Crystallographic analysis of freeze-fracture electron micrographs: application to the structure determination of cubic lipid-water phases

An original approach combining freeze-fracture electron microscopy and quan titative image processing has been developed as an alternative to X-ray ana lysis. It has been applied to the crystallographic study of different lipid -water cubic phases [bicontinuous or micellar and of type I (oil-in-water) or type II (water-in-oil)] and has enabled significant advances in the stud y of these phases, Freeze-fracture electron microscopy has revealed that th e cubic phases fracture preferentially along a few crystallographic directi ons which appeared on the images as noisy planar fracture surfaces containi ng periodic information. The visibility of the corresponding unit cells has been considerably improved by image-filtering techniques based on correlat ion averaging, allowing a quantitative analysis of the fracture images to b e made. This analysis yielded faithful information on the symmetries of the cubic structure (rotation axes and mirror planes) as well as on the struct ure of the cubic phase itself, Eventually, the different parameters that de termine the most favourable fracture pathways within the structures were es tablished. This novel approach constitutes a powerful tool of general inter est, complementary to X-ray diffraction, for solving complex ordered macrom olecular structures at low resolution.