Optical Activity of Amino Acids

Except glycine, all amino acids are optical active as all have an asymmetric carbon atom at position-2, the α-carbon atom. So, all amino acids except glycine, exist in D- and L-form which are nonsuperimpossible mirror image isomers. If the carbonyl group is written at the top, the D-form refers to the mirror image with the -NH₂ group on the right, the L- form is the isomer with the -NH₂ group on the left as shown below in figure. The reference compound for this assignment is glyceraldehyde.

Most of the natural occuring amino acids have L- configuration. Product of laboratorary synthesis are generally optically inactive due to the formation of racemates containing equal amounts of D- and L- forms.