The given structure of `alpha` - amino acid will exist at which pH ?
`overset(o+)(N)H_3-underset(R)underset(|)overset(COOH)overset(|)C-H`

Options

A. 7
B. 14
C. 6
D. 12

Answer

To determine the pH at which the given structure of the alpha-amino acid exists, we need to analyze the structure and the behavior of amino acids in different pH environments.

### Step-by-Step Solution:

1. **Identify the Structure**: The given structure represents an alpha-amino acid with a carboxylic acid group (-COOH) and an amino group (-NH2). The amino group has a lone pair of electrons.

2. **Understand Amino Acid Ionization**: Amino acids can exist in different ionic forms depending on the pH of the solution. At low pH (acidic conditions), the amino group tends to accept a proton (H+), becoming positively charged (-NH3+), while the carboxylic acid group remains protonated (-COOH).

3. **Protonation at Low pH**: In an acidic environment (low pH), the presence of excess H+ ions leads to the protonation of the amino group. This results in the structure being represented as:
\[ \overset{(o^+)}{N}H_3 - \underset{(R)}{C} - \overset{(COOH)}{C} - H \]
This indicates that the amino acid is in its cationic form.

4. **Determine the pH Range**: The pH at which the amino acid exists in this cationic form is typically below its pKa values. For most amino acids, the pKa of the carboxylic acid group is around 2-3, and the pKa of the amino group is around 9-10. Therefore, at a pH lower than the pKa of the amino group, the amino group will be protonated.

5. **Conclusion**: Since the structure shows the amino group as -NH3+, this indicates that the pH must be low (acidic). Therefore, the alpha-amino acid will exist in this form at a low pH, typically below 7.

### Final Answer:
The alpha-amino acid will exist at a low pH, specifically in acidic conditions (pH < 7).