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Galactosamine is an amino sugar derived from galactose by replacing its carbon-2 hydroxy group with an amino group. The conventional D compound is 2-amino-2-deoxy-D-galactose. Specifying the D configuration does not by itself specify the alpha or beta anomer of a cyclic form.^[ref]

The neutral free sugar has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{O}}_5$. As an original atom-counting exercise, begin with galactose, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. Replacing $\mathrm{OH}$ by $\mathrm{N}{\mathrm{H}}_2$ removes one oxygen and adds one nitrogen and one hydrogen overall. This gives the stated formula; it is structural bookkeeping, not a claim that this substitution is a one-step laboratory synthesis.^[ref]

Galactosamine and glucosamine differ in the configuration at carbon4 of their corresponding D-sugar frameworks. Adding an acetyl group to the nitrogen instead produces N-acetylgalactosamine, a different structural modification. Neither relationship is determined by a formula alone: epimers can share a formula, while nitrogen substitution can change it.

In chondroitin, the chain contains N-acetylgalactosamine-derived residues rather than simply a collection of free galactosamine molecules. Distinguishing the free sugar, its nitrogen-modified derivative and its residue within a polysaccharide prevents confusing composition with molecular identity.

Galactose is an aldohexose, a six-carbon monosaccharide whose open-chain reference form has an aldehyde group. It is the carbon-4 epimer of glucose: the corresponding structures differ at that one stereocenter. Its molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$.^[ref]

A Fischer projection makes the difference precise. With the aldehyde at the top, the carbon-2 through carbon-5 hydroxyl group pattern of D-glucose is right–left–right–right. Changing carbon 4 alone gives right–left–left–right for D-galactose. Reversing every stereocenter would instead make an enantiomer; the single-position change is an epimerization.

Neither the formula nor an accurate mass measurement distinguishes these two sugars, because moving a hydroxyl group from one side of a stereocenter to the other changes no atom counts. Their different spatial arrangements nevertheless present different binding surfaces to an enzyme. This is a useful example of why carbohydrate identification needs structural information in addition to elemental composition. The D designation fixes the reference stereochemistry; it does not itself select a single cyclic anomer or state the sign of optical rotation.

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]