WATER EXCHANGE AND ROTATIONAL-DYNAMICS OF THE DIMERIC GADOLINIUM(III)COMPLEX [BO(GD(DO3A)(H2O))(2)] - A VARIABLE-TEMPERATURE AND VARIABLE-PRESSURE O-17 NMR-STUDY

Rapid water exchange and slow rotation are essential for high relaxivi ty MRI contrast agents. A variable-temperature and -pressure O-17 NMR study at 14.1, 9.4, and 1.4 T has been performed on the dimeric BO(DO3 A)(2), y-4,9-dioxa-1,12-bis[1,4,7,10-tetraaza-4,7,10-tris (carboxymeth yl)cyclododecyl]dodecane, complex of Gd(III). This complex is of relev ance to MRI as an attempt to gain higher H-1 relaxivity by slowing dow n the rotation of the molecule compared to monomeric Gd(III) complexes used as contrast agents. From the O-17 NMR longitudinal and transvers e relaxation rates and chemical shifts we determined the parameters ch aracterizing water exchange kinetics and the rotational motion of the complex, both of which influence H-1 relaxivity. The rate constant and the activation enthalpy for the water exchange, k(ex) and Delta H dou ble dagger, are (1.0 +/- 0.1) x 10(6) s(-1) and (30.0 +/- 0.2) kJ mol( -1), respectively, and the activation volume, Delta V double dagger, o f the process is (+0.5 +/- 0.2) cm(3) mol(-1), indicating an interchan ge mechanism. The rotational correlation time becomes about three time s longer compared to monomeric Gd(III) polyamino-polyacetate complexes studied so far: tau(R) = (250 +/- 5) ps, which results in an enhanced proton relaxivity by raising the correlation time for the paramagneti c interaction.