The Binding of Calcium at Lipid‐Water Interfaces

Abstract

Measurements have been made of the binding of radioactive calcium to unimolecular films of purified lipids equivalent to those found in cell membranes. The adsorption of 45Ca on pure phospholipid films is largely independent of the chemical nature of the phospholipid. The affinity is controlled by Coulombic forces and is directly related to the nett excess negative charge on the lipid molecule. With pure triphosphoinositide films at collapse pressure on a subphase at pH 5.5 there is evidence that calcium adsorption is lower than that predicted assuming full ionisation of all five anionic sites. This is probably due to the high surface potential suppressing ionisation. After reducing the pressure or after dilution of the triphosphoinositide with lecithin the adsorption of calcium per molecule approaches that predicted from the other phospholipids. The electrophoretic mobility of lecithin particles containing small amounts of anionic phospholipids indicates that at a bulk pH of pH 5.5 both negative sites on phosphatidic acid and all five negative sites on triphosphoinositide are fully ionized at the ‘lecithin’‐water interface. The calcium adsorbed on anionic phospholipids and gangliosides is displaced by a large excess of Na+ or K+ but in no instance is one univalent cation significantly more effective than the other. The adsorbed calcium is also displaced by Mg2+ but with all films there is a greater preference for Ca2+ ranging from 4.3 fold with gangliosides to 21 fold with triphosphoinositide. The adsorption of calcium on stearic acid films is small until the subphase is adjusted to pH values above 9. This and the electrophoretic mobility data indicate that the carboxylic acid group at an interface is only fully ionized or orientated to the aqueous phase at a bulk pH greater than 11. The calcium adsorbed on stearic acid under alkaline conditions is less readily displaced by Na+, K+ and Mg2+ than from other lipids. Copyright © 1967, Wiley Blackwell. All rights reserved