Tautomer distributions of N-acetyl-D-glucosamine in the condition of commonly utilized solvents and catalysts for biorefinery: NMR study

Abstract

The tautomer structure of monosaccharides plays an important role in its conversion and utilization, which is mainly manifested in the selectivity of the product and the reaction mechanism. The tautomer distributions of N-acetyl-D-glucosamine (GlcNAc), a natural nitrogen-containing monosaccharide were investigated by quantitative 1H NMR (1H qNMR) and 13C NMR (13C qNMR) at 298 K under the condition commonly utilized for biorefinery. Catalysts used for biorefinery, i.e. BaCl2, KCl, ZnCl2, MgCl2, B(OH)3, SnCl4·5H2O and amino acid ionic liquid were introduced into D2O or DMSO‑d6 to explore the tautomer distributions of GlcNAc. The introduction of ZnCl2 and SnCl4·5H2O promoted the conversion of α-pyranose to β-pyranose. The introduction of amino acid ionic liquid has little effect on the percentage of tautomers in D2O. However, when the amino acid ionic liquid, such as [Thr]Cl, was introduced into DMSO‑d6, the proportion of β-pyranose increased to 29.06%, compared with no catalyst introduced. Besides, we also studied the tautomer distributions of GlcNAc in commonly utilized reaction solvents for biorefinery, such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidinone, ethylene and glycerol. In these solvents, the proportion of β-pyranose in N,N-dimethylformamide and glycerol was reduced to 25.84% and 38.38%, respectively. This study provides the basic experimental data of tautomer distributions of GlcNAc, and guided the conversion and utilization of GlcNAc in the future.