Zwitterion

A zwitterion is defined as the ion that contains two or more functional groups; This ion contains both negative and positive charges. Zwitterions are usually termed electrically neutral or almost zero charge ions due to this property. Conversely, the presence of both the electrical charges, positive and negative, is known to create a nullable effect, due to which the zwitterion might possess zero charge. Zwitterions are also known as dipolar salts or inner salts due to their hermaphrodite nature.

Let us understand zwitterion much more deeply with the help of the illustration. Take an example of the image shown below-

The most commonly occurring zwitterion in nature is amino acids. They contain ammonium or amino groups containing a positive electrical charge, and a carboxyl group gives it. We can also see this in the diagram given below.

We can see that the negative charge is formed on the oxygen atom by accepting an electron pair. On the other hand, the positive charge is formed on the amino acid group by donating an electron pair. This leads to the formation of amino acid zwitterion. Other examples of zwitterion include ricin, bicine, solid sulfamic acid, and alkaloids like psilocybin. One can also conclude that compounds containing an acid center or a base center can form a zwitterion.

Now, it is essential to determine the nature of these ions to be utilized when forming complex compounds with them. Hence, it is essential to calculate the pH value of the zwitterion or obtain the pH value at which the electrical charge on all the molecules is neutral.

Zwitterions have the isoelectric point property that helps measure the pH value of the ion or substance when the charges on them are neutral. It is represented as pI, pH(I), and IEP.

This point helps to provide the information of the pH value at which the charge in molecules is neutral. In general, the net charge on any molecule is greatly affected by the pH of its surrounding environment and several other environmental factors. However, in this case, the molecules can become more charged, i.e., more positively charged or more negatively charged, and as a result of gain or loss, several protons. Let us look at the example of the amino acid. The amino group is a very effective proton accepting group, whereas, on the other hand, the carboxyl group is an effective proton donating group.

Additionally, the pH value of the compound where the charge is neutral, i.e., the isoelectric point, is greatly affected by the solubility of the compound. To calculate the pH value at the isoelectric point from the equilibrium constants i.e., acid and base constants of the Zwitterion e use the formula-
pI = {pK_{a1} + pK_{a2}}/2
Where,

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

Characteristics of Zwitterion

Some of the characteristics of zwitterion are listed down below:-

1. Zwitterions are compounds containing both positive and negative regions of charges, i.e., acid or base groups—examples of compounds containing both acid and base groups like ampholytes.
2. In zwitterions, the positive and negative charge ions are held together through a covalent bond.
3. These compounds that contain both positive and negative electrical charges also have stability as the unit electrical charges are separated from the atoms.
4. Most zwitterions possess ammonium cations—for example, zwitterions of ammonium acids.

Applications of Zwitterions

Some of the applications of zwitterions are listed down below:-

1. Zwitterions are widely used in the marine industry. They help to prevent the sub-aquatic organisms from building up on surfaces of boats and piers.
2. They are also used to separate protein molecules through the sodium dodecyl sulfate-polyacrylamide gel electrophoresis method. It is also considered to be one of the best techniques in the field of microbiology.
3. Other common uses of zwitterions include their usage in blood-contact sensors or medical implants, utilizing separate membranes or any antifouling coating of medical implants that prevent the build-up of any microbial adhesion or biofilm formation.
4. They are also used in cell preservation, biomedical sensors, drug delivery, etc.