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Question #44986

Difficulty: 8/10 | 🟢 Free

The Question

An artificial electrophilic base analog, $p$ -nitrobenzenediazonium chloride, is incubated with 2'-deoxycytidine in an aqueous buffer at pH 8.5 and 273 K. Initially, a rapid reversible interaction is observed, but upon prolonged standing, a stable, irreversible covalent adduct is isolated, effectively bypassing standard DNA mismatch repair enzymes. Identify the structure of the major stable adduct and the dominant mechanistic control governing its formation.

Options

A.
5-( $p$ -nitrophenylazo)-2'-deoxycytidine; Thermodynamic control via Electrophilic Aromatic Substitution.
B.
$N^4$ -( $p$ -nitrophenyldiazenyl)-2'-deoxycytidine; Kinetic control via nucleophilic attack by the exocyclic amine.
C.
Non-covalent supramolecular complex; Thermodynamic control via Watson-Crick hydrogen bonding.
D.
6-( $p$ -nitrophenylazo)-2'-deoxycytidine; Thermodynamic control driven by steric relief from the deoxyribose moiety.

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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|Electrophilic Substitution Control|Azo Coupling Regioselectivity

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to differentiate between non-covalent supramolecular interactions (H-bonding) and covalent reactivity (EAS) in biological molecules.

The 'Chemical Exception' is the kinetic vs. thermodynamic trap: students often assume the exocyclic amine ( $NH_2$ ) will form a stable triazene (N-coupling) or that the molecule will simply H-bond (Watson-Crick mimicry).

However, the +M effect of the $NH_2$ group makes the C5 position highly nucleophilic.

The hidden variable is recognizing that while N-coupling is kinetically favored, C5-azo coupling is the irreversible thermodynamic sink, effectively cross-linking the base and bypassing standard non-covalent mismatch repair mechanisms.

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