Before You Begin

Question #44985

Difficulty: 8/10 | 🟢 Free

The Question

Consider a 1:1 aqueous complex of the DNA fragment $5'-d(ApC)-3'$ and the dipeptide L-Histidyl-L-Lysine at 298 K. The initial pH of the buffer is 7.4.

The pH is gradually lowered to 1.0 and the mixture is heated, resulting in the complete acid-catalyzed hydrolysis of all amide, phosphodiester, and N-glycosidic linkages. Assume all resulting monomeric units (bases, sugars, and amino acids) remain structurally intact without further thermal degradation.

Given the $pK_a$ values for the ionizable groups of the free amino acids: Histidine: $\alpha$ -COOH (1.8), $\alpha$ - $NH_3^+$ (9.2), imidazole side-chain (6.0) Lysine: $\alpha$ -COOH (2.2), $\alpha$ - $NH_3^+$ (9.0), $\epsilon$ - $NH_3^+$ (10.5)

Identify the correct combination of: (i) The predominant net charge of the intact dipeptide at $pH = 1.0$ (prior to hydrolysis). (ii) The total moles of chiral organic molecules produced per mole of the complex. (iii) The total moles of inorganic phosphate ( $H_3PO_4$ ) produced per mole of the complex.

Options

A.
(i) +3, (ii) 4, (iii) 1
B.
(i) +4, (ii) 4, (iii) 2
C.
(i) +3, (ii) 2, (iii) 2
D.
(i) +2, (ii) 3, (iii) 1

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

Base Pairing Stability|Nucleoside vs Nucleotide|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

The question tests three distinct traps: 1.

Peptide Charge Trap: Students often erroneously sum the charges of all free amino acid functional groups (+4 at pH 1.0) instead of recognizing that the peptide bond formation eliminates the ionizable $\alpha$ -COOH of His and $\alpha$ - $NH_2$ of Lys. 2.

Nucleoside vs Nucleotide Notation Trap: The notation $5'-d(ApC)-3'$ represents a dinucleoside monophosphate with no 5'-terminal phosphate.

Students who assume DNA fragments always have a 5'-phosphate will incorrectly count 2 moles of $H_3PO_4$ . 3.

Stereochemistry of Hydrolysis Products: Identifying which hydrolysis products retain chirality (sugars and amino acids) versus those that are achiral (aromatic nucleobases).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Base Pairing Stability
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free