isoelectric point

The isoelectric point (pI) is the specific pH at which a molecule, such as an amino acid or protein, has a net zero electrical charge. This means it carries an equal number of positive and negative charges. At this pH, the molecule exists predominantly as a zwitterion, a type of ion with both positive and negative charges.

Amino acids are amphoteric, meaning they can act as both acids and bases. They have a basic amino group and an acidic carboxyl group. The pKa values of these groups determine the pH at which they become protonated or deprotonated. For proteins, the isoelectric point is crucial in techniques like electrophoresis, where it dictates their migration behavior in an electrical field. Proteins will migrate towards the cathode (negative terminal) at low pH and towards the anode (positive terminal) at high pH. At their isoelectric point, they do not migrate. Isoelectric focusing is an electrophoretic technique that separates proteins based on their unique isoelectric points.123