zwitteriony

Zwitterions (zwitteriony in Polish) are molecules that bear both a positive and a negative charge at different atomic sites yet have an overall electrical neutrality. The term zwitterion comes from the German zwitter, meaning “hybrid” or “carrier,” reflecting the internal salt character of these species.

Most familiar are amino acids in aqueous solution: the amino group is protonated to form –NH3+ and the carboxyl group is deprotonated to form –COO−. This internal transfer of a proton creates a single, net-neutral molecule known as a zwitterion. In solid form, many amino acids also adopt zwitterionic structures.

Zwitterions tend to be highly polar with large dipole moments, which makes them very soluble in water and often poorly soluble in nonpolar solvents. Their charge state is pH-dependent; at a characteristic pH called the isoelectric point (pI), the molecule has zero net charge. For simple amino acids, pI lies between the pKa values of the amino and carboxyl groups and shifts with the presence of ionizable side chains.

Beyond amino acids, other molecules with internal acid–base groups can form zwitterions, including certain peptides, dyes, and pharmacological compounds. Zwitterions are important in biochemistry and analytical chemistry; they influence protein structure, enzyme activity, chromatography behavior, and electrospray ionization in mass spectrometry.