Zwitterionisiin

Zwitterion is a neutral molecule that contains both positive and negative formal charges on different parts of the same molecule. The term comes from the German Zwitterion, literally meaning “hybrid ion.” In solution, the best-known zwitterions are the amino acids at physiological pH, where intramolecular proton transfer converts the amino group to NH3+ and the carboxyl group to COO−, yielding NH3+-CHR-COO−. This internal salt form is typically the predominant species at pH around 7; the overall charge is zero, but the molecule behaves as a dipole with a sizable dipole moment.

The pH at which the net charge is zero is the isoelectric point (pI). Each amino acid has a characteristic pI depending on its side chain; glycine has a pI near 6.0, while acidic and basic amino acids differ.

Zwitterions influence solubility, reactivity, and transport properties. They are central to protein structure, enzyme activity, and chromatographic separation techniques such as isoelectric focusing, which separates species by pI. In chemistry and materials science, zwitterionic species and polymers are used to impart water compatibility and anti-fouling properties.

Examples extend beyond amino acids to certain dyes, drugs, and amino-alcohols, as well as zwitterionic polymers and surfaces. In nonpolar media, liquid-phase equilibria may shift toward non-zwitterionic forms.

In summary, zwitterions are dipolar, internally salt-like species that are common in biology and materials science, notable for their neutral charge but strong internal charge separation and pH-dependent behavior.