Zwitterions

Zwitterions are molecules that contain both a positive and a negative charge but have an overall neutral charge. The term comes from the German zwitter, meaning hybrid or hermaphrodite, reflecting the internal charge separation within the same molecule. Zwitterions form when a functional group acts as an acid and another as a base within the same molecule.

In amino acids, the most common zwitter ionic form in aqueous solution at neutral pH features a deprotonated carboxylate group (−COO−) and a protonated amino group (−NH3+). Glycine, the simplest amino acid, is often cited as a model zwitterion. At low pH, amino acids carry positive charges on both groups; at high pH, both are deprotonated and the molecule bears a negative charge. Between these extremes lies the isoelectric point, where the net charge is zero and the zwitterionic form is predominant.

Zwitterions are typically highly polar and water-soluble, and their internal charge separation influences solubility, reactivity, and binding interactions. Their net zero charge at the isoelectric point affects mobility in electric fields, a principle used in techniques such as electrophoresis and isoelectric focusing.

Beyond amino acids, many other molecules can exist as zwitterions, including certain betaines and biological metabolites. In nonpolar solvents, zwitterions are less stable and may exist as neutral molecules or form ion pairs depending on solvent and salt conditions.