The isoelectric point (pI) is the pH at which a molecule carries zero net charge; for an amino acid this is the pH at which the zwitterion — the species bearing simultaneous positive and negative formal charges on separate groups — predominates. Because the protonation state of each ionizable group is governed by its pKa, the negative logarithm of the acid dissociation constant, an amino acid exists as a cation at low pH where all basic sites are protonated, as an anion at high pH where all acidic sites are deprotonated, and as the neutral zwitterion at an intermediate pH computed as the average of the pKa values of the groups flanking the neutral species. This concept belongs to biochemistry and amino-acid chemistry, extending acid–base equilibrium theory from general chemistry into the prediction of protein charge, solubility, and behavior in electrophoretic and chromatographic separation.
The isoelectric point (pI) is the pH at which a molecule carries zero net charge; for an amino acid this is the pH at which the zwitterion — the species bearing simultaneous positive and negative for…