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GATE 2025 BT Q.56: solution and answer

GATE 2025 · BTQ.56 · 2 marks · MSQ

Which of the following amino acids contain(s) two chiral carbons?

  1. (A)

    L-Leucine

  2. (B)

    L-Threonine

  3. (C)

    L-Isoleucine

  4. (D)

    L-Asparagine

See the question in the paper
GATE 2025 BT question 56Tap to open full size

Answer and solution

ANSWER

Options (B) L-Threonine and (C) L-Isoleucine each contain two chiral carbons.

Official answer: B, CSource: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. A chiral carbon is one bonded to four different substituents.

  2. Every amino acid's α-carbon (bonded to -NH2, -COOH, -H and the side chain R) is chiral whenever R ≠ H, which is true for all four options, so each amino acid has at least one chiral carbon (Cα).

  3. L-Leucine: side chain is -CH2-CH(CH3)2. The branched carbon bears two identical methyl groups, so it is not a stereocentre. Total chiral carbons = 1 (Cα only). Option A is incorrect.

  4. L-Threonine: side chain is -CH(OH)-CH3. This β-carbon is bonded to -OH, -CH3, -H and the rest of the molecule (-CH(NH2)COOH) — four different groups, so it is a second stereocentre. Total = 2 (Cα and Cβ). Option B is correct.

  5. L-Isoleucine: side chain is -CH(CH3)-CH2-CH3. This β-carbon is bonded to -CH3, -CH2CH3, -H and the rest of the molecule — four different groups, so it is also a stereocentre. Total = 2 (Cα and Cβ). Option C is correct.

  6. L-Asparagine: side chain is -CH2-CONH2. The -CH2- carbon has two identical hydrogens, and the amide carbon is sp2 (trigonal planar), so neither can be chiral. Total = 1 (Cα only). Option D is incorrect.

  7. Hence only Threonine and Isoleucine have two chiral carbons each — consistent with each being known to have a diastereomeric 'allo' form (allo-threonine, allo-isoleucine) arising from the second stereocentre.

KEY IDEA

A carbon is chiral only if all four groups attached to it are different. In every amino acid the α-carbon (–NH2, –COOH, –H, side chain R) is chiral whenever R ≠ H; a second chiral centre exists only if some side-chain carbon also carries four distinct substituents.

WATCH OUT FOR

Forgetting to check the side-chain β-carbon for chirality and stopping after confirming only the α-carbon — this misses both Threonine and Isoleucine, or wrongly credits Leucine/Asparagine based on chain length alone rather than checking whether all four substituents on a given carbon are actually distinct.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 9 Assays & electrophoresis questions, with notes

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