Images of divalent cations in unstained symmetric and asymmetric lipid bilayers

Divalent cations have been microscopiccally visualized in association with simple lipid bilayers. Symmetric and asymmetric oriented bilayers were constructed from fatty acid monolayers and were cut in thin transverse sections for examination by bright field electron microscopy in the absence of stains, fixatives or embedding materials. It has been found that bilayers formed of lipid molecules having alkaline earth head groups exhibit natural electron contrast. The intrinsic image has been liked to local variations in the bilayer absolute electron density profile determined by X-ray diffraction analysis of the same specimens (McIntosh, T. J., Waldbillig, R. C. and Robertson, J. D. (1976) Biochim. Biophys. Acta 448, 15-33). By combining the microscopic, chemical and X-ray evidence it has been estimated that local increments of about 1 g/cm3 can produce detectable elelcron contrast in 500 A transverse sections of bilayers.