Anomeric proportions of d-glucopyranose at the equilibrium determined from 1H NMR spectra II. Effects of alkali metal chlorides, CaCl2 and BaCl2 on the anomeric equilibrium at 25.0 °C

Abstract

The isomeric proportions of saccharides are important information to discuss the properties of the saccharides such as the solvation states, structures, functions and so on in aqueous solutions. However, the isomeric composition, which is one of the fundamental properties of saccharides in the aqueous solution, has not been fully explored experimentally. Successively to our previous study on the anomeric proportion of d-glucopyranose in the aqueous solution [J. Mol. Liq., 232 (2017) 408–415.], the salt effect on these anomeric proportions at 25.0°C were investigated by 1H NMR measurement for the cases of all alkali metal chlorides, CaCl2 and BaCl2. The experimental conditions were; the glucose concentration was from 0.1wt% to 1.0wt% and the salt molality was from 0.1molkg−1 to 0.5molkg−1. At first, the optimal experimental conditions were confirmed in terms of the irradiation power and the target frequency of the presaturation pulse in 1H NMR measurement. The 1H NMR spectra measured under the optimal conditions with adequate accumulation number allowed us to determine the anomeric proportions within the error of 0.0004. The anomeric proportions of α-glucopyranose in the presence of each salts were determined from peak areas corresponding to the anomeric hydrogens of α- and β- d-glucopyranoses. For all kinds of the salts, the anomeric proportion increased consistently and the slope of the proportion decreased gradually with increasing glucose concentration at a given molality of the salts. The anomeric proportions at the infinite dilution of glucose were determined from these concentration dependences of glucose, and then the Gibbs energies of reaction for the anomerization ΔrG° were estimated from these results. The effects of the salts on ΔrG° were divided into two groups: (1) ΔrG° increased when LiCl, NaCl and CaCl2 were added, and (2) ΔrG° decreased when KCl, RbCl, CsCl and BaCl2 were added. The plots of ΔrG° against the ionic radius of the cations clearly illustrated this result, that is, ΔrG° for these two groups fell on the quite different cation radius dependences. ΔrG° of the first group of the salts were greater than that of the salt free system. ΔrG° for each molality of the salts linearly decreased with the cation radius, and became ΔrG° of the salt free glucose aqueous solution around 0.17nm irrespectively to the salt molality. On the other hand, ΔrG° of the second group of the salts were less than that of the salt free system. ΔrG° for each molality of the salts linearly decreased with the cation radius, and became ΔrG° of the salt free solution around 0.08nm irrespectively to the salt molality.