What happens to the charge of a protein when the pH is equal to its pI?

When the pH of the solution is equal to the isoelectric point (pI) of a protein, the protein has no net charge. The isoelectric point is the specific pH at which the number of positive charges on the protein equals the number of negative charges, leading to a neutral overall charge. This balance occurs because at the pI, the amino acid side chains, as well as the terminal amino and carboxyl groups of the protein, are fully ionized in such a way that their positive and negative charges cancel each other out.

This unique property at the pI is crucial in various biochemical contexts, such as protein purification techniques like isoelectric focusing. At the pI, proteins will not migrate in an electric field, which is key in separating proteins based on their charge characteristics.

Understanding this equilibrium helps clarify why proteins behave differently at various pH levels, influencing their solubility, stability, and interaction with other molecules in biochemical processes.