Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{pK}_{a}\) of the carboxyl group in valine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}\) \(\mathrm{CH} \mathrm{NH}_{2} \mathrm{COOH}\) is \(2.31\) and \(\mathrm{pK}_{\mathrm{a}}\) for the amino group of the acid is \(9.69 .\) The isoelectric point of the valine is

Step-by-Step Solution

Answer

The isoelectric point of valine is 6.00.

Step 1: Understanding the Isoelectric Point

The isoelectric point (pI) of an amino acid is the pH at which it carries no net electric charge. For amino acids with only two ionizable groups (a carboxyl group and an amino group), the pI is the average of the two pKa values.

Step 2: Identifying Ionizable Groups

Valine has two ionizable groups: a carboxyl group with a pKa of 2.31, and an amino group with a pKa of 9.69.

Step 3: Calculating the Isoelectric Point

To find the isoelectric point, calculate the average of the pKa values of the ionizable groups. Use the formula: \[ pI = \frac{{pK_{a1} + pK_{a2}}}{2} \] Substitute the given pKa values: \[ pI = \frac{{2.31 + 9.69}}{2} \]

Step 4: Performing the Calculation

Calculate the average: \[ pI = \frac{{2.31 + 9.69}}{2} = \frac{12.00}{2} = 6.00 \]

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins and play a crucial role in many biological processes. They are organic compounds composed of an amino group \(\text{(NH}_2\text{)}\) and a carboxyl group \(\text{(COOH)}\). Each amino acid also contains a unique side chain, which determines its specific properties. Collectively, amino acids help form proteins that are essential for cellular functions and the overall structure of organisms.
The unique structure of amino acids allows them to participate in a variety of chemical reactions. This property is essential for the diverse functions of proteins in the body. In particular, the charges of the amino and carboxyl groups can change with pH, impacting the amino acid's behavior and interaction with other molecules. This attribute is key when discussing concepts like the isoelectric point.

pKa Values

The pKa value of a substance is a measure of its acidity. Specifically, it tells us how easily an ionizable group can lose a proton. It is a crucial parameter in understanding the behavior of amino acids in different environments. The lower the pKa value, the more acidic the group, and the more likely it is to donate a proton.
Each amino acid has specific pKa values for its ionizable groups. This information is vital when calculating features like the isoelectric point, which gives insight into the amino acid's charge at different pH levels. Knowing the pKa values helps predict how the amino acid will react in various biochemical conditions.

• Pka values help in calculating the isoelectric point (pI) of amino acids.
• The pKa of the carboxyl group is usually lower than that of the amino group.

For valine, the pKa values are 2.31 for the carboxyl group and 9.69 for the amino group.

Ionizable Groups

Ionizable groups are parts of a molecule that can lose or gain protons. This ability makes them key players in a molecule's chemistry, including amino acids. In amino acids, the primary ionizable groups are the amino and carboxyl groups. These can accept or release protons, depending on the pH of the environment.
Understanding ionizable groups in amino acids is crucial when calculating the isoelectric point (pI). The pI occurs when an amino acid has no net charge, a point influenced significantly by its ionizable groups. In case of valine, which is an amino acid with two primary ionizable groups, understanding these groups helps in accurate pI determination.

• The carboxyl group \(\text{(COOH)}\) becomes negatively charged when it loses a proton (\(\text{COO}^-\)).
• The amino group \(\text{(NH}_2\text{)}\) becomes positively charged when it gains a proton (\(\text{NH}_3^+\)).

These shifts in charge are crucial in biochemical processes, protein structure, and function.