On the coupling between membrane bending and stretching in lipid vesicles

Håkan Wennerström, Emma Sparr, Joakim Stenhammar
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Abstract

The formation of a lipid vesicle from a lamellar phase involves a cost in bending energy of 100-1000 times the thermal energy for values of the membrane bending rigidity κ typical for phospholipid bilayers. The bending rigidity of a bilayer is however a strongly decreasing function of its thickness h, and the bilayer can thus reduce its bending energy by stretching (and thus thinning) the bilayer. In this paper, we construct a simple model to describe this mechanism for the coupling between bending and stretching and analyse its effect on the bending energy and thermal fluctuations of spherical lipid vesicles. We show that the bilayer thinning becomes significant for small vesicles, and for a vesicle with radius R0 ~ 15 nm there is a sizeable thinning of the bilayer compared to the planar state. We furthermore demonstrate how this thinning is associated with a significant decrease in free energy due to the thermally excited bending modes. We argue that this previously unexplored effect can explain the experimentally observed lower limit of achievable vesicle sizes, which eventually become unstable due to the thinning of the bilayer. We also sketch how this effect provides a potential generic mechanism for the strong curvature dependence of protein adsorption to lipid membranes.
关于脂质囊泡中膜弯曲与拉伸之间的耦合关系
在磷脂双分子层典型的膜弯曲刚度κ值下,从薄片相形成脂质囊泡所需的弯曲能是热能的 100-1000 倍。然而,双分子层的弯曲刚度是其厚度 h 的强烈递减函数,因此双分子层可以通过拉伸(从而变薄)来降低其弯曲能。在本文中,我们构建了一个简单的模型来描述这种弯曲与拉伸之间的耦合机制,并分析了它对球形脂质囊泡弯曲能和热波动的影响。我们的研究表明,对于小囊泡来说,双分子层变薄的现象非常明显,对于半径为 R0 ~ 15 nm 的囊泡来说,与平面状态相比,双分子层变薄的程度相当大。我们还进一步证明了这种减薄如何与热激发弯曲模式导致的自由能显著降低有关。我们认为,这种以前未曾探索过的效应可以解释实验观察到的可实现囊泡尺寸的下限,由于双分子层变薄,囊泡尺寸最终变得不稳定。我们还简述了这种效应如何为蛋白质吸附到脂膜上的强曲率依赖性提供了一种潜在的通用机制。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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