Developing high affinity oligosaccharide inhibitors: conformational pre-organization paired with functional group modification.

Intramolecular tethering combined with functional group modification has been investigated as an approach to design high affinity oligosaccharide ligands. The preceding paper reported successful tethering to constrain a trisaccharide in the conformation of its bound state with an antibody and thereby achieved a 15-fold increase in association constant. Here we report the synthesis of two beta-alanyl tethered derivatives that employ monochlorination and monodeoxygenation strategies to create inhibitors that should enhance the binding affinity of the target molecules by an additional 10-25-fold, provided that free energy changes are additive when tethering is paired with functional group changes. The binding parameters of the new ligands were measured by isothermal titration calorimetry and the results rationalized with molecular dynamics calculations and a simple docking analysis. The data indicate that while the alanine tether is a reasonable method to constrain trisaccharide , free energy gains obtained by pairing it with functional group modification are not additive and in one case counter-productive.