91Ó°ÊÓ

Stereoisomers that are non-superimposable mirror images of each other are called enantiomers. Enantiomers have the same physical and chemical properties and are optically active.

A racemic mixture of amino acids such as (R)- and (S)-leucine, the racemate is first treated with acetic anhydride to form N-acetyl amino acids.

Formation of N-acetyl amino acid

Both (R) and (S)-leucine enantiomers react with the R- isomer of the chiral amine form diastereomers. These salts have the same configuration around one stereogenic center but the opposite configuration about the other stereogenic center.

Formation of diastereomers

The diastereomers are separated by physical techniques, such as crystallization or distillation.

Diastereomers

The amides can be hydrolyzed with an aqueous base to regenerate the amino acids. The amino acids are now separated from each other. The optical activity of the amino acids can be measured and compared to their known rotations to determine the purity of each enantiomer.

Formation of S- and R-leucine