Adsorption of divalent cations to a variety of phosphatidylcholine bilayers

We have determined the degree of binding of divalent cations to several kinds of phosphatidylcholine (PC) bilayers. This has been done by measuring the electrostatic interbilayer repuslive force that results when multilamellar lattices are exposed to Me2+Cl2 solutions. Divalent cations bind to dipalmitoylphosphatidylcholine in the sequence Ca2+ approximately equal to Cd2+ approximately equal to Mn2+ greater than Ca2+ approximately equal to Mg2+ greater than Ba2+. Among the different synthetic lipids, preference for Ca2+ is in the sequence DOPC less than DLPC less than DMPC approximately equal to DPPC approximately equal to DSPC. The density of bound charge is proportional to the density of polar groups on the bilayer surface. Phosphatidylcholines with mixed hydrocarbon chains, such as egg PC or 1:1 mixtures of synthetic PC's, form two distinct lamellar phases in CaCl2 solutions. In all cases the electrostatic force between bilayers decays exponentially with their separation but more slowly than expected from ionic double-layer theory. We suggest that the electric fields from opposing surfaces perturb the zwitterionic charge-binding polar groups and continuously modify their ion binding affinities as the bilayers approach.