The isoelectric point (pI) is the pH at which a molecule has a net electric charge of zero. For simple amino acids, it is the average of the two pKa values that surround the neutral 'zwitterion' form. At pH values below the pI, the molecule carries a net positive charge; above the pI, it carries a net negative charge. This property is crucial in biochemistry for separating proteins via electrophoresis and for understanding protein solubility.
