The correct structure of glycine at given `pH` are :

Options

A. `H_(3) overset(oplus)(N)CH_(2)-underset(underset(O)(||))(C)-OH" "at pH=2.0`
B. `H_(3)overset(oplus)(N)CH_(2)-underset(underset(O)(||))(C)-O^(Theta)" "at pH=6.0`
C. `H_(2)NCH_(2)-underset(underset(O)(||))(C)-O^(Theta)`
D. `H_(2)NCH_(2)-underset(underset(O)(||))(C)-OH" "atpH=12`

Explanation

To determine the correct structure of glycine at various pH levels, we need to understand how glycine behaves in different pH environments. Glycine is an amino acid that contains both an amino group (-NH2) and a carboxylic acid group (-COOH).

### Step-by-Step Solution:

1. **Understanding Glycine Structure**:
- Glycine has the following structure:
\[ \text{NH}_2 - \text{CH}_2 - \text{COOH} \]
- At physiological pH (around 7), glycine exists predominantly in its zwitterionic form.

2. **Cationic Form at pH 2**:
- At a low pH (pH < 2), the carboxylic acid group (-COOH) is protonated, and the amino group (-NH2) is also protonated, forming a cation:
\[ \text{NH}_3^+ - \text{CH}_2 - \text{COOH} \]
- Therefore, at pH 2, glycine exists as a cation.

3. **Zwitterionic Form at pH 6**:
- At pH 6, which is close to the isoelectric point of glycine, the amino group is protonated, and the carboxylic acid group is deprotonated:
\[ \text{NH}_3^+ - \text{CH}_2 - \text{COO}^- \]
- This is the zwitterionic form of glycine.

4. **Anionic Form at pH 9**:
- At a higher pH (pH > 9), the amino group remains protonated, while the carboxylic acid group is fully deprotonated:
\[ \text{NH}_2 - \text{CH}_2 - \text{COO}^- \]
- Thus, at pH 9, glycine exists as an anion.

5. **Neutral Form at pH 12**:
- At very high pH (pH 12), the amino group would also lose a proton, leading to a neutral form:
\[ \text{NH}_2 - \text{CH}_2 - \text{COO}^- \]
- However, this is incorrect because at pH 12, glycine would still exist primarily in its anionic form.

### Summary of Structures at Different pH Levels:
- **pH 2**: Cationic form (NH3+ - CH2 - COOH)
- **pH 6**: Zwitterionic form (NH3+ - CH2 - COO-)
- **pH 9**: Anionic form (NH2 - CH2 - COO-)
- **pH 12**: Anionic form (NH2 - CH2 - COO-)

### Conclusion:
The correct structures of glycine at the given pH levels are:
- Cationic form at pH 2
- Zwitterion at pH 6
- Anionic form at pH 9
- The statement regarding the neutral form at pH 12 is incorrect.