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Question #44965

Difficulty: 8/10 | 🟢 Free

The Question

Consider the non-standard amino acid 2-amino-3-(4-aminophenyl)propanoic acid ( $p$ -aminophenylalanine) in an aqueous solution at 298 K. The $pK_a$ values for its three ionizable groups are 2.0, 4.6, and 9.2. Calculate the isoelectric point (pI) of this amino acid.

Options

A.
6.9
B.
3.3
C.
5.6
D.
5.3

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Amino Acid Zwitterion pH Sensitivity|Carboxylic Acid Strength|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

This question targets a common, fatal heuristic used by students: 'If an amino acid has two $pK_a$ values below 7, it acts like an acidic amino acid (e.g., Aspartic acid), and the pI is the average of the two lowest values.' Applying this blindly yields a pI of 3.3 (Option B).

The trap exploits the fundamental difference between a neutral acid ( $0 \rightarrow -1$ charge transition, like a side-chain carboxyl group) and a cationic acid ( $+1 \rightarrow 0$ charge transition, like the anilinium ion here).

Because the $pK_a$ of 4.6 belongs to a cationic acid, the molecule transitions from a $+1$ to a $0$ charge state at this pH, making 4.6 the lower bound of the zwitterion, not the upper bound.

Students must draw the actual protonation states rather than relying on numerical pattern matching.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Amino Acid Zwitterion pH Sensitivity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free