Two new DTPA-bis(amide) derivatives containing stable nitroxide free radicals, H3L2 and H3L3, were synthesized. The complexation properties of L2 and L3 with Gd3+ were compared with those of H3L1, synthesized earlier. The sums of the protonation constants ∑logK iH of the DTPA-bis(amide) derivative ligands with stable nitroxide free radicals are some-what higher than that for DTPA-bis(methylamide) (H3L4). The hgands L1, L2, and L3 form complexes with a metal-to-ligand ratio of 1:1 with Gd 3+ (GdL), while the species formed with Zn2+ are ZnL, ZnHL, ZnH2L, and Zn2L. The formation of the dinuclear complex Zn2L results in a lower selectivity of the ligands for Gd3+ over Zn2+ than was assumed for L4. The relaxivities of the complexes GdL1, GdL2, and GdL 3 are close to the sums of the relaxivities of the complex GdL 4 and of the free ligands L1, L2, and L 3. The relaxivities of GdL1, GdL2, and GdL 3 are practically independent of temperature in the 7-30 °C range, which is probably a result of the relatively long lifetime of the water molecules in the inner spheres of the [GdL(H2O)] complexes. The kinetic stabilities of the Gd3+ complexes were characterized by the rates of the exchange reactions taking place between the complexes GdL and Lu3+ or Zn2+. The reactions occur through the proton-assisted dissociation of the complexes, with the formation of a protonated intermediate. Another pathway is the direct reaction between the Gd3+ complex and Lu3+ or Zn2+, which occurs with the formation of a dinuclear intermediate, GdLM. The exchange reactions proceed about five times more rapidly with Zn2+ than with Lu 3+, indicating the importance of the formation of an intermediate containing an N-Zn2+, bond, which is stronger than the N-Lu 3+ bond in an intermediate with similar structure. The exchange reactions of the complex GdL3 formed with the disubstituted ligand L3 are significantly slower than those of the complexes of the monosubstituted ligands L1 and L2.
ASJC Scopus subject areas
- Inorganic Chemistry