Which of the following is the major solute species in a solution of lysine at `pH=10.5` ?

Options

A.
B.
C.
D.

Explanation

To determine the major solute species in a solution of lysine at pH 10.5, we need to analyze the structure of lysine and how it behaves in a basic medium.

### Step-by-Step Solution:

1. **Understand the Structure of Lysine**:
- Lysine is an amino acid with the following structure:
\[ \text{H}_2\text{N}-\text{(CH}_2)_4-\text{COOH} \]
- It contains an amino group (-NH2) and a carboxylic acid group (-COOH).

2. **Identify the pH Context**:
- The pH of the solution is 10.5, which indicates a basic environment. In basic solutions, the concentration of hydroxide ions (OH-) is high.

3. **Behavior of the Carboxylic Acid Group in Basic Medium**:
- In a basic medium, the carboxylic acid group (-COOH) can lose a proton (H+). This results in the formation of a carboxylate ion (-COO^-):
\[ \text{COOH} \rightarrow \text{COO}^- + \text{H}^+ \]

4. **Behavior of the Amino Group**:
- The amino group (-NH2) can remain protonated or deprotonated depending on the pH. At pH 10.5, it is likely that the amino group remains in its deprotonated state (-NH2), as the pKa of the amino group is typically around 9.0.

5. **Final Structure in Basic Medium**:
- Therefore, at pH 10.5, lysine predominantly exists in the form:
\[ \text{H}_2\text{N}-\text{(CH}_2)_4-\text{COO}^- \]
- This indicates that the carboxylic acid group is deprotonated, while the amino group remains unprotonated.

6. **Identify the Major Solute Species**:
- The major solute species in a solution of lysine at pH 10.5 is the deprotonated form of lysine with the carboxylate ion.

### Conclusion:
The major solute species in a solution of lysine at pH 10.5 is:
\[ \text{H}_2\text{N}-\text{(CH}_2)_4-\text{COO}^- \]