Elasticity of bilayer lipid membranes from their density correlation function

We study the density correlation function (DCF) of DPPC lipid bilayers. We compare Molecular Dynamics (MD) results with theoretical predictions obtained with a mesoscopic description, in terms of the lipid membrane elasticity. One key objective of our work is the quantification of the lipid membrane elasticity directly from the DCF, both for the membrane undulations and local membrane thickness. Our method does not require the definition of instantaneous surfaces or internal variables defining lipid orientations. Building on our previous work, here we focus on the intralayer correlations, i.e. the DCF of lipids residing on the same monolayer, by tracking only the position of the phosphorus atoms in a lipid head group. We demonstrate the relevance of the intralayer two-dimensional (2D) correlations to the total DCF. We further show that all-atom (AA) and coarse grained (CG) lipid forcefields, feature distintively different DCFs. The CG forcefield predicts results in good agreement with the mesoscopic predictions, for the entire wavevector range; the AA forcefield (CHARMM36) predict strong peristaltic fluctuations at long wavevectors q≳0.8 nm−1, which are absent in the CG lipid model (MARTINI).

本研究聚焦二棕榈酰磷脂酰胆碱（DPPC）脂双层的密度关联函数（DCF）。我们将分子动力学（MD）模拟结果与基于脂膜弹性的介观描述所得理论预测进行对比。本工作的核心目标之一是直接通过密度关联函数定量表征脂膜弹性，涵盖膜起伏与局部膜厚度两个维度。我们的方法无需定义瞬时曲面或描述脂类取向的内变量。基于前期研究基础，本文重点关注层内关联——即仅追踪脂头部基团中磷原子的位置，分析同一单分子层内脂类的密度关联函数。我们证实了层内二维（2D）关联对总密度关联函数的重要性。进一步研究表明，全原子（AA）与粗粒化（CG）脂类力场呈现特征性迥异的密度关联函数：粗粒化力场在全波矢范围内的预测结果与介观理论预测吻合良好；而全原子力场（CHARMM36）预测在长波矢$qgtrsim0.8 mathrm{nm}^{-1}$区域存在显著的蠕动波动，这一现象在粗粒化脂类模型（MARTINI）中并未出现。