Calorimetric, NMR, and UV investigations of aliphatic L-amino acids complexation by calix[4]arene bis-hydroxymethylphosphous acid

The stability constants, enthalpy Delta H-0, entropy Delta S-0, and Gibbs energy Delta G(0) were determined for the host-guest complexes (1:1) of calix[4]arene bis-hydroxymethylphosphous acid with glycine, L-alanine, L-valine, L-leucine, L-isoleucine residues in methanol solution with the aid of the titration experiments followed by calorimetric and spectroscopic (H-1 NMR, UV) methods. The experimental data indicated that the host-guest complexation was under control of the direct electrostatic interaction between negatively charged calixarene phosphoryl group and amino acid residue NH3+ group, modulated by the hydrophobic interaction, which drive the inclusion of the residue alkyl side-chain into the calixarene cavity. The stability of the inclusion complexes was found correlated with the size of the aliphatic amino acid's side-chain. The experimental data were additionally analyzed in the terms of the three state model corresponding to coexistence of 2:1 and 1:1 complexation equilibria.