Similarities and differences between the isoelectronic GdIII and EuII complexes with regard to MRI contrast agent applications
Résumé
In solution, the [EuII(DTPA)(H2O)]3−, [EuII(DOTA)(H2O)]2−, [EuII(ODDM)]2− and [EuII(ODDA)(H2O)] complexes are less stable towards oxidation than [Eu(H2O)8]2+, as shown by their redox potentials (E1/2=−1.35, −1.18, −0.92, −0.82 and −0.63 V versus Ag ∣ AgCl electrode, respectively) (DTPA5−=diethylenetriamine-N,N,N′,N″,N″-pentaacetate, DOTA4−=1,4,7,10-tetraazacyclododecane-1,4,7,10-tertraacetate, ODDM4−=1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-dimalonate and ODDA2−=1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate).
In the solid state, the [EuII(DTPA)(H2O)]3− complex is nine-coordinate with one inner-sphere water molecule. Its structure is practically identical with that obtained for the SrII analogue, and very similar to that of [Gd(DTPA)(H2O)]2− (the bond angles are equal, but the metal–nitrogen and metal–oxygen bond distances are longer for the EuII than for the GdIII complex).
In contrast to similar GdIII poly(amino carboxylate) complexes in general, the rate of water exchange for [EuII(DTPA)(H2O)]3− and [EuII(ODDA)(H2O)] is only slightly diminished as compared to the aqua ion (kex298=1.3×109, 0.43×109 and 4.4×109 s−1 for [EuII(DTPA)(H2O)]3−, [EuII(ODDA)(H2O)] and [Eu(H2O)8]2+, respectively). The mechanism of the water exchange, as evidenced by the activation volumes, is limiting associative (A) for [Eu(H2O)8]2+ (ΔV#=−11.3 cm3 mol−1), dissociative interchange (Id) for [EuII(DTPA)(H2O)]3− (ΔV#=+4.5 cm3 mol−1) and associative interchange (Ia) for [EuII(ODDA)(H2O)] (ΔV#=−3.9 cm3 mol−1).
Electron spin relaxation is faster for the EuII complexes studied than for corresponding GdIII analogues. Proton relaxivities of [Eu(H2O)8]2+, [EuII(DTPA)(H2O)]3− and [EuII(ODDA)(H2O)] are lower in the proton Larmor frequency range 0–200 MHz than those of the analogue GdIII chelates. The analysis of the NMRD profiles reveals that, beside fast rotation, for [Eu(H2O)8]2+ the extremely fast water exchange rate, whereas for [EuII(DTPA)(H2O)]3− and [EuII(ODDA)(H2O)] fast electron spin relaxation also limits proton relaxivity.