Step by Step
1
Below the pI
At a pH below its isoelectric point, an amino acid is positively charged (protonated).
2
Above the pI
At a pH above its isoelectric point, an amino acid is negatively charged (deprotonated).
3
At the pI itself
At exactly its isoelectric point, an amino acid exists as a zwitterion — carrying both positive and negative charges simultaneously, but with no net overall charge.
4
Practical application: electrophoresis
At its pI, an amino acid (or protein) has no net charge and therefore does not migrate in an electric field during gel electrophoresis — this property is specifically exploited to separate different proteins based on their distinct isoelectric points.
Applied Walkthrough
1
At a pH below a given amino acid's isoelectric point (pI), that amino acid carries an overall positive charge, since it's protonated relative to its neutral state.
2
At a pH above its pI, the same amino acid instead carries an overall negative charge, having become deprotonated.
3
Exactly at its pI, the amino acid exists as a zwitterion — simultaneously carrying both a positive and a negative charge internally, but with these charges exactly balancing to produce zero net charge overall.
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This zero-net-charge property is directly exploited in gel electrophoresis: since a molecule with no net charge doesn't migrate in an electric field, adjusting the pH of a solution to match a specific protein's pI allows that protein to be isolated and separated from others with different isoelectric points.
Exam Application
Exams test whether you understand what happens to an amino acid's charge above, below, and exactly at its isoelectric point, and whether you know how this property is exploited in gel electrophoresis for protein separation.
⚠ Common Trap
The most common trap is assuming a zwitterion at the pI has NO charges at all — it actually carries both a positive and a negative charge simultaneously, which happen to exactly cancel out, producing zero NET charge, rather than the complete absence of any charged groups.
✓ Quick Self-Check
1. What is the isoelectric point (pI)?
The pH at which an amino acid has no net charge.
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2. What charge does an amino acid carry below its pI?
Positive (protonated).
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3. What charge does an amino acid carry above its pI?
Negative (deprotonated).
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4. What is a zwitterion?
A molecule carrying both positive and negative charges simultaneously, with zero net overall charge — the state of an amino acid exactly at its pI.
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5. How is the pI used in gel electrophoresis?
A molecule at its pI has no net charge and doesn't migrate in an electric field, allowing separation of proteins based on differing isoelectric points.
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