Molecular recognition thermodynamics of pyridine derivatives by sulfonatocalixarenes at different pH values

The complex stability constants (Ka) and thermodynamic parameters (DeltaG degrees, DeltaH degrees, and TDeltaS degrees) for 1:1 complexation of water-soluble calix[4]arene, thiacalix[4]arene, and calix[5]arene sulfonates with pyridine and their methylated derivatives have been determined by means of isothermal titration calorimetry at pH 2.0 and 7.2 at 298.15 K, and their binding modes have been investigated by NMR spectroscopy. The results obtained show that sulfonatocalixarenes afford stronger binding ability toward pyridine guests at pH 2.0, attributable to the positive electrostatic interactions and the more extensive desolvation effects, but present higher molecular selectivity at pH 7.2 owing to the strengthened C-H...pi interactions. The pH-responsible binding ability and molecular selectivity are discussed from the viewpoint of electrostatic, pi-stacking, van der Waals interactions and size-fit relationship between host and guest. A close comparison further demonstrates that the C-H...pi interactions and van der Waals interactions play a more important role than pi...pi interactions in the present inclusion complexation.