Which of the following is not an `alpha`-amino acid?

Options

A. Glycina
B. Alanine
C. Histidine
D. Benzidine

Answer

To determine which of the given options is not an alpha-amino acid, we need to analyze the structures of each compound and identify if they meet the criteria of an alpha-amino acid. An alpha-amino acid is characterized by having an amino group (-NH2) and a carboxylic acid group (-COOH) attached to the same carbon atom (the alpha carbon).

Let's go through the options step by step:

### Step 1: Analyze Glycine
- **Structure**: Glycine has the structure NH2-CH2-COOH.
- **Analysis**: It has an amino group (NH2) and a carboxylic acid group (COOH) on the same carbon (the alpha carbon).
- **Conclusion**: Glycine is an alpha-amino acid.

### Step 2: Analyze Alanine
- **Structure**: Alanine has the structure NH2-CH(CH3)-COOH.
- **Analysis**: It has an amino group (NH2) and a carboxylic acid group (COOH) on the same carbon (the alpha carbon).
- **Conclusion**: Alanine is an alpha-amino acid.

### Step 3: Analyze Histidine
- **Structure**: Histidine has the structure NH2-CH(C3H3N)-COOH (with an imidazole side chain).
- **Analysis**: It has an amino group (NH2) and a carboxylic acid group (COOH) on the same carbon (the alpha carbon).
- **Conclusion**: Histidine is an alpha-amino acid.

### Step 4: Analyze Benzidine
- **Structure**: Benzidine has the structure NH2-C6H4-NH-C6H4-NH2 (two benzene rings connected by two amino groups).
- **Analysis**: Benzidine does not have a carboxylic acid group (-COOH) attached to the same carbon as the amino group (-NH2). Instead, it is an aromatic amine.
- **Conclusion**: Benzidine is not an alpha-amino acid.

### Final Answer
The compound that is not an alpha-amino acid is **Benzidine**.

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