Isoelectric

Isoelectric describes a state in which a molecule carries no net electric charge. The pH at which this occurs is called the isoelectric point, or pI. At pH values below the pI, a molecule tends to be positively charged; at pH values above the pI, it tends to be negatively charged. The concept applies to many substances in chemistry and biology, especially amino acids, peptides, and proteins, as well as colloidal particles.

For amino acids and proteins, the pI depends on the ionizable groups present: the amino and carboxyl termini and the side chains of acidic (such as aspartate and glutamate) and basic (such as lysine, arginine, and histidine) residues. Acidic groups lower the pI, basic groups raise it. The pI is the pH at which the molecule exists mostly as a zwitterion, with no net charge. Individual amino acids have characteristic pI values (e.g., acidic side chains yield lower pI; basic side chains yield higher pI), while proteins have a pI determined by their overall composition and structure.

Isoelectric focusing is a common analytical technique that separates proteins by their pI. In a gel containing a pH gradient and an electric field, proteins migrate until they reach the pH corresponding to their pI, where their net charge is zero and mobility stops. This technique is often used in combination with SDS-PAGE in two-dimensional protein separation.

In colloid chemistry, the isoelectric point of a particle is the pH at which its surface charge is zero, influencing stability and aggregation. Nucleic acids generally remain negatively charged across most pH ranges, so they do not have a practical isoelectric point like proteins do.