ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 37, Number 1
JKCSEZ 37(1)
February 20, 1993 

The stability constants for lanthanides complexes with optically active L-proline (1 : 1) were determined in aqueous solution in the ionic medium of 0.1 M NaClO4 at 25℃ using a pH titration method. The results show called "gadolinium break" between lighter and heavier lanthanides. The linear relation between the stability constant (logβ1) and the pKa values of ligands indicates that L-proline acts as a bidentate ligand in the complexation. The thermodynamic parameters (△H and △S) were also determined using an enthalpy titration method at the same condition. The positive endothermic enthalpy change and positive entropy change clearly indicate that the driving force for the complexation is an entropy effect. The comparison of the thermodynamic parameters of L-proline complexes with anthranilate complexes supports the conclusion that the heterocyclic nitrogen atom and carboxylate of L-proline are involved in the chleate formation. The enthalpy values for L-proline are more positive than the ones for anthranilate complex. The difference in enthalpy change for the complex formation between L-proline complex and anthranilate complex is explained in terms of the basicity of the nitrogen donor atom in the ligand. The relatively large entropy change may be described by the extra dehydration related to the rigidity of L-proline ring.