Synthesis of a Pentasaccharide Epitope for the Investigation of Carbohydrate - Protein Interactions

Article abstract of DOI:10.1021/jo00127a043

Pyranose residues of a polysaccharide that are not involved in the principal sugar - protein antibody combining site, filled by trisaccharide 1, cause a 50-fold reduction in intrinsic affinity.The antibody is crystallographically characterized, and the residue responsible for the lost binding energy has been identified as the terminal disaccharide Rha -> Gal of pentasaccharide 5.This disaccharide segment of 5 may avoid protein contact by adopting the "anti" conformer about the preceding Man-Rha glycosidic linkage.Monosaccharide thioglycoside synthons 6 - 9 were used in NIS-promoted glycosylations to synthesize the pentasaccharide as a glycoside that was suitable for binding and solution conformational studies, Disaccharide 29 was obtained upon the addition of rhamnose building unit 6 to the (trimethylsilyl)ethyl galactopyranoside 10 followed by protecting group manipulation.The sequential addition of 7 - 9 to 29 afforded the pentasaccharide derivative 35 bearing a 2-O-benzoate group suited for subsequent 1,2-trans-glycoside synthesis following its conversion to a glycosyl imidate.In order to preserve the integrity of the 3,6-dideoxyhexopyranosyl glycosidic bond during cleavage of the (trimethylsilyl)ethyl group leading to the imidate 39, it was essential to convert the benzylated pentasaccharide target 35 into its fully acylated derivative 37.Pentasaccharide 5 was obtained by transesterification of the protected glycoside 40 formed via 39.Qualitative NOE measurements suggests a predominant solution conformation for 5 that cannot be adopted in the bound state due to protein - oligosaccharide clashes at the periphery of the binding site.