NONAXISYMMETRIC VESICLE SHAPES IN A GENERALIZED BILAYER-COUPLE MODEL AND THE TRANSITION BETWEEN OBLATE AND PROLATE AXISYMMETRICAL SHAPES
NONAXISYMMETRIC VESICLE SHAPES IN A GENERALIZED BILAYER-COUPLE MODEL AND THE TRANSITION BETWEEN OBLATE AND PROLATE AXISYMMETRICAL SHAPES
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DOI：
10.1103/physreve.48.3112
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发表时间：
1993-10-01
影响因子：
2.4
通讯作者：
ZEKS, B
中科院分区：
文献类型：
--
作者：
HEINRICH, V;SVETINA, S;ZEKS, B
发展了一种计算磷脂微囊形状的理论方法。它是一般的，因为它不限于具有特殊对称属性的形状，并且包括形状的稳定性分析。与最近的实验结果一致，囊泡膜的描述是基于广义的双层耦合模型。根据这一概念，通过将膜的两个单层表示为具有恒定分离距离的闭合中性表面来模拟膜的双层结构。假定平衡形状对应于膜面积和囊泡体积不变时膜弹性能的最小值。弹性能由薄膜的局部弯曲能和非局部弯曲能组成。后一项表示单层相对面积变化的能量贡献。应用基于球谐展开的里兹方法解决了计算平衡形状的变分问题。数值计算集中在获得非轴对称形状的模型参数范围内。此外，还考察了在相同模型参数范围内得到的轴对称形状。结果表明，非局部弯曲模数(Q)与局部弯曲模数(Q)之比的微小差异可能会引起形状变换性质的显著变化。对于较高的Q值，非轴对称形状是稳定的，它们代表了扁平和长轴对称形状连续转变的中间状态。在磷脂双层膜特有的低Q值下，非轴对称形状是不稳定的。在这种情况下，扁平和长轴对称形状之间的过渡是不连续的。
A theoretical approach to calculate shapes of phospholipid vesicles is developed. It is general in the sense that it is not restricted to shapes with special symmetry properties, and it includes a stability analysis of the shapes. In keeping with recent experimental findings, the description of the vesicle membrane is based on the generalized bilayer-couple model. According to this concept, the bilayer structure of the membrane is modeled by representing its two monolayers as closed neutral surfaces with a constant separation distance. Equilibrium shapes are assumed to correspond to the minimum of the membrane elastic energy at constant values of the membrane area and the vesicle volume. The elastic energy is composed of the local and nonlocal bending energies of the membrane. The latter term represents the energy contribution of the relative area changes of monolayers. The variational problem to calculate equilibrium shapes is solved by applying a Ritz method based on an expansion in spherical harmonics. The numerical computations concentrate on the range of model parameters for which nonaxisymmetric shapes are obtained. In addition, axisymmetric shapes which are obtained in the same range of model parameters are examined. It is shown that small differences of the ratio between the nonlocal and local bending moduli (q) may cause significant changes in the nature of shape transformations. For high q values, nonaxisymmetric shapes are stable, and they represent the intermediate states in the continuous transformation between oblate and prolate axisymmetric shapes. At low q values characteristic for phospholipid bilayers, the nonaxisymmetric shapes are unstable. In this case, the transition between oblate and prolate axisymmetric shapes is discontinuous.