CBSE Class 12 Chemistry Notes: Biomolecules – Isoelectric Point

What happens when a solution of an amino acid is placed in an electric field depends upon the acidity or basicity of solution. In quite alkaline solution.

Anions (II) exceed cations (III), and there is a net migration of amino acid toward the anode. In quite acidic solution cations (III) are in excess, and there is a net migration of amino acid towards the cathode. If (II) and (III) are exactly balanced, there is no net migration; under such conditions any one molecule exists as a positive ion and as a negative ion for exactly the same amount of time and any small movement in the direction of one electrode is subsequently cancelled by an equal movement back towards the other electrode. The hydrogen ion concentration of the solution in which a particular amino acid does not migrate under the influence of an electric field is called the isoelectric point of that amino acid.

An amino acid shows its lowest solubility in a solution at the isoelectric point, since here there is the highest concentration of the dipolar ion. As the solution is made more alkaline or more acidic, the concentration of one of the more soluble ions, II or III increases.

If an amino acid has amino group and one carboxyl group, it has two pK values. The isoelectric point (PI) of this amino acid has the average value of the both pK values.

We take example of glycine.

H₃⁺N – CH₂ – COOH   H₃N⁺ – CH₂ ¾ COO^– + H⁺                                           …(1)

Conjugated acid (CA)                          Dipolar Ion (DI)

At equilibrium        

H₃N⁺ – CH₂ – COO^–   H₂N – CH₂ – COO^– + H⁺                 …(2)

DI  Conjugated Base (CB)

At equilibrium

[CA] =

[CB] =

At isoelectric point [CA] = [CB]

Where  = conc. of [ H⁺] at isoelectric point.

or, = K₁ K₂

or, 2log [H_i⁺] = log K₁ + log K₂

or –2 log (H_i⁺] = – log k₁ – logK₂

or 2pH_i = pK₁ + pK₂

or pH_i =

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