CBSE Class 12 Chemistry Notes: Biomolecules - Isoelectric Point

(i) When ionized form of amino acid is placed in an electric field it will migrate towards the opposite electrode.

(ii) Depending upon the pH of the medium following three thing may happen.

(a) In acidic medium, the cation move towards cathode.

(b) In basic medium, the anion move towards anode.

(iii) At a certain pH (i.e. H⁺ concentration), the amino acid molecules show no tendency to migrate towards any of the electrodes and exists as a neutral dipolar ion, when placed in electric field is known as isoelectric point.

(iv) All amino acids do not have the same isoelectric point & it depends upon the nature of R – linked to α- carbon atom.

Illustration 1. Pick out incorrect statement

(A) In an electrolysis experiment, α – amino acids do not towards any of the electrode at the isoelectric point.

(B) p-aminobenzenesulphonic acid is a dipolar ion; while p-aminobenzoic acid does not

(C) Sulphanilic acid is soluble in base, but not in acid

(D) H₃N⁺CH₂COOH (pka = 2.4) is more acidic than RCH₂COOH (pK_a = 4 – 5)

Solution: (A) The pH at which |Anion| = |Cation| is called isoelectric point. At isolectric point, a – amino acids to not migrate when field is applied.

(B) -SO₃H is strongly acidic and donates H⁺ to weakly basic arylamino group. ArCOOH is not acidic enough to transfer H⁺ to the arylamino group.

(C) In p- H₃N⁺C₆H₄SO₃^–, H₃N⁺ is acidic enough to transfer H⁺ to bases to give the soluble anion p – H₂N – C₆H₄SO₃^– – SO₃ is too feebly basic and cannot accept H⁺ from acids.

(D) α – H₂N⁺ group increases acidity, because of its electron – withdrawing inductive effect.

Hence (D) is correct.

Isoelectric point of some amino acid

Amino acid	Isoelectric point
Neutral amino acids	(pH 5.5 to 6.3)
Glycine	5.7
Alanine	6.1
Valine	6.0
Serine	5.7
Threonine	5.6
Acidic amino acids	(pH ≈ 3)
Aspartic acid	2.8
Glutamic acid	3.2
Basic amino acids	(pH ≈ 10)
Lysine	9.7
Arginine	10.8