Zwitterion

The term Zwitterion is originally derived from a German word zwitter which translates to the word hybrid or hermaphrodite. With that thought in mind, Zwitterion is normally described as an ion containing two functional groups. In simple terms, it is as ion possessing both positive and a negative electrical charge. Since the molecules contain two opposite charges, Zwitterion is most of the time electrically neutral or the net formal charge is zero.

Characteristics of Zwitterion

Some of the key characteristics of Zwitterion are;

• They can be formed from compounds like ampholytes which contain both acid and base groups in their molecules.
• In these 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.

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.

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 are 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;

$pI = \frac{pK_{a1} + pK_{a2}}{2}$

Where,

pI = isoelectric point,

Ka1 = the equilibrium constant of the acid.

Ka2 = the equilibrium constant of the base.

Applications of Zwitterions

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. They also have great potential to be applied in a wide range of medical and biological related fields. Some of the 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.

In the marine industry Zwitterionic polymers are used to prevent subaquatic organisms from building up on boats and piers.

Practise This Question

Which is not true about borax?