Q30. pKa value of the guanidinium group of Arginine is _____
(A) 4.30
(B) 7.40
(C) 9.20
(D) 12.50
The pKa value of the guanidinium group of Arginine is 12.50 (Option D).
This matches standard biochemistry references used in exams like CSIR NET Life Sciences.
Option Analysis
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(A) 4.30: This represents the side chain pKa of acidic amino acids like Aspartic acid or Glutamic acid (~4.0-4.3), not Arginine’s basic guanidinium group.
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(B) 7.40: Close to physiological pH (7.4) but incorrect; it approximates Histidine’s imidazole side chain pKa (~6.0), not Arginine.
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(C) 9.20: Matches Arginine’s α-amino group pKa (~9.0-9.2); the guanidinium side chain has a much higher value.
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(D) 12.50: Correct for Arginine’s guanidinium group, explaining its protonated (+1) state at physiological pH.
Arginine pKa Values
Arginine (R) has three ionizable groups with standard pKa values:
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α-COOH: ~2.17
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α-NH3+: ~9.04
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Side chain guanidinium: ~12.48-12.5
The high guanidinium pKa ensures it remains positively charged under most conditions, aiding protein interactions.
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Secondary Keywords: Arginine side chain pKa, CSIR NET amino acid pKa, guanidinium pKa 12.5, basic amino acids pKa
Introduction
The pKa value of the guanidinium group of Arginine is a key concept in biochemistry for CSIR NET Life Sciences aspirants. This value, approximately 12.50, determines Arginine’s role as the most basic amino acid, remaining protonated at physiological pH. Understanding it helps solve MCQs on protein charge, isoelectric points, and enzyme catalysis.
Why Guanidinium pKa is 12.50
The guanidinium group (-C(NH2)2+) in Arginine’s side chain features resonance delocalization across three nitrogens, stabilizing the protonated form. Standard tables list it as 12.48-12.5, distinct from research values (~13.8) used in protein modeling.
| Group | pKa Value | Charge at pH 7 |
|---|---|---|
| α-COOH | 2.17 | -1 (deprotonated) |
| α-NH3+ | 9.04 | +1 (protonated) |
| Guanidinium | 12.50 | +1 (protonated) |
This high pKa ensures net positive charge for Arginine (pI ~10.76).
Exam Relevance for CSIR NET
In competitive exams, 12.50 is the expected answer for Arginine’s guanidinium pKa, as seen in titration curves and charge calculations. Recent studies confirm ~13.8 experimentally, but textbooks standardize ~12.5 for MCQs.
Biological Significance
The elevated pKa value of the guanidinium group of Arginine enables hydrogen bonding in DNA-binding proteins and enzyme active sites, even in hydrophobic environments. It outperforms Lysine (pKa ~10.5) in basicity.
