Binding of metal ions to monolayers of lecithins, plasmalogen, cardiolipin, and dicetyl phosphate

Abstract

The surface pressure–area curves of synthetic (dipalmitoyl), egg, and yeast lecithins showed that the limiting areas depend on the degree of unsaturation of the fatty acid residues (dipalmitoyl < egg < yeast lecithin). The surface potential of phosphatidal choline (240 mv) was lower than that of the dipalmitoyl lecithin (380 mv), both at 60 A2/molecule. This difference in surface potentials is attributed to the presence of an additional induced dipole in the double bond of the vinyl ether linkage of the plasmalogen. The binding of metal ions to lecithin and phosphatidal choline resulted in an increase in the surface potential. With different lecithins, the binding of calcium varied with the degree of unsaturation, suggesting that steric characteristics of fatty acid residues significantly influence the phospholipid–metal ion interaction.The surface pressure–area curves of lecithin, phosphatidal choline, and dicetyl phosphate were not affected by the presence of the divalent metal ions in the subsolution whereas the surface pressure–area curve of cardiolipin showed 10–13% contraction of the film in their presence. The component dipoles of the dicetyl phosphate and that of the fatty acid are explained. The increase in the surface potential of phospholipids and the contraction–expansion effect of the cardiolipin monolayer is accounted for by postulating a position for the divalent metal ion in the dipole lattice.