Question

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

Short answer

Statement (c) is incorrect about the peptide bond.

Step-by-step solution

Analyze statement (a)

Statement (a) suggests that the \( \mathrm{C}-\mathrm{N} \) bond length in proteins is shorter than the usual bond length of a \( \mathrm{C}-\mathrm{N} \) bond. This is true because the peptide bond has partial double bond character, leading to a shorter bond length than a typical single \( \mathrm{C}-\mathrm{N} \) bond.

Analyze statement (b)

Statement (b) claims that spectroscopic analysis shows the \( \mathrm{C}-\mathrm{N} \) bond in proteins is planar. This is also true; the planarity is due to the partial double bond characteristic of the peptide bond, which restricts rotation.

Analyze statement (c)

Statement (c) contradicts statement (a), claiming that the \( \mathrm{C}-\mathrm{N} \) bond length in proteins is longer than a typical \( \mathrm{C}-\mathrm{N} \) bond. Given that statement (a) is correct, this statement is incorrect. The peptide bond's partial double bond nature means it is actually shorter.

Verify statement (d)

Statement (d) claims that none of the other statements are incorrect. Since we identified statement (c) as incorrect, statement (d) is also incorrect.

Key concepts

Protein Structure

Proteins are complex molecules that play many critical roles in the body. They are made up of chains of amino acids linked together by peptide bonds. The way these amino acid chains are structured and folded are key to a protein's function. Here's a simple guide to understanding this structure.

1. **Primary Structure**: This is the simplest level of protein structure. It involves the sequence of amino acids in the polypeptide chain, determined by genetic information.
2. **Secondary Structure**: Here, the sequence of amino acids starts to fold into characteristic shapes, often known as alpha-helices or beta-sheets. These structures form due to hydrogen bonding between the backbone components of the polypeptide chain.
3. **Tertiary Structure**: The secondary structures further fold into a three-dimensional shape. This structure is stabilized by various interactions, including hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic packing. The shape of the tertiary structure is directly related to the protein's functionality.

By understanding these different structure levels, we gain insights into how proteins function, how they fold, and how they interact with other molecules.

Bond Length

In chemistry, bond length is the average distance between the nuclei of two bonded atoms. For peptide bonds, this bond length holds special significance. A peptide bond forms between the carbon atom of one amino acid and the nitrogen atom of another.

Due to resonance, the peptide bond has a partial double-bond character. This means that electrons can be shared between carbon-nitrogen bonds more freely, creating a shorter, yet stronger bond. Typical single bond length between carbon and nitrogen might be about 1.47 Å (angstroms), whereas in a peptide bond, the length is closer to 1.32 Å.

The partial double-bond character of the peptide bond effectively results in a rigid and planar structure, limiting the rotation around the bond. This rigidity contributes significantly to the protein's overall structure and stability.

Spectroscopic Analysis

Spectroscopic analysis is a method used to understand the properties of molecules by examining how they interact with electromagnetic radiation. When applied to peptide bonds in proteins, spectroscopy can reveal essential information about molecular structures.

Spectroscopic methods like infrared spectroscopy provide insight into the planarity of the peptide bond. The spectroscopic analysis shows that peptide bonds have a planar structure. This is due to the partial double bond character of the C-NH linkage, which reduces any free rotation, thereby maintaining a fixed angle and planar orientation.

Additionally, such analysis can help identify secondary structural motifs, such as alpha helices and beta sheets, in proteins. Spectroscopy is thus crucial not only for confirming the presence of peptide bonds but also for understanding how these bonds influence the overall protein configuration.