Synthesis of three tethered trisaccharides to probe entropy contributions in carbohydrate—protein interactions

Crystal structure data show that the branched trisaccharide 1 constitutes the complete antigenic determinant of the Salmonella serogroup B antigen that is recognized by monoclonal antibody SE155.4. In an effort to characterize the entropic costs associated with immobilization of glycosidic torsional angles in the bound state, three distinct intramolecularly tethered analogues of this trisaccharide 2-4 have been synthesized. Two trisaccharides are tethered by a methylene acetal via the O-2 position of the 3,6-dideoxy-hexose, abequose to either O-2 of galactose ( 2 ) or O-4 of mannose ( 3 ). The third tether, α,α′-di-thio-p-xylene, spans the C-6 atoms of the mannose and galactose residues to create trisaccharide 4 . The acetal tethers of 2 and 3 span hydroxyl centers that are known to be involved in intramolecular sugar-sugar hydrogens bonds, but both trisaccharides are biologically inactive due to distorted conformations that cannot be accommodated in the antibody binding site. Trisaccharide 4 is active since both hydroxymethyl groups of galactose and mannose are solvent exposed in the bound state and the constrained conformation of 4 is virtually superimposable on the bound conformation of 1 . Despite the retained complimentary and significant reduction of torsional flexibility, trisaccharide 4 exhibits a ΔG° = -7.6 kcal mol^?1 compared to ΔG° = -7.1 kcal mol^?1 for 1 . The modest free energy gain for the tethered trisaccharide 4 arises from a small entropy gain (TΔΔS = 0.3 kcal mol^?1) and an even smaller enthalpic change (ΔΔH = -0.2 kcal mol^?1).