Modeling the Behavior of Coarse-Grained Polymer Chains in Charged Water Droplets: Implications for the Mechanism of Electrospray Ionization
Bibliographic record
Abstract
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.
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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 | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
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".