Tate M W, Gruner S M
Biochemistry. 1987 Jan 13;26(1):231-6. doi: 10.1021/bi00375a031.
The L alpha-HII phase transition behavior of many lipid-water liquid crystals is dominated by the competition between the tendency to curl the lipid layers to an intrinsic radius of curvature and opposing hydrocarbon packing constraints. In particular, packing constraints can increase the free energy of the inverted hexagonal (HII) phase as compared to that of the lamellar (L alpha) phase. This is especially true where the lipid molecule is not long enough to reach into the corners of the lattice in large hexagonal structures necessitated by a large intrinsic radius of curvature. In this paper it is shown that the addition of a minor fraction long-chain lipid to a system of otherwise uniform chain composition can also relax packing constraints, thereby lowering the lamellar to hexagonal transition temperature. For the specific systems used, dioleoylphosphatidylethanolamine (di-18:1c-PE) with minor fractions of 1,2-diacyl-sn-glycero-3-phosphocholines [di-n:1c-PC (n = 14, 18, 22, and 24)], the observed HII lattices systematically increased in size with increasing chain length, suggesting that the chain length also may affect the intrinsic curvature of the mixture. These experiments demonstrate that the lipid "shape concept", which is a qualitative expression of the concept quantitatively described by the intrinsic radius of curvature, is insufficient to understand the L alpha-HII transition. It is necessary to, at least, consider the competition between curvature and packing.
许多脂质 - 水液晶的Lα - HII相转变行为由脂质层卷曲至固有曲率半径的趋势与相反的烃堆积限制之间的竞争所主导。特别是，与层状（Lα）相相比，堆积限制会增加反相六角形（HII）相的自由能。当脂质分子不够长，无法在由大的固有曲率半径所必需的大六角形结构中到达晶格角时，情况尤其如此。本文表明，向具有均匀链组成的体系中添加少量长链脂质也可以缓解堆积限制，从而降低层状相到六角形相的转变温度。对于所使用的特定体系，即含有少量1,2 - 二酰基 - sn - 甘油 - 3 - 磷酸胆碱[di - n:1c - PC（n = 14、18、22和24）]的二油酰磷脂酰乙醇胺（di - 18:1c - PE），观察到的HII晶格尺寸随着链长增加而系统地增大，这表明链长也可能影响混合物的固有曲率。这些实验表明，脂质“形状概念”作为由固有曲率半径定量描述的概念的定性表达，不足以理解Lα - HII转变。至少有必要考虑曲率与堆积之间的竞争。
