Computational study of mutarotation in erythrose and threose.

For the first time the mutarotation mechanism of furanose rings has been investigated, with and without solvent. The transformations from open-chain furanose to D-erythrose and D-threose have been studied at B3LYP/6-311++G(d,p) and G3MP2B3 levels, in vacuum and in solution through continuum solvation models. We studied the catalytic influence of one, two or three water molecules, as well as simplified models of carbohydrates, that is, methanol and 1,2-ethanediol. Water molecules significantly reduce the energy barrier of the hemiacetal formation occurring between the open-chain and furanose configurations. The energy barrier is optimally reduced by two water molecules. Methanol yields a smaller transition state barrier than the one obtained with a single water molecule. In contrast, 1,2-ethanediol does not provide a lower transition state compared to the barrier in the presence of two water molecules.