Effective magnetic moment in cyclodextrin–polynitroxides: potential supramolecular vectors for magnetic resonance imaging

Abstract

Nitroxides have great potential as contrast agents for Magnetic Resonance Imaging (MRI). Two β-cyclodextrin (βCD) derivatives bearing one or seven (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) units on the small rim of βCD (CD3 and CD6 respectively) were synthesized. Their effective magnetic moments were measured by DC-SQUID magnetometry obtaining the values μ_eff/μ_B ≈ 1.7 and μ_eff/μ_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.