Oka Toshihiko, Tsuboi Taka-aki, Saiki Takahiro, Takahashi Tomoki, Alam Jahangir Md, Yamazaki Masahito
Nanomaterials Research Division, Research Institute of Electronics, ‡Department of Physics, Graduate School of Science, and §Integrated Bioscience Section, Graduate School of Science and Technology, Shizuoka University , Shizuoka 422-8529, Japan.
Langmuir. 2014 Jul 15;30(27):8131-40. doi: 10.1021/la5021719. Epub 2014 Jul 2.
Electrostatic interactions (EI) are an important factor for phase transitions between lamellar liquid-crystalline (L(α)) and inverse bicontinuous cubic (Q(II)) phases. We investigated the low pH-induced L(α) to double-diamond cubic (Q(II)(D)) phase transition in dioleoylphosphatidylserine (DOPS)/monoolein (MO) using time-resolved small-angle X-ray scattering. Using a stopped-flow apparatus, a suspension of liposomes (multilamellar vesicles (MLVs) or large unilamellar vesicles (LUVs)) of 20%-DOPS/80%-MO membrane at neutral pH was rapidly mixed with a low pH buffer, and then the structural change of the membranes in the resultant suspension was observed as a function of time (i.e., pH-jump experiment). At the initial step, the L(α) phase was directly transformed into the hexagonal II (H(II)) phase, and subsequently, the H(II) phase slowly converted into the Q(II)(D) phase. We obtained the rate constants of the initial step (i.e., the L(α) to H(II) phase transition) and of the second step (i.e., the H(II) to Q(II)(D) phase transition) using the non-negative matrix factorization method. The rate constant of the initial step was independent of the MLV concentration, indicating that single MLVs can convert into the HII phase without any interaction with other MLVs. On the other hand, the rate constant of the initial step increased with a decrease in pH, 0.041 s(-1) at pH 2.6 and 0.013 s(-1) at pH 2.8, and also exhibited a size dependence; for smaller vesicles such as LUVs and smaller MLVs with diameters of ~1 μm, the rate constant was smaller. They were reasonably explained by the classical nucleation theory. These results provide the first experimental evidence of the total kinetics of EI-induced L(α)/Q(II) phase transitions.
静电相互作用（EI）是层状液晶（L(α)）相与反相双连续立方（Q(II)）相之间相变的一个重要因素。我们使用时间分辨小角X射线散射研究了低pH诱导的二油酰磷脂酰丝氨酸（DOPS）/单油酸甘油酯（MO）中从L(α)相向双菱形立方（Q(II)(D)）相的转变。使用停流装置，将中性pH下20%-DOPS/80%-MO膜的脂质体（多层囊泡（MLV）或大单层囊泡（LUV））悬浮液与低pH缓冲液快速混合，然后观察所得悬浮液中膜的结构变化随时间的函数关系（即pH跃变实验）。在初始阶段，L(α)相直接转变为六方II（H(II)）相，随后，H(II)相缓慢转变为Q(II)(D)相。我们使用非负矩阵分解方法获得了初始阶段（即从L(α)相到H(II)相的转变）和第二阶段（即从H(II)相到Q(II)(D)相的转变）的速率常数。初始阶段的速率常数与MLV浓度无关，表明单个MLV可以在不与其他MLV相互作用的情况下转变为HII相。另一方面，初始阶段的速率常数随pH值降低而增加，在pH 2.6时为0.041 s⁻¹，在pH 2.8时为0.013 s⁻¹，并且还表现出尺寸依赖性；对于较小的囊泡，如直径约为1μm的LUV和较小的MLV，速率常数较小。经典成核理论对这些现象给出了合理的解释。这些结果首次提供了EI诱导的L(α)/Q(II)相变总动力学的实验证据。
