Q.54 For a 0.1 M aqueous solution of lysine, the pH at which it carries no net charge is
___________.
(pKa values for: α-carboxylic group = 3.1, α-amino group = 8.0, ε–amino group = 10.8)
At this point, the predominant lysine species has equal positive and negative charges. The pI is obtained by averaging the two highest pKa values: the α-amino (8.0) and ε-amino (10.8) groups. The α-carboxylic group (pKa 3.1) remains fully deprotonated (–1 charge) in this range.
Lysine Structure and Ionizable Groups
Lysine features a standard α-amino acid backbone with a side chain –(CH₂)₄–NH₃⁺. The ionizable groups are:
- α-COOH (pKa₁ = 3.1): deprotonates to –COO⁻ above pH 3.1
- α-NH₃⁺ (pKa₂ = 8.0): deprotonates to –NH₂ above pH 8.0
- ε-NH₃⁺ (pKa₃ = 10.8): deprotonates to –NH₂ above pH 10.8
Protonation States of Lysine
- Low pH (<3.1): +2 net charge (α-NH₃⁺, ε-NH₃⁺, α-COOH all protonated)
- pH 3.1–8.0: +1 net charge (α-COO⁻, both NH₃⁺ protonated)
- pH 8.0–10.8: 0 net charge (α-COO⁻ and ε-NH₃⁺ compensate)
- High pH (>10.8): –1 net charge (all groups deprotonated)
No net charge predominates between pKa₂ and pKa₃; the pI is their average.
pI Calculation Formula
For basic amino acids like lysine (side chain pKa > 7):
pI = (pKa₂ + pKa₃) / 2
Substitute values:
pI = (8.0 + 10.8) / 2 = 9.4
The 0.1 M concentration does not affect pI since it depends only on pKa values.
Common Calculation Errors
- Averaging all pKa values →
(3.1 + 8.0 + 10.8) / 3 = 7.3(incorrect: ignores basic nature) - Using α-COOH and α-NH₃⁺ →
(3.1 + 8.0)/2 = 5.55(used for neutral amino acids) - Correct for lysine →
(8.0 + 10.8)/2 = 9.4
Step-by-Step Lysine pI Calculation
- Identify groups: Acidic (pKa 3.1, –1 when deprotonated); two basic (pKa 8.0, 10.8, +1 when protonated).
- At zero net charge: α-COO⁻ (–1), ε-NH₃⁺ (+1), α-NH₂ (neutral).
- Average flanking pKa values:
pI = (8.0 + 10.8)/2 = 9.4.
This aligns with textbook and exam values; alternative pKa conditions may shift it slightly (e.g., 8.95/10.5 → pI ≈ 9.7).
Why Lysine pI Matters
The lysine pI calculation helps predict protein solubility and electrophoretic mobility. At pI = 9.4, lysine exists as a zwitterion and shows minimal migration in electric fields. Importantly, the pI remains independent of concentration (e.g., 0.1 M).
Exam Tips for CSIR NET Life Sciences
- Basic amino acids (Lys, Arg, His): Average two highest pKa values.
- Acidic amino acids (Asp, Glu): Average two lowest pKa values.
- Neutral amino acids: Average α-pKa pair (carboxyl & amino).
Practice with titration curves to visualize the plateau regions corresponding to each ionization step.
