FREEZE-FRACTURE ELECTRON-MICROSCOPE STUDY OF LIPID SYSTEMS - THE CUBIC PHASE OF SPACE GROUP PM3N

The cubic phase Q223 (space group Pm3n) of lipid-water systems has been studied by freeze-fracture electron microscopy. Four types of fracture planes were identified; all display highly ordered two-dimensional domains, each subdivided into subdomains related to each other by displacements and rotations connected to the symmetry of the space group. The images were filtered using cross-correlation averaging techniques and the filtered images were compared with the corresponding planar sections of the electron density map. The similarity of the two distributions was assessed via a mathematical parameter, the matching index, whose values were determined as a function of the apodization of the electron density map and of the position of the plane in the direction normal to the fracture. By definition, the best coincidence corresponds to an extremum of the matching index. Several conclusions were drawn. (1) The structure of the sample is well preserved in the replicas. (2) The "resolution" of the electron microscope experiments is remarkably close to that of the X-ray scattering study. (3) Of the two types of models that have been envisaged for phase Q223, the micellar is found to be in better agreement with the electron micrographs than the cage-like. (4) The symmetry of the space group is faithfully reflected in the electron microscope images; in particular, the presence of mirror planes rules out the non-centric of the two space groups that are compatible with the crystallographic data. (5) The crystallographic orientations of the fracture planes that are most frequently identified coincide with those of the most intense X-ray reflections; this observation was interpreted as an indication that the fracture propagates differently in the hydrocarbon and in the water volumes. (6) The thinner metal deposits in the replicas were found to coincide with the hydrocarbon regions of the fractures. (7) The fracture seems to be planar across the water matrix and the micelles seem to be removed from the replica and replaced by dips.