Investigation of the enolization and carbonyl group migration in reducing sugars by FTIR spectroscopy

The FTIR spectra of selected aldoses and ketoses were recorded in D2O as a function of temperature between 30 and 80°C. The analysis of the spectra has revealed the presence of two sets of temperature-sensitive bands, one centered in the carbonyl region (1700–1750 cm−1) and the other in the alkene absorption region (1630–1680 cm−1). The latter was assigned to the enediol species formed as a result of enolization of acyclic aldehydo and keto forms of the sugars. Both absorption frequencies were confirmed by studies with selected 13C-substituted sugars. The presence of α-hydroxyl groups in reducing sugars was found to shift the stretching frequencies of sugar carbonyl bands to higher values relative to simple alkyl-substituted carbonyl compounds. The relative concentration of enediol to that of carbonyl was also studied as a function of temperature and concentration. The ratio of intensities of enediol to carbonyl absorption bands was found to decrease with temperature and increase with concentration. Using d-[2-13C]ribose and d-[1-13C]ribose, further evidence for the occurrence of enolization was provided by observing the migration of carbonyl group from C-1 to C-2 in d-ribose and subsequent formation of d-erythro-pentulose (d-ribulose) as a result of reversal of enediol-carbonyl equilibrium through both hydroxyl groups. A mechanism is proposed for the enolization of sugars in neutral aqueous solutions involving hydrogen-bonded sugar dimers. These studies indicated that the relative concentrations of the enediols are as important as aldehydo or keto forms of reducing sugars in aqueous solutions.