Effect of different substituents on the water-solubility and stability properties of 1: 2 [60]fullerene derivative-gamma-cyclodextrin complexes

We have demonstrated that C-60 derivatives bearing a pyrrolidine moiety as well as a variety of other substituents can form 1 : 2 complexes with gamma-cyclodextrin (gamma-CDx) using a mechanochemical high-speed vibration milling apparatus. When the influence of the steric hindrance of the substituents on the formation of the complexes was negligible, the water-solubilities of the complexes were shown experimentally to be completely dependent on the hydrophobic properties of the substituent. Furthermore, the stabilities of the gamma-CDx-complexes of several different C-60 derivatives were found to be similar to or slightly higher than that of the C-60-gamma-CDx complex, with the solubilities of the complexes showing no correlation to the stabilities. Based on the results of a series of theoretical investigations, we have shown that the stabilities of the gamma-CDx-complexes can be affected not only by steric effects but also by the polarities of the substituent groups, which exist in the vicinity of the upper rim of gamma-CDx, because the water bound to the polar group can assist in the stabilisation of the complexes.