Structure studies of a phospholipid monolayer coupled to dextran sulfate

Abstract The negatively charged polymer dextran sulfate can couple to a non-charged phospholipid monolayer of 1,2-dipalmitoyl-phosphatidylethanolamine (DPPE) at the air–water interface via ionic bridges of calcium. This influences the lateral order of the lipid monolayer. The monolayer of DPPE on a Ca 2+ subphase was studied by isotherm measurements, grazing incidence X-ray diffraction (GIXD) and ellipsometry before and after the injection of dextran sulfate into the subphase. The coupled system is more condensed. The phase sequence did not change after the polymer binding, but all transition pressures and tilt angles are lowered. Because of the bridging mechanism and the multiple charges of each dextran unit, the whole system responds very flexibly to pressure changes.