Q103. Which of the following amino acids contain more than one chiral center?
(A) Leucine
(B) Isoleucine
(C) Serine
(D) Threonine
Option Analysis
Leucine (A)
Leucine has an isobutyl side chain (-CH₂-CH(CH₃)₂). Only the α-carbon is chiral, bonded to H, NH₂, COOH, and the side chain. The β-carbon has two identical methyl groups, so it is not chiral.
Isoleucine (B)
Isoleucine features a sec-butyl side chain (-CH(CH₃)CH₂CH₃). The α-carbon and the β-carbon in the side chain are both chiral, as each bonds to four distinct groups, yielding four stereoisomers.
Serine (C)
Serine’s side chain is -CH₂OH. The α-carbon is chiral, but the β-carbon bonds to two hydrogens, making it achiral.
Threonine (D)
Threonine’s side chain is -CH(OH)CH₃. The α-carbon and the β-carbon (with OH, H, CH₃, and the α-carbon) are both chiral.
The correct answer is (D) Threonine, as confirmed for CSIR NET-level questions.
Amino acids contain more than one chiral center in specific cases like threonine and isoleucine, crucial for understanding protein stereochemistry in molecular biology. These chiral centers influence enzyme function and genetic engineering applications.
Chiral Centers in Amino Acids
Standard amino acids have one chiral α-carbon, except glycine. Threonine and isoleucine uniquely possess a second chiral carbon in their side chains, leading to diastereomers.
Detailed Breakdown
| Amino Acid | Chiral Centers | Side Chain Structure |
|---|---|---|
| Leucine | 1 (α) | -CH₂CH(CH₃)₂ |
| Isoleucine | 2 (α, β) | -CH(CH₃)CH₂CH₃ |
| Serine | 1 (α) | -CH₂OH |
| Threonine | 2 (α, β) | -CH(OH)CH₃ |
This knowledge aids CSIR NET preparation in biochemistry and genetics.
