ACTIVITY-COEFFICIENTS OF CACL2 AND MGCL2 IN THE PRESENCE OF IPALMITOYLPHOSPHATIDYLCHOLINE-PHOSPHATIDYLINOSITOL VESICLES IN AQUEOUS-MEDIA

Mean activity coefficients of CaCl2 and MgCl2 salts have been measured in the presence of phospholipid vesicles, by use of the EMF method wi th ion-exchange membrane electrodes, as a novel procedure for studying the direct interactions between electrolytes and phospholipid vesicle s. Mixed lipid sonicated vesicles have been prepared from dipalmitoylp hosphatidylcholine (DPPC)-phosphatidylinositol (PI) mixtures, covering a range of composition. The CaCl2 and MgCl2 concentration range studi ed was 0.01-0.001 mol l(-1). For both salts studied, the activity coef ficients decrease linearly as the vesicle concentration in the suspens ion increases while the concentration of salt was kept constant. Maint aining a constant vesicle concentration, the decrease in the activity coefficients is more pronounced the more dilute the concentration of t he salts. Quantitative differences were observed in the behaviour of b oth salts which are explained on the basis of a higher affinity of the Ca2+, compared with that of Mg2+ ions, to the vesicle surface. Using a simple model for the vesicle-salt systems, the number of cations spe cifically bound to the vesicles has been calculated. In view of the re sults it can be concluded that estimation of the activity coefficients of the salts is a useful method for the study of the interactions tak ing place in vesicle-salt systems.