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

Area of Science:

• Medicinal Chemistry
• Carbohydrate Chemistry
• Structural Biology

Background:

• Sialyl Lewis^x (sLe^x) is a physiological E-selectin ligand crucial for cell adhesion.
• Its binding affinity is largely attributed to favorable entropy from pre-organization in the bound conformation.
• A specific non-classical hydrogen bond (C-H...O) between l-fucose and d-galactose moieties is vital for this pre-organization.

Purpose of the Study:

• To investigate the strengthening of the non-classical hydrogen bond in sLe^x derivatives.
• To enhance the pre-organization of sLe^x mimetics in their bioactive conformation.
• To improve the pharmacokinetic properties, including acid-stability, of sLe^x mimetics.

Main Methods:

• Synthesis of sLe^x derivatives incorporating electron-withdrawing groups, specifically 6,6,6-trifluoro-l-fucose.
• High-field Nuclear Magnetic Resonance (NMR) spectroscopy to analyze chemical shifts.
• X-ray diffraction analysis of co-crystallized glycomimetics with E-selectin.
• Isothermal titration calorimetry (ITC) to quantify binding thermodynamics.

Main Results:

• Replacing l-fucose with 6,6,6-trifluoro-l-fucose significantly strengthened the non-classical hydrogen bond from 7.4 kJ/mol to 14.9 kJ/mol.
• This stabilization led to improved pre-organization in the bioactive conformation of sLe^x mimetics.
• NMR, X-ray crystallography, and ITC confirmed the enhanced binding and pre-organization.
• The trifluoromethyl group improved acid-stability and other pharmacokinetic properties.

Conclusions:

• Electron-withdrawing groups effectively strengthen non-classical C-H...O hydrogen bonds.
• Enhanced hydrogen bonding and pre-organization improve the affinity and stability of sLe^x mimetics.
• Trifluorinated sLe^x derivatives show potential for improved therapeutic applications due to enhanced stability.