Electron microscopy of lipids: Effects of pH and fixatives on the appearance of a macromolecular assembly of lipid micelles in negatively stained preparations

SUMMARY Negatively stained preparations of macromolecular assemblies composed of lecithin, cholesterol and saponin have been examined by electron microscopy. Studies were made of the effects of different negative stains, pH and fixatives on the self‐assembly process that gives rise to helical and related structures, on the characteristic appearance of the structures that are formed, and on the stability of the different assemblies. A variety of different macromolecular assemblies, each apparently made of globular lipid subunits arranged in different ways, were observed under different experimental conditions. The structural appearance of the assemblies changes rapidly when the chemical constitution of the aqueous environment is altered. It is concluded that the negative‐staining technique normally provides an image of the hydrated specimen so that drying artifacts are not a major hazard, but the possibility of interactions between the specimen and the aqueous negative‐staining solution must not be ignored. The observations reported in this paper are relevant to an understanding of the factors that may control the formation, in vivo , of those biological membranes that contain discrete subunits composed of lipids or lipoproteins.