An H.P.L.C. Study of Cooperative Guest Binding by a Covalently Linked b-Cyclodextrin Dimer

Abstract

The complexation of the anions of Methyl Orange (MO¯) (1) and Indomethacin (Ind^-) (2) by the β-cyclodextrin dimer (3) has been studied by means of high-performance liquid chromatography. The cyclodextrin annuli are shown to jointly act as a single binding site for complexation of MO¯ (1). Thus the association constants of (3·3±0·3) × 10³ and (220±20) × 10³ dm³ mol⁻¹ for the 1 : 1 complexes with β-cyclodextrin and the dimer (3), respectively, indicate very strong cooperative binding by the annuli of the linked species. By contrast, the complexation of Ind¯ (2) by the cyclodextrin dimer (3) is characterized by two independent binding sites. This lack of cooperative binding is reflected in the association constant for the 1 : 1 complex in each binding site, which at (1·3±0·1) × 10³ dm³ mol⁻¹ is little greater than that of (0·7±0·05) × 10³dm³ mol⁻¹ for the complex with β-cyclodextrin.