Department of Chemistry, The University of Western Ontario, London, Ontario, Canada.
J Phys Chem B. 2012 Jan 12;116(1):104-12. doi: 10.1021/jp209344z. Epub 2011 Dec 27.
The mechanism whereby macromolecular analytes are transferred into the gas phase during the final stages of electrospray ionization (ESI) remains a matter of debate. In this work, we employ molecular dynamics simulations to examine the temporal behavior of nanometer-sized aqueous ESI droplets containing a polymer chain and excess ammonium ions. The polymer is modeled using a coarse-grained framework where a bead-string backbone is decorated with side chains that can be nonpolar, cationic, or anionic. Polymers that adopt compact conformations and that carry a large number of charged side chains remain close to the droplet center, where the charges are extensively hydrated. The ESI process for these compact/hydrophilic macromolecules must involve solvent evaporation to dryness. This behavior is consistent with the charged residue model (CRM). A completely different scenario is encountered for disordered (extended) chains that carry a large number of nonpolar side chains. In this case, the macromolecule tends to be rapidly expelled from the droplet surface in a stepwise sequential fashion. This process produces metastable structures where one end of the extended polymer chain remains connected with the droplet via charge solvation. Disruption of these last interactions will then produce a free gas phase macromolecular ion. The data of this work imply that the ESI process for unfolded/hydrophobic polymers proceeds via an ion evaporation (IEM)-like mechanism that is facilitated by hydrophobic solute/solvent interactions. Our model predicts that the ESI efficiency of the latter scenario is considerably higher than for the CRM. This prediction is verified experimentally through ESI mass spectrometry measurements on myoglobin.
在电喷雾电离（ESI）的最后阶段，大分子分析物进入气相的机制仍然存在争议。在这项工作中，我们采用分子动力学模拟来研究含有聚合物链和过量铵离子的纳米级水基 ESI 液滴的时间行为。聚合物采用粗粒度框架建模，其中珠串骨架用非极性、阳离子或阴离子侧链修饰。采用紧凑构象且带有大量带电侧链的聚合物仍然靠近带有大量电荷的液滴中心，这些电荷被充分水合。对于这些紧凑/亲水的大分子，ESI 过程必须涉及溶剂蒸发至干燥。这种行为与带电残基模型（CRM）一致。对于无序（扩展）链，其带有大量非极性侧链，会遇到完全不同的情况。在这种情况下，大分子倾向于以逐步顺序的方式从液滴表面快速排出。这个过程会产生亚稳态结构，其中扩展聚合物链的一端通过电荷溶剂化与液滴相连。最后这些相互作用被破坏，然后产生游离的气相大分子离子。这项工作的数据表明，展开/疏水分子聚合物的 ESI 过程通过类似于离子蒸发（IEM）的机制进行，这种机制受到疏水溶质/溶剂相互作用的促进。我们的模型预测，后一种情况的 ESI 效率比 CRM 高得多。通过对肌红蛋白进行 ESI 质谱测量，实验验证了这一预测。
