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 show(s) optical activity at 100 mM concentration in
water?
Options:
(A) Solution of NaCl
(B) Solution of D-Glucose
(C) Solution of Glycine
(D) Solution of L-Proline
Official Answer Key:
B;D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.24)**
**1. Governing Concept**:
- **Topic**: Stereochemistry & Optical Activity of Biomolecules
- A chemical species is **optically active** (rotates the plane of polarized light) if and only if it contains a chiral center (asymmetric carbon) and is not a meso compound or an equimolar racemic mixture.
- **Amino Acids**:
- Glycine ($\text{H}_2\text{N}-\text{CH}_2-\text{COOH}$) has two identical hydrogen atoms on the $\alpha$-carbon; it is **achiral** and optically inactive.
- $L$-Alanine has four distinct groups on the $\alpha$-carbon ($-\text{H}, -\text{NH}_2, -\text{COOH}, -\text{CH}_3$); it is **chiral** and optically active.
- **Carbohydrates**:
- $D$-Glucose has four asymmetric stereocenters; it is **chiral** and displays mutarotation/optical activity.
- Dihydroxyacetone ($\text{HOCH}_2-\text{CO}-\text{CH}_2\text{OH}$) possesses no stereocenter and is **achiral**.
**2. Option Analysis**:
- **Option (A) [Glycine] is INCORRECT**: Only non-chiral standard amino acid; lacks optical activity.
- **Option (B) [L-Alanine] is CORRECT**: Possesses a chiral $\alpha$-carbon and rotates plane-polarized light.
- **Option (C) [Dihydroxyacetone] is INCORRECT**: Symmetrical achiral ketotriose.
- **Option (D) [D-Glucose] is CORRECT**: Possesses multiple asymmetric chiral carbons with strong optical rotation ($[\alpha]_D \approx +52.7^\circ$).
**3. Final Answer**:
- **Correct Options**: **B, D**
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