The formation of amino acid and dipeptide complexes with α-cyclodextrin and cucurbit[6]uril in aqueous solutions studied by titration calorimetry

The complex stabilities and the thermodynamic data for the complexation of α-cyclodextrin and cucurbit[6]uril with some amino acids (glycine, l-alanine, l-valine, l-phenylalanine, 6-amino hexanoic acid, 8-amino octanoic acid, 11-amino undecanoic acid) and dipeptides (glycyl–glycine, glycyl–l-valine, glycyl–l-leucine and glycyl–l-phenylalanine) have been determined in aqueous solution by calorimetric titrations. The complex formation with α-cyclodextrin is mainly favoured by entropic contributions due to the release of water molecules from the cavity of the ligand. The values of the reaction enthalpies are small with the exception of 11-amino undecanoic acid. In case of the ligand cucurbit[6]uril, ion–dipole interactions between the protonated amino groups of the amino acids and the carbonyl groups take place. By steric reasons these interactions are lowered for native amino acids because the polar carboxylic groups are always located outside the hydrophobic cavity of cucurbit[6]uril. The complexes of both ligands with dipeptides in water are characterised by hydrophobic interactions and in case of cucurbit[6]uril by additional ion–dipole interactions.