The isoelectric point (pI) is the pH of a solution at which a molecule or a polyprotic species has no net electric charge. It can affect the solubility, stability, and behavior of biomolecules such as proteins, peptides, and amino acids. At the isoelectric point, a protein or peptide molecule tends to be least soluble and least reactive, as its surface charge is neutral and there is no net electrostatic repulsion or attraction between molecules. The isoelectric point is important for various biological applications, such as protein purification, protein-protein interactions, and protein crystallization.