SOLUTION THERMODYNAMICS OF AMINO-ACID 18-CROWN-6 AND AMINO-ACID CRYPTAND-222 COMPLEXES IN METHANOL AND ETHANOL - LINEAR ENTHALPY ENTROPY COMPENSATION EFFECT

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 acids 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.