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

• We reexamine the thermodynamic parameters of the anomerization for glucose and mannose.
• NMR spectroscopy enable us to estimate the population of the both epimers in D2O.
• The contribution of ΔS‡ to G‡ for glucose in water is clearly different for glucose and mannose.
• It is suggested that the anomerization pathway is not the same for glucose and mannose.

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 α- and β-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 (ΔG‡), enthalpy (ΔH‡), and entropy (ΔS‡). Although several research groups reported these quantities in aqueous solution, they have still been controversial especially for ΔS‡. In this paper, we employ 1H NMR spectroscopy for monitoring the population of both α- and β-anomers of glucose and mannose. The contribution of ΔS‡ to ΔG‡ for glucose in water is estimated to be ca. 30%, while that for mannose is 8.0%. The large difference in ΔS‡ suggests that the anomerization pathway is not the same for glucose and mannose.