Stereochemistry

Optical Activity of Amino Acids

Step 1: Understanding optical activity of amino acids. All naturally occurring -amino acids, except for glycine, are optically active because they contain a chiral carbon atom (except glycine, which has two hydrogen atoms on the chiral center, making it achiral).

Step 2: L-configuration of amino acids. Most naturally occurring amino acids are in the L-configuration, which is why they are optically active (except glycine).

Step 3: Conclusion. Thus, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A), making option (A) the correct answer.

(i) Enantiomers:

Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. They have identical physical and chemical properties except for their interaction with plane-polarized light (optical activity) and reactions with other chiral molecules. Enantiomers rotate plane-polarized light in equal magnitudes but opposite directions (one being dextrorotatory [+] and the other levorotatory [−]).

Example: The two forms of lactic acid (D-lactic acid and L-lactic acid) are enantiomers.

(ii) Racemic Mixture:

A racemic mixture (or racemate) is a 1:1 mixture of two enantiomers of a chiral molecule. Because the optical activities of the enantiomers cancel each other out, a racemic mixture is optically inactive (does not rotate plane-polarized light). Racemic mixtures are often formed in chemical reactions where a chiral product is generated from achiral reactants without the use of a chiral catalyst or enzyme.

Example: Racemic tartaric acid is an equal mixture of D-tartaric acid and L-tartaric acid.