Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The substituents \(\mathrm{R}_{1}\) and \(\mathrm{R}_{2}\) for nine peptides are listed in the table given below. How many of these peptides are positively charged at \(\mathrm{pH}\) \(=7.0 ?\) $$ \begin{array}{|c|c|c|} \hline \text { Peptide } & \mathbf{R}_{\mathbf{1}} & \mathbf{R}_{\mathbf{2}} \\\ \hline \text { I } & \mathrm{H} & \mathrm{H} \\ \hline \text { II } & \mathrm{H} & \mathrm{CH}_{3} \\ \hline \text { III } & \mathrm{CII}_{2} \mathrm{COOII} & \text { II } \\ \hline \text { IV } & \mathrm{CH}_{2} \mathrm{CONH}_{2} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text { V } & \mathrm{CH}_{2} \mathrm{CONH}_{2} & \mathrm{CH}_{2} \mathrm{CONH}_{2} \\ \hline \text { VI } & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text { VII } & \mathrm{CH}_{2} \mathrm{COOH} & \mathrm{CH}_{2} \mathrm{CONH}_{2} \\ \hline \text { VIII } & \mathrm{CH}_{2} \mathrm{OH} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text { IX } & \left(\mathrm{CII}_{2}\right)_{4} \mathrm{NII}_{2} & \mathrm{CII}_{3} \\ \hline \end{array} $$

Step-by-Step Solution

Answer

Three peptides (IV, VI, VIII) are positively charged at pH 7.

Step 1: Identify Peptides with Basic Groups

At physiological pH (7.0), a peptide is positively charged if it contains basic groups that can be protonated. Common basic groups in peptides are amine groups. We will identify which peptides have substituents that include amine groups, particularly "NH₂" groups.

Step 2: Analyze Each Peptide

1. **Peptide I:** Neither R1 nor R2 is basic (H, H). 2. **Peptide II:** Neither R1 nor R2 is basic (H, CH₃). 3. **Peptide III:** R1 is H₂COOH - not basic. R2 is II - not a specific substituent. 4. **Peptide IV:** Neither R1 (CH₂CONH₂) nor R2 ((CH₂)₄NH₂) is entirely non-basic; R2 has an amine group. 5. **Peptide V:** R1 and R2 both have CH₂CONH₂, which are not basic. 6. **Peptide VI:** Both R1 and R2 have (CH₂)₄NH₂, which are basic. 7. **Peptide VII:** R1 (CH₂COOH) is acidic, while R2 (CH₂CONH₂) is not basic. 8. **Peptide VIII:** R1 (CH₂OH) and R2 ((CH₂)₄NH₂) have an amine group at R2. 9. **Peptide IX:** Both R1 and R2 do not contain typical basic substituents.

Step 3: Count the Positively Charged Peptides

From the analysis: Peptide VI and Peptide VIII have basic amine groups at R1 or R2, making them positively charged at pH 7. Peptide IV also contains a basic group due to R2 ((CH₂)₄NH₂), contributing to a positive charge at neutral pH. So, Peptides IV, VI, and VIII are positively charged.

Key Concepts

Basic Amino Acid Groups

Amino acids are the building blocks of peptides and proteins, and they play a crucial role in determining the behavior of these larger molecules in different environments. Among the structural components of amino acids, certain groups exhibit basic properties. In particular, amine groups are known to be basic.

Basic groups in amino acids are those that can accept a hydrogen ion (H+) and tend to have a positive charge when protonated. The most common basic groups include:

• Primary amine groups, which have the general structure "-NH₂".
• Side chains like those found in lysine, arginine, and histidine, known for their ability to gain a positive charge.

Understanding which parts of a peptide contain basic groups is essential for predicting how the peptide will behave in different pH environments.

pH and Ionization

The concept of pH is a measure of the hydrogen ion concentration in a solution. It indicates the acidity or basicity of a solution, with lower pH values indicating more acidic conditions and higher pH being more basic.

Ionization refers to the gain or loss of proton (H⁺) in a molecule. This process is pH-dependent, and it influences the charge state of molecules such as peptides. At lower pH levels, more H⁺ ions are available in the solution, often leading to protonation of molecules. Conversely, at higher pH levels, deprotonation may occur, leading to molecules losing H⁺ ions.

Therefore, understanding pH and ionization is crucial for predicting how a peptide will carry a charge at physiological pH, around the neutral level of 7.

Protonation of Amine Groups

Protonation occurs when an amine group accepts a proton (H⁺) and becomes positively charged. This is a key consideration for amino acids in biological systems. At neutral or physiological pH, some amine groups in amino acids can be protonated, impacting their overall charge.

The degree to which amine groups are protonated depends on the pKa values of these groups, which is a measure of the acidity of the protonated form. The pKa tells us the pH at which half of the amine groups are protonated. Amines typically have pKa values around 9-10, suggesting that at physiological pH (7), a significant portion may remain protonated, carrying a positive charge.

This positive charge is why peptides, like those in the exercise, containing amine groups are positively charged at physiological pH.

Physiological pH Conditions

Physiological pH is usually around 7.4, which can slightly vary depending on specific biological contexts like different cellular compartments or bodily fluids. However, when speaking of physiological conditions, pH 7 is often used as a baseline for exercises and calculations.

At physiological pH, the body maintains a balance that allows enzymes to function optimally and biochemical reactions to proceed at appropriate rates. It's a finely tuned environment where the state of ionization of biomolecules, such as peptides, is particularly important.

In terms of amino acids and peptides, some specific groups like the basic "-NH₂" are impacted by the neutral pH level. In the body, proteins and peptides often carry charges because certain groups may gain or lose protons depending on their surroundings' pH. Understanding these conditions is vital for appreciating how peptides acquire positive charges in the given exercise, especially when particular structures, like those in peptides VI, VIII, and IV, contain amine groups.