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Zwitterion is derived from the German word 'zwitter', which can be translated as 'hybrid' or 'hermaphrodite'. A zwitterion is an ion consisting of two active groups. In simple terms, it is like an ion that has both positive and negative charges. Therefore, zwitterion is neutral. Zwitterions are sometimes called "internal salts”. Usually, dipolar compounds are not classified as zwitterions.
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Key Terms: Zwitterionic, isoelectric point, amino group, equilibrium constant, protein molecules, electrostatic interactions, hermaphrodite, internal salts
What is Zwitterion?
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A zwitterion is a molecule that has both positive and negative charges. A zwitterion is an ion or molecule that contains an equal amount of well-charged and poorly charged functional groups. It is also called a neutral ion or internal salt. Zwitterionic polymers are also available containing active groups that are poorly charged. Zwitterion is naturally amphoteric and during electrolysis, it does not migrate to the cathode or anode.
Zwitterion Charge
α - amino acids are excellent examples of zwitterions. The amino group contains the positive charge whereas the carboxylic acid group contains the negative charge. α - amino acids are crystalline due to the formation of zwitterion.
It forms a zwitterion when dissolved in an aqueous solution. For example, when R-CH-NH2-COOH is dissolved in an aqueous solution, the oxygen atom takes up electrons from hydrogen in the carboxyl group. Thus, oxygen gains a negative charge and results in the formation of H+ ions. This H+ gets bonded with a lone pair of Nitrogen forming NH3+ ions.
Amino Acid Zwitterion
Amino acids contain both basic and acid groups. Here, H+ gets bonded with a lone pair of Nitrogen and forms NH3+ ions which are basic while COO- ion leaves from the acidic carboxylic acid group resulting in the formation of Zwitterion.
Any compound containing acid and subcutaneous compounds can form a Zwitterion form. Other examples include tricine, bicine, solid sulfamic acid, and alkaloids such as psilocybin.
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Isoelectric Point
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Zwitterion has an isoelectric point which is represented by pI, pH (I), IEP. This point is the pH value at which the charge in molecules is neutral. In general, the net charge of a molecule is greatly affected by the pH of the surrounding environment.
In this case, the molecules become more positively charged by accepting electrons in an acidic medium while it becomes more negatively charged in a basic medium. If we look at amino acids, the amino group is the most efficient proton receptor and the carboxyl group is the most active proton supplier.
Isoelectric Point in Amino Acid
In addition, the solubility of a molecule at a given pH is affected by the pI value. There would be less to no solubility in aqueous solutions at the determined isoelectric point. Therefore, the pH at which the zwitterion of a molecule is formed is called the cell point of that molecule. Isoelectric point affects various molecular structures such as dissolving solutions.
Calculation of pH value
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The pl value of zwitterion is calculated by using the following formula.
\(pI = \frac{pKa_1 + pKa_2}{2}\)
Where,
pI → Isoelectric point.
Ka1 → The equilibrium constant of the acid.
Ka2 → The equilibrium constant of the base.
Application of Zwitterion
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- A zwitterion is widely used in the process of separating protein molecules by the SDS PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) method. This is one of the most popular methods used in molecular biology. They have great potential for use in many different medical and biological fields.
- Other popular uses include medical implants, drug delivery, blood-sensing nerves, dissecting membranes, and biopsies that help prevent bacterial adhesions and biofilm formation.
- In the maritime industry, Zwitterionic polymers are used to prevent underwater organisms from forming algae or moss on boats and pillars.
Things to Remember
- A zwitterion is an active group molecule where at least one has a positive electrical charge and the other has a negative electrical charge.
- Amino acids are the best-suited example of zwitterion.
- Zwitterions provide strong hydration by electrostatic interactions with water molecules and provide a stronger barrier to adsorption.
- Therefore, Zwitterions are electrically neutral and do not migrate in an electric field.
- A zwitterion is widely used in the process of separating protein molecules by the SDS PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) method.
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Sample Questions
Ques. Give some other examples of zwitterion. (2 marks)
Ans. Some other examples of zwitterion are trimethylglycine, psilocybin, Cocamidopropyl betaine, etc.
Ques. Is lysine a zwitterion? (2 marks)
Ans. In all eukaryotes, from yeast to individuals, L-Lysine occurs. An L-lysine zwitterion is an L-alpha-amino acid zwitterion caused by the transfer of a carboxy proton to the L-lysine amino group; prominent species in pH 7.3. As a human metabolite, it has a function.
