The D- and L- families of amino acids.

The amino acids of which proteins are composed are related to L-alanine but have various side chains (R groups) in place of the methyl group of alanine. In the preceding section the structure of L-alanine was given in four different ways. To recognize them all as the same structure, we can turn them in space to an orientation in which the carboxyl group is at the top, the side chain (–CH3) is down, and both project behind the paper. The amino group and hydrogen atom will then project upward from the paper at the sides as shown below. According to a convention introduced at the beginning of this century by Emil Fischer, an amino acid is L if, when oriented in this manner, the amino group lies to the left and D if it lies to the right.



Fischer further proposed that the amino acid in this orientation could be projected onto the paper and drawn with ordinary lines for all the bonds. This gives the previously shown Fischer projection formula of L-alanine.
Although the D and L system of designating configuration is old it is still widely used. Remember that D and L refer to the absolute configurations about a selected reference atom in the molecule; for an amino acid this is the number 2 or α-carbon. A quantity that is related to the asymmetry of molecules is the experimentally measurable optical rotation. The sign of the optical rotation (+ or –) is sometimes given together with the name of a compound, e.g., D(+)-glucose. The older designations d (dextro) and l (levo) indicated + and–, respectively. However, compounds with the D configuration may have either + or – optical rotation.
In older literature optical isomerism of the type represented by D and L pairs was usually discussed in terms of “asymmetric carbon atoms” or “asymmetric centers.” Now the terms chiral (pronounced ki-ral) molecules, chiral centers, and chirality (Greek: “handedness”) are preferred.