Thermodynamic study on isomerization equilibria between aldohexoses and ketohexoses: evaluation of Gibbs energy and enthalpy of formation for hexoses and their tautomers

Abstract The systematic synthesis of all hexoses has been established recently by new enzymes that catalyze isomerization and epimerization between hexoses. This provides an opportunity to investigate the thermodynamic properties of all hexoses, including rare sugars. This study determined equilibrium constants for isomerization between various aldohexoses and ketohexoses in the temperature range of 10 to 60 °C, and the Gibbs energy and enthalpy of the reaction were evaluated. These data were used to determine the Gibbs energies and enthalpies of formation for all hexoses in aqueous solution. Hexoses in aqueous solution are equilibrium mixtures of 5 tautomers: chain, α-pyranose, β-pyranose, α-furanose, and β-furanose. The equilibrium composition of each tautomer was employed to estimate the Gibbs energies of formation for each tautomer. The formation Gibbs energies of α-pyranose and β-pyranose were compared with the steric energy of pyranoses calculated using the molecular mechanics algorithm MM3. For ketohexoses, the enthalpy of formation of each tautomer was also evaluated from the temperature dependence of tautomeric composition. Further, the enthalpy of formation in the solid-state ketohexoses was determined from the heat of solution in water. The obtained thermodynamic parameters provide basic information on the stability of hexoses in aqueous solution and the solid state.