Kusube Masataka, Goto Masaki, Tamai Nobutake, Matsuki Hitoshi, Kaneshina Shoji
Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan.
Chem Phys Lipids. 2006 Jul;142(1-2):94-102. doi: 10.1016/j.chemphyslip.2006.03.004. Epub 2006 Apr 4.
The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.
通过差示扫描量热法（DSC）和高压透光率观察了四种具有不同大小极性头部基团的不饱和磷脂，即二油酰磷脂酰乙醇胺（DOPE）、二油酰磷脂酰 - N - 甲基乙醇胺（DOMePE）、二油酰磷脂酰 - N，N - 二甲基乙醇胺（DOMe2PE）和二油酰磷脂酰胆碱（DOPC）的双层相变。每种磷脂囊泡溶液的DSC热谱图和透光率曲线在常压下分别仅表现出一个相变。在高于234 MPa的压力下，DOPC溶液的透光率在两个温度下突然逐步增加，这对应于除液晶相之外在高压下稳定亚凝胶相和层状凝胶相的出现。比较了这些磷脂构建的温度（T）-压力（p）相图。磷脂的相变温度通过头部基团的N - 甲基化逐步降低。发现DOPE、DOMePE和DOMe2PE双层的T - p相边界斜率（dT/dp）（0.127 - 0.145 K MPa⁻¹）与DOPC双层从层状晶体（或亚凝胶；Lc）相到液晶（Lα）相的转变斜率（0.131 K MPa⁻¹）接近。另一方面，DOPC双层从层状凝胶（Lβ）相到Lα相的主要转变的dT/dp值（0.233 K MPa⁻¹）与Lc/Lα转变的dT/dp值显著不同，因此两条曲线在234 MPa处相交。与DOPE、DOMePE和DOMe2PE双层相变相关的热力学量也与DOPC双层Lc/Lα转变的热力学量具有相似的值。考虑到转变温度、dT/dp和热力学量的值，并与饱和磷脂的相应结果进行比较，我们确定在DOPE、DOMePE和DOMe2PE双层中观察到的相变是从Lc相到Lα相的转变，尽管在先前的研究中它们被视为主要转变。Lβ相对于DOPE、DOMePE和DOMe2PE双层在所有压力下可能都是不稳定的，它在低于234 MPa的压力下作为亚稳相存在，而在DOPC双层中在高于234 MPa的压力下作为稳定相存在。磷脂双层之间相稳定性的差异源于极性头部基团水化结构的差异。
