HYDROXYL PROTON MAGNETIC RESONANCE IN RELATION TO RING SIZE, SUBSTITUENT GROUPS, AND MUTAROTATION OF CARBOHYDRATES

Some magnetic resonance properties of hydroxyl protons are described for a variety of sugars and derivatives in dimethyl sulfoxide. Spectral information of this kind is shown to be useful, particularly for differentiating between certain types of interconvertible molecular species. For example, a reducing sugar may be characterized as pyranose or furanose depending upon whether it exhibits a resonance signal that can be ascribed to OH-4 or OH-5, respectively. According to the data obtained, α-D-glucose, α-D-xylose, β-D-arabinose, α-D-lyxose, and α- and β-D-ribose are pyranoses when freshly dissolved, whereas the 2,3-carbonate of D-mannose or D-lyxose is in a furanose form. Selective deuteration and spin decoupling were of primary importance in making some of the spectral assignments.Although arabinose, ribose, and galactose exist partly as furanoses in mutarotated aqueous solutions, their tendency to adopt the structure of the five-membered ring is much greater in dimethyl sulfoxide. Such variability may be due to an interplay of effects; for example, the furanose forms are relatively stable, but this property is masked in water by preferential solvent stabilization of the pyranose structures.Depending on the number and kinds of hydroxyl proton signals detected, it is possible to distinguish between cyclic and acyclic forms of certain sugar derivatives, as illustrated with hydrazones and with dialdehydes obtained by oxidative glycol scission. Hydroxyl proton magnetic resonance data are of value also for characterizing partially acetylated sugars.