BIOLOGICAL EVALUATION OF RATIONALLY MODIFIED ANALOGS OF THE H-TYPE-II BLOOD-GROUP TRISACCHARIDE - A CORRELATION BETWEEN SOLUTION CONFORMATION AND BINDING-AFFINITY

The role of local steric influences on the solution conformation and the biological activity of the II-type II blood group determinant 1 has been evaluated using structurally modified trisaccharides 2-4 and their corresponding C1-substituted C-glycosides 5-8 as conformational models. The preference of the C-glycosidic bond to adopt the gauche ''exo-anomeric'' conformation and the removal of destabilizing 1,3-diaxial-like interactions on the C-aglyconic bond have been used to create predictable conformational characteristics in C-trisaccharides 5-8. Vicinal coupling constants from H-1 NMR spectroscopy and 2D NOESY spectroscopy demonstrate that structural modifications in the C-trisaccharides result in large changes in their conformational preferences. To test the impact of solution conformation on receptor-ligand recognition, the affinities of compounds 1-8 toward the lectin I of Ulex europaeus (UEA-I) have been investigated using a quantitative binding assay. The binding affinities of the II-type II trisaccharide 1 and the corresponding carbon analog 5 are virtually identical. The activities of the structurally modified C-trisaccharides 6-8 decrease sharply relative to the unmodified C-trisaccharide 5, correlating conformation to binding affinity. A parallel gradient in binding affinity is observed for the O-trisaccharides 1-4. The selectivity of UEA-I for epitopes 1-8 validates the assumption that its receptor site largely defines a bound conformation for the substrates, and establishes that the conformational behavior of O-glycosides such as 1-4 is similar to that of C-glycosides such as 5-8.