A DFT study of solvation effects on the tautomeric equilibrium and catalytic ylide generation of thiamin models
Copyright 2007 Wiley Periodicals, Inc.
| Veröffentlicht in: | Journal of computational chemistry. - 1984. - 29(2008), 7 vom: 30. Mai, Seite 1037-47 |
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| Format: | Online-Aufsatz |
| Sprache: | English |
| Veröffentlicht: |
2008
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| Zugriff auf das übergeordnete Werk: | Journal of computational chemistry |
| Schlagworte: | Journal Article Research Support, Non-U.S. Gov't Cyclohexanes Ethers Ethylene Dichlorides Gases Solvents Water 059QF0KO0R Cyclohexane mehr... |
| Zusammenfassung: | Copyright 2007 Wiley Periodicals, Inc. Thiamin diphosphate (ThDP) is the biologically active form of vitamin B1 and an essential cofactor for a number of enzymes. The effect of solvent polarity on the tautomeric equilibria of ThDP using three model systems of the 4'-aminopyrimidine ring is studied by density functional theory calculations (B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)) in the gas phase and selected solvents (cyclohexane, ether, dichloroethane, and water). Solvation effects are investigated using three different schemes: implicit solvation by a continuum model, explicit solvation by inclusion of three water molecules mimicking the first solvation shell of the enzymatic environment, and by a mixed implicit/explicit solvation model. The 4'-aminopyrimidine tautomer is more stable than the 1',4'-iminopyrimidine tautomer in all solvation schemes employed; however, the trend for the stabilities of the 1',4'-iminopyrimidine tautomer in the solvents depends on the specific ThDP-model. Formation of the catalytic important ylide for ThDP-dependent enzymes by deprotonation of ThDP(C2) is also investigated by localization of transition states for two possible pathways. Only the less stable tautomer, 1',4'-iminopyrimidine ThDP, is able to form the catalytic active ylide. Generation of the ylide through a direct intramolecular proton transfer from ThDP(C2) to the ThDP(N4') nitrogen lone pair is favored by 6 kcal/mol in the gas phase, as compared to a water-mediated ylide generation. However, inclusion of a dielectric medium reduces this difference dramatically. Furthermore, inclusion of two water molecules to model the apoenzymatic environment lowers the activation energies of both direct and water-mediated ylide generation |
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| Beschreibung: | Date Completed 11.06.2008 Date Revised 21.11.2013 published: Print Citation Status MEDLINE |
| ISSN: | 1096-987X |