zwitterion

An amino acid, in its (1) nominal (unionized) and (2) usual, zwitterionic forms.

A zwitterion (first part pronounced TSVIT-ər, from German "Zwitter" ([ˈtsvɪtɐ]) = hybrid, hermaphrodite) is a chemical compound that carries a total net charge of 0 and is thus electrically neutral, but carries formal charges on different atoms.

Some chemists restrict the term to compounds with the charges on non-adjacent atoms. ^[1] This would exclude compounds such as N-oxides and ylides, which have a charge on adjacent atoms, specificately, nitrogen and carbon being the cations, respectively. Zwitterions are polar and are usually very water-soluble, but poorly soluble in most organic solvents.

Ampholytes are molecules that contain both acidic and basic groups (and are therefore amphoteric) and will exist mostly as zwitterions in a certain range of pH. The pH at which the average charge is zero is known as the molecule's isoelectric point.

Applications

Ampholytes are used to establish a stable pH gradient for use in isoelectric focusing.

Typical examples of zwitterions are:

• Most amino acids at physiological pH are zwitterionic.
• Used as buffering agents in Good's buffers:
  • The amino-sulfonic acid based MES, MOPS, HEPES, PIPES or CAPS
  • The amino-carboxylic acid (amino acid) based glycine, its derivatives bicine and tricine, and alanine
• Used as detergents:
  • CHAPSO
• Natural products like the alkaloids psilocybin and lysergic acid.
• Betaines

Less common examples of zwitterions are:

• Quinonoid zwitterions.
• Drugs such as Fexofenadine (Allegra) and Cephaloridine.
• decaphenylferrocene [(η⁵-C₅Ph₅)₂Fe] has been shown to have a zwitterionic linkage isomer [(η⁵-C₅Ph₅)Fe⁺(η⁶-C₆H₅C₅Ph₄⁻)].

References

1. ^ International Union of Pure and Applied Chemistry. "zwitterionic compounds/zwitterions". Compendium of Chemical Terminology Internet edition.