Which of the following statements about the peptide bond is (are) correct?

Options

A. a) The `C-N` bond of the peptide linkage is longer than that of an amide.
B. b) The `-overset(O) overset(||)(C)-NH-` portion of the peptide linkage is planar
C. c) The `C-N` bond length of peptide is shorter than that of an amine
D. d) The rotation about `CO-N` bond of peptide is restricted.

Explanation

To determine which statements about the peptide bond are correct, let's analyze each statement step by step.

### Step 1: Understanding Peptide Bond Formation
A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another. This reaction results in the release of a water molecule (condensation reaction).

### Step 2: Analyzing the Statements
Now, let's evaluate each of the statements provided:

1. **Statement 1**: The C-N bond of the peptide linkage is not a C=O bond.
- **Analysis**: This statement is incorrect because while the peptide bond does involve a carbonyl (C=O) group, the C-N bond is indeed a single bond with some double bond character due to resonance. However, the statement seems to imply a misunderstanding about the nature of the bond.

2. **Statement 2**: The bond portion of the peptide linkage is planar.
- **Analysis**: This statement is correct. The peptide bond has a partial double bond character due to resonance, which restricts rotation and results in a planar structure around the bond.

3. **Statement 3**: The C-N bond length of the peptide bond is shorter than that of an amine.
- **Analysis**: This statement is correct. The C-N bond in a peptide bond has partial double bond character, making it shorter than the C-N bond in a typical amine (R-NH2), which is a single bond.

4. **Statement 4**: The rotation about the C-N bond of the peptide bond is restricted.
- **Analysis**: This statement is correct. The presence of partial double bond character in the peptide bond restricts rotation, similar to the restriction seen in double bonds.

### Conclusion
Based on the analysis:
- **Correct Statements**: 2, 3, and 4
- **Incorrect Statement**: 1

### Final Answer
Only the first statement is incorrect; the rest (2, 3, and 4) are correct.

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