The synthesis of chemically modified disaccharide derivatives of the Shigella flexneri Y polysaccharide antigen

Characterization of functional oligosaccharide mimics of the Shigella flexneri serotype 2a O-antigen: implications for the development of a chemically defined glycoconjugate vaccine

Protection against reinfection with noncapsulated Gram-negative bacteria, such as Shigella, an enteroinvasive bacterium responsible for bacillary dysentery, is mainly achieved by Abs specific for the O-Ag, the polysaccharide part of the LPS, the major bacterial surface Ag. The use of chemically defined glycoconjugates encompassing oligosaccharides mimicking the protective determinants carried by the O-Ag, thus expected to induce an efficient anti-LPS Ab response, has been considered an alternative to detoxified LPS-protein conjugate vaccines. The aim of this study was to identify such functional oligosaccharide mimics of the S. flexneri serotype 2a O-Ag. Using protective murine mAbs specific for S. flexneri serotype 2a and synthetic oligosaccharides designed to analyze the contribution of each sugar residue of the branched pentasaccharide repeating unit of the O-Ag, we demonstrated that the O-Ag exhibited an immunodominant serotype-specific determinant. We also showed that elongating the oligosaccharide sequence improved Ab recognition. From these antigenicity data, selected synthetic oligosaccharides were assessed for their potential to mimic the O-Ag by analyzing their immunogenicity in mice when coupled to tetanus toxoid via single point attachment. Our results demonstrated that induction of an efficient serotype 2a-specific anti-O-Ag Ab response was dependent on the length of the oligosaccharide sequence. A pentadecasaccharide representing three biological repeating units was identified as a potential candidate for further development of a chemically defined glycoconjugate vaccine against S. flexneri 2a infection.

Precise structure activity relationships in asymmetric catalysis using carbohydrate scaffolds to allow ready fine tuning: dialkylzinc–aldehyde additions

The ready construction of 24 stereochemically and functionally diverse carbohydrate ligand structures from a core D-glucosamine scaffold has allowed the evaluation of broad ranging structure activity relationships in ligand accelerated zincate additions to aldehydes, with variations in deltadeltaG+/+(R-S) of up to 5650 J mol(-1) that create opposing senses of asymmetric induction and that are consistent with models based on several ligand X-ray structures and molecular mechanics analysis. Factorial analysis of enantioselectivity using key dihedral angles and steric volume on N-2 also highlight the potential for the use of factorial design in ligand construction.

Synthesis of disaccharide congeners of the Trichinella spiralis glycan and binding site mapping of two monoclonal antibodies

The tetrasaccharide epitope, β-D-Tyvp(1[Formula: see text]3)β-D-GalNAcp(1[Formula: see text]4)[α-L-Fucp(1[Formula: see text]3)]β-D-GlcNAcp (1) is the major constituent of the N-glycan expressed on the cell surface of the parasite Trichinella spiralis. Two monoclonal antibodies (Mabs 9D4 and 18H1) that protect rats against infection by T. spiralis bind the terminal disaccharide epitope β-D-Tyvp(1[Formula: see text]3)β-D-GalNAcp conjugated to BSA. The syntheses of disaccharide congeners containing mono-deoxy, mono-methyl, as well as modifications to replace the acetamido group are reported. These target disaccharides were assayed for binding to the protective MAbs. For each antibody different clusters of three hydroxyl groups, that include C-2 and C-4 of tyvelose and for 18H1, the GalNAc acetamido group, provide the key polar interactions with the antibody binding sites. Mapping of the sites by functional group replacement revealed a similar pattern of recognition for the dideoxyhexose by the two MAbs while each recognizes distinct surfaces of the GalNAc residue. Consequently although both antibodies bury the 4-OH of tyvelose, the principal contact surface occurs on opposite sides of the 3,6-dideoxyhexose.Key words: β-tyveloside, 3,6-dideoxy-D-arabino-hexose, Trichinella carbohydrate antigen, antibody mapping, Trichinella spiralis, N-glycans, molecular recognition of carbohydrates, antigen topology, functional group replacement.

Synthesis of specifically deoxygenated disaccharide derivatives of the Shigella dysenteriae type 1 O-antigen

The synthesis of methyl O-alpha-L-rhamnopyranosyl-(1-->2)-alpha-D-galactopyranosides specifically deoxygenated at position 2 (31), or 4 (21) of the rhamnopyranosyl residue was accomplished using methyl 3,4,6-tri-O-benzoyl-alpha-D-galactopyranoside (18) as the glycosyl acceptor. Phenyl thionocarbonate activation of the penta-O-benzoylated disaccharide precursor followed by Barton reduction and Zemplén transesterification gave 31, while 21 was obtained via condensation of the deoxygenated monosaccharide donor with 18, and subsequent debenzoylation of the product.