Structural XAFS investigation of Eu2+ and Sr2+ poly(amino carboxylates):: Consequences for water exchange rates on MRI-relevant complexes

The structure of the Eu2+ and Sr2+ DOTA(4-) (1,4,7, 10-tetraazacyclododecane- 1,4,7,10-tetraacetate), DTPA(5-)(diethylenetriamine-N,N,N',N'',N"-pentaacetate), and ODDA (2-) (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate) complexes were characterized using XAFS in the solid state and in aqueous solution. The results show the structural similarity between the highly paramagnetic and MRI-relevant Eu2+ poly(amino carboxylate) complexes with their diamagnetic Sr2+ homologues in each state as well as the overall conservation of the solid-state structure in aqueous solution. The DOTA(4-) ligand adopts a twisted-square antiprism conformation in aqueous solution to accommodate the large Eu2+ and Sr2+ ions, leading to metal ion-to-coordinated water distances 0.2 Angstrom longer in the [M-II(DOTA)(H2O)](2-) complexes than in the [M-II(DTPA)(H2O)](3-) complexes (M-II = Eu2+, Sr2+). The different structures adopted by the complexes in aqueous solution were found to be responsible for their different water exchange mechanisms: changing from D (DTPA(5-), DOTA(4-)) for Gd3+ to I-d (DTPA(5-)), 1 (DOTA(4-)), and l(a) (ODDA(2-)) for the Eu2+ complexes. Finally, a lower charge density and substantially longer M-O-w distances for the Eu2+ ion explain the 3 orders of magnitude higher water exchange rates observed for the Eu2+ poly(amino carboxylates) over the corresponding Gd3+ complexes. Such high water exchange rates could be valuable in designing more efficient responsive contrast agents for magnetic resonance imaging.