Freeze–fracture electron microscopy of lyotropic lipid systems Quantitative analysis of cubic phases of space group Ia3d (Q230)

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Abstract
The cubic phases Q230 (space group Ia3d) of type I (oil in water) and type II (water in oil) have been studied by freeze-fracture electron microscopy. The preservation of the sample structure during cryofixation was verified by low temperature X-ray diffraction. Three types of fracture planes were clearly identified in the type I system and two in the type II system. These fracture planes showed two-dimensionally ordered domains, each subdivided into subdomains related to each other by displacements and rotations compatible with the symmetry of the space group. The images were filtered using correlation averaging techniques and the filtered images were compared with the corresponding sections of the electron density map. Several conclusions were drawn: (i) The structures of the samples had been preserved after quenching, fracturing and replicating processes, (ii) The electron microscope images were fully compatible with the postulated space group symmetries. (iii) The fracture planes coincide with the planes giving rise to the most intense X-ray reflections and containing the rod-like structure elements that seem to facilitate the propagation of the fractures. This approach constitutes a new and powerful tool of general interest for the study of liquid crystals.