Thermodynamics and NMR studies of DTPA-bis(methoxyethylamide) and its derivatives. Protonation and complexation with Ln(III)

The thermodynamic parameters (Delta G, Delta H, Delta S) of protonation and of complexation of trivalent La, Nd, Gd, Dy and Lu with DTPA-bis(methoxyethylamide) (MEA), DTPA-bis(methylmethoxyethylamide) (MEA) and DTPA-bis(hydroxyethylmethoxyethylamide) (HEMEA) have been measured by potentiometric and calorimetric titration at I=0.1 M and 25 degrees C. Especially in the first two protonation steps, there are significant differences in the pK(a) and the protonation enthalpy among the three DTPA-bis( amides) and DTPA. The fractional proton population determined by NMR shifts for each basic site of MEA and MMEA is compared with the thermodynamic parameters of those compounds and discussed in terms of the role of NH and CO of the amide groups in the protonation. The complexation parameters are compared with other Ln-polyaminopolycarboxylate complexes to understand the role of the carboxylate, amino and methoxyethylamide groups of the DTPA-bis( amides). The luminescence lifetime of the Eu(III) complexes of the bisamide ligands indicated the coordination of a single water molecule to the metal in the aqueous complexes which is evidence that the DTPA-bis(amide) ligands have octadentate coordination with the Ln(III) ions.