Department of Chemistry, The University of Western Ontario , London, Ontario N6A 5B7, Canada.
J Am Chem Soc. 2015 Oct 7;137(39):12667-76. doi: 10.1021/jacs.5b07913. Epub 2015 Sep 25.
The mechanism whereby gaseous protein ions are released from charged solvent droplets during electrospray ionization (ESI) remains a matter of debate. Also, it is unclear to what extent electrosprayed proteins retain their solution structure. Molecular dynamics (MD) simulations offer insights into the temporal evolution of protein systems. Surprisingly, there have been no all-atom simulations of the protein ESI process to date. The current work closes this gap by investigating the behavior of protein-containing aqueous nanodroplets that carry excess positive charge. We focus on "native ESI", where proteins initially adopt their biologically active solution structures. ESI proceeds while the protein remains entrapped within the droplet. Protein release into the gas phase occurs upon solvent evaporation to dryness. Droplet shrinkage is accompanied by ejection of charge carriers (Na(+) for the conditions chosen here), keeping the droplet at ∼85% of the Rayleigh limit throughout its life cycle. Any remaining charge carriers bind to the protein as the final solvent molecules evaporate. The outcome of these events is largely independent of the initial protein charge and the mode of charge carrier binding. ESI charge states and collision cross sections of the MD structures agree with experimental data. Our results confirm the Rayleigh/charged residue model (CRM). Field emission of excess Na(+) plays an ancillary role by governing the net charge of the shrinking droplet. Models that envision protein ejection from the droplet are not supported. Most nascent CRM ions retain native-like conformations. For unfolded proteins ESI likely proceeds along routes that are different from the native state mechanism explored here.
在电喷雾电离（ESI）过程中，带电荷的溶剂液滴释放气态蛋白离子的机制仍然存在争议。此外，电喷雾蛋白质在多大程度上保留其溶液结构尚不清楚。分子动力学（MD）模拟为蛋白质系统的时间演化提供了深入了解。令人惊讶的是，迄今为止，还没有针对蛋白质 ESI 过程的全原子模拟。目前的工作通过研究携带过量正电荷的含蛋白质水纳米液滴的行为来填补这一空白。我们专注于“天然 ESI”，其中蛋白质最初采用其具有生物活性的溶液结构。ESI 过程在蛋白质被捕获在液滴内的情况下进行。溶剂蒸发至干燥时，蛋白质会释放到气相中。随着液滴的收缩，电荷载体（对于这里选择的条件为 Na(+)）被喷出，使液滴在其整个生命周期中保持在瑞利极限的 85%左右。当最后一个溶剂分子蒸发时，任何剩余的电荷载体都会与蛋白质结合。这些事件的结果在很大程度上与初始蛋白质电荷和电荷载体结合方式无关。MD 结构的 ESI 电荷状态和碰撞截面与实验数据一致。我们的结果证实了瑞利/带电残基模型（CRM）。过量 Na(+) 的场发射通过控制收缩液滴的净电荷起到辅助作用。不支持将蛋白质从液滴中喷射出来的模型。大多数新生 CRM 离子保留类似天然的构象。对于展开的蛋白质，ESI 可能沿着与这里探索的天然状态机制不同的途径进行。
