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Tautomeric and epimeric equilibria of aldo- and ketohexoses studied by the MD simulations and QM calculations

Gaweda, Karolina, Plazinski, Wojciech
Carbohydrate research 2019 v.474 pp. 8-15
aldohexoses, aqueous solutions, energy, moieties, molecular models, monosaccharides
The article is devoted to the problem of molecular modeling of tautomeric and epimeric equilibria in aqueous solutions of unfunctionalized d-aldo- and d-ketohexoses. We have applied the computational protocol proposed in our previous article [Gaweda, Plazinski, Phys. Chem. Chem. Phys., 2017, 19, 20760–20772, doi:10.1039/c7cp02920a], originally designed to study the conformational features of saccharides, in order to check whether it can be extended to the case of tautomeric/epimeric equilibria of monosaccharides. The results show that the most important trends are correctly reflected in a qualitative manner, i.e. within the limits of ‘chemical accuracy’ (∼±4 kJ/mol). Insight into the calculated conformational energies provides a molecular interpretation of the tautomeric preferences of aldohexoses, according to which the pyranose/furanose ratio is determined mainly by the energy level of pyranose forms, whereas the energies of furanose forms are approximately constant along the series. The investigated paths of epimerization suggest that epimerization of aldohexopyranoses at any center favors the equatorial arrangements of the hydroxyl group. The energetic effects of epimerization in furanoses are significantly lower and do not exhibit related systematic trends.