Electrostatic interactions at charged lipid membranes. Electrostatically induced tilt.

The changes in bilayer structure induced by surface charges in the case of an ionizable lipid were studied by X-ray diffraction, Raman spectroscopy, and film-balance measurements. With increasing surface charge in the ordered phase, the X-ray results show a decrease in bilayer thickness, whereas the hydrocarbon chain packing stays essentially constant, the Raman data signify that the internal chain ordering does not change, and the monolayer studies show a lateral expansion of the bilayer. These results are interpreted in terms of a tilt of the chains caused by the surface charges on the polar heads. The tilt angle between the direction of the chains and the bilayer normal is obtained by a detailed theoretical evaluation. The tilt allows for a better understanding of the electrostatically induced shift of the phase transition temperature and of the shift induced by the binding of water in the case of lecithin in contrast ethanolamine.