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Solvent effects on glycine. I. A supermolecule modeling of tautomerization via intramolecular proton transfer
Author(s) -
Balta Bülent,
Aviyente Viktorya
Publication year - 2003
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.10341
Subject(s) - zwitterion , supermolecule , chemistry , tautomer , solvation , intramolecular force , natural bond orbital , molecule , computational chemistry , solvent effects , valence bond theory , intermolecular force , solvent , chemical physics , hydrogen bond , photochemistry , density functional theory , molecular orbital , stereochemistry , organic chemistry
The relative stabilities of glycine tautomers involved in the intramolecular proton transfer are investigated computationally by considering glycine‐water complexes containing up to five water molecules. The supermolecule results are compared with continuum calculations. Specific solute‐solvent interactions and solvent induced changes in the solute wave function are considered using the natural bond orbitals (NBO) method. The stabilization of the zwitterion upon solvation is explained by the changes in the wave functions localized on the forming and breaking bonds as well as by the different interaction energies in the zwitterionic and neutral clusters. Only the neutral species exist in mono‐ and dihydrated clusters and in the gas phase. In the smaller clusters, zwitterions are mainly stabilized by conformational effects, whereas in larger clusters, in particular when glycine is solvated on both sides of its heavy atom backbone, polarization effects dominate the stability of a given tautomer. Generally, the strength of the solute‐solvent interactions is governed by the intermolecular charge transfer interactions. As the solvation progresses, the hypothetical gaseous zwitterion is better solvated than the gaseous neutral, making zwitterion to neutral tautomerization progressively less exothermic for clusters containing up to three water molecules, and endothermic for larger clusters. The neutral isomer does not exist for some solvent arrangements with five water molecules. Only solvent arrangements in which water molecules do not interact with the reactive proton are considered. Hence, the experimentally observed double well potential energy surface may be due to such an interaction or to a different reaction mechanism. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1789–1802, 2003