X-RAY-DIFFRACTION STUDY OF BILAYER TO NONBILAYER PHASE-TRANSITIONS INAQUEOUS DISPERSIONS OF DI-POLYENOIC PHOSPHATIDYLETHANOLAMINES

The low temperature phase properties of aqueous dispersions of di-18:2 and di-18:3 phosphatidylethanolamine are strongly influenced by the p resence of ice. In the presence of cryoprotectants to inhibit ice form ation, these lipids persist in the H-II phase down to at least -50 deg rees C. Ice formation, however, leads to a drastic reduction in the am ount of available free water and a rapid reduction in the diameter of the inverted cylindrical micelles of the H-II phase. The resulting inc rease in surface curvature of the micelles induces an imbalance in the forces acting in the lipid surface and the hydrophobic core which is relieved by formation of the L alpha phase. On reheating the lipid sam ples undergo an abrupt L alpha --> H-II phase transition at about -20 degrees C. The radius of the water core of the inverted micelles at th eir point of formation is estimated to be 0.9 nm. This increases with temperature as more unfrozen water becomes available until the normal equilibrium radius of about 2.3 nm is reached at 0 degrees C when the bulk water in the sample finally melts. A small proportion of the H, p hase lipid enters an as yet unidentified cubic phase on freezing. The spacings of the (10) planes of the H-II phase, the (111) planes of the cubic phase and the d-spacing of the L-alpha phase were found to be a lmost identical at the phase transition temperature. The cubic phase a ppears to disappear at low temperature but to reform on heating. Freez e-fracture studies revealed no unequivocal evidence for cubic phase li pid but the presence of residual non-bilayer lipid structures was obse rved even at temperatures as low as -80 degrees C. The presence of int ersecting stacks of lamellar sheets in the replicas strongly suggest t he existence of an epitaxial relationship between the L-alpha and H-II phases in these systems.