Tenchov B, Koynova R, Rapp G
Institute of Biophysics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
Biophys J. 1998 Aug;75(2):853-66. doi: 10.1016/S0006-3495(98)77574-5.
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.
通过X射线衍射，我们发现几种钠盐以及二糖蔗糖和海藻糖在磷脂酰乙醇胺（PE）分散体系温度循环通过层状液晶-反相六角（Lα-HII）相变时，能强烈加速立方相的形成。乙二醇则没有这种效果。加速程度随溶质浓度增加而增大。对于二油酰磷脂酰乙醇胺（DEPE）、二己酰磷脂酰乙醇胺和二棕榈酰磷脂酰乙醇胺都观察到了这种加速现象。我们对DEPE分散体系进行了详细研究。对于1M溶质浓度的DEPE（脂质含量10wt%）水分散体系，10 - 50次温度循环通常会使Lα相完全转变为立方相。最有效的是通过激光闪光T跳跃进行温度循环。在这种情况下，10 - 15个循环内转变就可完成。然而，与以10℃/min的线性、均匀温度循环产生的相当规则的立方结构相比，激光T跳跃产生的立方相有序度较低。1 - 3℃/min扫描速率的温度循环也能诱导立方相的快速形成。所有使用的溶质在10wt%的DEPE分散体系中都能诱导Im3m（Q229）立方相的形成。在最初的温度循环中出现的Im3m相，其晶格参数较大，在Lα相消失后继续循环时会松弛到较小值。在大约85℃会发生协同的Im3m→Pn3m转变，将Im3m相转变为共存的Pn3m（Q224）和Im3m相的混合物。Im3m/Pn3m晶格参数比为1.28，这与分别用原始和金刚石无限周期最小表面表示Im3m和Pn3m相所预期的结果一致。在较高的DEPE含量（约30wt%）下，20 - 30次温度循环后立方相的形成受到阻碍。转变没有完全完成，而是达到一个共存Ia3d（Q230）和Lα相的阶段。加热时，Ia3d相在大约Lα-HII转变温度协同转变为可能是Im3m和Pn3m相的混合物。这种转变随温度很容易逆转。DEPE立方相的晶格参数在Lα温度范围内对温度不敏感，而在HII相范围内随温度降低。
