Soltani Sepideh, Oh Myong In, Consta Styliani
Department of Physics and Astronomy, The University of Western Ontario, London, Ontario N6A 3K7, Canada.
Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
J Chem Phys. 2015 Mar 21;142(11):114307. doi: 10.1063/1.4914923.
We examine the effect of solvent on the charging mechanisms of a macromolecule in a droplet by using molecular dynamics simulations. The droplet contains excess charge that is carried by sodium ions. To investigate the principles of the charging mechanisms of a macromolecule in a droplet, we simulate aqueous and methanol droplets that contain a poly(ethylene glycol) (PEG) molecule. We find that the solvent plays a critical role in the charging mechanism and in the manner that the sodiated PEG emerges from a droplet. In the aqueous droplets, the sodiated PEG is released from the droplet while it is being charged at a droplet charge state below the Rayleigh limit. The charging of PEG occurs on the surface of the droplet. In contrast to the aqueous droplets, in the methanol droplet, the sodiated PEG resides in the interior of the droplet and it may become charged at any location in the droplet, interior or surface. The sodiated PEG emerges from the droplet by drying-out of the solvent. Even though these two mechanisms appear to be phenomenologically similar to the widely accepted ion-evaporation and charge-residue mechanisms, they have fundamental differences from those. An integral part of the mechanism that the macromolecular ions emerge from droplets is the droplet morphology. Droplet morphologies give rise to different solvation interactions between the solvent and the macromolecule. In the water-sodiated PEG system, we find the extrusion of the PEG morphology, while in methanol-sodiated droplet, we find the "pearl-on-the-necklace" morphology and the extrusion of the sodiated PEG in the last stage of the desolvation process. These findings provide insight into the mechanisms that macromolecules acquire their charge in droplets produced in electrospray ionization experiments.
我们通过分子动力学模拟研究了溶剂对液滴中大分子充电机制的影响。该液滴含有由钠离子携带的过量电荷。为了研究液滴中大分子充电机制的原理，我们模拟了含有聚乙二醇（PEG）分子的水滴和甲醇液滴。我们发现溶剂在充电机制以及钠化PEG从液滴中出现的方式中起着关键作用。在水滴中，钠化PEG在低于瑞利极限的液滴电荷状态下充电时从液滴中释放出来。PEG的充电发生在液滴表面。与水滴不同，在甲醇液滴中，钠化PEG位于液滴内部，并可能在液滴内部或表面的任何位置带电。钠化PEG通过溶剂的干燥从液滴中出现。尽管这两种机制在现象学上似乎与广泛接受的离子蒸发和电荷残留机制相似，但它们与那些机制有根本区别。大分子离子从液滴中出现的机制的一个组成部分是液滴形态。液滴形态导致溶剂与大分子之间产生不同的溶剂化相互作用。在水 - 钠化PEG系统中，我们发现了PEG形态的挤出，而在甲醇 - 钠化液滴中，我们发现了“项链上的珍珠”形态以及在去溶剂化过程的最后阶段钠化PEG的挤出。这些发现为大分子在电喷雾电离实验中产生的液滴中获得电荷的机制提供了见解。
