Lipid polymorphism: a correction. The structure of the cubic phase of extinction symbol Fd-- consists of two types of disjointed reverse micelles embedded in a three-dimensional hydrocarbon matrix

The X-ray scattering study of a cubic phase of extinction symbol Fd--, recently performed on a lipid extract (PFL) from Pseudomonas fluorescens [Mariani et al. (1990) Biochemistry 29, 6799-6810] has been extended to several other systems, all consisting of mixtures of water-miscible (MO, PC, PE, oleate) and of water-immiscible (FA, DG) lipids, plus water. In all of these systems the cubic phase was observed in the presence of excess water. Some inconsistencies observed between PFL and the other systems, the fact that in PFL one of the reflections of the cubic phase happened to coincide with the strongest reflection of the hexagonal phase, and the finding, in one of the original cubic samples of PFL kept in the cold for more than 3 years, that the intensity of one of the reflections had decreased dramatically all indicated that a nonnegligible amount of a hexagonal impurity was in fact present in the samples of PFL originally thought to contain a pure cubic phase. The intensities were corrected for that impurity and analyzed again using a pattern recognition approach based upon the axiom that the histogram of the electron density maps is invariant with respect to physical structure, when different phases are compared whose chemical composition is the same. The hexagonal phase provided the reference phase for the comparison. The moments <(DELTA-rho)n> were used to compare the histograms. All the phase combinations (the phi-sets) compatible with the data were generated and were screened using the distance between the points of the cubic and the hexagonal phases in the 6D space of the moments ([<DELTA-rho)n>]1/n), with n = 3-8. The result of the analysis is a structure formed by two types of disjointed micelles of type II (water-in-oil), quasi-spherical in shape: eight (per F-centered cubic cell), of symmetry 4BAR3m, are centered at positions a, and 16, of symmetry 3BARm, are centered at positions d [according to the International Tables (1952)]. This structure has previously been proposed by Charvolin and Sadoc (1988) on the basis of formal geometric arguments. The study of the cubic phase of the other lipid systems is hindered by the lack of a reference phase; the structure was analyzed by reference to the cubic phase of PFL and found to be the same in all the systems. This cubic phase is the first example, among lipid-containing phases, of a 3D periodically ordered micellar organization of type II.