| Abstract
| - The structure of the Eu2+ and Sr2+ DOTA4- (1,4,7,10-tetraazacyclododecane-1,4,7,10-tertraacetate), DTPA5-(diethylenetriamine-N,N,N‘,N‘ ‘,N‘ ‘-pentaacetate), and ODDA2- (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate) complexes were characterized using XAFS in the solid state and in aqueous solution. Theresults show the structural similarity between the highly paramagnetic and MRI-relevant Eu2+ poly(aminocarboxylate) complexes with their diamagnetic Sr2+ homologues in each state as well as the overall conservationof the solid-state structure in aqueous solution. The DOTA4- ligand adopts a twisted-square antiprismconformation in aqueous solution to accommodate the large Eu2+ and Sr2+ ions, leading to metal ion-to-coordinated water distances 0.2 Å longer in the [MII(DOTA)(H2O)]2- complexes than in the [MII(DTPA)(H2O)]3-complexes (MII = Eu2+, Sr2+). The different structures adopted by the complexes in aqueous solution werefound to be responsible for their different water exchange mechanisms: changing from D (DTPA5-, DOTA4-)for Gd3+ to Id (DTPA5-), I (DOTA4-), and Ia (ODDA2-) for the Eu2+ complexes. Finally, a lower chargedensity and substantially longer M−Ow distances for the Eu2+ ion explain the 3 orders of magnitude higherwater 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 formagnetic resonance imaging.
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