X-ray diffraction study of bilayer to non-bilayer phase transitions in aqueous 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 presence of ice. In the presence of cryoprotectants to inhibit ice formation, these lipids persist in the H-II phase down to at least -50 degrees C. Ice formation, however, leads to a drastic reduction in the amount of available free water and a rapid reduction in the diameter of the inverted cylindrical micelles of the H-II phase. The resulting increase 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 samples 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 their 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, phase 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 almost identical at the phase transition temperature. The cubic phase appears to disappear at low temperature but to reform on heating. Freeze-fracture studies revealed no unequivocal evidence for cubic phase lipid but the presence of residual non-bilayer lipid structures was observed even at temperatures as low as -80 degrees C. The presence of intersecting stacks of lamellar sheets in the replicas strongly suggest the existence of an epitaxial relationship between the L-alpha and H-II phases in these systems.