Question

Select the correct options:
(A) Isoelectric point is the pH at which an amino acid exists primarily in its neutral form.
(B) Isoelectric point is the average of ${\text{p}}{{\text{K}}_{\text{a}}}$ value of ${\alpha - COOH}$ amino ${\alpha - NH}_{\text{3}}^{\text{ + }}$ groups [valid only for neutral amino acid].
(C) Glycine is characterised by two ${\text{p}}{{\text{K}}_{\text{a}}}$ values.
(D) For neutral amino acids, the concentration of zwitterion is maximum at its isoelectric point.

Answer 1

To solve this we must know that the isoelectric point is the intermediate pH at which the amino acid shows no tendency to migrate towards any electrode when placed in an electric field. Also, we must now consider the composition of the amino acid.

Complete step-by-step solution: We know that the isoelectric point is the intermediate pH at which the amino acid shows no tendency to migrate towards any electrode when placed in an electric field.
Thus, at an isoelectric point, the net electric charge on the amino acid is zero.
Thus, the statement ‘isoelectric point is the pH at which an amino acid exists primarily in its neutral form’ is correct.

We know that amino acids are organic compounds that combine to form proteins. The amino acids are made up of basic amino $\left( { - {\text{N}}{{\text{H}}_2}} \right)$ group and an acidic carboxylic $\left( { - {\text{COOH}}} \right)$ group. One end of the amino acid is basic in nature while the other end is acidic in nature.
Thus, the statement ‘isoelectric point is the average of ${\text{p}}{{\text{K}}_{\text{a}}}$ value of ${\alpha - COOH}$ amino ${\alpha - NH}_{\text{3}}^{\text{ + }}$ groups [valid only for neutral amino acid]’ is correct.
Glycine is an amino acid. We know that amino acids are made up of basic amino $\left( { - {\text{N}}{{\text{H}}_2}} \right)$ group and an acidic carboxylic $\left( { - {\text{COOH}}} \right)$ group. Thus, glycine has two ${\text{p}}{{\text{K}}_{\text{a}}}$ values of amine and carboxylic groups.
Thus, the statement ‘glycine is characterised by two ${\text{p}}{{\text{K}}_{\text{a}}}$ values’ is correct.

The zwitterion is an ion or molecule that contains an equal number of positively and negatively charged functional groups. At isoelectric point, the concentration of zwitterions is maximum.
Thus, the statement ‘for neutral amino acid, the concentration of zwitterion is maximum at its isoelectric point’ is correct.
Thus,all the statements are correct.
Thus, the correct options are (A), (B), (C) and (D).

Note: We know that amino acid contains both an acidic and a basic group. The amino $\left( { - {\text{N}}{{\text{H}}_2}} \right)$ group is the basic group and carboxylic $\left( { - {\text{COOH}}} \right)$ group is an acidic group. Thus, amino acids form zwitterions.