A neutron diffraction study of the influence of ions on phospholipid membrane interactions

Abstract

Neutron diffraction is used to examine the effects of Ca 2+ and ClO 4 − ions on interactions and some structural features of dipalmitoylphosphatidylcholine membranes in both solid and fluid lamellar phases. The results are described within the framework of Derjaguin-Landau-Verwey-Overbeek (DLVO)_theory with reference to electrostatic, van der Waals, and hydration components of disjoining pressure. The Hamaker constants are evaluated under equilibrium conditions. Addition of 100 mM CaCl 2 to the aqueous phase substantially increases the lamellar repeat spacing ( d), which is interpreted in terms of adsorption of Ca 2+ ions to bilayers followed by electrostatic repulsion between membranes. The rise of NaClO 4 concentration in the presence of 100 mM CaCl 2 leads to gradual decrease in d, evidently resulted from the diminution of Ca 2+-induced positive surface potential by both electrostatic screening and binding of ClO 4 − ions. In the absence of CaCl 2, elevation of NaClO 4 concentration to 100–300 mM drastically enhances the repeat spacing and then dramatically decreases d at about 1 M NaClO 4. Estimation of the hydration coefficients showed that the pronounced decrease of the repeat spacing at high NaClO 4 concentrations was resulted mainly from the (partial) disruption of the structure of intermembrane bound water by chaotropic ClO 4 − ions and subsequent decrease in hydration repulsive pressure. Moreover, in the case of solid membranes (20°C) high concentrations of ClO 4 − induced formation of interdigitated phase paralleled with marked reduction in bilayer thickness and corresponding increase in the effective cross-sectional area per lipid molecule.