molecules, even if only partial 3-O-acetylation seems necessary to maintain the flexibility of the molecules.3 So, the modification of position 6 should not influence the biological activity provided that the conformational properties at the alpha-(1→4) glycosidic linkage, as well as the overall geometry of the polymer, are maintained. Herein we will report the conformational analysis and a stereoselective synthesis of a Vi antigen sulfated-analogue dimeric fragment.