Synthesis and conformational analysis of a conformationally constrained trisaccharide, and complexation properties with concanavalin A

The trisaccharide beta-D-GlcNAc(1 --> 2)alpha-D-Man(1 --> 3)D-Man is a frag ment of a biantennary glycan that is recognized by alpha-D-Man-specific lec tins such as concanavalin A (ConA), Lathyrus ochrus lectin, lentil lectin, and can adopt several conformations upon binding. To probe the importance o f loss of flexibility of a saccharide during binding with ConA this trisacc haride has been synthesized in a conformationally constrained form where a methylene acetal bridge mimics a GlcNAc-O-6 "...Man-O-4 intramolecular hydr ogen bond. Microcalorimetry measurements revealed that the conformationally constrained compound has a more favorable entropy term but this term is of fset by a smaller enthalpy term. NMR spectroscopic studies have shown that the cyclic compound is indeed considerably less flexible than the linear co mpound and both compounds adopt mainly one conformation. SYBYL software tog ether with energy parameters appropriate for carbohydrates was used for a s ystematic conformational search. The linear compound is very flexible. A cl ustering method determined seven main conformational families. Six possible conformational families were identified for the cyclic compound when consi dering the orientations of the beta GlcNAc(1 --> 2)Man and alpha beta Man-( 1 --> 3)Man glycosidic bonds. The central mannose residue was docked in the binding site of ConA and the complex was refined. The results are compared with crystal structures of legume lectin oligosaccharide complexes and wit h the NMR and thermodynamic data.