A direct-dynamics study of the zwitterion-to-neutral interconversion of glycine in aqueous solution
Bibliographic record
Abstract
The mechanism of interconversion between the neutral and zwitterionic forms of glycine in aqueous solution is studied theoretically. It is argued that indirect transfer via a water bridge is a plausible alternative to the generally assumed direct transfer mechanism. The argument is based on model calculations in which the system glycine-water is represented by a 1:6 supermolecule embedded in a dielectric continuum. Optimized geometries and vibrational frequencies are obtained at the Hartree–Fock level with a 6-31G* basis set, and at the second-order Mo/ller–Plesset frozen-core level with the 6-31+G* basis set for the neutral and zwitterionic forms, and for their transition state. At both levels the energetics are corrected by single-point quadratic configuration interaction calculations, including single and double substitutions with frozen-core inner-shell orbitals. Both models reproduce the observed endothermicity of the transfer better than models that use only a limited number of discrete water molecules without a continuum and models solely based on the continuum approximation. In the optimized structures of this complex and of complexes with fewer water molecules, one of the water molecules always bridges the two functional groups. In the 1:6 complex, two of the other water molecules form hydrogen bonds with the amino hydrogens, two others with the carboxyl oxygens, and the sixth water molecule forms a bridge between the two water molecules attached to the amino group. The interaction of this supermolecule with the bulk solvent is treated by means of the Onsager model. The transition state calculated with the two models implies that the mechanism of interconversion is concerted transfer of two protons along the amino–water–carboxyl bridge. The dynamics calculations are performed with a multidimensional instanton model that includes solvent reorganization. For both models the calculated transfer rate constants are about an order of magnitude larger than the observed rate constants, indicating that the indirect mechanism can easily account for the observed dynamics. These results confirm the plausibility of the indirect mechanism of proton transfer via a water bridge in aqueous solutions of glycine.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| 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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".