Effective magnetic moment in cyclodextrin-polynitroxides: Potential supramolecular vectors for magnetic resonance imaging

Nitroxides have great potential as contrast agents for Magnetic Resonance Imaging (MRI). Two ?-cyclodextrin (?CD) derivatives bearing one or seven (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) units on the small rim of ?CD (CD3 and CD6 respectively) were synthesized. Their effective magnetic moments were measured by DC-SQUID magnetometry obtaining the values ?eff/?B ? 1.7 and ?eff/?B ? 4.2 for CD3 and CD6 respectively. Interestingly, while isothermal magnetization data of CD3 were well described by a Brillouin function for a S = 1/2 single spin system, those associated with CD6 could not be explained in the framework of a non-interacting spins model. For this reason, four different configurations for the seven interacting nitroxides were considered and modeled. The numerical results evidenced that only the configurations with a privileged central spin could take into account the experimental observations, thus justifying the reduced effective magnetic moment of CD6. The water relaxivity (r1) in DMSO-d6-water (9:1 v:v) solutions was also measured for both the derivatives obtaining the values r1 = 0.323 mM^-1 s^-1 and r1 = 1.596 mM^-1 s^-1 for CD3 and CD6 respectively.