Which one of the following is `beta` - amino acid ?

Options

A. `CH_3 - underset(NH_2)underset(|)(CH)-COOH`
B. `NH_2CH_2-CH_2-COOH`
C. `NH_2-(CH_2)_3-COOH`
D.

Answer

To determine which of the given options is a beta-amino acid, we first need to understand the definition of beta-amino acids and how they differ from alpha-amino acids.

### Step-by-Step Solution:

1. **Understanding Amino Acids**:
- Amino acids are organic compounds that contain both an amino group (-NH2) and a carboxylic acid group (-COOH).
- The position of the amino group relative to the carboxylic acid group determines whether the amino acid is classified as alpha, beta, or gamma.

2. **Definition of Beta-Amino Acid**:
- A beta-amino acid has its amino group attached to the second carbon atom (beta carbon) adjacent to the carboxylic acid group.
- In contrast, an alpha-amino acid has its amino group attached to the first carbon atom (alpha carbon) next to the carboxylic acid.

3. **Analyzing the Options**:
- Let's examine each option to identify the structure of the amino acids presented.

**Option A**:
- Structure: CH3-CH(NH2)-COOH
- Here, the amino group is on the alpha carbon. Therefore, this is an alpha-amino acid.

**Option B**:
- Structure: NH2-CH2-CH2-COOH
- The amino group is on the beta carbon (the second carbon from the carboxylic acid). This is a beta-amino acid.

**Option C**:
- Structure: CH2-CH2-CH2-COOH with NH2 on the alpha carbon.
- The amino group is not on the beta carbon, so this is not a beta-amino acid.

**Option D**:
- Structure: Similar to option C, with the amino group not on the beta carbon.
- This option also does not qualify as a beta-amino acid.

4. **Conclusion**:
- After analyzing all the options, we find that **Option B** is the only beta-amino acid.

### Final Answer:
**Option B is the beta-amino acid.**

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