Which statement is incorrect about peptide bond?

Options

A. C - N bond length in proteins is longer than usual bond length of C - N bond
B. Spectroscopic analysis shows planar structure of `-underset(O)underset(||)C-NH-` bond
C. C - N bond length in proteins is smaller than usual bond length of C - N bond
D. None of these

Answer

To determine which statement is incorrect about peptide bonds, we need to analyze the properties and characteristics of peptide bonds. Let's go through the statements step by step.

### Step 1: Understand the Structure of Peptide Bonds
Peptide bonds are formed between the carboxyl group of one amino acid and the amino group of another. The structure can be represented as:
\[ \text{C(=O) - N(H)} \]
This indicates that the bond involves a carbonyl (C=O) and an amine (N-H) group.

**Hint:** Remember that peptide bonds have a specific structure that influences their properties.

### Step 2: Analyze the Bond Length
One of the key characteristics of peptide bonds is that the carbon-nitrogen (C-N) bond has a partial double bond character due to resonance. This occurs because the lone pair of electrons on the nitrogen can delocalize, leading to a shorter bond length compared to a typical C-N single bond.

**Hint:** Consider how resonance and conjugation affect bond lengths.

### Step 3: Evaluate the Statements
Now, let's evaluate the statements provided in the options:

1. **C-N bond length in proteins is longer than the usual C-N bond.**
- This statement is incorrect because the C-N bond in peptide bonds is shorter due to the partial double bond character.

2. **Spectroscopic analysis shows planar structure of CONH bond.**
- This statement is true. The planar structure is a result of the partial double bond character.

3. **C-N bond length in proteins is smaller than the usual bond length of C-N.**
- This statement is true, as explained above.

4. **Peptide bonds are rigid and do not allow rotation.**
- This statement is also true, as the partial double bond character restricts rotation around the bond.

### Step 4: Identify the Incorrect Statement
From the analysis, we can conclude that the incorrect statement is:
- **C-N bond length in proteins is longer than the usual C-N bond.** (This is false; it is actually shorter.)

### Final Answer
The incorrect statement about peptide bonds is:
- **C-N bond length in proteins is longer than the usual C-N bond.**