Configuration of Carbohydrates

A carbohydrate molecule is almost always optically active because it possesses one or more chiral (asymmetric) carbon atoms -- carbons attached to four different groups. The total number of optical isomers a given carbohydrate can have follows the rule 2ⁿ, where n is the number of chiral carbons present in the molecule; a sugar with two chiral centres can therefore exist as four stereoisomers, one with four chiral centres as sixteen, and so on.

To describe these isomers unambiguously, Emil Fischer devised the Fischer projection formula, which draws an open-chain carbohydrate as a vertical carbon skeleton with horizontal bonds understood to point towards the viewer and vertical bonds away from the viewer. Fischer related every carbohydrate's configuration to one of the two enantiomeric forms of the simplest possible sugar, glyceraldehyde -- the D-form and the L-form -- and on this basis every carbohydrate is assigned the prefix D or L. The assignment rule is to compare the configuration of the highest-numbered chiral carbon (the one nearest the terminal -CH₂OH group) with the reference carbon of glyceraldehyde: for example, D-glucose earns its 'D' label because the arrangement of H and OH on its C5 carbon exactly matches the arrangement of H and OH on the C2 carbon of D-glyceraldehyde. …