Adsorption of neutral polymers on negatively charged liposomes. A novel quantitative method to measure the rate of polymer adsorption on the liposomal surface

We studied the adsorption of two neutral polymers [poly(vinyl pyrrolidone) and poly(vinyl alcohol) (PVA)] on negatively charged liposomes composed of 25:2:3 (molar ratio) soy lecithin/dicetyl phosphate/cholesterol. The liposo mes were prepared in buffered solution at pH 7.4 and were mixed with the so lution of the polymers in the desired polymer/lipid ratios. Adsorption was measured by determination of the equilibrium bulk concentration of the poly mer. Protamine hydrochloride was used to aggregate the liposomes with polym ers adsorbed on their surface and to facilitate their separation from the e quilibrium bulk solution. In the case of PVA, quantitative adsorption measu rements with a specific reagent were possible. Adsorption isotherms were re corded at 25 +/- 0.2 degreesC. It was concluded that adsorbed and non-adsor bed PVA molecules are in equilibrium even at low polymer/lipide ratios. The results were confirmed by dynamic laser light scattering, X-ray diffractio n and thermal activity monitoring experiments.