Isoelectric Point

Synonyms

IEP; pI

Definition

The isoelectric point is the pH at which a molecule or surface carries no net electrical charge. Many molecules are zwitterions, containing both positive and negative charges. The net charge on the molecule is governed by the pH of the surrounding medium. The pI is the pH value at which the molecule carries no net electrical charge and thus the negative and positive charges are equal. For a doubly charged species containing both negative and positive charges, for example an amino acid with only one amine and one carboxyl group, the pI is simply the sum of the pKa’s of the two ionizable groups divided by 2. The terms isoelectric point (IEP) and point of zero charge (PZC) are often used interchangeably, although they have subtle differences.

For surfaces, in the case when the surface charge-determining ions are H⁺/OH⁻, the net surface charge is affected by the pH of the surrounding liquid medium. The pI then is the pH value of the solution at which the...