Complexation of α-cyclodextrin with carborane derivatives in aqueous solution

Abstract

Although the complexation of carborane derivatives with β-cyclodextrin (β-CD) is well-known, we present here the first observation of the complexation of carborane derivatives with α-CD in aqueous solution. The stoi-chiometry and association constant (Ka) of the complexes were estimated from Job's plots and NMR titration studies, respectively. The carborane: α-CD stoichiometry was 1:1 in all cases. The complexation ability and selectivity of the carborane derivatives for α-CD are markedly decreased compared with those for β-CD. The interaction between the carborane cage and the hydrophobic cavity of α-CD appears to be weak, probably because the hydrophobic cavity of α-CD is too small to accommodate the carborane cage. The C-H hydrogen and the polar substituents of carborane cage may form hydrogen bonds with secondary alcohol groups at the rim of α-CD. The orientation of the carborane cage influences the inclusion process, and o- and m-carborane derivatives showed moderately stronger association constants than p-carborane derivatives. © 2009 Pharmaceutical Society of Japan.