Anion influence on the binding of divalent cations to phosphatidylcholine

We have used the osmotic pressure technique of Rand, Parsegian and co-workers (Nature 259 (1976) 601-603) to investigate the effect of anion species on the binding of M2+ to dipalmitoylphosphatidylcholine bilayers. Calcium and magnesium salts show a complex behavior which is consistent with both anion binding and screening. We observe virtually no change, within the accuracy of our experiment, in the decay of repulsive pressure with inter-bilayer separation for the acetate and nitrate salts of magnesium and calcium; however, the chloride salt does show a different pressure decay. At any given bilayer separation, 35 A less than dw less than 75 A, with calcium and magnesium salts present, the anions produce a decrease in the repulsive pressure in the order acetate- greater than Cl- greater than NO-3.