The effect of hydrophobic character of drugs and helix-coil transition of kappa-carrageenan on the polyelectrolyte-drug interaction

Purpose. The extent of adsorption of different drug molecules to K-carrageenan was investigated in order to evaluate the effect of drug hydrophobicity on the adsorption isotherm. Method. Dialysis experiments were used to determine the amount of drug adsorbed to the polyelectrolyte. The amount of drug on both sides of the membrane was determined spectrophotometrically after attaining equlibrium. CMC for the drugs were determined by the dye solubilisation method. Results. It is shown that the small differences in structure between the drug molecules used in this study still leads to considerable difference in adsorption properties, especially the onset of adsorption. It was also found that the slope of the adsorption isotherms among the drug molecules followed the same pattern as the CMC values for drugs. The extent of adsorption of drugs to the helix form of K-carrageenan was much higher than to the coil form. Conclusions. These results suggest that the adsorption of charged drug molecules to an oppositely charged polymer is effected not only by the coulombic interactions, but also by the hydrophobicity of the drug. Furthermore, the adsorption of drug molecules to K-carrageenan in the helix form is higher than for the coil form because of the shorter distance between the charges and the thereby enhanced hydrophobic interaction between bound drug molecules.