Zwitterion

Zwitterion is a molecule that consists of an equal number of positive (cation) and negative (anion) charges within its structure. As zwitterions contain an equal number of opposite charges the net charge on it nullifies and becomes zero. They are also referred to as inner salts or dipolar salts. Amino acid is the most common example of zwitterion. Zwitterions have a wide scope of application in fields like marine, medical, and biological fields.

Further in this article, we will learn more about zwitterion, the structure of zwitterion, its examples, properties, and uses.

What is a Zwitterion?

Zwitterions are polyelectrolytes that contain two or more functional groups. They contain an equal number of positive and negative charges, therefore, they are electrically neutral. ‘Zwitterion’ is a term coined from the German word ‘Zwitter’ which means ‘hybrid’, so the zwitterion is a hybrid of a positive (cation) and negative (anion) charge. Therefore, zwitterion can also be referred to as half-cation or haf-anion. It can also act as both an acidic group and a basic group, for example, the amino acid is a zwitterion with an amino group(which is basic) and a carboxyl group (which is acidic).

Zwitterion Definition

Zwitterion is a molecule that consists of both positive (cation) and negative (anion) charges within its structure. The net charge on zwitterion is zero as it is nullified by the presence of equal number of opposite charges.

Examples of Zwitterion

Amino acid is the most common example of zwitterion. Alanine, Tetracycline, bicine, solid sulfamic acid, phosphatidylcholine, and anthranilic acid are some other examples of zwitterions. But to check whether a chemical is zwitterion or not its pH range must be calculated because the zwitterion becomes an anion in the presence of excess alkaline solution, and it acts as a cation in the presence of sufficient acid solution.

Structure of Zwitterion

The structure of zwitterion varies from one compound to another. The compounds that contain both an acidic and a basic group can form a zwitterion, for example, an amino acid. The structure of amino acid as a zwitterion is described below:

Amino Acid as Zwitterion

Amino acids act as the building blocks of proteins in living organisms. Its structure contains an alkaline amino group (-NH₂) an acidic carboxyl group (-COOH) and a side chain known as R-group all attached to a single carbon atom. When an amino acid becomes a zwitterion, the amino group has a positive charge, and the carboxyl group has a negative charge, nullifying the net charge. Hence, amino acids is a zwitterion that is electrically neutral and has both acidic and basic group within its molecule.

Zwitterion Structure of Alanine

Alanine is an amino acid that occurs in high levels in its free state and is used to make proteins. Alanine, in its zwitterion structure, has both a positive and a negative charge on it. Its zwitterion structure consists of an amino group (NH₃⁺) which is positively charged and a negatively charged carboxyl group (COO^–). The structure of alanine as a zwitterion is described below:

Zwitterion Structure of Glycine

Glycine is the simplest and most stable form of amino acid. Its structure contains an amino group (-NH₂), a carboxyl group (-COOH), and a side chain with a single H-atom attached to the central carbon. It transfers a proton from the carboxyl group making it negative (COO^–) to the amino group (NH₃⁺) making it positively charged. This results in glycine to produce a zwitterion. The structure of glycine as a zwitterion is described below:

Zwitterion Polymers

Zwitterion polymers are polymers (molecular chains) that have equal cation and anion groups. It contains both negative and positive charges in its structure. Their charge distribution makes them unique and versatile materials which results in their wide applications. They are used in drug delivery, water purification, sensors, wastewater treatment, etc.

Isoelectric Point

The isoelectric point is a point where the net charge on a molecule is neutral (i.e. it is neither positive nor negative). Zwitterions have an isoelectric point that helps to calculate the pH value of the ion. It is represented as pI, pH(I), or IEP. The solubility of a molecule is also affected by its isoelectric point. Amino acids have an isoelectric point of 5.5, this means that amino acid has a neutral charge at pH 5.5.

Calculation of pH value

We can calculate the pH value of zwitterions at the isoelectric point (i.e. the point where charge is neutral) from the equilibrium constants (i.e. acid and base constants) of the zwitterion by the formula:

pI = (Pka₁ + Pka₂)/2

where,

• pI is Isoelectric point of Zwitterion
• Ka₁ is the Acidic equilibrium constant of the Zwitter Ion
• Ka₂ is the Basic equilibrium constant of the Zwitterion

Properties of Zwitterion

The properties of Zwitterions are mentioned below:

• Zwitter ions have an isoelectric point that helps to calculate their pH value.
• Zwitterions consist of both positive and negative charges held together by covalent bonds.
• Zwitterions are highly soluble in water, the interaction with water molecules is done by hydrogen bonding.
• The atoms of zwitte rions have a distinct and stable unit electrical charge.
• Zwitterions generally have a greater melting point, however, it varies depending on their chemical structure.
• Quaternary ammonium cations are also present in Zwitterions.

Uses of Zwitterion

Zwitterions have a vast scope of application in different fields such as:

• Marine industry: It is used in the marine industry to prevent the growth of aquatic organisms like algae and moss on the surface of boats, jetties, and, pillars.
• Chemical agent: It is used as a chemical agent in various personal care products like shampoos, soaps, etc as it acts as a mild cleansing agent.
• Medical industry: It has vast usage in the medical and biological fields. It is used in medical implants, blood contact sensors, biopsies, cell preservation, biomedical sensors, etc.
• Drug delivery: Its unique properties help in improving drug stability and solubility, and thus helpful in targeting particular cells or tissues.
• Separation and analysis: It is widely used in various techniques like chromatography for separating and analyzing various complex compounds.

Zwitterions – FAQs

1. What are Zwitter Ions?

Zwitterions are polyelectrolytes that contain two or more functional groups. It consists of an equal number of positive (cation) and negative (anion) charges within its structure which nullifies the net charge on it.

2. What is the Melting of Zwitterions?

Zwitterions generally have a greater melting point, however, it varies depending on their chemical structure.

3. Are Zwitter Ions Soluble in Water?

Yes, Zwitterions are highly soluble in water because of its polar nature, the interaction with water molecules in zwitterions is done by hydrogen bonding.

4. Are Zwitterions Acidic or Basic?

Zwitterion can act as both acidic and basic, for example, the amino acid is a zwitterion with an amino group(which is basic) and a carboxyl group (which is acidic).

5. What is an Examples of Zwitterions?

Amino acid, Tetracycline, bicine, phosphatidylcholine, and anthranilic acid are some examples of zwitterions.

6. Why is a Zwitter Ion Neutral?

Zwitterions have both positive(+) and negative(-) charged groups, and thus the overall charge on a Zwitterion is neutral.