Binding of divalent cations to dipalmitoylphosphatidylcholine bilayers and its effect on bilayer interaction

Abstract

CaCl2 swells the multilayer lattice formed by dipalmitoylphosphatidylcholine (DPPC) in an aqueous solution. Specifically, at room temperature 1 mM CaCl2 causes these lipid bilayers to increase their separation, dw from 19 .ANG. in pure water to > 90 .ANG., CaCl2 concentrations > 40 mM cause less swelling. The new repulsive force between the bilayers in 30 mM CaCl2 at T = 25.degree. C (below the acyl chain freezing temperature). For interbilayer separations between 30 and 90 .ANG., the dominant repulsion between bilayers is probably electrostatic; Ca2+ binds to DPPC lecithin bilayers, imparting a charge to them. The addition of NaCl to CaCl2 solutions decreases this repulsion. For dw < 20 .ANG., the bilayer repulsion appears to be dominated by the hydration forces observed previously between both neutral and charged phospholipids. From the electrostatic repulsive force, the extent of Ca2+ binding to the bilayer surface was estimated. The desorption of bound Ca2+, apparent when bilayers are pushed together, is more rapid than expected if an association constant governed Ca2+ binding. The association affinity does not appear to be a fixed quantity but rather a sensitive function of ionic strength and bilayer separation.