With the exception of glycine, all α-amino acids are asymmetric molecules. They can be obtained in two forms called enantiomers. The D-enantiomer (D: dexter) represents the mirror image of the L-enantiomer (L: laevus); image and mirror image cannot be super-imposed, which is possible when mirroring symmetric structures.
D- and L-enantiomer do not differ in their physical properties, but they rotate the plane of linearly polarized light. Most L-amino acids rotate the light counterclockwise. The rotation angle for a given concentration, solvent, temperature, wavelength, and light path (length of the measuring cell) is specific for an amino acid derivative.
e.g. determined for batches of both enantiomers of Fmoc-phenylalanine measured at 589 nm (NaD line), 24°C, 0.1% in DMF
Fmoc-Phe-OH D24 -39.1°
Fmoc-D-Phe-OH D24 + 39.0°
Enantiomers differ considerably in their biological activity.