Which of the following structures is not a dipeptide ?

Options

A.
B.
C.
D.

Explanation

To determine which structure is not a dipeptide, we need to understand the characteristics of a dipeptide.

### Step-by-Step Solution:

1. **Understanding Dipeptides**:
- A dipeptide is formed when two amino acids are linked together by a peptide bond.
- The peptide bond is characterized by the presence of a carbonyl group (C=O) adjacent to a nitrogen atom (N) from an amino group (NH).

2. **Analyzing the Options**:
- We will examine each option provided to identify the presence of the peptide bond (C=O and N).
- Look for the structure that lacks the characteristic C=O and NH group.

3. **Identifying the Non-Dipeptide Structure**:
- If an option does not show the C=O and NH linkage, it cannot be classified as a dipeptide.
- For example, if an option presents only a single amino acid or lacks the necessary functional groups, it is not a dipeptide.

4. **Conclusion**:
- After reviewing all options, select the structure that does not exhibit the C=O and NH bond characteristic of dipeptides.

### Final Answer:
- The structure that does not contain the C=O and NH bond is not a dipeptide.