17O NMR Study of Diamagnetic and Paramagnetic Lanthanide(III)-DOTA Complexes in Aqueous Solution.

The complexes between the polyaminocarboxylate DOTA ligand and the whole series of stable lanthanide(III) metal ions, except Gd3+, were studied in aq. soln. by 17O NMR. For all of the paramagnetic systems, the 17O NMR signals of both the nonchelating (O1) and chelating (O2) O atoms could be detected, and for some of them, the signals of both the SAP and TSAP (TSAP') conformational isomers were also obsd. Line width data anal. reveals that signal broadening is not dominated by paramagnetic relaxation enhancement, as it was believed to be. Quadrupole relaxation and, for some complexes, chem. exchange between the SAP and TSAP isomers are the major contributions to the 17O NMR line width at 25°. Besides, the Fermi contact and pseudocontact contributions to the obsd. lanthanide-induced shifts could be extd. The 17O hyperfine coupling consts. detd. for O2 in the SAP and TSAP isomers are similar to each other and to the values reported for several Gd(III) complexes comprising fast-exchanging ligands. Probably 17O NMR should prove to should be useful for the study of highly paramagnetic Gd(III) complexes of nonlabile ligands. [on SciFinder(R)]