All common amino acids except one react with cold nitrous acid `(HNO_(2))` and evolve nitrogen gas, that amino acid is

Options

A. cysteine
B. proline
C. histidine
D. None of these

Explanation

To solve the question regarding which amino acid does not evolve nitrogen gas when reacted with cold nitrous acid (HNO2), we can follow these steps:

### Step 1: Understand the Reaction
Cold nitrous acid reacts with amino acids, leading to the evolution of nitrogen gas (N2) when the amino acid contains a primary amine group (NH2). However, if the amino acid contains a secondary amine group (NH), it will not produce nitrogen gas but instead form a nitrosamine.

### Step 2: Identify the Amino Acids
The amino acids given in the options are:
1. Cysteine
2. Proline
3. Histidine

### Step 3: Analyze the Structure of Each Amino Acid
- **Cysteine**: Contains a primary amine group (NH2) and will evolve nitrogen gas when reacted with nitrous acid.
- **Proline**: Contains a secondary amine group (NH) in its structure. Therefore, it will not evolve nitrogen gas but will form nitrosamine.
- **Histidine**: Contains a primary amine group (NH2) and will also evolve nitrogen gas when reacted with nitrous acid.

### Step 4: Conclusion
Based on the analysis, the amino acid that does not evolve nitrogen gas upon reaction with cold nitrous acid is **Proline**.

### Final Answer
The amino acid that does not react with cold nitrous acid to evolve nitrogen gas is **Proline**.

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