Department of Chemistry, The University of Western Ontario, London, ON, N6A 5B7, Canada.
J Am Soc Mass Spectrom. 2017 Nov;28(11):2262-2279. doi: 10.1007/s13361-017-1754-4. Epub 2017 Aug 11.
We study systematically the charging and release mechanisms of a flexible macromolecule, modeled by poly(ethylene glycol) (PEG), in a droplet by using molecular dynamics simulations. We compare how PEG is solvated and charged by sodium Na ions in a droplet of water (HO), acetonitrile (MeCN), and their mixtures. Initially, we examine the location and the conformation of the macromolecule in a droplet bearing no net charge. It is revealed that the presence of charge carriers do not affect the location of PEG in aqueous and MeCN droplets compared with that in the neutral droplets, but the location of the macromolecule and the droplet size do affect the PEG conformation. PEG is charged on the surface of a sodiated aqueous droplet that is found close to the Rayleigh limit. Its charging is coupled to the extrusion mechanism, where PEG segments leave the droplet once they coordinate a Na ion or in a correlated motion with Na ions. In contrast, as PEG resides in the interior of a MeCN droplet, it is sodiated inside the droplet. The compact macro-ion transitions through partially unwound states to an extended conformation, a process occurring during the final stage of desolvation and in the presence of only a handful of MeCN molecules. For charged HO/MeCN droplets, the sodiation of PEG is determined by the HO component, reflecting its slower evaporation and preference over MeCN for solvating Na ions. We use the simulation data to construct an analytical model that suggests that the droplet surface electric field may play a role in the macro-ion-droplet interactions that lead to the extrusion of the macro-ion. This study provides the first evidence of the effect of the surface electric field by using atomistic simulations. Graphical Abstract ᅟ.
我们通过分子动力学模拟系统地研究了柔性聚合物聚乙二醇（PEG）在液滴中的荷电和释放机制。我们比较了 PEG 在水（HO）、乙腈（MeCN）液滴及其混合物中被钠离子 Na 溶剂化和荷电的情况。首先，我们研究了在不带净电荷的液滴中大分子的位置和构象。结果表明，与中性液滴相比，电荷载体的存在并不影响 PEG 在水相和 MeCN 液滴中的位置，但大分子的位置和液滴的大小确实会影响 PEG 的构象。PEG 在靠近瑞利极限的带正电荷的水合钠离子液滴表面荷电。它的荷电与挤出机制有关，其中 PEG 片段一旦与 Na 离子配位或与 Na 离子协同运动，就会离开液滴。相比之下，由于 PEG 存在于 MeCN 液滴的内部，它在液滴内部被钠化。致密的大分子离子通过部分展开状态过渡到伸展构象，这一过程发生在去溶剂化的最后阶段，并且只有少量的 MeCN 分子存在。对于带电荷的 HO/MeCN 液滴，PEG 的钠化取决于 HO 成分，这反映了其蒸发速度较慢，并且比 MeCN 更倾向于溶剂化 Na 离子。我们使用模拟数据构建了一个分析模型，该模型表明液滴表面电场可能在导致大分子离子挤出的大分子-液滴相互作用中发挥作用。这项研究首次通过原子模拟提供了表面电场影响的证据。
