Question

Choose the optically inactive amino acid from the following:

• A. Glycine
• B. Alanine
• C. Leucine
• D. Isoleucine

Hint:

Glycine is the only common \(\alpha\)-amino acid that is optically inactive because its \(\alpha\)-carbon contains two identical hydrogen atoms.

Answer 1

The Correct Option is A

Concept: Optical activity arises due to the presence of a chiral carbon atom attached to four different groups. Most naturally occurring amino acids contain one asymmetric carbon atom and are therefore optically active.

Step 1: Recall the general structure of amino acids.
\[ R-CH(NH_2)-COOH \] If the carbon atom is attached to four different groups, the amino acid becomes optically active.

Step 2: Examine glycine.
For glycine, \[ R=H \] Hence its structure is \[ H_2N-CH_2-COOH \] The central carbon is attached to: \[ H,\ H,\ NH_2,\ COOH \] Since two substituents are identical hydrogen atoms, the carbon atom is not chiral. Therefore glycine is optically inactive.

Step 3: Examine the remaining amino acids.
Alanine, leucine and isoleucine possess asymmetric carbon atoms and therefore are optically active.

Step 4: Final answer.
\[ \boxed{\text{Glycine}} \]