Reconsidering the activation entropy for anomerization of glucose and mannose in water studied by NMR spectroscopy

The anomerization of monosaccharides is a very important process to understand how their stereoisomers are stabilized in aqueous solutions. For glucose and mannose, it has been known that alpha- and beta-anomers of hexopyranose exist as the major components. In order to examine the anomerization pathway for glucose and mannose in aqueous solutions, it is indispensable to determine the thermodynamic parameters such as the activation energy, the activation Gibbs free energy (AG(double dagger)), enthalpy (Delta H-double dagger) and entropy (Delta S-double dagger). Although several research groups reported these quantities in aqueous solution, they have still been controversial especially for Delta S-double dagger. In this paper, we employ H-1 NMR spectroscopy for monitoring the population of both alpha- and beta-anomers of glucose and mannose. The contribution of Delta S-double dagger to Delta G(double dagger) for glucose in water is estimated to be ca. 30%, while that for mannose is 8.0%. The large difference in Delta S-double dagger suggests that the anomerization pathway is not the same for glucose and mannose. (C) 2015 Elsevier B.V. All rights reserved.