Transition states for glucopyranose interconversion.

Glucose is a central molecule in biology and chemistry, and the anomerization reaction has been studied for more than 150 years. Transition-state structure is the last impediment to an in-depth understanding of its solution chemistry. We have measured kinetic isotope effects on the rate constants for approach of alpha-glucopyranose to its equilibrium with beta-glucopyranose, and these were converted into unidirectional kinetic isotope effects using equilibrium isotope effects. Saturation transfer 13C NMR spectroscopy has yielded the relative free energies of the transition states for the ring-opening and ring-closing reactions, and both transition states contribute to the experimental kinetic isotope effects. Both transition states of the anomerization process have been modeled with high-level computational theory with constraints from the primary, secondary, and solvent kinetic isotope effects. We have found the transition states for anomerization, and we have also concluded that it is forbidden for the water molecule to form a hydrogen bond bridge to both OH1 and O5 of glucose simultaneously in either transition state.