Specific Binding and Cooperative Binding of Dodecylpyridinium Chloride to Water-Soluble Calix[n]arenes and Effect of Cerium and Uranyl Cations

Abstract

Abstract Binding of dodecylpyridinium chloride (DPC) to water-soluble calix[n]arenes (n = 4, 6, 8) (CALXSn) has been studied by potentiometric titration. The binding isotherms were found to be composed of two phases; one is strong specific binding to one site and the other is cooperative binding to residual sites. Thermodynamic parameters for the specific binding suggests that the complex is stabilized by van der Waals force between the alkyl chain of DPC and hydrophobic cavity of CALXSn in addition to electrostatic force between opposite charges of DPC and CALXSn. The specific site was highly reduced in the presence of cerium cation for CALXS4 and of uranyl cation for CALXS6 and by the pH change from 7.0 to 12.5 for CALXS8. These effects were interpreted by competitive binding of the metal cations and the pyridinium cation of DPC to the specific sites of CALXS4 and CALXS6, and by deprotonation of hydroxyl group having pKa = 10.1 and/or accompanying conformational change of CALXS8. The cooperative binding takes place in one stage for CALXS4 and CALXS6 but in two stages for CALXS8. The multiple-stage cooperativity was consistent with ‘inverted double cone’ conformation of CALXS8. The results were discussed as a model of ligand binding to protein local structure.