13C Solid-state NMR chemical shift anisotropy analysis of the anomeric carbon in carbohydrates Original Research Article

13C NMR solid-state structural analysis of the anomeric center in carbohydrates was performed on six monosaccharides: glucose (Glc), mannose (Man), galactose (Gal), galactosamine hydrochloride (GalN), glucosamine hydrochloride (GlcN), and N-acetyl-glucosamine (GlcNAc). In the 1D 13C cross-polarization/magic-angle spinning (CP/MAS) spectrum, the anomeric center C-1 of these carbohydrates revealed two well resolved resonances shifted by 3–5 ppm, which were readily assigned to the anomeric α and β forms. From this experiment, we also extracted the 13C chemical shift anisotropy (CSA) tensor elements of the two forms from their spinning sideband intensities, respectively. It was found out that the chemical shift tensor for the α anomer was more axially symmetrical than that of the β form. A strong linear correlation was obtained when the ratio of the axial asymmetry of the 13C chemical shift tensors of the two anomeric forms was plotted in a semilogarithmic plot against the relative population of the two anomers. Finally, we applied REDOR spectroscopy to discern whether or not there were any differences in the sugar ring conformation between the anomers. Identical two-bond distances of 2.57 Å (2.48 Å) were deduced for both the α and β forms in GlcNAc (GlcN), suggesting that the two anomers have essentially identical sugar ring scaffolds in these sugars. In light of these REDOR distance measurements and the strong correlation observed between the ratio of the axial asymmetry parameters of the 13C chemical shift tensors and the relative population between the two anomeric forms, we concluded that the anomeric effect arises principally from interaction of the electron charge clouds between the C-1–O-5 and the C-1–O-1 bonds in these monosaccharides.