Question

If pI (isoelectric pH) of serine and leucine are 5.68 and 5.98 respectively then in a solution of pH 3.0

• A. Net charge of both Serine and leucine will be positive
• B. Net charge of serine will be positive and net charge of leucine will be negative
• C. Net charge of leucine will be positive and net charge of serine will be negative
• D. Net charge of both serine and leucine sill be negative

Hint:

Always remember this simple rule:
At a pH lower than the pI, the excess $H^+$ ions in the acidic solution protonate the amino acid, resulting in a net positive charge.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The isoelectric point (pI) is the specific pH at which an amino acid carries no net electrical charge (it exists as a neutral zwitterion).
The net charge of an amino acid in solution is determined by the relationship between the pH of the medium and the pI of the amino acid:
1. If $\text{pH} < \text{pI}$, the amino acid accepts protons and carries a net positive charge.
2. If $\text{pH} > \text{pI}$, the amino acid loses protons and carries a net negative charge.
Detailed Explanation:
Given values:
- pI of Serine = $5.68$
- pI of Leucine = $5.98$
- pH of the solution = $3.0$
Let us evaluate the net charge of each amino acid at this pH:
- For Serine: The pH of the solution ($3.0$) is less than its pI ($5.68$).
Since the medium is more acidic than its isoelectric point, the carboxylic acid groups and amino groups will be protonated ($-\text{COOH}$ and $-\text{NH}_3^+$), giving Serine a net positive charge.
- For Leucine: The pH of the solution ($3.0$) is also less than its pI ($5.98$).
Similarly, Leucine will accept protons and carry a net positive charge.
Therefore, at pH 3.0, both serine and leucine will carry a net positive charge.

Step 2: Final Answer:
The net charge of both serine and leucine will be positive. This corresponds to option (A).