Strengthening an Intramolecular Non-Classical Hydrogen Bond to Get in Shape for Binding

In this research article, we report on the strengthening of a non-classical hydrogen bond (C-H⋅⋅⋅O) by introducing electron withdrawing groups at the carbon atom. The approach is demonstrated on the example of derivatives of the physiological E-selectin ligand sialyl Lewis^x (1, sLe^x). Its affinity is mainly due to a beneficial entropy term, which is predominantly caused by the pre-organization of sLe^x in its binding conformation. We have shown, that among the elements responsible for the pre-organization, the stabilization by a non-classical hydrogen bond between the H-C5 of l-fucose and the ring oxygen O5 of the neighboring d-galactose moiety is essential and yields 7.4 kJ mol^-1. This effect could be further strengthened by replacing l-fucose by 6,6,6-trifluoro-l-fucose leading to an improved non-classical H-bond of 14.9 kJ mol^-1, i.e., an improved pre-organization in the bioactive conformation. For a series of glycomimetics of sLe^x (1), this outcome could be confirmed by high field NMR-shifts of the H-C5^Fuc, by X-ray diffraction analysis of glycomimetics co-crystallized with E-selectin as well as by isothermal titration calorimetry. Furthermore, the electron-withdrawing character of the CF₃-group beneficially influences the pharmacokinetic properties of sLe^x mimetics. Thus, acid-stability, a prerequisite for gastrointestinal stability, could be substantially improved.