Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Which statement is incorrect about peptide bond? (a) C-N bond length in proteins is smaller than usual bond length of \(\mathrm{C}-\mathrm{N}\) bond (b) spectroscopic analysis shows planar structure (c) C-N bond length in proteins is longer than usual bond length of C-N bond (d) none of these

Step-by-Step Solution

Answer

Statement (c) is incorrect as peptide bond C-N is shorter.

Step 1: Understanding Peptide Bond

A peptide bond is formed between two amino acids and involves a chemical bond where the carboxyl group of one amino acid reacts with the amino group of another, releasing a molecule of water. This bond is characterized by partial double-bond character due to resonance.

Step 2: Analyze Bond Length

The typical C-N bond length is around 1.47 Å. However, in peptide bonds, the C-N bond length is about 1.32 Å due to resonance and partial double-bond character, making it shorter than a usual single C-N bond.

Step 3: Spectroscopic Analysis

Spectroscopic analysis, such as X-ray crystallography and infrared spectroscopy, shows that the peptide bond is planar. This planarity is due to the resonance structure which contributes to the rigidity and stability of the bond.

Step 4: Evaluate Statements (a) and (b)

Statement (a) indicates the C-N bond length is smaller than usual, which is correct as peptide bonds are shorter. Statement (b) suggests peptides have a planar structure, which is also correct based on spectroscopic data.

Step 5: Evaluate Statement (c)

Statement (c) claims the C-N bond length in proteins is longer than the usual C-N bond, which is incorrect. In reality, the peptide bond is shorter due to its partial double-bond character.

Step 6: Consider Statement (d)

Statement (d) is 'none of these', suggesting that all other statements are correct. Since statement (c) is incorrect, statement (d) is not the right choice as the answer.

Key Concepts

Amino Acid

Amino acids are the building blocks of proteins. They are organic compounds composed of an amino group (dalized NH), a carboxyl group (COOH), a hydrogen atom, and an R group each connected to a central  carbon atom. The amino group and carboxyl group are key in peptide bond formation, a vital connection holding together long chains of amino acids known as polypeptides or proteins.
Amino acids vary based on the R group attached, which dictates their properties and categorization. These categories include:

• Non-polar amino acids
• Polar amino acids
• Acidic amino acids
• Basic amino acids

The unique properties of amino acids, due to their variable R groups, lend to diverse protein structures and functions. Comprehending amino acids deepens the understanding of the formation and characteristics of peptide bonds.

Spectroscopic Analysis

Spectroscopic analysis involves techniques that help identify and analyze molecular structures using light or electromagnetic radiation. These methods are crucial in chemistry for revealing the structures and characteristics of molecules such as proteins and the peptide bonds that hold them together.
Two commonly used spectroscopic techniques for studying peptide bonds are:

• **X-ray Crystallography**: Provides information about the 3D structure of proteins, illustrating a planar arrangement due to resonance in peptide bonds.
• **Infrared Spectroscopy**: Identifies specific bond vibrations, highlighting functional groups like the amino and carboxyl groups integral to peptide bond formation.

These techniques prove indispensable in understanding protein structures and verifying their fundamental properties, like the planarity of peptide bonds. The application of spectroscopic analysis helps elucidate the microscopic structures that underlie the macromolecular behavior of proteins.

Chemical Bond Length

Chemical bond length refers to the distance between the nuclei of two bonded atoms. It is a critical factor in understanding chemical structures and their stability. In a typical C-N single bond, the length is approximately 1.47 Å. However, in peptide bonds, the bond length is shorter at around 1.32 Å.
This shortened bond length in peptide bonds is attributed to the partial double-bond character imparted by resonance. The presence of resonance causes the electron density to be more evenly distributed among atoms, strengthening the bond due to added stability and rigidity. As a result, the typical characteristics of a single bond are altered, making the C-N bond length in peptide bonds notably shorter.

Resonance in Chemistry

Resonance in chemistry refers to a phenomenon where a molecule can be represented by multiple structures called resonance structures. These structures are important in explaining the behavior of bonds, especially in complex molecules like peptides.
For peptide bonds, resonance allows the electrons to be delocalized over the oxygen, carbon, and nitrogen atoms involved in the bond. This delocalization imparts a partial double-bond character to the C-N bond. Some crucial effects of resonance in peptide bonds include:

• Increased stability: The peptide bond is notably more robust than ordinary single bonds due to electron delocalization.
• Planar structure: The resonance induces a planar conformation, vital for protein secondary structures like alpha-helices and beta-sheets.

Resonance is a fundamental concept that enhances understanding of molecular stability and structural properties, making it a significant topic when studying peptide bonds and proteins.