NMR conformational study of the lanthanide(III) complexes of DOTA in aqueous solution

Variable temperature NMR spectroscopy studies of lanthanide(III) complexes with 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA) provide evidence of the presence of two isomeric forms in aqueous solution. An NMR study was performed in order to gain structural information on the two isomeric species, using a fitting procedure of calculated to experimental dipolar contributions to the paramagnetic shifts. The two isomers show to have identical macrocyclic ring conformations and differ only in the coordination or orientation of the acetate arms of the ligand, yielding either square prismatic and square anti-prismatic coordination geometries, or octadentate and heptadentate coordinations of the ligand relative to the lanthanide ion. Analysis of the dipolar shifts is consistent with both types of acetate conformational equilibria.