Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

Which statements are correct about peptide bond? (1) - CONH-group is planar (2) \(\mathrm{C}-\mathrm{N}\) bond length in protein is longer than usual bond length of \(\mathrm{C}-\mathrm{N}\) bond (3) C-N bond length in protein is smaller than usual bond length of \(\mathrm{C}-\mathrm{N}\) bond (a) 2 and 3 (b) 1 and 2 (c) 2 only (d) 1 and 3

Step-by-Step Solution

Answer

The correct option is (d) 1 and 3.

Step 1: Understand the Peptide Bond

The peptide bond is the chemical bond that links amino acids together in a protein. It is formed between the carboxyl group (COOH) of one amino acid and the amino group (NH2) of another, releasing a molecule of water. The resulting bond is a -C(O)-N- linkage, often represented as the -CONH- group.

Step 2: Evaluate Statement 1

Statement 1 involves the planarity of the -CONH- group in the peptide bond. Due to resonance between the C=O data and C-N data bonds, the peptide bond adopts a partial double-bond character, which restricts rotation and results in a planar structure. Therefore, Statement 1 is correct.

Step 3: Investigate Statement 2

Statement 2 suggests that the C-N data bond length in proteins is longer than a typical C-N data single bond. Due to the partial double-bond character mentioned earlier, the actual C-N data bond length in peptide bonds is shorter than a regular single C-N data bond. Hence, Statement 2 is incorrect.

Step 4: Assess Statement 3

Statement 3 suggests that the C-N data bond length in proteins is smaller than the usual bond length of a C-N data bond. This is true because the peptide bond exhibits partial double-bond character, which shortens the C-N data bond length compared to a typical single bond. Therefore, Statement 3 is correct.

Step 5: Identify Correct Options

Based on the correct evaluations, only Statement 1 and Statement 3 are correct. This means the correct answer must include these two statements. From the options provided, option (d) '1 and 3' is the correct choice.

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins. They are small organic compounds that contain both an amino group \(\text{(-NH}_2)\) and a carboxyl group \(\text{(-COOH)}\). In nature, there are 20 standard amino acids, each differing by their side chain, also known as the \'R\' group. This side chain determines the characteristics and role of each amino acid in a protein structure.

Amino acids link together via peptide bonds to form peptides and proteins, with the sequence of amino acids in a protein being critical for its function. These chains fold into specific shapes, which are essential for their biological activity. The properties of the amino acids, such as polarity and pH, significantly influence this folding process.

Protein Structure

Proteins have complex structures divided into four levels: primary, secondary, tertiary, and quaternary.

• **Primary Structure**: This is the sequence of amino acids in a single polypeptide chain.
• **Secondary Structure**: The folding or coiling of the chain into structures like alpha-helices and beta-sheets, stabilized by hydrogen bonds.
• **Tertiary Structure**: This is the overall three-dimensional structure of a single polypeptide, formed by various interactions like hydrogen bonds, ionic bonds, and van der Waals forces.
• **Quaternary Structure**: Involves more than one polypeptide chain, forming a multi-subunit protein like hemoglobin.

Protein structure determines its function, and even slight changes can significantly impact how well a protein performs its role.

Bond Length

Bond length refers to the average distance between the nuclei of two bonded atoms. In the case of the peptide bond, the \(\text{C-N}\) bond length is shorter than a typical carbon-nitrogen single bond. This is because the peptide bond has partial double-bond character due to resonance.

Typical \(\text{C-N}\) single bond lengths are approximately 1.47 Å, whereas, in a peptide bond, the \(\text{C-N}\) bond length is about 1.32 Å. The shorter bond length in peptides is an essential characteristic because it gives stability to protein backbones and affects the protein structure's rigidity and flexibility.

Resonance

Resonance is a concept used to describe delocalization of electrons in molecules where more than one valid electron structure can represent the molecule. In the peptide bond, resonance occurs between the carbonyl \(\text{C=O}\) group and the nitrogen atom of the amide bond. This results in the \(\text{C-N}\) bond displaying partial double-bond character.

This partial double-bond character significantly impacts the behavior and properties of the peptide bond:

• **Rigidity**: Reduces rotational freedom around the \(\text{C-N}\) bond.
• **Planarity**: Ensures the atoms involved are co-planar, allowing for a stable and specific arrangement of atoms.
• **Stability**: Provides additional stability to the backbone of proteins.

Resonance is crucial for understanding the unique properties that make peptide bonds important in protein structure and function.

Planarity in Chemistry

Planarity in chemistry refers to the geometric arrangement of atoms in a molecule, where all involved atoms lie in the same plane. The peptide bond exemplifies this planarity. Due to the resonance and partial double-bond character of the \(\text{C-N}\) bond, peptide bonds are typically planar.

This planarity is essential because:

• It stabilizes the protein’s overall three-dimensional structure.
• It influences the way proteins interact with other molecules.
• It affects the way proteins fold and function in biological systems.

Therefore, understanding planarity helps in grasping how proteins maintain their structures and how modifications can affect their biological roles.