Hyperfine Coupling Constants on Inner-Sphere Water Molecules of Gd-III-Based MRI Contrast Agents
Herein we present a theoretical investigation of the hyperfine coupling constants (HFCCs) on the inner-sphere water molecules of [Gd(H2O)(8)](3+) and different Gd-III-based magnetic resonance imaging contrast agents such as [Gd(DOTA)(H2O)](-), [Gd(DTPA)(H2O)](2-), [Gd(DTPA-BMA)(H2O)] and [Gd(HP-DO3A)(H2O)]. DFT calculations performed on the [Gd(H2O)(8)](3+) model system show that both hybrid-GGA functionals (BH&HLYP;, B3PW91 and PBE1PBE) and the hybrid meta-GGA functional TPSSh provide O-17 HFCCs in close agreement with the experimental data. The use of all-electron relativistic approaches based on the DKH2 approximation and the use of relativistic effective core potentials (RECP) provide results of essentially the same quality. The accurate calculation of HFCCs on the [Gd(DOTA)(H2O)](-), [Gd(DTPA)(H2O)](2-), [Gd(DTPA-BMA)(H2O)] and [Gd(HP-DO3A)(H2O)] complexes requires an adequate description of solvent effects. This was achieved by using a mixed cluster/continuum approach that includes explicitly two second-sphere water molecules. The calculated isotropic O-17 HFCCs (A(iso)) fall within the range 0.40-0.56 MHz, and show deviations from the corresponding experimental values typically lower than 0.05 MHz. The Aiso values are significantly affected by the distance between the oxygen atom of the coordinated water molecule and the GdIII ion, as well as by the orientation of the water molecule plane with respect to the Gd-O vector. H-1 HFCCs of coordinated water molecules and O-17 HFCCs of second-sphere water molecules take values close to zero.