Question

The correct statement about isoelectric point is:

• A. If the pH is greater than the isoelectric point then the amino acid will move towards the cathode
• B. At this point, the amino acid is present in the form of zwitter ion
• C. If the pH is lower than the isoelectric point, then the amino acid will move towards the anode
• D. At this point, the amino acids are basic in nature

Answer 1

The correct option is B At this point, the amino acid is present in the form of zwitter ion

The isoelectric point of an amino acid is the pH at which it has a neutral charge.
a) When the pH is greater than the isoelectric point, the solution is basic in nature. So the amine group present cannot abstract a proton but the proton from the carboxylic acid part will be abstracted by the hydroxyl ions present in the solution. Thus the amino acid possesses a net negative charge and so the amino acid will move towards the anode.
b) At the isoelectric point, the amino acid is present in the zwitter ion form.
c) If the pH is lower than the isoelectric point, the solution contains more number of $H^{+}$ ions. So the amine group will accept a proton and acquire a positive charge. Thus the amino acid will move towards the cathode.
d) At this point, amino acids are neutral.