Amino acids contain both a basic group $(-N{H}_2)$  and an acidic group  $(-C{O}_{2}H)$. In the dry solid state, amino acids exist as dipolar ions, a form in which the carboxylic group is present as a carboxylate ion,  $-C{O}_2^{-}$ and the amino group is present as on ammonium ion,  $-N{H}_3^{\oplus }$. In aqueous solution, an equilibrium exists between the dipolar ion (zwitter ions)and the anionic and cationic forms of an amino acid.

In strongly acidic solutions (pH = 0) all amino acids are present primarily as cation, in strongly basic solution they are present as anions. At some intermediate pH, called the isoelectric point (pI), the concentration of the anions and cations is equal. Each amino acid has particular isoelectric point.