Observation of the keto tautomer of D-fructose in D2O using 1H NMR spectroscopy

D-Fructose was analysed by NMR spectroscopy and previously unidentified H-1 NMR resonances were assigned to the keto and alpha-pyranose tautomers. The full assignment of shifts for the various fructose tautomers enabled the use of H-1 NMR spectroscopy in studies of the mutarotation (5-25 degrees C) and tautomeric composition at equilibrium (5-50 degrees C). The mutarotation of p-pyranose to furanose tautomers in D2O at a concentration of 0.18 M was found to have an activation energy of 62.6 kJ mol(-1). At tautomeric equilibrium (20 degrees C in D2O) the distribution of the beta-pyranose, beta-furanose, alpha-furanose, alpha-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 degrees C there was a linear relationship between the change in concentration and temperature for all forms. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.