Energy landscape for the insertion of amphiphilic nanoparticles into lipid membranes: a computational study

Amphiphilic, monolayer-protected gold nanoparticles (NPs) have been shown to enter cells via a non-endocytic, non-disruptive pathway that could be valuable for biomedical applications. The same NPs were also found to insert into a series of model cell membranes as a precursor to cellular uptake, but the insertion mechanism remains unclear. Previous simulations have demonstrated that an amphiphilic NP can insert into a single leaflet of a planar lipid bilayer, but in this configuration all charged end groups are localized to one side of the bilayer and it is unknown if further insertion is thermodynamically favorable. Here, we use atomistic molecular dynamics simulations to show that an amphiphilic NP can reach the bilayer midplane non-disruptively if charged ligands iteratively ``flip'' across the bilayer. Ligand flipping is a favorable process that relaxes bilayer curvature, decreases the nonpolar solvent-accessible surface area of the NP monolayer, and increases attractive ligand-lipid electrostatic interactions. Analysis of end group hydration further indicates that iterative ligand flipping can occur on experimentally relevant timescales. Supported by these results, we present a complete energy landscape for the non-disruptive insertion of amphiphilic NPs into lipid bilayers. These findings will help guide the design of NPs to enhance bilayer insertion and non-endocytic cellular uptake, and also provide physical insight into a possible pathway for the translocation of charged biomacromolecules.

两亲性单层保护金纳米颗粒（amphiphilic, monolayer-protected gold nanoparticles，NPs）已被证实可通过非内吞、无破坏的途径进入细胞，该特性在生物医学应用中具备重要价值。此类纳米颗粒还被发现可插入一系列模型细胞膜，作为细胞摄取的前置步骤，但其具体插入机制仍不明确。此前的模拟研究已证明，两亲性纳米颗粒可插入平面脂质双层的单叶层，但在该构型下，所有带电端基均局限于双层的一侧，目前尚不清楚进一步插入是否具备热力学可行性。本研究采用全原子分子动力学模拟，证实若带电配体可迭代地“翻转”穿过脂质双层，两亲性纳米颗粒可无破坏地抵达双层中平面。配体翻转是一个有利过程，可缓解双层曲率、降低纳米颗粒单层的非极性溶剂可及表面积，并增强配体与脂质间的静电吸引相互作用。对端基水合作用的分析进一步表明，迭代配体翻转可在与实验相关的时间尺度上发生。基于上述研究结果，我们构建了两亲性纳米颗粒无破坏插入脂质双层的完整能量图景。这些发现将有助于指导纳米颗粒的设计，以增强其双层插入能力与非内吞细胞摄取效率，同时也为带电生物大分子的易位提供了潜在途径的物理层面的见解。