Enantiomers of the amino acid alanine.
Enantiomers are molecules that are optical isomers, or mirror images, of one another. They can be distinguished by the direction in which they rotate the plane of polarization of polarized light and are referred to, therefore, as being dextrorotatory (d-) or levorotatory (l-).
Enantiomers can exist when there is an asymmetric carbon atom within the molecule, i.e., a carbon that is attached to four different structures. The example illustrated is the amino acid alanine, which occurs as l-alanine (left in the picture) and d-alanine (right). Chemical synthesis in the laboratory usually produces equal amounts of the two enantiomers, known as a racemic mixture. However, in living organisms there is a rigid preference for one over the other.