Which of the following is an example of acidic amino acid?

Options

A. `CH_3-underset(COOH)underset(|)(CH)-NH_2`
B. `CH_3-underset(H)underset(|)overset(CH_2COOH)overset(|)(C )-underset(COOH)-NH_2`
C. `underset(NH_2)underset(|)(CH_2)-COOH`
D. `NH_2 - COOH`

Answer

To determine which of the given options is an acidic amino acid, we need to analyze the structures of each option based on the definitions of acidic and basic amino acids.

### Step-by-Step Solution:

1. **Understanding Amino Acids**:
- Amino acids have a general structure that includes an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a variable R group (side chain).
- The amino group is basic, while the carboxylic acid group is acidic.

2. **Identifying Acidic Amino Acids**:
- An amino acid is classified as acidic if it has more carboxylic acid groups than amino groups. This means that if the number of -COOH groups exceeds the number of -NH2 groups, the amino acid is acidic.

3. **Analyzing the Options**:
- **Option A**: CH3CH(NH2)COOH
- Contains 1 NH2 and 1 COOH. This is a neutral amino acid.
- **Option B**: CH3C(CH2COOH)(NH2)COOH
- Contains 1 NH2 and 2 COOH groups. Since there are more carboxylic acid groups than amino groups, this is an acidic amino acid.
- **Option C**: CH2(NH2)COOH
- Contains 1 NH2 and 1 COOH. This is a neutral amino acid.
- **Option D**: NH2-COOH
- This structure is not a complete amino acid as it lacks the alpha carbon that connects the amino and carboxylic groups properly.

4. **Conclusion**:
- Based on the analysis, **Option B** is the only one that qualifies as an acidic amino acid because it has more carboxylic acid groups than amino groups.

### Final Answer:
**Option B** is the correct answer as it represents an acidic amino acid.