Alanine at its isoelectric point, exist in solution as:

Options

A. `H_2 N- underset(CH_3)underset(|)CH-COO^(ө)`
B. `H_3 overset(o+)N- underset(CH_3)underset(|)CH-COOH`
C. `H_3 overset(o+)N -underset(CH_3)underset(|)CH-COO^(ө)`
D. `H_2 N - underset(CH_3)underset(|)CH-COOH`

Explanation

To determine how alanine exists in solution at its isoelectric point, we need to understand the concept of isoelectric point (pI) and the structure of alanine.

### Step-by-Step Solution:

1. **Understanding the Structure of Alanine**:
- Alanine is an amino acid with the following structure:
- An amino group (-NH2)
- A carboxyl group (-COOH)
- A side chain (methyl group -CH3)
- The full structure can be represented as:
```
NH2-CH(CH3)-COOH
```

2. **Concept of Isoelectric Point**:
- The isoelectric point (pI) is the pH at which an amino acid exists in a form that has no net charge. This occurs when the positive and negative charges balance each other out.

3. **Ionization States of Alanine**:
- At low pH (acidic conditions), alanine exists predominantly in its cationic form:
```
NH3+-CH(CH3)-COOH
```
- At high pH (basic conditions), alanine exists predominantly in its anionic form:
```
NH2-CH(CH3)-COO-
```

4. **Alanine at Its Isoelectric Point**:
- At the isoelectric point, alanine will have one protonated amino group (NH3+) and one deprotonated carboxyl group (COO-). Therefore, the structure at the isoelectric point is:
```
NH3+-CH(CH3)-COO-
```
- This structure has a net charge of zero, indicating that it is neutral.

5. **Conclusion**:
- Thus, at its isoelectric point, alanine exists in solution as a dipolar ion (zwitterion) with the structure:
```
NH3+-CH(CH3)-COO-
```

### Final Answer:
Alanine at its isoelectric point exists in solution as a zwitterion (dipolar ion) with the structure:
```
NH3+-CH(CH3)-COO-
```