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) \(\mathrm{C}-\mathrm{N}\) 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

Option (d) 1 and 3 are correct.

Step 1: Understanding Peptide Bonds

In a peptide bond, the -CONH- group essentially links amino acids together within a protein. The peptide bond is known for having a planar structure due to resonance, which allows electrons to be shared between the carbonyl carbon and the nitrogen, creating partial double bond character.

Step 2: Evaluating Statement 1

Statement 1 claims that the - CONH- group is planar. This is accurate because the resonance in peptide bonds leads to a planar conformation, which restricts rotation around the bond and maintains a flat shape.

Step 3: Evaluating Statement 2

Statement 2 suggests that the C−N bond length in proteins is longer than the usual bond length of a C−N bond. The C-N bond in peptide bonds has partial double-bond character due to resonance, leading to a shorter bond length compared to a typical single C−N bond.

Step 4: Evaluating Statement 3

Statement 3 claims that the C−N bond length in proteins is smaller than the usual bond length of a C−N bond. Given the partial double-bond character from resonance in peptide bonds, this statement is true as it results in a shorter bond length.

Step 5: Identifying the Correct Answer

From the evaluations, statement 1 is true and statement 3 is true. Therefore, option (d) 1 and 3 is the correct answer.

Key Concepts

Resonance

Resonance is a fundamental concept in chemistry that describes the delocalization of electrons within certain molecules or polymers. In the case of peptide bonds, resonance is the key reason behind their strong and stable form. The peptide bond occurs between amino acids, where a carbon atom of one amino acid links with a nitrogen atom of another, forming the

• -CONH- group.

The shared electrons between carbon and nitrogen can be visualized through resonance structures. These structures illustrate how electrons can shift, creating a "blend" of possible electron locations. This contributes to a balance that enhances bond stability.
Importantly, resonance in peptide bonds explains why these bonds don't behave like typical single or double bonds. The electrons are not confined to a single location; they distribute themselves over the carbonyl and amide nitrogen atoms.
Because of this electron spread, peptide bonds exhibit unique behavior, including restricted rotation and a tendency to form planar structures. Understanding resonance helps us grasp why peptide bonds are such crucial building blocks in proteins, affecting their structure and function.

Partial Double Bond Character

In peptide bonds, the concept of partial double bond character is quite significant. It arises due to the resonance mentioned earlier. When electrons shift and share between the carbonyl carbon and the nitrogen, the traditional single bond becomes more like a double bond. This shift in electron density leads to a few essential outcomes:

• The bond is stronger than a typical single bond.
• The length of the bond is shorter than a single C-N bond, but not as short as a true double bond. It falls somewhere in between.

Because of this partial double bond character, the peptide bond gains strength, which is vital for maintaining the integrity of protein structures. This bond characteristic limits rotation, as the more rigid bond locks the atoms in a planar arrangement across the peptide.
By preventing free rotation, the partial double bond ensures proteins have a defined three-dimensional shape necessary for proper function. This feature is integral to many biological processes where precise protein shapes are crucial for their roles in the body.

Bond Length in Proteins

The bond length in proteins, especially concerning peptide bonds, is another intriguing area of study. Typically, the C-N bond in a peptide bond is shorter than what you might expect for a simple single C-N bond. Why does this happen?

• The resonance effect and the resultant partial double bond character are the driving forces.
• These factors compress the bond to a length shorter than usual single bonds.

It's critical to remember that this shortened bond length has practical implications. Shorter bonds are stronger and less prone to breaking, which contributes to the protein's overall stability. That’s why proteins, despite being complex and varied in structure, rely on these shorter, stronger bonds to maintain their shape and function under different conditions.
Notably, the consistency of bond length due to resonance ensures that proteins form predictable and stable structures, essential for the vast range of biological functions they perform. Understanding this bond length variation is crucial for those studying protein dynamics and their applications in fields like biochemistry and molecular biology.