Utilization of a Model Copolymer to Evaluate the Contribution of Hydrophobic Bonding in Drug Binding

A copolymer, having an overall composition of 1 unit of vinylpyridine to 26 units of vinylpyrrolidone, was synthesized. Several alkyl copolymers were made by quaternizing the nitrogen of vinylpyridine units of the copolymer with normal alkyl bromides, ethyl through hexyl. Using an equilibrium dialysis procedure, the interaction of p-toluene sulfonic acid sodium with each alkyl copolymer was studied over a temperature range of 15-45°, and the binding constants and thermodynamic parameters were determined. The quaternized vinylpyridine units of the alkyl copolymers constituted the binding sites for the p-toluene sulfonate ions. The stability of the complex formed in the reaction increased with an increase in temperature from 15-37° but decreased at 45°. The binding process was found endothermic and associated with positive entropy effects, indicating that the polymeric model system described here demonstrated properties expected for hydrophobic bonding. The effect of urea on the binding of sulfonate ion by one of the alkyl copolymers was studied and found to be in keeping with the claim that urea breaks hydrophobic bond.