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Question #44999

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The Question

An aqueous solution at 298 K contains free ascorbic acid (Vitamin C, $pK_{a1} = 4.2$ ) and Vitamin $B_{12}$ tightly bound to a carrier tripeptide, Asp-Gly-His. The $pK_a$ values for the ionizable groups of the tripeptide are 2.0 ( $\alpha$ -COOH), 4.0 (Asp side-chain), 6.0 (His side-chain), and 9.0 ( $\alpha$ - $NH_3^+$ ). The solution is initially at pH 7.5. Dilute $\ce{HCl}$ is added slowly until the macroscopic solubility of the Vitamin $B_{12}$ complex reaches an absolute minimum. Which of the following statements correctly identifies the pH at which this occurs and the respective states of the vitamins in the mixture?

Options

A.
At pH 5.0, the $B_{12}$ -peptide complex precipitates due to minimized electrostatic repulsion, while Vitamin C remains in the aqueous supernatant primarily as an ascorbate anion.
B.
At pH 7.5, the $B_{12}$ -peptide complex precipitates due to the zwitterionic nature of the His residue, while Vitamin C remains soluble.
C.
At pH 5.0, both the $B_{12}$ -peptide complex and Vitamin C precipitate because Vitamin C ( $pK_a = 4.2$ ) becomes predominantly un-ionized and loses its hydration shell.
D.
At pH 4.0, the $B_{12}$ -peptide complex precipitates due to the protonation of the Asp residue, trapping the hydrophilic Vitamin $B_{12}$ in the solid phase.

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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

Vitamin Solubility Logic|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of macromolecular carrier dependence in vitamin solubility.

Students often memorize the heuristic that 'Vitamins B and C are water-soluble' and apply it blindly, assuming they will always remain in the aqueous phase.

The trap exploits the Isoelectric Point (pI) shift: when Vitamin B12 is tightly bound to a peptide, the complex's solubility is governed by the peptide's net charge, not the vitamin's polarity.

At the pI, intermolecular hydrophobic interactions dominate, causing the complex to precipitate and dragging the 'soluble' vitamin out of solution.

Distractors test miscalculations of the pI and the misapplication of the Henderson-Hasselbalch equation to Vitamin C.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Vitamin Solubility Logic
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free