Ques. Why do zwitterions have a high melting point? (2 marks)
Ans. There is a stronger ionic attraction that exists between ions and their neighbour, instead of the weaker hydrogen bonds and other intermolecular forces. Zwitterions require more energy to split these ionic attractions and thus, the amino acids have high melting points for the size of the molecules.
Ques. Write a short note on zwitterion. (2 marks)
Ans. A zwitterion is a molecule with at least two active groups: one with a positive charge and the other with a negative charge, charged entirely at zero. Zwitterions can form from other molecules called ampholytes, or amphoteric compounds, and can act as an acid and base. Polyelectrolyte Zwitterions contain both positive or negatively charged groups, and total charges are neutral.
Ques. How does a zwitterion form? (2 marks)
Ans. A zwitterion is a dipolar ion formed by the combination of a carboxyl group with an amino group. As a result, it is highly composed of amino acids. The carboxyl group on zwitterion carries a negative charge, while the amine group carries a positive charge.
Ques. Amino acids behave like salts rather than simple amines or carboxylic acids. Explain. (2 marks)
Ans. In an aqueous solution, the -COOH group of an amino acid loses a proton and the -NH2 group accepts a proton to form zwitterion (salt).
Ques. When a protein in its native form is subjected to a physical change like a change in temperature or a chemical change like a change in pH, denaturation of protein takes place. Explain the cause. (2 marks)
Ans. Due to physical or chemical change, hydrogen bonding and various other attractive forces are disturbed, globules unfold and the helix gets uncoiled to form a thread-like molecule. Therefore, the secondary and tertiary structure of protein loses all or part of its biological activity. This is called the denaturation of proteins.
Ques. The activation energy for the acid catalysed hydrolysis of sucrose is 6.22 kJ mol-1, while hydrolysis is catalysed by the enzyme sucrase. Explain. (2 marks)
Ans. Enzymes are biocatalysts. They reduce the magnitude of activation energy by providing alternative paths. In the hydrolysis of sucrose, the enzyme sucrase reduces the activation energy from 6.22 kJ mol-1 to 2.15 kJ mol-1. As a result, enzyme catalysed reactions occur at a much faster rate than ordinary chemical reactions using conventional catalysts.
Ques. How do you explain the presence of an aldehydic group in a glucose molecule? (3 marks)
Ans. Glucose reacts with hydroxylamine to form a monoxime and adds one molecule of hydrogen cyanide to give cyanohydrin.
Therefore, it contains a carbonyl group which can be an aldehyde or a ketone. On mild oxidation with bromine water, glucose gives gluconic acid which is a carboxylic acid-containing six carbon atoms.
This indicates that the carbonyl group present in glucose is an aldehydic group.
Ques. What are glycosidic linkages? In which type of biomolecules are they present? (2 marks)
Ans. Two molecules of monosaccharides are joined together by an oxide linkage formed by the loss of water molecules. Such a linkage between two monosaccharide units through an oxygen atom is called glycosidic linkage.
Glycosidic linkage is present in disaccharides, trisaccharides and polysaccharides, etc.
Ques. Describe the terms D- and L-configuration used for amino acids with examples. (3 marks)
Ans. The sugars are divided into two families: the D-family and L-family which
have definite configurations. These configurations are represented with respect to glyceraldehyde as the standard. The glyceraldehydes may be presented in two forms:
D-(+)-Glyceraldehyde L-(-)-Glyceraldehyde The D-configuration has -OH attached to the carbon adjacent to -CH2OH on the right while L-configuration has -OH attached to the carbon adjacent to -CH2OH on left. The sugars are called D- or L-depending upon whether the configuration of the molecule is related to D-glyceraldehyde or L-glyceraldehyde. It has been found that all naturally occurring sugars belong to D-series, e.g D-glucose, D-ribose and D-fructose.
Ques. How will you distinguish 1° and 2° hydroxyl groups present in glucose? Explain with reactions. (3 marks)
Ans. Glucose on treatment with acetic anhydride in presence of pyridine or a few drops of con. H2SO4 forms a penta-acetyl derivative indicating the presence of 5 -OH groups. Out of which one -OH group is primary (1°) alcoholic and four (C2, C3, C4 and C5) -OH groups are secondary (2°) alcoholic groups.
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