Lysine is least soluble in water in the `pH` range.

Options

A. 3 to 4
B. 5 to 6
C. 6 to 7
D. 8 to 9

Explanation

To determine the pH range in which lysine is least soluble in water, we need to analyze the properties of lysine as an amino acid.

### Step-by-Step Solution:

1. **Identify the Nature of Lysine**:
- Lysine is an amino acid classified as a basic amino acid due to the presence of an amino group (-NH2) in its side chain.

2. **Understand Zwitterionic Form**:
- Amino acids can exist in a zwitterionic form, which is a dipolar ion that has both positive and negative charges. This occurs at a specific pH known as the isoelectric point (pI).

3. **Define Isoelectric Point (pI)**:
- The isoelectric point is the pH at which the amino acid has no net charge and does not migrate in an electric field. At this point, the solubility of the amino acid in water is at its lowest.

4. **Determine the pI of Basic Amino Acids**:
- For basic amino acids like lysine, the isoelectric point typically falls within the range of pH 7.6 to 10.8.

5. **Find the Specific pI for Lysine**:
- The specific isoelectric point for lysine is approximately pH 9.7.

6. **Identify the pH Range of Least Solubility**:
- Since lysine is least soluble at its isoelectric point, we can conclude that lysine will be least soluble in the pH range around its pI, which is approximately 8 to 9.

### Final Answer:
Lysine is least soluble in water in the pH range of **8 to 9**.

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