Water Exchange and Rotational Dynamics of the Dimeric Gadolinium(III) Complex [BO{Gd(DO3A)(H 2 O)} 2 ]: A Variable-Temperature and -Pressure 17 O NMR Study 1

International audienceRapid water exchange and slow rotation are essential for high relaxivity MRI contrast agents. A variable-temperature and -pressure 17O NMR study at 14.1, 9.4, and 1.4 T has been performed on the dimeric BO(DO3A)2, 2,11-dihydroxy-4,9-dioxa-1,12-bis[1,4,7,10-tetraaza-4,7,10-tris(carboxymethyl)cyclododecyl]dodecane, complex of Gd(III). This complex is of relevance to MRI as an attempt to gain higher 1H relaxivity by slowing down the rotation of the molecule compared to monomeric Gd(III) complexes used as contrast agents. From the 17O NMR longitudinal and transverse relaxation rates and chemical shifts we determined the parameters characterizing water exchange kinetics and the rotational motion of the complex, both of which influence 1H relaxivity. The rate constant and the activation enthalpy for the water exchange, kex and ΔH⧧, are (1.0 ± 0.1) × 106 s-1and (30.0 ± 0.2) kJ mol-1, respectively, and the activation volume, ΔV⧧, of the process is (+0.5 ± 0.2) cm3 mol-1, indicating an interchange mechanism. The rotational correlation time becomes about three times longer compared to monomeric Gd(III) polyamino−polyacetate complexes studied so far: τR = (250 ± 5) ps, which results in an enhanced proton relaxivity by raising the correlation time for the paramagnetic interaction