Unravelling the Effects of Cholesterol on the Second-Order Nonlinear Optical Responses of Di-8-ANEPPS Dye Embedded in Phosphatidylcholine Lipid Bilayers,The Journal of Physical Chemistry B

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Unravelling the Effects of Cholesterol on the Second-Order Nonlinear Optical Responses of Di-8-ANEPPS Dye Embedded in Phosphatidylcholine Lipid Bilayers
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2021-09-07 , DOI: 10.1021/acs.jpcb.1c05630
Charlotte Bouquiaux ₁ , Frédéric Castet ₂ , Benoît Champagne ₁

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Cholesterol is known for its role in maintaining the correct fluidity and rigidity of the animals cell membranes and thus their functions. Assessing the content and the role of cholesterol in lipid bilayers is therefore of crucial importance for a deeper understanding and control of membrane functioning. In this computational work, we investigate bilayers built from three types of glycerophospholipid phosphatidylcholine (PC) lipids, namely dipalmitoylphosphatidylcholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and dioleoylphosphatidylcholine (DOPC), and containing different amounts of cholesterol by analyzing the second-harmonic generation (SHG) nonlinear optical (NLO) response of a probe molecule, di-8-ANEPPS, inserted into the membranes. This molecular property presents the advantage to be specific to interfacial regions such as lipid bilayers. To unravel these effects, Molecular Dynamics (MD) simulations have been performed on both DPPC and DOPC lipids by varying the cholesterol mole fraction (from 0 to 0.66), while POPC was only considered as a pure bilayer. In the case of the structural properties of the bilayers, all the analyses converge toward the same conclusion: as the mole fraction of cholesterol increases, the systems become more rigid, confirming the condensing effect of cholesterol. In addition, the chromophore is progressively more aligned with respect to the normal to the bilayer. On the contrary, addition of unsaturation disorders the lipid bilayers, with barely no impact on the alignment of the chromophore. Then, using the frames obtained from the MD simulations, the first hyperpolarizability β of the dye in its environment has been computed at the TDDFT level. On the one hand, the addition of cholesterol induces a progressive increase of the diagonal component the β tensor parallel to the bilayer normal. On the other hand, larger β values have been calculated for the unsaturated than for the saturated lipid systems. In summary, this study illustrates the relationship between the composition and structure of the bilayers and the NLO responses of the embedded dye.

中文翻译：

阐明胆固醇对嵌入磷脂酰胆碱脂质双层中的 Di-8-ANEPPS 染料的二阶非线性光学响应的影响

胆固醇以其在维持动物细胞膜的正确流动性和刚性及其功能方面的作用而闻名。因此，评估胆固醇在脂质双层中的含量和作用对于更深入地了解和控制膜功能至关重要。在这项计算工作中，我们研究了由三种类型的甘油磷脂磷脂酰胆碱 (PC) 脂质构建的双层，即二棕榈酰磷脂酰胆碱 (DPPC)、1-棕榈酰-2-油酰-sn-甘油-3-磷酸胆碱 (POPC) 和二油酰磷脂酰胆碱 (DOPC)，并通过分析插入的探针分子 di-8-ANEPPS 的二次谐波产生 (SHG) 非线性光学 (NLO) 响应而含有不同量的胆固醇进入膜。这种分子特性具有专用于脂质双层等界面区域的优势。为了阐明这些影响，通过改变胆固醇摩尔分数（从 0 到 0.66）对 DPPC 和 DOPC 脂质进行了分子动力学 (MD) 模拟，而 POPC 仅被视为纯双层。在双层结构特性的情况下，所有分析都趋向于相同的结论：随着胆固醇摩尔分数的增加，系统变得更加僵硬，证实了胆固醇的凝聚效应。此外，生色团相对于双层法线逐渐对齐。相反，不饱和度的添加会扰乱脂质双层，对发色团的排列几乎没有影响。然后，使用从 MD 模拟中获得的帧，在 TDDFT 级别计算了染料在其环境中的第一个超极化率 β。一方面，胆固醇的添加诱导β张量平行于双层正常的对角线分量逐渐增加。另一方面，计算出的不饱和脂质系统比饱和脂质系统的 β 值更大。总之，这项研究说明了双层的组成和结构与嵌入染料的 NLO 响应之间的关系。

更新日期：2021-09-16

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