The structure and dynamics of the lanthanide(III) complexes of DTPA-BGLUCA3- (DTPA-bis(glucamide)), DTPA-BENGALAA3- (DTPA-bis(ethylenegalactamine−amide)), DTPA-BEA3- (DTPA-bis(ethanolamide)), and DTPA-BPDA3- (DTPA-bis(propanediolamide)) in water have been investigated. These complexes are of relevance as potential MRI contrast agents. 13C relaxation times of the Nd(III) complexes show octadentate binding of the organic ligand via the three amines, the three carboxylates, and the two amide oxygens. 17O NMR measurements indicate that the coordination sphere is completed by one water ligand. Eight diastereomeric pairs of isomers are observed for the DTPA-bis(sugaramides). Data sets obtained from variable-temperature and -pressure 17O NMR at 9.4 T and variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) on the Gd(III) complexes were fitted simultaneously to give insight into the parameters governing the water 1H relaxivity. The water exchange rates, k ex 298, on [Gd(DTPA-BPDA)(H2O)], [Gd(DTPA-BGLUCA)(H2O)] and [Gd(DTPA-BENGALAA)(H2O)] are 3.6 ± 0.3, 3.8 ± 0.2, and 2.2 ± 0.1 × 105 s-1, and the activation volumes are +6.7, +6.8, and +5.6 cm3 mol-1 (±0.2 cm3 mol-1), respectively, indicating a strongly dissociatively activated mechanism. The sugar moieties have no significant influence on the coordination of the Gd(III) ion and on the parameters governing the relaxivity, apart from the expected increase in the rotational correlation time. The relaxivity under the usual MRI conditions is limited by the water exchange rate and the electronic relaxation. The data obtained are used to explain the relaxivity of conjugates of polysaccharides and Gd(DTPA).

The lanthanide coordination of the macrocyclic ligands cy(DTPA-EN) and cy(DTPA-EN-DTPA-EN) was studied in aqueous solution. 17O NMR measurements on the DyIII complexes showed that, in both complexes, the first coordination sphere of the LnIII ion contains one water molecule, leaving eight coordination sites for the ligand molecule. 89Y and 139La NMR analysis confirmed that the coordination mode of cy(DTPA-EN-DTPA-EN) is similar to that of the acyclic DTPA-bis(amide) derivatives. However, as a result of the cyclic nature of the ligands considered, the number of isomers in solution is lower than for the acyclic compounds. From variable-temperature 1H and 13C NMR data, we conclude that, in the respective LnIII complexes in solution, the cy(DTPA-EN) ligand is present in two rapidly interconverting isomers, whereas the cy(DTPA-EN-DTPA-EN) ligand exists in four isomeric forms. Two types of exchange processes are observed for the cy(DTPA-EN-DTPA-EN) complexes; one is fast on the NMR time scale and does not require decoordination of the ligand, and the second is relatively slow and decoordination is necessary to realize the interconversion. The complexes of cy(DTPA-EN) and the lighter LnIII ions (Ln = La → Eu) precipitated, probably due to the formation of binuclear complexes. Comparisons are made with the previously studied acyclic DTPA-bis(amides).

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