Department of Chemical and Biological Physics, Weizmann Institute of Science, P.O. Box 26, Rehovot 7610001, Israel.
Department of Chemistry and Biochemistry and Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA.
J Chem Phys. 2018 Mar 28;148(12):123333. doi: 10.1063/1.5010341.
Supported lipid bilayers (SLBs) have been studied extensively as simple but powerful models for cellular membranes. Yet, potential differences in the dynamics of the two leaflets of a SLB remain poorly understood. Here, using single particle tracking, we obtain a detailed picture of bilayer dynamics. We observe two clearly separate diffusing populations, fast and slow, that we associate with motion in the distal and proximal leaflets of the SLB, respectively, based on fluorescence quenching experiments. We estimate diffusion coefficients using standard techniques as well as a new method based on the blur of images due to motion. Fitting the observed diffusion coefficients to a two-leaflet membrane hydrodynamic model allows for the simultaneous determination of the intermonolayer friction coefficient and the substrate-membrane friction coefficient, without any prior assumptions on the strengths of the relevant interactions. Remarkably, our calculations suggest that the viscosity of the interfacial water confined between the membrane and the substrate is elevated by ∼10 as compared to bulk water. Using hidden Markov model analysis, we then obtain insight into the transbilayer movement of lipids. We find that lipid flip-flop dynamics are very fast, with half times in the range of seconds. Importantly, we find little evidence for membrane defect mediated lipid flip-flop for SLBs at temperatures well above the solid-to-liquid transition, though defects seem to be involved when the SLBs are cooled down. Our work thus shows that the combination of single particle tracking and advanced hydrodynamic modeling provides a powerful means to obtain insight into membrane dynamics.
支持的脂质双层 (SLB) 已被广泛研究，作为细胞膜的简单但强大的模型。然而，SLB 的两个叶层的动力学之间的潜在差异仍未得到很好的理解。在这里，我们使用单粒子跟踪技术，获得了关于双层动力学的详细图片。我们观察到两个明显分开的扩散群体，快和慢，我们根据荧光猝灭实验将其分别与 SLB 的远端和近端叶层的运动相关联。我们使用标准技术和基于由于运动而导致的图像模糊的新方法来估计扩散系数。将观察到的扩散系数拟合到双层膜流体动力学模型中，可以同时确定层间摩擦系数和基底-膜摩擦系数，而无需对相关相互作用的强度进行任何先验假设。值得注意的是，我们的计算表明，与体相水相比，被膜和基底之间的界面水的粘度升高了约 10 倍。然后，我们使用隐马尔可夫模型分析，深入了解脂质的跨膜运动。我们发现脂质翻转动力学非常快，半衰期在秒范围内。重要的是，我们发现，在远高于固-液转变温度的温度下，SLB 中不存在由膜缺陷介导的脂质翻转的证据，尽管当 SLB 冷却时，缺陷似乎会涉及到。因此，我们的工作表明，单粒子跟踪和先进的流体力学建模的结合提供了一种深入了解膜动力学的强大手段。
