Which structural feature distinguishes proline from other natural `alpha-`amino acids?

Options

A. It is optically inactive.
B. It contains armatic group.
C. It is a dicarboxylic acid
D. It has a secondary amine.

Answer

To determine which structural feature distinguishes proline from other natural alpha-amino acids, we need to analyze the structure of proline in comparison to other amino acids.

### Step-by-Step Solution:

1. **Understanding Amino Acids**:
- Amino acids are the building blocks of proteins and have a general structure that includes an amino group (-NH2), a carboxylic acid group (-COOH), and a side chain (R group) attached to an alpha carbon (Cα).

2. **Identifying Proline**:
- Proline is unique among the amino acids because its amino group is part of a cyclic structure. In proline, the side chain (R group) is connected to the amino group, forming a five-membered ring.

3. **Examining the Amino Group**:
- In most natural alpha-amino acids, the amino group attached to the alpha carbon is a primary amine (-NH2), meaning it is connected to one carbon atom.
- In proline, however, the amino group is a secondary amine because it is bonded to two carbon atoms (the alpha carbon and the carbon in the side chain).

4. **Conclusion**:
- The distinguishing structural feature of proline compared to other natural alpha-amino acids is that it contains a secondary amine group due to its cyclic structure, making it unique among the amino acids.

### Final Answer:
The structural feature that distinguishes proline from other natural alpha-amino acids is that it has a secondary amine group.