Based on the context given below, answer the following questions:
Glycine is most often used as a buffering agent in biochemical experiments. This diprotic amino acid has a pKa value of 2.4 owing to carboxyl group and 9.6 owing to amino group. Glycine can exist as a zwitterion which can exist in either protonated form (NH₃⁺) or as a free base (–NH₂) because of the reversible reaction:
NH₃⁺ – CH₂ – COOH ⇌ NH₃⁺ – CH₂ – COO⁻ ⇌ NH₂ – CH₂ – COO⁻
Q70.The characteristic pH, at which the net electric charge is zero, achieved at
-
2.4
-
6.0
-
7.5
-
9.6
The correct answer is 6.0.
Isoelectric Point Explanation
Glycine’s pI (pH where net charge is zero) is the average of its pKa values: pI = (pKa₁ + pKa₂)/2 = (2.4 + 9.6)/2 = 6.0. At pI, predominant zwitterion (NH₃⁺–CH₂–COO⁻) has +1 and -1 charges balancing to zero. Below pI, cationic form (+1 net); above, anionic (-1 net).
Option Analysis
-
2.4: pKa₁ (carboxyl deprotonation); here, fully protonated form (NH₃⁺–CH₂–COOH, +1 net) predominates.
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6.0: pI—zwitterion maximum, zero net charge (equal + and – species).
-
7.5: Neutral pH range but above pI; slight anionic shift (net -0.2 to -0.5).
-
9.6: pKa₂ (amino deprotonation); fully deprotonated (NH₂–CH₂–COO⁻, -1 net).
Introduction: Characteristic pH Net Electric Charge Zero for Glycine
NEET biochemistry tests amino acid buffers like glycine’s “characteristic pH at which the net electric charge is zero” (pI). Calculated as 6.0 from pKa 2.4/9.6 average, it maximizes zwitterion form. This solves the MCQ, Henderson-Hasselbalch details, and exam tips.
Glycine Ionization and pI
Diprotic:
-
pKa₁=2.4: COOH ⇌ COO⁻ + H⁺ (zwitterion forms).
-
pKa₂=9.6: NH₃⁺ ⇌ NH₂ + H⁺.
pI formula (neutral AA): (pKa₁ + pKa₂)/2 = 6.0. At pH=pI, [cation] = [anion], net zero.
Why 6.0? Option Elimination
NEET Buffer Context
Glycine buffers pH 2-10; best near pKa (±1). Zwitterion solubility peaks at pI, ideal for experiments.
NEET Tip: Mnemonic—”pI Midway: Average pKas.” Practice: Asp (acidic) pI=(pKa₁+pKa₂)/2 <7; Lys (basic) =(pKa₂+pKa₃)/2 >7.
