9. The three pK’s of the amino acid lysine are approximately 2.1, 9.2 and 10.8. Which
of the following is true in an electrophoresis experiment:
a. Lysine does not migrate at pH 1.2
b. Lysine does not migrate at pH 3.1
c. Lysine does not migrate at pH 9.5
d. Lysine does not migrate at pH 13.6
Lysine does not migrate in electrophoresis at its isoelectric point (pI), where the net charge is zero. The pI for lysine, with pKa values of 2.1 (α-COOH), 9.2 (α-NH3+), and 10.8 (ε-NH3+), is calculated as the average of the two higher pKa values: (9.2 + 10.8)/2 = 10.0. None of the given pH options exactly match this pI, but option c (pH 9.5) is closest among them, making it the intended correct choice for CSIR NET-style questions.
Option Analysis
pH 1.2 (Option a): At pH 1.2 (< pKa1 2.1), all groups are protonated: α-COOH neutral, both NH3+ charged (+2 net). Lysine migrates to cathode.
pH 3.1 (Option b): At pH 3.1 (> pKa1 but < pKa2), α-COO- (-1), both NH3+ (+2 net +1). Lysine migrates to cathode.
pH 9.5 (Option c): At pH 9.5 (≈ pI 10.0, between pKa2 and pKa3), α-COO- (-1), α-NH2 neutral, ε-NH3+ (+1 net ≈0). Lysine does not migrate.
pH 13.6 (Option d): At pH 13.6 (> pKa3), all deprotonated: α-COO- (-1), both NH2 neutral (net -1). Lysine migrates to anode.
Amino acids like lysine exhibit zero net charge at their isoelectric point (pI), halting migration in electrophoresis experiments crucial for CSIR NET Life Sciences. Lysine’s three pKa values—2.1 (carboxyl), 9.2 (α-amino), 10.8 (side chain)—yield pI = (9.2 + 10.8)/2 = 10.0, the pH where positive and negative charges balance.
Electrophoresis Migration Rules
-
pH < pI: Net positive charge → cathode migration.
-
pH = pI: Net zero charge → no migration.
-
pH > pI: Net negative charge → anode migration.
This principle separates proteins in gels, with lysine’s basic side chain shifting pI higher than neutral amino acids.
CSIR NET Question Breakdown
| Option | pH | Net Charge | Migration | Correct? |
|---|---|---|---|---|
| a | 1.2 | +2 | Cathode | No |
| b | 3.1 | +1 | Cathode | No |
| c | 9.5 | ~0 | None | Yes |
| d | 13.6 | -1 | Anode | No |
For CSIR NET prep, practice pI calculations: average pKa flanking zwitterion form for basic amino acids like lysine.
