The capsular polysaccharide Vi from Salmonella typhi: synthesis and molecular dynamic simulations of short analogue fragments

Vi capsular polysaccharide (Vi antigen) is 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. Vaccination with purified Vi antigen from Salmonella typhi can protect against typhoid fever, although many aspects of the mechanism of action have yet to be established. It has been demonstrated that the immunogenicity of Vi is strongly related to the content of O-acetyl groups and seems not related to the presence of the carboxyl groups. In fact, the acetate groups dominate the molecular surface of the polysaccharide and confer hydrophobic properties to it, probably shielding the carboxylic groups from interaction with other molecules, even if only partial 3-O-acetylation seems necessary to maintain the flexibility of the molecules. Being interested in the study of the role of the negative charge of the Vi biopolymer on the biological activity, we have planned the preparation of analogues where the carboxylic group has been substituted with a pH-independent ionizable group, i.e. the sulfate group. The sulfate group has been selected after preliminary investigations through molecular dynamics simulations on a hexasaccharide Vi antigen fragment, that showed similarities between the conformational behavior of the natural antigen and the sulfate analogue, where the galacturonic residues have been replaced with 6-O-sulfo-galactoses. Herein we will report our results on the conformational analysis and the stereoselective synthesis of Vi antigen sulfated-analogue fragments.