Lipid Chain Upturns and Orientational Potential in Membrane Liquid Crystals
Lipid Chain Upturns and Orientational Potential in Membrane Liquid Crystals
复制标题
膜液晶中的脂质链翻转和取向潜力
DOI：
10.1016/j.bpj.2020.11.2049
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发表时间：
2021
影响因子：
3.4
通讯作者：
Petrache, Horia I.
中科院分区：
文献类型：
--
作者：
Ramkumar, Abhinav;Brown, Michael F.;Petrache, Horia I.
在流体状态下，脂双层的特征是酰基链的显著无序。在实验上，这可以从2H核磁共振波谱[1，2]测量的低阶参数和其他方法，如x射线散射法，测量与有序相相比减少的膜厚度。此外，红外光谱和EPR光谱都测量一个特征序参数。脂类双层的全原子分子动力学(MD)模拟表明，可以使用作用于碳段水平的平均场取向势(平均扭矩势)来解析地描述酰链无序[3]。最简单的近似是一阶项，其中排列势用ε1来量化，我们称之为扭矩强度。正如我们和其他人所表明的那样，这种近似足以计算膜厚度和每种脂类的横截面积等几何量。然而，在膜的生物物理中还有其他令人感兴趣的物理参数。影响膜自由能的一个重要参数是取向熵。在这里，我们表明，一阶平均扭矩近似需要包括一个额外的项(带有一个额外的参数ε1‘)来解释链条的上升。我们发现，向上的碳段经历了更强的对齐场，因为向上的碳段必然会将剩余的碳段放置在膜的法线上更高的位置，即靠近头基。这种脂质膜结构的统计模型可以更好地解释离子通道对调节细胞信号等生物功能的膜自由能的贡献。
Lipid bilayers in the fluid state are characterized by significant disorder of the acyl chains. Experimentally this can be seen by the low order parameters measured by 2H NMR spectroscopy [1, 2] and by other methods such as x-ray scattering that measures a reduced membrane thickness compared to ordered phases. In addition, IR and EPR spectroscopy that each measure a characteristic order parameter. All-atom Molecular Dynamics (MD) simulations of lipid bilayers show that acyl chain disorder can be described analytically using a mean-field orientational potential (potential of mean-torque) acting at the level of carbon segments [3]. The simplest approximation is a first-order term in which the alignment potential is quantified by a ε1 that we call a torque-strength. As shown by us and others, this approximation is sufficient for the calculation of geometric quantities such as the membrane thickness and cross-sectional area per lipid. However, there are other physical parameters of interest in the biophysics of membranes. One important parameter that contributes to the membrane free energy is the orientational entropy. Here we show that the first order mean-torque approximation needs to include an extra term (with an additional parameter ε1′) to account for chain upturns. We find that upturned carbon segments experience a stronger alignment field because upturns necessarily place the remaining carbon segments higher on the membrane normal, ie close to the headgroups. Such statistical models of lipid membrane structure can better account for the contributions to membrane free energy that governs biological functions such as cellular signaling by ion channels.