Phosphorylated calixarenes as receptors of L-amino acids and dipeptides: Calorimetric determination of Gibbs energy, enthalpy and entropy of complexation

Thermodynamics of 5,17bis(dihydroxyphosphorylhydroxymethyl)-25,27-dipropoxy-calix[4]arene in pure Racemic form and 1:1 mixture of the Meso and Racemic forms (host molecule) interacting in methanol solution with free amino acids (guest molecule), and additionally with five dipeptides, has been studied using isothermal titration calorimetry. A moderate variation in the changes of enthalpy, entropy and Gibbs free energy, depending on the nature of the guest molecule as well as the stereomeric form of the host molecule, was observed. The stability constants in the range of 3000-17000 M-1 (25000-45000 M-1 for dipeptides) and enthalpy changes in the range of -10- - 2 kjmol(-1) (-10.5- - 5.9 for dipeptides) were evaluated experimentally by ITC. The decreased variation in the estimated Gibbs free energy (-25- - 20 kjmol(-1) for amino acids, and -26.5- - 25.3 for dipeptides, respectively) was attributed to the effect of enthalpy-entropy compensation. The complexation phenomenon was found driven by electrostatic interactions between protonated N-terminal amino group of the guest and calixarene phosphoryl groups. The complex stability correlates with the hydrophobicity of amino acid residues, indicating significant partition of the solvatophobic interactions.