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Structure and Dynamics of a Trinuclear Gadolinium(III) Complex: The Effect of Intramolecular Electron Spin Relaxation on Its Proton Relaxivity1

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posted on 17.07.1998, 00:00 authored by Éva Tóth, Lothar Helm, André E. Merbach, Roman Hedinger, Kaspar Hegetschweiler, András Jánossy
The trinuclear [Gd3(H-3taci)2(H2O)6]3+ complex has been characterized in aqueous solution as a model compound from the point of view of MRI: the parameters that affect proton relaxivity have been determined in a combined variable temperature, pressure, and multiple-field 17O NMR, EPR, and NMRD study. The solution structure of the complex was found to be the same as in solid state: the total coordination number of the lanthanide(III) ion is 8 with two inner-sphere water molecules. EPR measurements proved a strong intramolecular dipole−dipole interaction between Gd(III) electron spins. This mechanism dominates electron spin relaxation at high magnetic fields (B > 5 T). Its proportion to the overall relaxation decreases with decreasing magnetic field and becomes a minor term at fields used in MRI. Consequently, it cannot increase the electronic relaxation rates to such an extent that they limit proton relaxivity. [Gd3(H-3taci)2(H2O)6]3+ undergoes a relatively slow water exchange (kex298 = (1.1 ± 0.2) × 107 s-1) compared to the Gd(III) aqua ion, while the mechanism is much more associatively activated as shown by the activation volume (ΔV = (−12.7 ± 1.5) cm3 mol-1). The lower exchange rate, as compared to [Gd(H2O)8]3+ and [Gd(PDTA)(H2O)2]-, can be explained with the higher rigidity of the [Gd3(H-3taci)2(H2O)6]3+ which considerably slows down the transition from the eight-coordinate reactant to the nine-coordinate transition state. The unexpectedly low rotational correlation time of the complex is interpreted in terms of a spherical structure with a large hydrophobic surface avoiding the formation of a substantial hydration sphere around [Gd3(H-3taci)2(H2O)6]3+.

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