Chapter 24, Problem 1PP

Interpretation Introduction

Interpretation:

The predominant species of glutamic acid in strongly acidic and basic solutions and at its isoelectric point is to be predicted, and the higher isoelectric point of glutamine than that of glutamic acid is to be explained.

Concept introduction:

Amino acids contain both acidic ($\text{COOH}$) and basic (${\text{NH}}_2$) group.

In solid state, it exists as dipolar or zwitter ionic state, where carboxylic acid group exists as carboxylate ion and amino group as aminium ion.

In aqueous state, equilibrium exists between cationic and anionic form. The predominant form depends on the pH and the nature of amino acid.

Answer to Problem 1PP

Solution:

a)

b)

c)

d)

Glutamic acid is a dicarboxylic acid, whereas glutamine with only one carboxyl group has a resemblance with glycine or phenylalanine. Moreover, its isoelectric point exists at a higher $\text{pH}$.

Explanation of Solution

a) Strongly acidic solution

In strongly acidic solution, all amino acids are present as cations. Here, amine group is protonated and bears a formal positive charge and carboxylic acid group is neutral (has no formal charge).

b) Strongly basic solution

In strongly basic solution, all amino acids are present as anions. The addition of base causes removal of proton from both carboxylic acid groups, resulting in a dianion. The amine group is electrically neutral.

c) At the isoelectric point (pl=3.2)

Each amino acid has a particular intermediate pH at which the concentration of dipolar ion will be maximum, and the concentration of cations and anions will be equal. This point is called isoelectric point and will be different for each amino acid.

d) The isoelectric point of glutamic is consider higher than that of glutamic acid

The structure of glutamine and glutamic acid is as follows:

Glutamine contains only one carboxylic acid group, whereas glutamic acid has two. At isoelectric point, dipolar state exists that is there will be a carboxylate anion and an ammonium group. In order to maintain the dipolar form in glutamic acid, only one carboxylate ion is required. To suppress the ionization of second carboxylic acid, more acid is required than that for glutamine. That is pH of glutamic acid is maintained lower than that of glutamine to achieve isoelectric point. (Isoelectric point is the average of ${\text{pK}}_{\text{a}}$ of dipolar form and ${\text{pK}}_{\text{a}}$ of cationic form.)

Hence, isoelectronic point of glutamine is higher than that of glutamic acid.