Low-pH-Induced Lamellar to Bicontinuous Primitive Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein Membranes
Low-pH-Induced Lamellar to Bicontinuous Primitive Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein Membranes
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二油酰磷脂酰丝氨酸/单油精膜中低 pH 诱导的层状到双连续原始立方相变
DOI：
10.1021/acs.langmuir.7b02512
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
Yamazaki Masahito
中科院分区：
文献类型：
--
作者：
Oka Toshihiko;Hasan Moynul;Islam Md. Zahidul;Moniruzzaman Md.;Yamazaki Masahito
静电相互作用（EI）在脂质膜反双连续立方（QII）相的结构和稳定性中发挥着重要作用。我们使用小角 X 射线散射 (SAXS) 研究了低离子强度下 pH 对二油酰磷脂酰丝氨酸 (DOPS)/单油酸甘油酯 (MO) 膜相的影响。我们发现，当膜悬浮液的 pH 值从中性值降低时，在最终 pH 值为 2.75 及以下的含有 50 mM NaCl 的缓冲液中，DOPS/MO（2/8 摩尔比）膜中会发生从层状液晶 (Lα) 到原始立方 (QIIP) 相的相变。使用时间分辨 SAXS 和停流装置揭示了这种转变的动力学途径。第一步是从 Lα 相快速转变为六方 II (HII) 相，第二步是从 HII 相缓慢转变为 QIIP 相。通过定量分析 SAXS 强度的时间过程，我们确定了第一步的速率常数 k1 和第二步的速率常数 k2。 k1值随温度增加而增加。对该结果的分析提供了其表观活化能的值，该值随着温度的变化而保持恒定，但随着 pH 值的增加而增加。这可以通过过渡态自由能的 EI 效应来解释。相反，k2值随温度降低，表明真实活化能随温度升高。当过渡态的自由能取决于温度时，使用脂质膜相变的活化能理论对这些实验结果进行了分析。在这些结果的基础上，我们讨论了这种相变的机制。
Electrostatic interactions (EIs) play important roles in the structure and stability of inverse bicontinuous cubic (QII) phases of lipid membranes. We examined the effect of pH on the phase of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes at low ionic strengths using small-angle X-ray scattering (SAXS). We found that the phase transitions from lamellar liquid-crystalline (Lα) to primitive cubic (QIIP) phases in DOPS/MO (2/8 molar ratio) membranes occurred in buffers containing 50 mM NaCl at and below the final pH of 2.75 as the pH of the membrane suspension was decreased from a neutral value. The kinetic pathway of this transition was revealed using time-resolved SAXS with a stopped-flow apparatus. The first step is a rapid transition from the Lαphase to the hexagonal II (HII) phase, and the second step is a slow transition from the HIIphase to the QIIPphase. We determined the rate constants of the first step,k1, and of the second step,k2, by analyzing the time course of SAXS intensities quantitatively. Thek1value increased with temperature. The analysis of this result provided the values of its apparent activation energy, which were constant over temperature but increased with pH. This can be explained by an EI effect on the free energy of the transition state. In contrast, thek2value decreased with temperature, indicating that the true activation energy increased with temperature. These experimental results were analyzed using the theory of the activation energy of phase transitions of lipid membranes when the free energy of the transition state depends on temperature. On the basis of these results, we discussed the mechanism of this phase transition.