The three-dimensional arrangement of the groups attached to the chiral carbon determines the absolute configuration (D- and L-) of the carbohydrate.

Stereoisomers are compounds that have the same chemical formula but their molecules differ from one another in terms of the spatial arrangement of their component atoms. Molecules that are mirror images of each other are enantiomers; these are molecules that contain chiral carbons and no internal planes of symmetry. All carbohydrates contain at least one asymmetrical (chiral) carbon. The chiral carbon furthest from the carbonyl group determines the absolute configuration L or D of the sugar. These two configurations are the enantiomers. Take a look at the figure below to see an example of the two enantiomers of the carbohydrate glyceraldehyde.

As seen above, there is only one chiral carbon present, and so these are the only two stereoisomers that exist for glyceraldehyde.

The number of possible stereoisomers of a compound can be calculated by:

For example, for a molecule with 1 chiral carbon, the number of stereoisomers is 2¹ = 2 and for a molecule with 2 chiral carbons, the number of stereoisomers is 2² = 4.

The Fischer projection is a simple two-dimensional drawing of a molecule’s stereoisomers. If the OH group on the chiral carbon furthest from the carbonyl is pointing left, then it’s described as the L form. If it’s pointing right, then it’s the D form.

Keep in mind that enantiomers are the same sugars in different optical families (such as D-glucose and L-glucose). Diastereomers are two sugars that are in the same family, but are not identical and are not mirror images of each other.  Epimers are a special subtype of diastereomers that differ in configuration at exactly one chiral center. D-ribose and D-arabinose are epimers (at carbon number 2), as shown below.

Key Points

• The chiral carbon furthest from the carbonyl group determines the absolute configuration L or D of the sugar.

• In the Fischer projection, all D-sugars have the OH group of their highest-numbered chiral center on the right, and all L-sugars have that OH on the left.

• Enantiomers are the same sugars in different optical families (such as D-glucose and L-glucose).

• Diastereomers are two sugars that are in the same family, but are not identical and are not mirror images of each other.

• Epimers are a special subtype of diastereomers that differ in configuration at exactly one chiral center.

Key Terms

Chiral carbon (asymmetric carbon):A carbon that is attached to four different types of atoms or groups of atoms.

Absolute configuration: The three-dimensional arrangement of the groups attached to the chiral carbon in a carbohydrate.

Stereoisomers (aka optical isomers): Any of two or more compounds with identical molecular formulas and arrangements of atoms that only differ in the spatial arrangement of their atoms; they have non-superimposable mirror images.

Enantiomers: the same sugars in different optical families (D-glucose and L-glucose).

Fischer projection: A two-dimensional representation of a molecule that maintains information about its absolute configuration.

Diastereomers: Two sugars that are in the same family (that are stereoisomers), but are not identical and are not mirror images of each other.

Epimers: A type of stereoisomer that differs in configuration at a single stereogenic center (the anomeric carbon).