Nature of interaction of dextran sulfate with lecithin dispersions and lysolecithin micelles.

Lecithin and lecithin/cholesterol dispersions as well as lysolecithin micelles were used to provide basic information on the mechanism of the interaction of zwitterionic phospholipids with dextran sulfate. The addition of dextran sulfate to lecithin dispersions or lysolecithin micelles in the presence of Ca2+ produced insoluble complexes. At each Ca2+ concentration, the amount of insoluble complex formed was maximal at the equivalence dextran sulfate/phospholipid weight ratio. An increase in CaCl2 concentration up to 10 mM progressively increased the equivalence ratios for the phospholipids. Further increase in Ca2+ concentration did not influence the equivalence ratio for maximal complex formation. The conversion of lecithin dispersions into insoluble complexes was very effective even at low Ca2+ concentrations. Approximately 70% of the lecithin was converted to the insoluble complex at CaCl2 concentrations as low as 0.5 mM and the complete conversion was observed at CaCl2 concentrations above 2.5 mM. In the presence of cholesterol, the precipitation curve for insoluble complex formation became broader than that of lecithin alone, indicating the enhancing effect of cholesterol on the insoluble complex formation both below and above the equivalence ratio. The stoichiometry of the interaction of the zwitterionic phospholipids with dextran sulfate inthe presence of calcium was determined using lysolecithin micelles. At the CaCl2 concentrations above 20 mM, the insoluble complex possessed a ratio of lysolecithin/calcium/sulfate group of approximately 2/1/3 (mol/mol/mol). It appears that the formation of the insoluble complex of lecithin or lysolecithin with dextran sulfate represents the mutually enhancing interactions involving both positive and negative charges of the zwitterionic phospholipids. These are the direct electro static interaction between the phospholipid choline nitrogen and the sulfate groups of dextran sulfate and the calcium cross-linking of the phosphate groups to the sulfate groups or to the phosphate groups of neighboring phospholipids.