Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

If in an amino acid, the carboxylic group ionizes at \(\mathrm{pKa}_{1}=2.34\) and the ammonium ion at \(\mathrm{pKa}_{2}=9.60 .\) The isoelectric point of the amino acid is (a) \(2.36\) (b) \(5.97\) (c) \(6.86\) (d) \(7.58\)

Step-by-Step Solution

Answer

The isoelectric point (pI) of the amino acid is 5.97.

Step 1: Understanding the Problem

The exercise requires finding the isoelectric point (pI) of an amino acid, given the pKa values of its carboxylic group and ammonium ion. The isoelectric point is where the amino acid has no net electric charge.

Step 2: Formula for Isoelectric Point

For amino acids with two ionizable groups, the isoelectric point is calculated as the average of the pKa values of those groups. The formula is: \( pI = \frac{\mathrm{pKa}_1 + \mathrm{pKa}_2}{2} \) where \( \mathrm{pKa}_1 \) is for the carboxylic group and \( \mathrm{pKa}_2 \) is for the ammonium ion.

Step 3: Substitute the Values

Substitute the given values into the formula: \( \mathrm{pKa}_1 = 2.34 \) and \( \mathrm{pKa}_2 = 9.60 \).

Key Concepts

pKa values in amino acids

In the world of amino acids, understanding the concept of pKa values is crucial. The term "pKa" stands for "acid dissociation constant," which measures how easily a hydrogen ion (H+) can be removed from a molecule. For amino acids, these values typically pertain to the ionizable groups: the carboxylic acid group and the amino group.

For instance, if an amino acid's carboxyl group has a pKa value of 2.34, it suggests that at a pH of 2.34, the carboxyl group is half-ionized, making it equally balanced between its protonated and deprotonated forms. This balance is crucial for understanding the behavior of amino acids in different environments.

Knowing these pKa values allows us to predict how an amino acid will react or bond with other molecules in various pH environments. The pKa value corresponds to the pH at which half of a substance is ionized.

Ionizable groups

Amino acids are fascinating molecules with specific features, notably their ionizable groups. These groups, primarily the carboxyl group and the amino group, can hold or release hydrogen ions (H+), thus influencing the net charge of the amino acid.

🌟 **Ionizable Groups to Note:**

• **Carboxylic Group**: Typically releases H+ in aqueous solutions, resulting in a negatively charged COO^- group. It often has a lower pKa value, indicating that it ionizes first.
• **Amino Group**: Accepts H+ in solutions, forming a positively charged
  4 NH3^+ group, commonly with a higher pKa value, causing it to ionize later compared to the carboxylic group.

These ionizable groups are essential for the chemical characteristics of amino acids and are key in processes such as protein folding, enzyme activity, and molecular binding.

Understanding which groups ionize at particular pH levels helps us predict the behavior of amino acids in diverse biochemical pathways.

Isoelectric point calculation

The isoelectric point (pI) is a special pH level where an amino acid carries no net electrical charge. Calculating this point involves the ionizable groups, as these groups can gain or lose protons based on the surrounding pH.

Calculating the isoelectric point is straightforward for amino acids with two ionizable groups. You can find it by taking the average of their pKa values. For example, if you have a carboxyl group with a pKa of 2.34 and an aminogroup with a pKa of 9.60, the formula is:

\[pI = \frac{\mathrm{pKa}_1 + \mathrm{pKa}_2}{2} = \frac{2.34 + 9.60}{2} = 5.97\]
This average represents the pH at which the positive and negative charges on the amino acid balance out to zero. This state is especially relevant for amino acids in solutions, as it influences protein behavior in biological systems.

Knowing how to calculate the isoelectric point is essential for biologists, as it aids in understanding protein solubility and stability.