New phases induced by sucrose in saturated phosphatidylethanolamines: an expanded lamellar gel phase and a cubic phase.

A new lamellar gel phase (L beta *) with expanded lamellar period was found at low temperatures in dihexadecylphosphatidylethanolamine (DHPE) and dipalmitoylphosphatidylethanolamine (DPPE) dispersions in concentrated sucrose solutions (1-2.4 M). It forms via a cooperative, relatively broad transition upon cooling of the L beta gel phase of these lipids. According to the X-ray data, the transformation between L beta and L beta * is reversible, with a temperature hysteresis of 6-10 degrees C and a transition width of about 10 degrees C. No specific volume changes and a very small heat absorption of about 0.05 kcal/mol accompany this transition. The L beta *-L beta transition temperature strongly depends on the disaccharide concentration. From a value of about 10 degrees C below the melting transition of DHPE, it drops by 25 degrees C with decrease of sucrose concentration from 2.4 M to 1 M. The low-temperature gel phase L beta * has a repeat spacing by 8-10 A larger than that of the L beta gel phase and a single symmetric 4.2 A wide-angle peak. It has been observed in 1, 1.25, 1.5 and 2.4 M solutions of sucrose, but not in 0.5 M of sucrose. The data clearly indicate that the expanded lamellar period of the L beta * phase results from a cooperative, reversible with the temperature, increase of the interlamellar space of the L beta gel phase. Other sugars (trehalose, maltose, fructose, glucose) induce similar expanded low-temperature gel phases in DHPE and DPPE. The L beta * phase is osmotically insensitive. Its lamellar period does not depend on the sucrose concentration, while the lattice spacings of the L alpha, L beta and HII phases decrease linearly with increase of sucrose concentration. Another notable sugar effect is the induction of a cubic phase in these lipids. It forms during the reverse HII-L alpha transition and coexists with the L alpha phase in the whole temperature range between the HII and L beta phases. The cubic phase has only been observed at sucrose concentrations of I M and above. In accordance with previous data, sucrose suppresses the L alpha phase in both lipids and brings about a direct L beta-HII phase transition in DHPE. A raid, reversible gel-subgel transformation takes place at 17 degrees C in both DPPE and DHPE. Its properties do not depend on the sucrose concentration. The observed new effects of disaccharides on the properties of lipid dispersions might be relevant to their action as natural protectants.