The bilayer phase transitions of medium-chain phosphatidylcholines with linear saturated acyl chains (Cn = 12, 13 and 14) were measured by high-pressure light-transmittance measurements and differential scanning calorimetry to investigate the formation of intermediate gel-liquid crystalline phase called Lx phase. The constructed phase diagrams showed that there existed a distinct region of the Lx phase between ripple gel (Pβ') and liquid crystalline (Lα) phase for multilamellar vesicle bilayers of C12PC and C13PC. The Lx phase of the C12PC bilayer was metastable at all pressures and disappeared at a higher pressure. In the C13PC bilayer, the Lx phase was stable and also disappeared at a higher pressure but its region markedly shrunk. By contrast, the Lx phase was not detected for the C14PC bilayer. Effects of other factors such as vesicle size and solvent substitution on the Lx phase of the C13PC bilayer were also examined. A decrease in vesicle size and solvent substitution from water to 50 wt% ethylene glycol solution promoted the Lx-phase formation as opposed to the effects of acyl-chain elongation and pressurization. The fluorescence data of the C13PC bilayer with different vesicle sizes showed that the Lx phase is caused by the difference of local packing in the bilayer. Considering these facts, we concluded that the Lx phase is an intermediate gel-Lα phase that has gel-phase monolayers with negative curvature and Lα-phase monolayers with positive curvature. The formation mechanism of the Lx-phase in stacked bilayers and dispersed vesicles is also explainable by this difference in packing state.

中文翻译：

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在中链磷脂酰胆碱双层膜上形成中间凝胶-液晶相：相变取决于双层堆积。

通过高压透光率测量和差示扫描量热法测量具有线性饱和酰基链（Cn = 12、13和14）的中链磷脂酰胆碱的双层相变，以研究称为Lx的中间凝胶-液晶相的形成相。所构建的相图表明，对于C12PC和C13PC的多层囊泡双层，在波纹凝胶（Pβ'）和液晶（Lα）相之间存在一个明显的Lx相区域。C12PC双层的Lx相在所有压力下都是亚稳态的，在较高压力下会消失。在C13PC双层中，Lx相是稳定的，并且在较高压力下也消失了，但其区域明显收缩。相反，未检测到C14PC双层的Lx相。还检查了其他因素（如囊泡大小和溶剂取代）对C13PC双层Lx相的影响。与酰基链伸长和加压的作用相反，囊泡尺寸的减小和从水到50 wt％乙二醇溶液的溶剂取代促进了Lx相的形成。不同囊泡大小的C13PC双层的荧光数据表明，Lx相是由双层中局部堆积的差异引起的。考虑到这些事实，我们得出结论，Lx相是中间凝胶-Lα相，具有凝胶层单层的负曲率和Lα相单层的正曲率。堆积双层和分散囊泡中Lx相的形成机理也可以通过这种堆积状态的差异来解释。