Solution thermodynamics of amino acid–18-crown-6 and amino acid–cryptand 222 complexes in methanol and ethanol. Linear enthalpy–entropy compensation effect

Abstract

Stability constants, free energies, enthalpies and entropies of complexation of amino acids with synthetic macrocyclic ligands (18-crown-6 and cryptand 222) in methanol and ethanol are reported. No significant variations are found in the free energies of complexation of the various amino acids with these two ligands as a result of a remarkable enthalpy–entropy compensation effect. Free energies of transfer of amino acids and recently isolated amino acid–18-crown-6 and amino acid–cryptand 222 complexes have been mostly obtained from solution thermodynamic data of the free and complexed amino acids in the appropriate solvents. Analysis of transfer-free-energy data indicates that in the amino acid–macrocyclic complex, most of the amino acid molecule is fully exposed to the reaction medium. Previous suggestions on the linear enthalpy–entropy relationship for complexation reactions involving crown ethers and cryptands with metal and organic cations in different reaction media are discussed. The implications of the results obtained for amino acid–macrocyclic complexation reactions to processes of biological importance are considered.