A water-soluble aza-adamantyl nitroxide radical and its complexes with β-cyclodextrin derivatives

We describe the synthesis and characterization of a hydroxylated aza-adamantyl nitroxide radical Z-3, with the solubility in water of 170 mM at room temperature. We find that hydroxylation has no significant effect on the electronic structure and stability of aza-adamantyl nitroxides. We prepare a series of beta-cyclodextrin (beta-CD) derivatives which can form stoichiometric 1 : 1 inclusion complexes with Z-3, to investigate their effects on water solubility and stability of the radical upon complexation. The results indicate that water solubility of Z-3 increases by 1.4-3 times, while the radical stability somewhat decreases, depending on different beta-CD substituents. Overall, sodium disulfobutylether-beta-CD shows the best result in complexation with Z-3, providing water solubility >0.35 mol L-1, while maintaining the stability of the nitroxide radical with a half-life of 13.5 days at room temperature in water.