The interaction of various lanthanide ions and some anions with phosphatidylcholine vesicle membranes. A 31P NMR study of the surface potential effects

Abstract

The interaction of various lanthanide ions with vesicles of phosphatidylcholine from egg yolk has been followed by 31P NMR at 30 degrees C. From known magnetic properties of these ions, separation of the paramagnetic shift into a pure contact and a pseudo-contact part was carried out. Binding curves for the contact contribution (F curves) were obtained from vesicles in solutions of sodium salts with monovalent anions over a wide concentration range. These curves should be insensitive to any conformational effects due to ion binding. Indication of a conformational change in the lipid head group at low ion binding was obtained by studying the ratio between the contact and the pseudo-contact contributions. Besides the adsorption of lanthanide ions, specific anion binding to the surface was introduced to account for the enhanced chemical shifts (Cl- < Br- < NO3-). The results were analyzed in terms of the theory for the diffuse double layer (Gouy-Chapman-Grahame) with equilibrium conditions for the adsorbing cations and anions. Simulations of the titration curves furnished parameters for the ion-lipid interactions. The synergism between the cations and anions follows from the potential effects. Comparison of results with lanthanide ions and Ca2+ indicates that the anion adsorption probably depends on the nature of the adsorbed cation. Lanthanide ion binding to L-glycerophosphorylcholine is not influenced by sodium salts. The binding constant for this complex is weaker than with phosphatidylcholine. The chemical shifts for the lanthanide ion complexes with these two phosphorus compounds seem to be about the same.