Raman spectroscopy and x-ray diffraction are used to investigate the influence of surface charges on the structure of ionizable lipid membranes of dimyristoylmethylphosphatidic acid. The membrane surface charge d. is regulated by varying the pH of the aq. phase. Changes of the conformational order of the lipid chains are detd. from the intensity of the C-C stretch chain vibrations around 1100 cm-1 in a lipid Raman spectrum. In going from an elec. neutral to a neg. charged membrane, the conformational order is reduced by 5% in the ordered and by 9% in the fluid membrane phase, corresponding to 0.6 and 0.8 C-C bonds, resp., which change from a trans to a gauche conformation. The electrostatically induced conformational change is mainly concd. at the lipid chain ends as indicated by the spectral variations of the 890 cm-1 CH3 rocking band of the chain termini. The x-ray diffraction expts. show that increasing the surface charge d. in the ordered membrane phase leads to a lateral expansion of the packing of the lipid polar groups, whereas the packing of the lipid chains in a plane perpendicular to the chain axes remains const., indicating an increase of the tilt of the lipid chains from d = 10 Deg (pH 3) to d = 27 Deg (pH 9). [on SciFinder (R)]