Lipid bilayer ultrastructure. Electron density profiles and chain tilt angles as determined by X-ray diffraction.

High resolution (6A) electron density profiles have been computed on an absolute electron density scale for bilayers composed of both saturated fatty acids and fatty acids associated with the alkaline earth series of divalent cations. Lowangle X-ray diffraction data have been interpreted by an isomorphous replacement technique. The position on the X-ray film of discrete wide-angle reflections has provided direct information on the hydrocarbon chain packing and chain tilt in these bilayers. These results have been correlated to an electron microscopy study of the same bilayers (Waldbilling, R. C., Robertson, J.D. and McIntosh, T. J. (1976) Biochim. Biophys. Acta 448, 1-14) and also to X-ray diffraction studies of fatty acid crystals. A method for forming and structurally analyzing bilayers of well defined chemical asymmetry is also described.