Formation of isomers of anionic hemiesters of sugars and carbonic acid in aqueous medium

Hemiesters of carbonic acid can be freely formed in aqueous media containing HCO3-/CO2 and mono- or poly-hydroxy compounds. Herein, C-13 NMR spectroscopy was used to identify isomers formed in aqueous solutions of glycerol (a prototype compound) and seven carbohydrates, as well as to estimate the equilibrium constant of formation (K-eq). Although both isomers are formed, glycerol 1-carbonate corresponds to 90% of the product. While fructose and ribose form an indistinct mixture of isomers, the anomers of D-glucopyranose 6-carbonate correspond to 74% of the eight isomers of glucose carbonate that were detected. The values of K-eq for the disaccharides sucrose (4.3) and maltose (4.2) are about twice the values for the monosaccharides glucose (2.0) and fructose (2.3). Ribose (K-eq = 0.89)-the only sugar without a significant concentration of a species containing a -CH2OH group in an aqueous solution-resulted in the smallest K-eq. On the basis of the K-eq value and the concentrations of HCO3- and glucose in blood, one can anticipate a concentration of 2-4 mu mol L-1 for glucose 6-carbonate, which corresponds to ca. of 10% of its phosphate counterpart (glucose 6-phosphate). (C) 2016 Elsevier Ltd. All rights reserved.