An amino acid usually shows its lowest solubility in water :

Options

A. in acidic solution
B. in basic solution
C. at pH 7
D. at isoelectric point

Explanation

To determine when an amino acid shows its lowest solubility in water, we need to understand the concept of the isoelectric point (pI) and how it relates to the solubility of amino acids in different pH environments.

### Step-by-Step Solution:

1. **Understanding Amino Acids**: Amino acids can exist in different ionic forms depending on the pH of the solution. They can be cationic (positively charged), anionic (negatively charged), or zwitterionic (neutral with both positive and negative charges).

2. **Isoelectric Point (pI)**: The isoelectric point is the pH at which an amino acid has no net charge. At this point, the concentrations of the cationic and anionic forms of the amino acid are equal. This results in the amino acid existing predominantly in its zwitterionic form.

3. **Solubility and pH**:
- In **acidic solutions** (pH < pI), amino acids tend to exist as cations (positively charged), which can increase their solubility due to interaction with water.
- In **basic solutions** (pH > pI), amino acids exist as anions (negatively charged), which can also increase solubility.
- At the **isoelectric point (pI)**, the amino acid exists primarily in its zwitterionic form, which has a neutral charge. This form is less soluble in water compared to its charged forms.

4. **Conclusion**: Therefore, an amino acid usually shows its lowest solubility in water at its isoelectric point (pI). At this point, the amino acid has no net charge and is less likely to interact favorably with water molecules.

### Final Answer:
An amino acid usually shows its lowest solubility in water at its isoelectric point (pI).