Insights into the Mechanism of Protein Electrospray Ionization From Salt Adduction Measurements
Bibliographic record
Abstract
The mechanisms whereby protein ions are liberated from charged droplets during electrospray ionization (ESI) remain under investigation. Compact conformers electrosprayed from aqueous solution in positive ion mode likely follow the charged residue model (CRM), which envisions analyte release after solvent evaporation to dryness. The concentration of nonvolatile salts such as NaCl increases sharply within vanishing CRM droplets, promoting nonspecific pairing of Cl(-) and Na(+) with charged groups on the protein surface. For unfolded proteins, it has been proposed that ion formation occurs via the chain ejection model (CEM). During the CEM proteins are expelled from the droplet long before complete solvent evaporation has taken place. Here we examine whether salt adduction levels support the view that folded and unfolded proteins follow different ESI mechanisms. Solvent evaporation during the CEM is expected to be less extensive and, hence, the salt concentration at the point of protein release should be substantially lower than for the CRM. CEM ions should therefore exhibit lower adduction levels than CRM species. We explore the adduction behavior of several proteins that were chosen to allow comparative studies on folded and unfolded structures in the same solution. In-source activation eliminates chloride adducts via HCl release, generating protein ions that are heterogeneously charged because of sodiation and protonation. Sodiation levels measured under such conditions provide estimates of the salt adduction behavior experienced by the "nascent" analyte ions. Sodiation levels are significantly reduced for unfolded proteins, supporting the view that these species are indeed formed via the CEM.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".