13C NMR studies of isomerization of D-glucose in an aqueous solution of Ca(OH)2.: The effect of molecular oxygen

Isomerization of D-glucose to fructose and mannose in aqueous solutions of Ca(OH)(2) with the initial pH 11.4 in a temperature interval of 20-90 degrees C was studied by C-13 NMR spectroscopy in the presence and absence of dissolved oxygen. In the presence of oxygen. the apparent equilibrium isomerization constant is much lower than that in the absence of oxygen. This is related to the oxidation of monosaccharides to formic and aldonic acids, a decrease in the pH of solutions, and cessation of isomerization at pH < 9.