Anomeric proportions of d-glucopyranose at the equilibrium determined from 1H-NMR spectra I. Investigation of experimental conditions and concentration dependence at 25.0 °C.

Anomeric proportions of d -glucopyranose, defined as the ratio of the amount of substance of α- d -glucopyranose to the sum of that of α- and β- d -glucopyranoses, in aqueous solution were determined from 1 H NMR spectra in the concentration range between 0.1 wt% and 10 wt% at 25.0 °C. First of all, experimental conditions were investigated in terms of a time required to equilibrate anomerization and an irradiation power and a target frequency of the presaturation pulse in 1 H NMR measurement. Using the optimal condition determined by us, we were able to obtain spectra whose resonant peaks of anomeric protons of d -glucopyranoses were symmetric and baseline were non-distorted. These spectra allowed us to determine the anomeric proportions within experimental error of 0.005. Anomeric proportions of α-glucopyranose in the concentration range from 0.1 wt% to 10 wt% were determined from peak areas corresponding to anomeric hydrogens of α- and β- d -glucopyranose in 1 H NMR spectra. The anomeric proportion increased with increasing glucose concentration, though the degree of its increment was not uniform. The steep increase of the proportion was observed in the range between 0.1 wt% and 1 wt% and the increase was gentle between 1 wt% and 6 wt%. The proportion at infinite dilution determined by the least square method using a quadratic curve for the proportions was 0.3598 with a standard error of ± 0.0003, which lead that the Gibbs energy of anomerization of − 1.43 kJ mol − 1 . The concentration dependence of the ratio of the activity coefficient for α-glucopyranose to that for β-glucopyranose was estimated from the anomeric proportions and equilibrium constant of the anomerization. The ratio increased with increasing concentration, which shows that the activity coefficient for β-glucopyranose was slightly greater than those for α-glucopyranose at this concentration range. [ABSTRACT FROM AUTHOR]