ADSORPTION OF DIVALENT-CATIONS TO A VARIETY OF PHOSPHATIDYLCHOLINE BILAYERS

The degree of binding of divalent cations to several kinds of phosphatidylcholine (PC) bilayers determined by measuring the electrostatic interbilayer repulsve force that results when multilamellar lattices are exposed to Me2+Cl2 solutions. Divalent cations bind to dipalmitoylphosphatidylcholine [DPPC] in the sequence Ca2+ .simeq. Cd2+ .simeq. Mn2+ > Ca2+ .simeq. Mg2+ > Ba2+. Among the different synthetic lipids, preference for Ca2+ is in the sequence DOPC [dioleoyl PC] < DLPC [dilauryl PC] < DMPC [dimyristoyl PC] .simeq. DPPC .simeq. DSPC [distearoyl PC]. 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, form 2 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. The electric fields from opposing surfaces may perturb the zwitterionic charge-binding polar groups and continuously modify their ion binding affinities as the bilayers approach.