Q.2 Match List-I with List-II
| List-I | List-II |
|---|---|
| (Amino Acid) | (pK value) |
| (A) α-Carboxyl groups | (I) 1.8 – 2.9 |
| (B) α-Amino groups | (II) 8.8 – 10.8 |
| (C) Acidic Amino Acid – Asp | (III) 12.5 |
| (D) Basic Amino Acid – Arg | (IV) 3.9 |
Choose the correct answer from the options given below:
- (A) – (I), (B) – (II), (C) – (III), (D) – (IV)
- (A) – (I), (B) – (II), (C) – (IV), (D) – (III)
- (A) – (I), (B) – (III), (C) – (IV), (D) – (II)
- (A) – (III), (B) – (IV), (C) – (I), (D) – (II)
α-Carboxyl groups of amino acids typically have pKa values of 1.8–2.9, while α-amino groups range from 8.8–10.8. The correct matching identifies Aspartic acid (Asp) side chain at pKa ~3.9 and Arginine (Arg) side chain at ~12.5, making option (B) correct.
Question Breakdown
List-I items represent ionizable groups in amino acids, matched to their characteristic pKa (pK) values in List-II. Amino acids ionize based on pH relative to pKa: below pKa, protonated; above, deprotonated.
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(A) α-Carboxyl groups: pKa 1.8–2.9 (I). This matches the -COOH group ionizing at low pH.
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(B) α-Amino groups: pKa 8.8–10.8 (II). The -NH3+ group deprotonates at higher pH.
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(C) Acidic Amino Acid – Asp: pKa 3.9 (IV). Asp’s side chain -COOH is more acidic than α-carboxyl.
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(D) Basic Amino Acid – Arg: pKa 12.5 (III). Arg’s guanidino group stays protonated until very high pH.
Option Analysis
Option A→ B→ C→ D→ Correct? Reason First (I) (II) (III) (IV) No Swaps C (Asp pKa 3.9) and D (Arg pKa 12.5) . Second (I) (II) (IV) (III) Yes Matches all standard pKa ranges . Third (I) (III) (IV) (II) No Mismatches B (amino ~9–10, not 12.5) . Fourth (III) (IV) (I) (II) No Wrong for A (carboxyl not 12.5) . Amino acid pK values matching is crucial for GATE Life Sciences aspirants tackling biochemistry questions on ionization and isoelectric points. This guide solves a classic matching question on α-carboxyl groups, α-amino groups, acidic Asp, and basic Arg pKa ranges.
Understanding pKa in Amino Acids
Amino acids have ionizable groups with specific pK values dictating charge at physiological pH ~7. α-Carboxyl (pK 1.8–2.9) loses H+ first, becoming -COO-. α-Amino (pK 8.8–10.8) follows as -NH2. Side chains like Asp (3.9) add extra acidity; Arg (12.5) extreme basicity.
Solved Matching Question
Match List-I (amino acid groups) to List-II (pK values):
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(A) α-Carboxyl → (I) 1.8–2.9
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(B) α-Amino → (II) 8.8–10.8
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(C) Asp → (IV) 3.9
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(D) Arg → (III) 12.5
Correct Option: (A)-(I), (B)-(II), (C)-(IV), (D)-(III).
Why These Matches?
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α-Carboxyl pKa stays low due to electron withdrawal.
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Asp side chain pKa ~3.9 confirms acidity.
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Arg’s high pKa ensures + charge at pH 7.
Practice this for GATE: pH < pKa = protonated form dominates.
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