Question

If you had a mixture of 4 and 5 sugar molecules linked together, how could one distinguish the polymers by 13C-NMR. Assume some carbons will overlap and you can’t just count total carbons.

Answer 1

Introduction of anomeric carbon: Carbohydrate exists in a either cyclic or an acyclic form. During cyclization, the carbonyl carbon transforms into a new stereocenter. Cyclization causes the formation of 2 new diasteriomers. They differ in the position of the attachment of a certain group to the new stereocenter. The new stereocenter is referred to as the anomeric carbon. In a cyclic carbohydrate, the carbon that was the carbonyl carbon in acyclic form. In the cyclic form, the anomeric carbon can be found next to the oxygen atom in the pyranose or furanose ring, but on the opposite side from the carbon that carries the acyclic CH₂O group.

All the sugar molecules have anomeric carbon and have certain characteristic chemical shift values. Generally, 13C chemical shift value of anomeric carbon is somewhere in between 90-100 ppm.

For example, five different peaks will appear in the region between 90-100 ppm in 13C-NMR spectra. This means the sugar polymer have five different type of sugar molecules.

On the other way (for example), the sugar polymer containing five different sugars like Glucose, Fructose, Xylose, Mannitol, and Galactose will show five signal in the region between 90-100 ppm in 13C-NMR spectra corresponding to five different anomeric carbons. Now, 13C value of anomeric carbon of sugar polymer can be compared with that of the indivisual sugar molecules (i.e. Glucose, Fructose, Xylose, Mannitol, and Galactose; this can be done by recording 13C-NMR of each sugars indivisually).