MRI contrast agents based on water-soluble Gd-metallofullere

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The relaxivities of each Gd-Merenols (pure ; Rl=62 mM-1 sec-1 , R2=69 mM-1 sec-1 , mixture ; Rl=56 rnM-1 sec-1, R2=67 mM-1 sec-1) were much higher than that of a commercial MRI contrast agent, MagnevistR (Rl=3.8 mM-1 sec-1, R2=4.3 mM-1 sec-1).Gd-fullerenols showed high Rl throughout the range of magnetic fields. Further, the peak of Rl appeared nearby 1T. This field strength coincides with the range of common clinical MRI.A Gd3+ ion with 7 unpaired electrons in half filled 4f orbital has been known to exhibit a strong relaxation capability of proton nuclear spin of nearby water molecules. The conventional MRI contrast agents consisting of the Gd3+ -chejate complexes have one direct coordination window to a water molecule that rapidly exchanges with the bulk water molecules surrounding the Gd3+ ion, which leads to an efficient nuclear spin relaxation of solvent water molecules. In contrast, in the Gd-fullerenols, there are no water molecules which can form a direct coordination to the encaged Gd3+ ion. Although the details of the electronic and magnetic properties of the current Gd-fullerenols is not known at present, it is probable that Gd@C82(OH)p has the paramagnetic property due to three electron transfer from Gd to the fullerene cage [ Gd3+@C82(OH)n3-] resulting in seven unpaired electrons in the Gd-4f orbitals inside the C82 cage and one on the cage.