Ionization and mutarotation of hexoses in aqueous alkaline solution as studied by 13C NMR spectroscopy

A 13C NMR spectroscopic study of glucose, mannose and fructose in aqueous alkaline solution has been carried out. Some of the 13C resonances are found to shift substantially with increasing pH. The 13C NMR titration curves allow of the determination of both the pKa values of the hexoses and the differences in acid strength of the α/β-anomeric forms. The shifts observed cannot be explained by a simple ionization; the ionized species is considered either as a rapid equilibrium between cyclic and pseudo-cyclic ionized species or as a non-classical anion. It is concluded that base catalysis in the mutarotation reaction originates from a weakening of the ring C-O bond in the generated sugar anion.