Accelerated formation of cubic phases in phosphatidylethanolamine dispersions.

By means of x-ray diffraction we show that several sodium salts and the disaccharides sucrose and trehalose strongly accelerate the formation of cubic phases in phosphatidylethanolamine (PE) dispersions upon temperature cycling through the lamellar liquid crystalline-inverted hexagonal (Lalpha-HII) phase transition. Ethylene glycol does not have such an effect. The degree of acceleration increases with the solute concentration. Such an acceleration has been observed for dielaidoyl PE (DEPE), dihexadecyl PE, and dipalmitoyl PE. It was investigated in detail for DEPE dispersions. For DEPE (10 wt% of lipid) aqueous dispersions at 1 M solute concentration, 10-50 temperature cycles typically result in complete conversion of the Lalpha phase into cubic phase. Most efficient is temperature cycling executed by laser flash T-jumps. In that case the conversion completes within 10-15 cycles. However, the cubic phases produced by laser T-jumps are less ordered in comparison to the rather regular cubic structures produced by linear, uniform temperature cycling at 10 degrees C/min. Temperature cycles at scan rates of 1-3 degrees C/min also induce the rapid formation of cubic phases. All solutes used induce the formation of Im3m (Q229) cubic phase in 10 wt% DEPE dispersions. The initial Im3m phases appearing during the first temperature cycles have larger lattice parameters that relax to smaller values with continuation of the cycling after the disappearance of the Lalpha phase. A cooperative Im3m --> Pn3m transition takes place at approximately 85 degrees C and transforms the Im3m phase into a mixture of coexisting Pn3m (Q224) and Im3m phases. The Im3m/Pn3m lattice parameter ratio is 1. 28, as could be expected from a representation of the Im3m and Pn3m phases with the primitive and diamond infinite periodic minimal surfaces, respectively. At higher DEPE contents ( approximately 30 wt%), cubic phase formation is hindered after 20-30 temperature cycles. The conversion does not go through, but reaches a stage with coexisting Ia3d (Q230) and Lalpha phases. Upon heating, the Ia3d phase cooperatively transforms into a mixture of, presumably, Im3m and Pn3m phases at about the temperature of the Lalpha-HII transition. This transformation is readily reversible with the temperature. The lattice parameters of the DEPE cubic phases are temperature-insensitive in the Lalpha temperature range and decrease with the temperature in the range of the HII phase.