A molecule with two personalities.
A 2-amino acid contains an amino group and a carboxyl group on the framework NH₂–CH(R)–COOH. Proton transfer can produce a zwitterion with NH₃⁺ and COO⁻: it has both charges but may have zero net charge.
Below its isoelectric point (pI), an amino acid has a net positive charge. Above pI it has a net negative charge. At pI its average net charge is zero, so it shows no net electrophoretic migration in the ideal model.
In an electric field, positive ions move towards the negative electrode (cathode); negative ions move towards the positive electrode (anode). The buffer pH therefore controls separation.
Read the chromatogram
Illustrative TLC plate with a solvent-front distance of 60 mm. Measure from the same baseline. Rf has no units.
Explore: Follow the amino acid’s charge as pH changes and predict its movement during separation.
Explain why an amino acid at its pI may contain charged groups but not migrate overall.
- The zwitterion contains NH₃⁺ and COO⁻ groups.
- Balanced charge gives no net migration in the ideal model.
Answer: Positive and negative charges balance to give zero average net charge, so there is no net electric-field force.
Assumed knowledge
Opposite charges attract; use the specified pI for each amino acid.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Recognise amino and carboxyl groups in 2-amino acids.
- Relate pH and isoelectric point to net charge.
- Explain electrophoretic movement.