The hydration of monosaccharides—an NMR study

At temperatures close to 0°C proton exchange between sugar hydroxyl groups and water is slow, and separate proton resonance peaks can be detected for the hydroxyl protons. All are shifted downfield of the water resonance, the anomeric hydroxyl proton shift being the greatest. Axial anomeric hydroxyl protons are shifted less than corresponding equatorial protons. Proton exchange with water is strongly acid and base catalyzed, but, at least in some cases, there seems to be an additional pH-independent mechanism involved. From the temperature effect on the shifts, and the effect of added dimethyl sulfoxide, we conclude that each hydroxyl group is bonded on average to two water molecules. This estimate of the hydration number for monosaccharides is far greater than those previously deduced from relaxation studies. It is suggested that the source of this difference lies in the residence times of the bound water molecules. Shifts of the hydroxyl proton resonances for sugars in methanol are compared with those for aqueous solutions and are found to be very similar. Hence it is concluded that these shifts do not reveal any special effects due to water structure. There are quite marked differences in the shifts for different sugars, and, in particular, the anomeric hydroxyl proton shifts for ketoses are smaller than those for aldoses.Taken as solvation spectra, Part 56.