Isoelectric point

The pH at which a molecule has zero average net electrical charge. It depends on the molecule’s ionizable groups and the surrounding conditions.

Acid–base equilibrium: The balance between protonated and deprotonated forms of acids and bases in solution. The isoelectric point follows from how a molecule’s groups gain or lose protons as pH changes.
Isoelectric focusing: An electrophoretic method that separates molecules along a pH gradient until each reaches its isoelectric point. It turns differences in isoelectric point into distinct positions in a gel or capillary.
Net charge: The algebraic sum of the positive and negative charges in a system. The defining condition is zero average net charge, not an absence of charged groups.
Isoelectric point of amino acids: The pH at which an amino acid has zero average net charge. Simple amino acids can have calculable isoelectric points from their ionizable groups’ pKa values.
pH: A logarithmic measure of hydrogen-ion activity in a solution. The isoelectric point is a particular pH value, not a charge intrinsic to the molecule alone.
Electrophoresis: The movement of charged particles through a medium under an applied electric field. A molecule’s charge relative to its isoelectric point affects the direction and strength of its migration.
Formal charge: A bookkeeping charge assigned to an atom using a particular electron-allocation convention. Formal charges on individual atoms can remain nonzero when a molecule’s net charge is zero.
Electrophoretic mobility: The velocity of a particle in an electric field divided by the field strength. Mobility is a measured response that depends on charge and other factors, rather than the zero-charge pH itself.
Amino acid: An organic molecule containing amino and carboxyl groups, often with an additional variable side chain. Its ionizable groups provide a familiar example of charge changing with pH.
Protein purification: The isolation of a target protein from other molecules in a biological sample. Protein charge near its isoelectric point can guide separation and precipitation conditions.

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