SYNTHESIS OF A PENTASACCHARIDE EPITOPE FOR THE INVESTIGATION OF CARBOHYDRATE-PROTEIN INTERACTIONS

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 suggest 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.