Observation of the keto tautomer of d-fructose in D ₂O using ¹H NMR spectroscopy

d-Fructose was analysed by NMR spectroscopy and previously unidentified ¹H NMR resonances were assigned to the keto and α-pyranose tautomers. The full assignment of shifts for the various fructose tautomers enabled the use of ¹H NMR spectroscopy in studies of the mutarotation (5-25 °C) and tautomeric composition at equilibrium (5-50 °C). The mutarotation of β-pyranose to furanose tautomers in D ₂O at a concentration of 0.18 M was found to have an activation energy of 62.6 kJ mol ^-1. At tautomeric equilibrium (20 °C in D ₂O) the distribution of the β-pyranose, β-furanose, α-furanose, α-pyranose and the keto tautomers was found to be 68.23%, 22.35%, 6.24%, 2.67% and 0.50%, respectively. This tautomeric composition was not significantly affected by varying concentrations between 0.089 and 0.36 M or acidification to pH 3. Upon equilibrating at 6 temperatures between 5 and 50 °C there was a linear relationship between the change in concentration and temperature for all forms.