Question

In an electric field, if an amino acid migrates towards cathode, the pH of solution is said to be –
A.Less than pI
B.More than pI
C.Equal to pI
D.7

Answer 1

Hint:
Amino acid are basic building blocks of proteins. They can act as both, a negative charge as well as a positive. This is because it has an amino group as well as a carboxylic acid group.

Complete step by step answer:
All amino acids have one amino group and one carboxylic acid group which is bonded to the same carbon atom (known as alpha carbon). The basic structure of amino acid can be represented as –

The carboxylic acid group is responsible for acidity, whereas the amino group is responsible for basicity. Since amino acids can act as both – an anion as well as a cation, it is known as a dipolar or zwitter ion.
So, when amino acids are placed in an electric field, they may move towards cathode or anode depending on the pH of the solution. In an acidic solution, amino acids have a tendency to exist as cation. So, amino acids move towards cathode (negative electrode) under the influence of an electric field.

Isoelectric point is the pH at which amino acid has no net charge. In order to behave as a cation, the solution needs to be acidic. Therefore, the pH of the solution should be less than isoelectric pH.
Therefore, the answer is – option (a)

Additional Information: The isoelectric point varies for different amino acids.

Note:
Remember that cations move towards cathode and anion move towards anode. In alkaline or basic medium, amino acids exist as anions. Therefore, under the influence of an electric field, they migrate towards anode.