COMPLEXATION STUDIES OF ZWITTERIONIC AMINO-ACIDS WITH CROWN AND GUANIDINIUM COMPOUNDS USING TITRATION CALORIMETRY

Log K, Delta G, Delta H, and T Delta S were determined by titration ca lorimetry for the reaction of amino acids in their zwitterionic form w ith bicyclic guanidinium salts as anchor groups for the carboxylate an d crown compounds as ammonium binding moiety in methanol at 25 degrees 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 highes t enthalpic contribution accompanied by a negative entropic term was o btained with 18-crown-6, The substitution of ether oxygen by both amin e and pyridine nitrogen leads to a decrease of enthalpic contribution. The introduction of bulky substituents into the macrocyclic ring syst em increases the reaction entropy. Yielding a medium value of complexa tion enthalpy and a slightly positive value of T Delta S the highest s tability constant was observed with pyridino 18-crown-6. (C) 1998 Else vier Science B.V.