Question

Given below are two statements:
Statement I: If pH of a protein solution is more than its pI then the number of negatively charged protein molecules is more than the positively charged protein molecules
Statement II: If pH of a protein solution is equal to its pI then the net charge of protein is zero
In the light of the above statements, choose the most appropriate answer from the options given below

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Protein Charge Rules:
$\text{pH} > \text{pI} \implies$ Anion (Net Negative, migrates to Anode).
$\text{pH} = \text{pI} \implies$ Zwitterion (Net Zero).
$\text{pH} < \text{pI} \implies$ Cation (Net Positive, migrates to Cathode).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The isoelectric point (pI) is the specific pH at which a zwitterionic polypeptide possesses zero net electrical charge.
Key Formula or Approach:
\[ \text{pH} \text{pI} \implies \text{Net Negative Charge} \]

Step 2: Detailed Explanation:
1. Statement I: When the ambient $\text{pH} > \text{pI}$, basic amino groups deprotonate ($-\text{NH}_3^+ \rightarrow -\text{NH}_2$) and acidic carboxyl groups remain deprotonated ($-\text{COO}^-$), imparting an overall net negative charge to the protein population. Hence, Statement I is true.
2. Statement II: By definition, the Isoelectric Point (pI) is the pH at which the sum of positive charges equals the sum of negative charges, resulting in a net charge of zero (minimum solubility and zero electrophoretic mobility). Hence, Statement II is true.

Step 3: Final Answer:
Thus, Both Statement I and Statement II are true, matching option (A).