Conn Charlotte E, Ces Oscar, Squires Adam M, Mulet Xavier, Winter Roland, Finet Stephanie M, Templer Richard H, Seddon John M
Department of Chemistry, Imperial College London, SW7 2AZ, UK.
Langmuir. 2008 Mar 18;24(6):2331-40. doi: 10.1021/la7023378. Epub 2008 Feb 8.
In the past two decades, the geometric pathways involved in the transformations between inverse bicontinuous cubic phases in amphiphilic systems have been extensively theoretically modeled. However, little experimental data exists on the cubic-cubic transformation in pure lipid systems. We have used pressure-jump time-resolved X-ray diffraction to investigate the transition between the gyroid QGII and double-diamond QDII phases in mixtures of 1-monoolein in 30 wt % water. We find for this system that the cubic-cubic transition occurs without any detectable intermediate structures. In addition, we have determined the kinetics of the transition, in both the forward and reverse directions, as a function of pressure-jump amplitude, temperature, and water content. A recently developed model allows (at least in principle) the calculation of the activation energy for lipid phase transitions from such data. The analysis is applicable only if kinetic reproducibility is achieved, at least within one sample, and achievement of such kinetic reproducibility is shown here, by carrying out prolonged pressure-cycling. The rate of transformation shows clear and consistent trends with pressure-jump amplitude, temperature, and water content, all of which are shown to be in agreement with the effect of the shift in the position of the cubic-cubic phase boundary following a change in the thermodynamic parameters.
在过去二十年中，两亲体系中反双连续立方相之间转变所涉及的几何路径已得到广泛的理论建模。然而，关于纯脂质体系中立方相 - 立方相转变的实验数据却很少。我们使用压力阶跃时间分辨X射线衍射来研究30 wt%水的1 - 单油酸甘油酯混合物中螺旋状QGII相和双菱形QDII相之间的转变。对于该体系，我们发现立方相 - 立方相转变发生时没有任何可检测到的中间结构。此外，我们已经确定了正向和反向转变的动力学，作为压力阶跃幅度、温度和水含量的函数。一个最近开发的模型（至少在原则上）允许根据此类数据计算脂质相变的活化能。只有当至少在一个样品内实现动力学可重复性时，该分析才适用，并且通过进行长时间的压力循环，在此展示了这种动力学可重复性的实现。转变速率随压力阶跃幅度、温度和水含量呈现出清晰且一致的趋势，所有这些都表明与热力学参数变化后立方相 - 立方相边界位置的移动效应一致。
