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

Question

The \(\mathrm{pK}_{\text {a }}\) 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 Isoelectric Point

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. For amino acids, if there are two ionizable groups, the pI is calculated as the average of the pKa values of the amino and carboxyl groups.

Step 2: Identify pKa Values

Valine has two pKa values: the pKa of the carboxyl group is 2.31, and the pKa for the amino group is 9.69.

Step 3: Calculate Isoelectric Point

For valine, the isoelectric point (pI) is the average of its pKa values. Calculate the pI using the formula:\[ ext{pI} = \frac{\text{pK}_{\text{a, carboxyl}} + \text{pK}_{\text{a, amino}}}{2} = \frac{2.31 + 9.69}{2} = \frac{12.00}{2} = 6.00.\]

Step 4: Conclusion

The isoelectric point of valine is 6.00. This is the pH where valine has no net charge.

Key Concepts

Understanding pKa Values

pKa is a key concept when studying the chemistry of amino acids like valine. It helps describe the acidity of a particular hydrogen atom in a molecule. Essentially, the pKa value tells you how easily a hydrogen ion can be removed from the molecule. It's the pH at which half of the molecules of a certain type are charged, and half are uncharged.
This value is crucial when considering molecules like amino acids, which can donate or accept protons at different pH levels. A lower pKa value indicates a stronger acid, meaning it can release its hydrogen ion more easily. Conversely, a higher pKa means the substance is a weaker acid. Knowing the pKa values of an acid's functional groups is essential to determine its behavior in different environments.

Exploring Amino Acids

Amino acids, the building blocks of proteins, are compounds containing both an amino group (-NH₂) and a carboxyl group (-COOH). They form proteins through peptide bonds, critical for numerous biological functions such as enzyme reactions and transport processes. Each amino acid has a unique side chain or "R" group that determines its properties and role within a protein.
Besides serving as parts in proteins, amino acids also have various roles on their own:

• Serve as precursors to neurotransmitters.
• Participate in biosynthesis of hormones.
• Act as intermediates in metabolic pathways.

Amino acids are categorized by their ability to be synthesized by organisms (essential and non-essential), polarity, charge, and structure, making their study diverse and integral to understanding biological processes.

Focusing on Valine

Valine is one of the twenty standard amino acids and is classified as a non-polar, aliphatic amino acid. The structure of valine includes a distinct branched side chain, \((\text{CH}_3)_2\text{CH} \text{CH}- \), giving it a bulky hydrophobic character. This nature often influences protein structure, as valine will preferentially locate within protein interiors to avoid water.
In the human body, valine serves critical roles including:

• Acting as a key player in muscle growth and tissue repair.
• Participating in energy production during exercise.
• Aiding in nitrogen balance in the body.

Essential for human health, valine must be obtained through diet, as the body cannot synthesize it.