NMR, relaxometric, and structural studies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands

Research was conducted to examine the solution structure and dynamics of metal-bound water exchange in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. The relative proportions of the cationic gadolinium complexes of 12-N4-based tetraamide complexes that exist in solution as two isomers were governed by the nature of the amide substituents. Results indicate that new contrast agents may be developed by devising 'tetraazacyclododecane tetraacetate-like' complex structures wherein the proportion of the isomer in solution is maximized.