Which of the following amino acids contains a thiol group in the side chain?

Options

A. methionine
B. cysteine
C. valine
D. serine

Answer

To determine which of the given amino acids contains a thiol group in its side chain, we will analyze the structures of each amino acid provided in the options.

### Step-by-Step Solution:

1. **Understand the Structure of Amino Acids**:
- All amino acids have a general structure that includes an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group).

2. **Identify the Side Chains of the Given Amino Acids**:
- **Methionine**: The side chain is -CH2-CH2-S-CH3 (contains a sulfur atom but not a thiol group).
- **Cysteine**: The side chain is -CH2-SH (contains a thiol group).
- **Valine**: The side chain is -CH(CH3)2 (no sulfur, hence no thiol group).
- **Serine**: The side chain is -CH2-OH (no sulfur, hence no thiol group).

3. **Identify the Thiol Group**:
- A thiol group is characterized by the presence of a -SH group. In this case, cysteine has the -CH2-SH side chain, which clearly shows the presence of the thiol group.

4. **Conclusion**:
- Among the options provided, cysteine is the only amino acid that contains a thiol group in its side chain.

### Final Answer:
The amino acid that contains a thiol group in the side chain is **Cysteine** (Option 2).