Resolution of Amino acids

Enantiomers cannot be separated by using ordinary methods like filtration, distillation, crystallization, normal chromatography etc. as enantiomers have the same physical properties. On the other hand, diastereomers have different physical properties therefore they can be separated by general separating methods. Naturally, amino acids exist in S-configuration. Whereas the synthesis of amino acids results in the formation of both enantiomers. The separation of these two enantiomers is termed as resolution of racemic mixture. The oldest and still applied method for resolving a racemic mixture is to convert the enantiomers into its corresponding diastereomers. Diastereomers can be separated using ordinary separating methods. This method can be performed in three steps.

• a) Formation of Diastereomers:

React the racemic mixture with an enantiopure resolving agent to form two diastereomers having different physical properties.

• b) Separation of Diastereomers:

Since diastereomers have different physical properties therefore they can be separated from each other using ordinary separating methods like crystallization and fractional distillation.

• c) Separation of pure Enantiomer:

Finally, the enantiomer is separated from the resolving agent by chemical means. Once separated the two different compounds are then separated by physical means.

Following scheme shows the separation of a racemic mixture of valine. First the racemic mixture of valine is reacted with acetic anhydride to form a racemic mixture of N-acetyl amino acids. This step is necessary to protect the -NH₂ group of amino acids.

The enantiomers are still not separable therefore they are further treated with enantiopure chiral reagents like (R)-α-methylbenzylamine. This will result in the formation of diastereomeric salts.

Next the diastereomers formed are separated from each other.

In the last step the enantiomers of valine are regenerated by hydrolysis reactions.

Another strategy employed for the separation of amino acids is to react the racemic mixture of amino acids with enzymes. Enzymes function as chiral reagents. For example, when the racemic mixture of N-acetyl alanine is treated with acylase enzyme the amide bonds are hydrolyzed. The acylase will only hydrolyze the amide bond of S-enantiomer of alanine.

The two products formed can be separated easily. Since the enzyme is selectively reacting with a single enantiomer therefore this type of separation technique is termed as kinetic resolution.