COORDINATION EQUILIBRIA AND WATER EXCHANGE KINETICS OF LANTHANIDE(III) PROPYLENEDIAMINETETRAACETATES AND OTHER MAGNETIC-RESONANCE-IMAGING RELATED COMPLEXES

The UV-Vis absorption spectra of aqueous solutions of Eu3+ complexes with the hexadentate polyaminocarboxylate ligands EDTA(4-), CDTA(4-), HDTA(3-) and PDTA(4-) were measured as a function of temperature and pressure in the frequency region corresponding to the F-7(0)-->D-5(0) transition of Eu3+. The results can be explained in terms of equilibria between nine-coordinate and eight-coordinate species where the eight-coordinate species contain one less inner sphere water molecule than the nine-coordinate species. The thermodynamic parameters, including the reaction volume, for these equilibria were determined. O-17 NMR transverse relaxation rates and chemical shifts were measured for aqueous solutions of the eight-coordinate complexes [Ln(PDTA)(H2O)(2)](-) (Ln = Tb, Dy, Er, Tm, Yb) and [Er(EDTA)(H2O)(2)](-) as a function of temperature, pressure and magnetic field. The results were analysed in terms of the water exchange kinetics on the complexes. The water exchange rate on [Ln(PDTA)(H2O)(2)](-) decreases dramatically with decreasing ionic radius across the lanthanide series from k(ex)(298) = (2.4 +/- 0.1) X 10(7) s(-1) for Ln = Tb to k(ex)(298) = (2.8 +/- 0.3) X 10(5) s(-1) for Ln = Yb. The activation volumes show that this is accompanied by a change of exchange mechanism from associatively activated for Ln = Tb (Delta V-# = -7.6+/-0.3 cm(3) mol(-1)) to dissociatively activated for Ln = Yb (Delta V-# = +7.4 +/- 0.8 cm(3) mol(-1)). Water exchange on [Er(EDTA)(H2O)(2)](-) (k(ex)(298) = (9.8 +/- 1.9) X 10(6) s(-1)) is more than an order of magnitude faster than on [Er(PDTA)(H2O)(2)](-) (k(ex)(298) = (5.6 +/- 0.5) X 10(6) S-1). These kinetic results can be interpreted in terms of the equilibria measured by UV-Vis spectrophotometry. The implications of these observations for the design of new MRI contrast agents are discussed.