Formation of Gaseous Proteins via the Ion Evaporation Model (IEM) in Electrospray Mass Spectrometry.
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The mechanisms whereby protein ions are released into the gas phase from charged droplets during electrospray ionization (ESI) continue to be controversial. Several pathways have been proposed. For native ESI the charged residue model (CRM) is favored; it entails the liberation of proteins via solvent evaporation to dryness. Unfolded proteins likely follow the chain ejection model (CEM), which involves the gradual expulsion of stretched-out chains from the droplet. According to the ion evaporation model (IEM) ions undergo electrostatically driven desorption from the droplet surface. The IEM is well supported for small precharged species such as Na+. However, it is unclear whether proteins can show IEM behavior as well. We examined this question using molecular dynamics (MD) simulations, mass spectrometry (MS), and ion mobility spectrometry (IMS) in positive ion mode. Ubiquitin was chosen as the model protein because of its structural stability which allows the protein charge in solution to be controlled via pH adjustment without changing the protein conformation. MD simulations on small ESI droplets (3 nm radius) showed CRM behavior regardless of the protein charge in solution. Surprisingly, many MD runs on larger droplets (5.5 nm radius) culminated in IEM ejection of ubiquitin, as long as the protein carried a sufficiently large positive solution charge. MD simulations predicted that nonspecific salt adducts are less prevalent for IEM-generated protein ions than for CRM products. This prediction was confirmed experimentally. Also, collision cross sections of MD structures were in good agreement with IMS data. Overall, this work reveals that the CRM, CEM, and IEM all represent viable pathways for generating gaseous protein ions during ESI. The IEM is favored for proteins that are tightly folded and highly charged in solution and for droplets in a suitable size regime.