Complexation studies of zwitterionic amino acids with crown and guanidinium compounds using titration calorimetry

Log K, ΔG, ΔH, and TΔS were determined by titration calorimetry for the reaction of amino acids in their zwitterionic form with bicyclic guanidinium salts as anchor groups for the carboxylate and crown compounds as ammonium binding moiety in methanol at 25°C. Under the experimental conditions chosen, bicyclic guanidines show very low enthalpic contribution (<-2 kJ mol^-1) due to their strong solvation by methanol. The complexation of the ammonium form of amino acids is strongly influenced by the ring size, nature and position of donor atoms of the macrocyclic crown compound investigated. The highest enthalpic contribution accompanied by a negative entropic term was obtained with 18-crown-6. The substitution of ether oxygen by both amine and pyridine nitrogen leads to a decrease of enthalpic contribution. The introduction of bulky substituents into the macrocyclic ring system increases the reaction entropy. Yielding a medium value of complexation enthalpy and a slightly positive value of TΔS the highest stability constant was observed with pyridino 18-crown-6.