| Abstract
| - The water-soluble endohedral gadofullerene derivatives, Gd@C60(OH)x and Gd@C60[C(COOH)2]10,have been characterized with regard to their MRI contrast agent properties. Water-proton relaxivities havebeen measured in aqueous solution at variable temperature (278−335 K), and for the first time forgadofullerenes, relaxivities as a function of magnetic field (5 × 10-4 to 9.4 T; NMRD profiles) are alsoreported. Both compounds show relaxivity maxima at high magnetic fields (30−60 MHz) with a maximumrelaxivity of 10.4 mM-1 s-1 for Gd@C60[C(COOH)2]10 and 38.5 mM-1 s-1 for Gd@C60(OH)x at 299 K. Variable-temperature, transverse and longitudinal 17O relaxation rates, and chemical shifts have been measured atthree magnetic fields (B = 1.41, 4.7, and 9.4 T), and the results point exclusively to an outer sphere relaxationmechanism. The NMRD profiles have been analyzed in terms of slow rotational motion with a long rotationalcorrelation time calculated to be τR298 = 2.6 ns. The proton exchange rate obtained for Gd@C60[C(COOH)2]10is kex298 = 1.4 × 107 s-1 which is consistent with the exchange rate previously determined for malonic acid.The proton relaxivities for both gadofullerene derivatives increase strongly with decreasing pH (pH: 3−12).This behavior results from a pH-dependent aggregation of Gd@C60(OH)x and Gd@C60[C(COOH)2]10, whichhas been characterized by dynamic light scattering measurements. The pH dependency of the protonrelaxivities makes these gadofullerene derivatives prime candidates for pH-responsive MRI contrast agentapplications.
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