Unexpected differences in the dynamics and in the nuclear and electronic relaxation properties of the isoelectronic [EuII(DTPA)(H2O)]3- and [GdIII(DTPA)(H2O)]2- complexes (DTPA = diethylenetriamine pentaacetate)

The [Eu-II(DTPA)(H2O)](3-) complex (DTPA = diethylenetriamine pentancetate) has been prepared by controlled potential coulometry from [Eu-III(DTPA)(H2O)](2-). [Eu-II(DTPA)(H2O)](3-) is less stable toward oxidation than Eu-(aq)(2+), as shown by its more negative redox potential (E-1/2 = -1.34 V in comparison to E-1/2 = -0.61 V vs calomel electrode, respectively). Nevertheless, the rate of oxidation was found to be reasonably slow in highly concentrated solutions. Variable-temperature and -pressure, multiple-field O-17 NMR and nuclear magnetic relaxation dispersion (NMRD) measurements have been performed on [Eu-II(DTPA)(H2O)](3-) in aqueous solution. The water-exchange rate (k(ex)(298) = 1.3 x 10(9) S-1) is 3 orders Of magnitude higher than that on the corresponding Gd(mj complex, and it is only slightly smaller than that on the Eu(II) aqua ion. The positive activation volume (Delta V-double dagger = +4.5 cm(3) mol(-1)) indicates a dissociatively activated water-exchange process. The rotational correlation time is slightly longer for [Eu-II(DTPA)(H2O)](3-) as compared to that for [Gd-III(DTPA)(H2O)](2-), which is explained by the higher number of water molecules hydrogen-bended to the carboxylates of the ligand in the highly charged Eu(II) chelate. The electronic relaxation parameters obtained from NMRD and low-field transverse O-17 relaxation rates indicate that electron spin relaxation is considerably faster on [Eu-II(DTPA)((HO)-O-2)](3-) than on EU(aq)2+ or on the isoelectronic [Gd-III(DTPA)(H2O)](2-). Possibilities to use Eu-II complexes as MRI contrast agents are discussed.