At isoelectric point, amino acid is present as

Options

A. `H_(2)N - CHR - COOH`
B. `H_(2)N - CHR - COOH`
C. `NH_(2) - CHR - COO^(-)`
D. `H_(2)N^(+) - CHR - COO^(-)`

Answer

To answer the question "At isoelectric point, amino acid is present as," we need to understand the concept of the isoelectric point and the behavior of amino acids at this point.

### Step-by-Step Solution:

1. **Define Isoelectric Point (pI)**:
- The isoelectric point is the pH at which an amino acid has no net charge. At this point, the positive and negative charges balance each other out.

2. **Understanding Zwitterions**:
- At the isoelectric point, amino acids exist predominantly in a form known as a zwitterion. A zwitterion is a molecule that has both positive and negative charges but is overall neutral.

3. **Structure of Zwitterion**:
- In the zwitterionic form, the amino group (-NH2) of the amino acid can accept a proton (H+), becoming positively charged (-NH3+), while the carboxyl group (-COOH) can lose a proton, becoming negatively charged (-COO-). Thus, at the isoelectric point, the amino acid can be represented as:
- H3N+ -CHR - COO−
- Here, "R" represents the side chain of the amino acid.

4. **Analyzing the Options**:
- **Option A**: H2N - CHR - COOH (This shows no charge, hence incorrect)
- **Option B**: Same as Option A (Also incorrect)
- **Option C**: H2N - CHR - COO− (This shows a negative charge but lacks a positive charge, hence incorrect)
- **Option D**: H3N+ - CHR - COO− (This shows both positive and negative charges, hence correct)

5. **Conclusion**:
- Therefore, at the isoelectric point, the amino acid is present as a zwitterion, which has both a positive and a negative charge.

### Final Answer:
At isoelectric point, amino acid is present as a zwitterion (H3N+ - CHR - COO−).