The Gadolinium(III)−Water Hydrogen Distance in MRI Contrast Agents

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Abstract
The ion−nuclear distance of Gd(III) to a coordinated water proton, rGd-H, is central to the understanding of the efficacy of gadolinium-based MRI contrast agents. The dipolar relaxation mechanism operative for contrast agents has a 1/r6 dependence. Estimates in the literature for this distance span 0.8 Å (2.5−3.3 Å). This study describes a direct determination of rGd-H using the anisotropic hyperfine constant T⊥ determined from pulsed ENDOR spectra. Five Gd(III) complexes were examined: [Gd(H2O)8]3+, [Gd(DTPA)(H2O)]2-, [Gd(BOPTA)(H2O)]2-, MS-325, and [Gd(HP-DO3A)(H2O)]. The distance, rGd-H, was the same within error for all five complexes: 3.1 ± 0.1 Å. These distance estimates should aid in the design of new contrast agents, and in the interpretation of other molecular factors influencing relaxivity.