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) \(7.58\)

Short answer

The isoelectric point of the amino acid is 5.97.

Step-by-step solution

Understand the Isoelectric Point Concept

The isoelectric point (pI) of an amino acid is the pH at which the amino acid has no net electric charge. For an amino acid with one carboxylic group and one ammonium group, the pI can be estimated by averaging the pKa values of these two groups.

Identify the pKa Values

In the given problem, we are provided with the pKa values for ionization of the carboxylic group (\(\mathrm{pKa}_{1}=2.34\)) and the ammonium ion (\(\mathrm{pKa}_{2}=9.60\)).

Calculate the Isoelectric Point

The formula to find the isoelectric point for an amino acid with these two groups is given by\[pI = \frac{\mathrm{pKa}_{1} + \mathrm{pKa}_{2}}{2}\]Substitute the provided pKa values into the formula: \[pI = \frac{2.34 + 9.60}{2} = \frac{11.94}{2} = 5.97\]

Choose the Correct Isoelectric Point

From the calculated value in the previous step, the isoelectric point is 5.97. Therefore, the correct answer is (b) 5.97.

Key concepts

Amino Acid Chemistry

Amino acids are the building blocks of proteins, and they play crucial roles in numerous biological processes. Each amino acid is composed of a central carbon atom bonded to a hydrogen atom, a carboxylic group, an amino group, and a distinct side chain, or R-group, which determines its properties.

In aqueous solutions, amino acids can act as both acids and bases, making them amphoteric. This dual nature comes from the presence of the carboxylic group (-COOH) that can donate a proton (acting as an acid), and the amino group (-NH₂) that can accept a proton (acting as a base).

This unique property allows amino acids to exist in various forms, such as the zwitterionic form, where the molecule carries both a positive and a negative charge but has no net overall charge.

pKa Values

The term 'pKa' refers to the negative logarithm of the acid dissociation constant (Ka) of a molecule. It provides insight into how easily an acidic group on a molecule donates its proton to the surrounding solution. In the context of amino acids, pKa values are critical for understanding the ionization of both the carboxylic and ammonium groups.

For each amino acid, there are typical pKa values associated with these functional groups:

• The pKa of the carboxylic group, usually around 2-3, indicates the acidic strength of this group.
• The pKa of the ammonium group, generally around 9-10, reflects how readily the group accepts a proton, making it a weak base.

By understanding the pKa values, one can predict the behavior of amino acids at different pH levels, including ionization states and charge distributions.

Ionization of Carboxylic Group

The ionization of the carboxylic group in amino acids is a fundamental aspect of their chemistry. At lower pH levels, the carboxyl group (-COOH) remains protonated. As the pH increases and surpasses the group's pKa (usually around 2-3), the carboxylic group loses a proton to form a negatively charged carboxylate ion (-COO^-).

This change in charge is significant because it affects how amino acids interact with the environment, influencing properties like solubility and reactivity.

• Before ionization, the carboxylic group is neutral, while after ionization, it's negatively charged.
• The ionized carboxylate plays a key role in biochemical interactions and reactions, such as enzyme catalysis and substrate binding.

Ionization of Ammonium Group

The ammonium group, more formally the amino group, undergoes ionization differently compared to the carboxylic group. In acidic solutions, the amino group is protonated to form an ammonium ion (-NH₃⁺). As the pH rises and exceeds the pKa of the ammonium group (usually around 9-10), the group loses its extra proton, reverting to its neutral form (-NH₂).

The ionization state of the ammonium group influences the overall charge and behavior of the amino acid:

• In its protonated form, the ammonium group is positively charged.
• Once deprotonated, it becomes neutral, contributing to the zwitterionic form when combined with a deprotonated carboxylic group.

Understanding this ionization process helps in predicting how amino acids function in different pH environments, which is critical for protein structure and function.