Zwitterion or Zwitter ion is an important topic of organic chemistry. Every year many questions are asked from this topic in various examinations. So, in this article we are discussing zwitterion in detail. The word zwitterion is derived from the German language word Zwitter which means hermaphrodite.  

What is Zwitterion? 

Zwitterion is an ion or molecule which contains an equal number of positively charged functional group’s and negatively charged functional group’s. It is also called neutral ion or inner salts. Zwitterionic polymers are also found which contain negatively and positive charged functional groups at either end of the chain. Zwitter ion is amphoteric in nature and during electrolysis it neither migrates to cathode nor to anode. 

α – amino acids are best examples to understand the zwitterions. α – amino acids show crystalline properties due to zwitterion formation. When we dissolve . α – amino acids in aqueous solution, it gives zwitterion. For example, if we dissolve R-CH-NH2-COOH in an aqueous solution then the oxygen atom pulls electrons from hydrogen in the carboxyl function group. Thus, oxygen gets negative charge and formation of H+ ion takes place. Now this H+ ion gains a lone pair of nitrogen and gets bonded with it. Thus, nitrogen gains a positive charge with formation of NH3. Structure is given below –

Amino acids contain both basic and acidic groups. In them amine is basic while carboxylic is an acidic group. Amine being a stronger basic group gains H+ ion from COOH and leaves the carboxylic group as COO- ion. Thus, zwitterion forms. 

Apart from amino acids sulfamic acid also crystallizes in the zwitterion form. Sulfamic acid is also called amidosulfonic acid, amidosulfuric acid, aminosulfonic acid or sulfamidic acid. It is an inorganic compound with the formula H3NSO3. In solid state its structure is explained by its zwitterionic form. Its zwitterionic form is given below –

Betaine – It is also an example of zwitterion. It is a neutral chemical compound with positively charged functional group quaternary ammonium or phosphonium and with a negatively charged functional group such as carboxylate group. Betaine is a specific type of zwitterion as it does not have any hydrogen atom on the atom which is bearing a formal positive charge. 

Other examples of Zwitterion include trimethyl glycine, psilocybin, cocamidopropyl betaine etc. The only amino acid which is optically inactive is glycine. In Glycine, a carbon atom is bonded with two same groups or two hydrogen atoms due to which it lacks a chiral carbon atom, so it is optically inactive. Structure of glycine is given below –

If a chemical species is resonance stabilized then it can be called a zwitterion, only if its all forms of resonance follow the definition of zwitterion. 

Effect of pH 

pH of the solution affects the zwitterions. If a solution of amino acids is prepared by using a low pH medium. Then, the amino group of amino acid accepts a proton and forms a positively charged ion. It happens due to the presence of H+ ions in acidic medium. During electrolysis this cation goes towards the negatively charged electrode. 

If a solution of amino acids is prepared by using a high pH medium. Then, deprotonation takes place in amino acid and a carboxylic group of amino acid donates a proton and forms the negatively charged ion and H2O molecule. It happens due to the presence of OH- ions in the basic medium. During electrolysis, this anion goes towards the positively charged electrode. 

Depending upon the level of pH of the solution amino acid can be found as anion, cation, or zwitterion in its aqueous solution. 

Isoelectric point 

After understanding all three forms of amino acids in their aqueous solution. We are in the condition of understanding the isoelectric point of zwitterions. Each and every zwitterion has an isoelectric point. The isoelectric point is the pH at which a molecule (a polyatomic ion) carries no net electric charge. It is represented as pI, pH(I) or IEP although standard nomenclature to represent the isoelectric point is pH(I). pH of the surrounding environment of the molecule affects its net electrical charge. In an acidic medium it can become more positively charged by accepting protons. While in the basic medium it can become more negatively charged by donating protons. The pI value of zwitterion can affect the solubility of the molecule. The molecule will show less or even minimum solubility in water or salt solutions at the pH that corresponds to their pI. Generally, molecules form precipitate in the solution at their pI. 

Thus, the pH at which zwitterion of a molecule is formed is called isoelectric point of that molecule. Isoelectric point affects various properties of the molecule such as solubility in a solution. 

Calculating pI values 

pI values are calculated by taking the average of the pKas that involve the zwitterion. pKa is a negative log of Ka and Ka is acid dissociation constant. pI value of zwitterion can be represented by the following formula –

\[{\text{pl}} = \frac{{p{K_{a1}} + p{K_{a2}}}}{2}\]

Let's understand it by taking an example of alanine. For alanine value of pK1 is 2.34 and pK2 is 9.64 then what will be the value of pI for alanine? 

As we know,    \[{\text{pl}} = \frac{{p{K_{a1}} + p{K_{a2}}}}{2}\]

On keeping the values given in the questions = \[\frac{{2.34 + 9.64}}{2}\]

pI = 5.99

It means in an electrolytic solution at pH 5.99 alanine will neither migrate to cathode nor to anode. This question came in NEET 2016. 

Applications of Zwitterion 

Few applications of zwitterions are listed below –

  • SDS PAGE is a common practical procedure of molecular biology. It is used to separate protein molecules by molecular mass. Glycine is used in this method and zwitterionic properties of glycine play a vital role for separating proteins using this method. 

  • Zwitterionic polymers possess antifouling properties and prevent microbial adhesion and biofilm formation.

  • These are useful in the medical field, drug delivery and medical implants etc. 

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