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).

本研究针对二棕榈酰磷脂酰胆碱（DPPC）脂质双分子层的密度相关函数（density correlation function, DCF）开展分析。我们将分子动力学（Molecular Dynamics, MD）模拟结果与基于脂质膜弹性的介观描述得到的理论预测进行对比。本研究的核心目标之一是直接通过DCF完成脂质膜弹性的定量表征，覆盖膜起伏与局部膜厚度两个方面。我们提出的方法无需定义瞬时曲面或描述脂质取向的内禀变量。基于前期研究基础，本研究仅追踪脂质头部基团中的磷原子位置，重点聚焦层内相关性——即同一单分子层内脂质的DCF。我们证实了层内二维（2D）相关性与总DCF的关联性。进一步研究发现，全原子（AA）与粗粒度（CG）脂质力场所对应的DCF存在显著差异。在全波矢范围内，粗粒度力场的预测结果与介观理论预测结果吻合良好；而全原子力场（CHARMM36）在长波矢q≳0.8 nm⁻¹区域会预测出强烈的蠕动涨落，这一现象在粗粒度脂质模型（MARTINI）中并未出现。