Mannose

Mannose is an aldohexose, a six-carbon monosaccharide. The commonly discussed biological form, D-mannose, has neutral molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. Like other sugars, it can have distinct cyclic forms; specifying D configuration does not by itself identify an alpha or beta anomer.^[ref]

A Fischer projection provides an original structural comparison with D-glucose. Reading hydroxyl positions from carbon2 to carbon5, D-glucose has the pattern right–left–right–right, while D-mannose has left–left–right–right. The difference is at carbon2 alone, so they are epimers. Reversing all four stereocenters would be a different operation, producing the corresponding enantiomer.

Changing that one spatial arrangement changes neither the number of carbon atoms nor the total hydrogen and oxygen counts. A molecular-mass measurement therefore cannot, by itself, distinguish the two sugars. An identifying experiment must supply additional structural discrimination, for example through an appropriately interpreted nuclear magnetic resonance spectrum.

The useful distinction is between composition and configuration. The D label concerns a reference stereocenter; alpha/beta labels concern the anomeric center in cyclic forms. Neither label should be read as a direct statement about sweetness or the sign of optical rotation.^[ref]