Determination of Amide cis/trans Isomers in N-Acetyl-d-glucosamine: Tailored NMR Analysis of the N-Acetyl Group Conformation.

N-Acetyl-d-glucosamine (GlcNAc) is one of the most common amino sugars in nature, but the conformation of its N-acetyl group has drawn little attention. We report herein the first identification of NH protons of the amide cis forms of α- and β-GlcNAc by NMR spectroscopy. Relative quantification and thermodynamic analysis of both cis and trans forms was carried out in aqueous solution. The NH protons were further utilized by adapting protein NMR experiments to measure eight J-couplings within the N-acetyl group, of which six are sensitive to the H2-NH conformation and two are sensitive to the amide conformation. For amide cis and trans forms, the orientation between H2 and NH was determined as anti conformation, while a small percentage of syn conformation was predicted for the amide trans form of β-GlcNAc. This approach holds great promise for the detailed conformational analysis of GlcNAc in larger biomolecules, such as glycoproteins and polysaccharides.