A comparison of the binding of linear chain-linked nitroxides to cyclodextrins and cucurbiturils

This study reports the synthesis of six new nitroxides with a paramagnetic moiety attached to a linear alkyl chain, their analyses by electron paramagnetic resonance spectroscopy, and their interaction with two classes of host molecules: cyclodextrins (α- and β-CD) and cucurbiturils (CB[n], n = 6-8). The series of new nitroxides consists of two PROXYL radicals, two TEMPO radicals and two β-phosphorylated nitroxides. As evidenced by changes in the hyperfine coupling constants, the PROXYL and TEMPO spin probes form complexes with CDs by threading of the alkyl chain and/or by inclusion of the paramagnetic moiety. The binding constants of the host-guest complexes, evaluated from the variation in the rotational correlation time with the concentration of the host, fall in the range of 10²-10³ M^-1. The β-phosphorylated nitroxides present low water solubility but are solubilised by interaction with CDs at high concentrations, while complexation with CB[n] has not been evidenced for either probe. The interaction of the cationic nitroxides CAT1 and CAT16 with CDs and CB[n] was also considered in order to elucidate the role played by charged species in the complexation. Both CAT1 and CAT16 form complexes with CB[n]. In the particular case of CAT1/CB[6], the initial association of the complexes at high concentrations of CB[6] was evidenced by the appearance of the spin-spin interaction, followed by oxidation of iodine and separation of red crystals characterized by X-ray diffraction.