JEE Chemistry 2025 Question with Solution

Given: Four statements about naturally occurring amino acids are to be checked.

Find: The correct statement.

The solution evaluates all four statements one by one.

1. For statement A, glycine is achiral, but threonine and isoleucine have two chiral centres, so the claim that all amino acids except glycine contain one chiral centre is incorrect.

2. For statement B, glycine is not optically active because its $\alpha$-carbon has two hydrogen atoms, so this statement is incorrect.

3. For statement C, aspartic acid and glutamic acid both contain a side-chain –COOH group, so this statement is incorrect.

4. For statement D, cysteine contains a free –SH group that undergoes oxidation to form a disulfide-linked dimer, cystine:

Therefore, cysteine easily undergoes dimerization due to the presence of the free –SH group.

The correct option is D.

The solution's shows a discrepancy because the answer key says A, but the worked solution concludes that D is correct.

Given: Statements on chirality, optical activity, acidic side chains, and thiol reactivity of amino acids.

Find: Which statement is true.

Step 1: Check chirality. Most amino acids have a chiral $\alpha$-carbon, but threonine and isoleucine each have an additional chiral carbon in the side chain. Hence statement A is false.

Step 2: Check optical activity. Glycine is achiral because its $\alpha$-carbon is attached to two hydrogen atoms. Hence all naturally occurring amino acids are not optically active, so statement B is false.

Step 3: Check acidic side chains. Both aspartic acid and glutamic acid have a side-chain carboxyl group. Hence statement C is false.

Step 4: Check cysteine reactivity. Cysteine contains a thiol group –SH, and two cysteine molecules can be oxidized to form a disulfide bond –S–S–, giving cystine. Hence statement D is true.

Therefore, the correct option is D.