What is the isoelectric point of an amino acid?

The isoelectric point (pI) of an amino acid is the specific pH value at which the molecule carries a net zero charge. At this point, the amino acid exists as a zwitterion, meaning it possesses an equal number of positive and negative charges.1

While the molecule is electrically neutral in terms of its net charge at the isoelectric point, it is important to note that it does not have an absolute zero charge; it remains in an ionic state with charged groups present. Because amino acids are amphoteric—meaning they can act as either an acid or a base—their charge state shifts depending on the surrounding environment's pH.1

In practical applications like electrophoresis, the isoelectric point is the pH at which the molecule will not migrate in an electrical field, as it lacks a net positive or negative charge to drive movement toward the cathode or anode.2