Thiol group is present in

Options

A. Cystine
B. Cysteine
C. Methionine
D. Cytosine

Answer

To determine which compound contains a thiol group (–SH), we will analyze the structures of the given compounds one by one.

### Step 1: Understand the Thiol Group
The thiol group is characterized by the presence of a sulfur atom bonded to a hydrogen atom (–SH). Compounds containing this functional group are known as thiols.

### Step 2: Analyze Compound A (Cysteine)
1. **Structure of Cysteine**: The structure of cysteine is:
- It has an amino group (–NH2), a carboxylic acid group (–COOH), and a thiol group (–SH).
- The structure can be represented as:
```
NH2-CH(COOH)-SH
```
2. **Identify the Thiol Group**: In cysteine, the –SH group is present. Thus, cysteine contains a thiol group.

### Step 3: Analyze Compound B (Cysteine)
1. **Structure of Cysteine**: The question seems to refer to cysteine again, but we will assume it is a different compound for the purpose of analysis.
2. **Identify the Thiol Group**: If this compound has an –SH group, it would also be classified as a thiol. However, if it is indeed cysteine, it has already been established that it contains a thiol group.

### Step 4: Analyze Compound C (Methionine)
1. **Structure of Methionine**: The structure of methionine is:
- It has an amino group (–NH2), a carboxylic acid group (–COOH), and a thioether group (–S–CH3).
- The structure can be represented as:
```
NH2-CH(COOH)-S-CH3
```
2. **Identify the Thiol Group**: Methionine does not contain a thiol group; it contains a thioether group instead.

### Step 5: Analyze Compound D (Cytosine)
1. **Structure of Cytosine**: The structure of cytosine is:
- It contains nitrogen atoms and does not have any sulfur atoms.
- The structure can be represented as:
```
C4H5N3O
```
2. **Identify the Thiol Group**: Cytosine does not contain a thiol group.

### Conclusion
After analyzing all the compounds:
- **Cysteine** (Compound A) is the only compound that contains a thiol group (–SH).
- **Methionine** and **Cytosine** do not contain thiol groups.

Thus, the correct answer is **Cysteine**.