Isoelectric Point
Definition and meaning of Isoelectric Point in chemistry.
Isoelectric point (pI) is the pH at which a molecule, ion, or protein carries no net electric charge because the total positive charge equals the total negative charge.
In more detail
At the isoelectric point, the positive charges contributed by protonated groups (such as ammonium groups) and the negative charges contributed by deprotonated groups (such as carboxylate groups) sum to zero across the whole molecule, even though individual ionizable groups are not necessarily half-ionized. For each protein or amino acid, this characteristic pH depends on the pKa values of its ionizable functional groups. At this pH, proteins have minimum solubility and greatest tendency to precipitate out of solution, making this concept essential for protein purification and electrophoresis separation.
Key facts
| Field | Biochemistry |
|---|---|
| Symbol | pI |
| Key Property | Proteins have minimum solubility and maximum precipitation tendency at their pI |
| Determination | Calculated from pKa values or measured by isoelectric focusing electrophoresis |
Alanine has an isoelectric point at pH 6.0; at this pH, the ionized amino and carboxyl groups on the molecule have equal magnitudes of charge.
Frequently asked questions
Why does a protein precipitate at its isoelectric point?
At the pI, the protein carries no net charge and loses the electrostatic repulsion that normally keeps it dispersed in solution, causing it to aggregate and precipitate.
How do you calculate the isoelectric point of an amino acid?
For amino acids with no ionizable side chain, the pI equals the average of the two nearest pKa values: pI = (pKa1 + pKa2) / 2.