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 const.; Ramam data show that the internal chain ordering changes, and 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 theor. evaluation. The tilt allows for a better understanding of the electrostatically induced shift of the phase transition temp. and of the shift induced by the binding of water in the case of lecithin in contrast to ethanolamine. [on SciFinder (R)]