`alpha`-Amino acids behave as crystalline ionic solid and have high
melting point due to the presence of:

Options

A. `-NH_2 ` group
B. `-COOH` group
C. both - `NH_2 and -COOH`
D. None of these

Explanation

To answer the question regarding why alpha-amino acids behave as crystalline ionic solids with high melting points, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Alpha-Amino Acids**:
- Alpha-amino acids are organic compounds that contain both an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon atom (the alpha carbon).

2. **Formation of Zwitterions**:
- In solution, alpha-amino acids can exist as zwitterions. A zwitterion is a molecule that has both positive and negative charges but is overall electrically neutral.
- The carboxyl group (-COOH) can donate a proton (H+) to the amino group (-NH2), resulting in the formation of a zwitterion: RCH(NH3+)COO-.

3. **Properties of Zwitterions**:
- The zwitterionic form of alpha-amino acids has a dipolar nature, meaning it has both a positive and a negative charge. This dipolar nature contributes to strong ionic interactions between zwitterions.

4. **Crystalline Ionic Solid**:
- Due to the ionic interactions between the zwitterions, alpha-amino acids tend to form a crystalline structure. The strong electrostatic forces between the oppositely charged parts of the zwitterions lead to the formation of a stable lattice structure.

5. **High Melting Points**:
- The presence of these strong ionic bonds in the crystalline structure results in high melting points. To melt the solid, a significant amount of energy is required to overcome these ionic interactions.

6. **Conclusion**:
- Therefore, the high melting points and crystalline nature of alpha-amino acids can be attributed to the presence of both the amino group (-NH2) and the carboxyl group (-COOH), which facilitate zwitterion formation and strong ionic interactions.

### Final Answer:
Alpha-amino acids behave as crystalline ionic solids and have high melting points due to the presence of both the amino group (-NH2) and the carboxyl group (-COOH), which allow for zwitterion formation and strong ionic interactions.