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Question #44968

Difficulty: 8/10 | 🟢 Free

The Question

Consider two synthetic amino acids, $X$ and $Y$ :

$X$ : 2-amino-3-(4-aminophenyl)propanoic acid $Y$ : 2-amino-3-(2-aminophenyl)propanoic acid

For both $X$ and $Y$ , the $pK_a$ values of the $-\ce{COOH}$ and $\alpha-\ce{NH3+}$ groups are 2.0 and 9.0, respectively. The $pK_a$ values of the conjugate acids of the aromatic $-\ce{NH2}$ groups in $X$ and $Y$ are 5.1 and 4.4 (not necessarily in that order).

A mixture of $X$ and $Y$ is subjected to paper electrophoresis in a buffer at $pH = 6.85$ at $298\ K$ .

Which of the following correctly describes the isoelectric points ( $pI$ ) and the direction of migration of $X$ and $Y$ ?

Options

A.
$pI_X = 7.05$ , $pI_Y = 6.70$ ; $X$ migrates to the cathode, $Y$ migrates to the anode
B.
$pI_X = 5.50$ , $pI_Y = 5.50$ ; Both $X$ and $Y$ migrate to the anode
C.
$pI_X = 3.55$ , $pI_Y = 3.20$ ; Both $X$ and $Y$ migrate to the anode
D.
$pI_X = 6.70$ , $pI_Y = 7.05$ ; $X$ migrates to the anode, $Y$ migrates to the cathode

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Amino Acid Zwitterion pH Sensitivity|Ortho-Effect in Anilines|Sandmeyer vs Gattermann

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of the Ortho-Effect in anilines and the calculation of isoelectric points for multi-basic amino acids.

The primary trap is failing to recognize that the ortho-isomer ( $Y$ ) is less basic than the para-isomer ( $X$ ) due to steric hindrance to solvation, which lowers its $pK_a$ .

Students who reverse this effect will choose Option D.

Additionally, students must correctly identify that for an amino acid with an extra basic group, the $pI$ is the average of the two highest $pK_a$ values.

Averaging the wrong $pK_a$ values leads to Options B or C.

Finally, students must correctly correlate the relationship between $pH$ , $pI$ , net charge, and electrode migration (positive to cathode, negative to anode).

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