Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
Which of the following amino acids contain(s) two chiral carbons?
Options:
(A) L-Leucine
(B) L-Threonine
(C) L-Isoleucine
(D) L-Asparagine
Official Answer Key:
B;C
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.56)**
**1. Governing Concept**:
- **Topic**: Biochemistry & Stereochemistry of Standard Amino Acids
- Among the 20 standard proteinogenic amino acids:
- 18 amino acids possess **exactly one chiral center** (the $\alpha$-carbon $C_\alpha$).
- **Glycine** has no chiral center (**0 chiral carbons**).
- Exactly **two amino acids** possess **two chiral centers**:
1. **Isoleucine**: Has a chiral $C_\alpha$ and a chiral $\beta$-carbon ($C_\beta$) bearing four distinct groups ($-\text{H}, -\text{CH}_3, -\text{CH}_2\text{CH}_3$, and $-C_\alpha$).
2. **Threonine**: Has a chiral $C_\alpha$ and a chiral $\beta$-carbon ($C_\beta$) bearing four distinct groups ($-\text{H}, -\text{OH}, -\text{CH}_3$, and $-C_\alpha$).
**2. Option Analysis**:
- **Option (A) [Leucine] is INCORRECT**: Contains only 1 chiral carbon ($C_\alpha$); its $C_\gamma$ is bonded to two identical methyl groups ($-\text{CH}_3$), so it is achiral.
- **Option (B) [Isoleucine] is CORRECT**: Possesses 2 chiral carbons ($C_\alpha$ and $C_\beta$).
- **Option (C) [Threonine] is CORRECT**: Possesses 2 chiral carbons ($C_\alpha$ and $C_\beta$).
- **Option (D) [Valine] is INCORRECT**: Contains only 1 chiral carbon ($C_\alpha$); its $C_\beta$ is bonded to two identical methyl groups, so it is achiral.
**3. Final Answer**:
- **Correct Options**: **B, C**
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