PHYSICAL STUDIES OF PHOSPHOLIPIDS .11. CA2+ BINDING TO MONOLAYERS OF PHOSPHATIDYLSERINE AND PHOSPHATIDYLINOSITOL

Measurements have been made of the binding of radioactive Ca2+ to unimolecular films of purified phosphatidylserine and phosphatidylinositol at various film pressures. Three theoretical treatments based on the mass equation have been applied to the experimental data to evaluate the binding of Ca2+ in terms of KA, the apparent association constant, and n, the fraction of phospholipids molecules acting as binding sites for a Ca2+. At low film pressures (or low charge densities) the binding of Ca2+ follows the mass equation. Deviations from the mass equation are observed with increasing film pressure (charge density) and are attributed to electrostatic interaction between adjacent binding sites. The ratio of Ca2+ bound/lipid reaches a maximum at about 30 dynes/cm when the spacing between phospholipid molecules is most favourable for two-point electrostatic attachment of Ca2+. Ca2+ bound to phosphatidylserine and phosphatidylinositol form a Stern layer and do not penetrate into the plane of the Gouy potential. © 1969.