The interaction of double-charged metal ions with monolayers and bilayers of phospholipids

Electrophoretic mobilities of hexadecane/water emulsions containing dimyristoylphosphatidylcholine (DMPC) or egg yolk lecithin (EYL) monolayers at the interface and those of liposomes prepared from the same lipids were measured as functions of the concentrations of Ca2+, Mn2+, Cu2+, and Ni2+ cations in the aqueous phase. The surface potentials, surface charge densities (sigma), and the Langmuir adsorption isotherms for various distances from the charged surface to the slip plane (6) were calculated on the basis of the Gouy-Chapman theory for 1:2 electrolytes and the values of zeta-potentials. The binding constants (K) and parameters d were determined under the assumption that the maximum sigma values correspond to one ion per phospholipid molecule at the interface. In the case of DMPC, the ion binding constants (L mol-L) at 25 degrees C are 230 and 87 for Ca2+, 31.5 and 21 for Mn2+, 11 and 6 for Cu2+, and 7.5 and 5.3 for Ni2+ in liposomes and emulsions, respectively. The affinities of Cu2+ and Ni2+ ions for En monolayers and bilayers are lower than those for DMPC mono- and bilayers. The d parameters for all ions are smaller than the radii of the hydrated ions. In the case of Ca2+, Cu2+, and Ni2+, th, d values for mono- and bilayers are different. The differences in K values between monolayers and bilayers as well as those between DMPC and EYL mono- and bilayers can be attributed to the differences in the local environment and orientation of the interfacial phosphate groups in these systems.