WANI Dictionary

Isoelectric point

Isoelectric point.

The isoelectric point (pI) is the pH of a solution at which a molecule of interest has no net charge.

The isoelectric point is typically used to characterize an amino acid, a peptide, and a protein.

Definitions in the literature

• The isoelectric point, pI, is the pH of an aqueous solution of an amino acid (or peptide) at which the molecules on average have no net charge. In other words, the positively charged groups are exactly balanced by the negatively charged groups. For simple amino acids such as alanine, the pI is an average of the pKa's of the carboxyl (2.34) and ammonium (9.69) groups. Thus, the pI for alanine is calculated to be: (2.34 + 9.69)/2 = 6.02, the experimentally determined value [1].