The term ‘Zwitterion’ is originally derived from the German word ‘zwitter’, which can be roughly translated as ‘hybrid’ or ‘hermaphrodite’. A zwitterion is an ion that contains two functional groups. In simple terms, it is as ion possessing both positive and negative electrical charges. Therefore, zwitterions are mostly electrically neutral (the net formal charge is usually zero).

Zwitterions are sometimes referred to as “inner salts“. Usually, dipolar compounds are not classified as zwitterions. The distinction lies in the fact that the plus and minus signs on the amine oxide signify formal charges. Zwitterions may be worthy of medicinal chemistry design considerations when working with acid, basic or neutral leads.

Zwitterion Definition

“A zwitterion is a molecule that has both positive and negative regions of charge.” In the solid state, amino acids exist as dipolar ions called zwitterions. While discussing whether a substance is zwitterionic or not, the pH range in which the information is required must be specified (because a sufficiently alkaline solution will change the zwitterion to an anion, and a sufficiently acid solution will change it to a cation).

Some key Characteristics of Zwitterion are;

  • They can be formed from compounds like ampholytes which contain both acid and base groups in their molecules.
  • In this type of ions, the charged atoms are usually held together by one or more covalent bonds.
  • Zwitterionic compounds have stable, separated unit electrical charges on atoms.
  • These compounds contain quaternary ammonium cations.

Let us further understand the topic by looking at an example of Zwitterion.

Zwitterion Structure

Amino Acids

Amino acids are the most common example of zwitterions. They are made up of an ammonium or amino group which contains a positive charge as well as a carboxyl group which contains a negative charge. The zwitterion form of an amino acid is given below.

Zwitterion form of an amino acid

Zwitterion Structure

Apart from amino acids, any compound that contains acid and base centres can obtain a Zwitterion form. Some more examples include tricine, bicine, solid sulfamic acid, alkaloids like psilocybin amongst others.

Isoelectric Point

  • Another main property of a Zwitterion is that it has an isoelectric point (represented as pI, pH(I), IEP).
  • This point is the pH value at which the charge in molecules is neutral.
  • Usually, the net charge on a molecule is greatly affected by the pH of its surrounding environment.
  • In this case, molecules can become more charged (positively or negatively) as a result of gain or loss in the number of protons.
  • If we look at amino acid, the amino group is a very effective proton acceptor and the carboxyl group is an effective proton donor.
  • In addition, the solubility of a molecule at a given pH is also affected by the pI value.

Calculation of pH Value

The pH value at the isoelectric point can be calculated from the equilibrium constants (acid and base) of the Zwitterion. It is represented by the formula;

\(pI = \frac{pK_{a1} + pK_{a2}}{2}\)


  • pI = isoelectric point,
  • Ka1 = the equilibrium constant of the acid.
  • Ka2 = the equilibrium constant of the base.

Applications of Zwitterions

  1. Zwitterions are widely applied in the process of separating protein molecules via SDS PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) method which is one of the most popular techniques used in molecular biology.
  2. They also have great potential to be applied in a wide range of medical and biological related fields.
  3. Some popular uses include medical implants, drug delivery, blood contacted sensor, separation membrane, as well as antifouling coatings of biomedical implants that help prevent the build-up of microbial adhesion and biofilm formation.
  4. In the marine industry, Zwitterionic polymers are used to prevent subaquatic organisms from building up on boats and piers.

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