Question

Which of the following amino acids possess two chiral centres?

• A. Leucine
• B. Valine
• C. Serine
• D. Threonine

Hint:

Amino acids with two chiral centres are relatively rare. Threonine and isoleucine are examples of amino acids with two chiral centres, which result in more stereoisomeric forms.

Answer 1

The Correct Option is D

Amino acids are classified based on the number of chiral centres they possess. A chiral centre is a carbon atom bonded to four different groups, which gives rise to stereoisomerism.

• Leucine: Leucine has only one chiral centre, which is the central carbon attached to the amino group, the carboxyl group, the hydrogen atom, and the isopropyl side chain.
• Valine: Valine also has only one chiral centre, which is the central carbon attached to the amino group, the carboxyl group, the hydrogen atom, and a branched alkyl group (isopropyl).
• Serine: Serine has only one chiral centre, which is the central carbon attached to the amino group, the carboxyl group, the hydrogen atom, and the hydroxymethyl side chain.
• Threonine: Threonine, however, has two chiral centres. One is the central carbon attached to the amino group, the carboxyl group, the hydrogen atom, and the side chain (hydroxymethyl). The second chiral centre is the carbon atom attached to the hydroxyl group in the side chain.

Therefore, the correct option is (D) Threonine.