Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Natural proteins are composed primarily of 20 a-amino acids. Which one of the following
statements is true for any of these amino acids in a solution of pH 1.5?
Options:
(A) Only the amino group is ionized
(B) Only the carboxylic acid group is ionized
(C) Both amino and carboxylic acid groups are ionized.
(D) Both amino and carboxylic acid groups are neutral.
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.02)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biochemistry (Amino Acid Acid-Base Chemistry & Ionization States)
**2. Detailed Step-by-Step Derivation**:
- In standard $\alpha$-amino acids, there are two primary ionizable backbone functional groups:
1. The $\alpha$-carboxylic acid group ($-\text{COOH} \rightleftharpoons -\text{COO}^- + \text{H}^+$), with typical $pK_{a1} \approx 1.8 - 2.5$.
2. The $\alpha$-amino group ($-\text{NH}_3^+ \rightleftharpoons -\text{NH}_2 + \text{H}^+$), with typical $pK_{a2} \approx 9.0 - 10.5$.
- Using the Henderson-Hasselbalch equation:
$\text{pH} = pK_a + \log \frac{[\text{Conjugate Base}]}{[\text{Acid}]}$
- At a strongly acidic $\text{pH} = 1.5$:
- Since $\text{pH} < pK_{a1}$ (e.g., $1.5 < 2.0$), the hydronium ion concentration is high, driving the equilibrium toward the fully protonated, neutral carboxylic acid form ($-\text{COOH}$). Thus, the carboxyl group remains unionized (neutral).
- Since $\text{pH} \ll pK_{a2}$ (e.g., $1.5 \ll 9.5$), the amino group is almost $100\%$ protonated into the positively charged ammonium ion form ($-\text{NH}_3^+$). Thus, the amino group is fully ionized.
- Therefore, in a solution at $\text{pH} = 1.5$, **only the amino group is ionized** ($-\text{NH}_3^+$).
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: At $\text{pH} = 1.5$, the carboxylic acid group ($pK_a \approx 2$) is protonated ($-\text{COOH}$, unionized), while the basic amino group ($pK_a \approx 9.5$) is protonated into a charged cation ($-\text{NH}_3^+$, ionized).
- **Option (B) [INCORRECT]**: The carboxylate form ($-\text{COO}^-$) only predominates when $\text{pH} > pK_{a1}$.
- **Option (C) [INCORRECT]**: Both groups are ionized only in the zwitterionic state at intermediate pH ($pK_{a1} < \text{pH} < pK_{a2}$).
- **Option (D) [INCORRECT]**: The amino group is a strong base and is fully protonated and positively charged at pH 1.5.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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