TSMS-PC-007
Peptide Charge, pKa, and Isoelectric Point Study Guide
School of Peptide Chemistry · Peptide Chemistry Foundations · Intermediate
Open full lessonLearning objectives
- Identify major ionizable groups in peptides.
- Explain how pH changes net charge.
- Interpret pKa conceptually.
- Define isoelectric point.
- Connect charge with solubility, chromatography, electrophoresis, and MS behavior.
Executive summary
Peptide charge is not fixed. It changes with pH as amino, carboxyl, and ionizable side-chain groups gain or lose protons.
The **pKa** of an ionizable group describes the pH at which its protonated and deprotonated forms are present in equal proportions. The **isoelectric point**, or pI, is the pH at which the peptide has approximately zero net charge.
Charge strongly affects solubility, molecular interactions, ion-exchange retention, capillary electrophoresis, aggregation, and ionization during mass spectrometry.
The **pKa** of an ionizable group describes the pH at which its protonated and deprotonated forms are present in equal proportions. The **isoelectric point**, or pI, is the pH at which the peptide has approximately zero net charge.
Charge strongly affects solubility, molecular interactions, ion-exchange retention, capillary electrophoresis, aggregation, and ionization during mass spectrometry.
Key takeaways
- Peptide charge depends on pH.
- pKa describes protonation behavior of individual groups.
- pI describes the pH of approximately zero net charge.
- Charge influences solubility, chromatography, electrophoresis, and MS.
- Local sequence can shift ionization behavior.
- Ionic strength modifies electrostatic interactions.
Self-review questions
- What happens to an acidic group as pH rises above its pKa?
- Why can terminal amidation change net charge?
- What is the pI?
- Why may solubility decrease near pI?
- How does charge affect ion-exchange chromatography?
Use the full lesson to verify your answers.