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Question #44028

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The Question

Consider an aqueous buffer solution containing equimolar amounts of the purine base Guanine and the amino acid L-Arginine, maintained at $\text{pH} = 7.4$ and $298\text{ K}$ . Guanine contains multiple nitrogen atoms (labeled N1, N2, N3, N7, N9 by standard IUPAC purine numbering) and exhibits lactam-lactim tautomerism. The side chain of Arginine features a guanidino group containing both $sp^2$ and $sp^3$ hybridized nitrogen atoms. Which of the following statements correctly identifies the thermodynamic protonation states, the most basic sites, and the underlying electronic effects governing these molecules under the specified conditions?

Options

A.
In Guanine, N7 is the most basic site but remains unprotonated at $\text{pH} = 7.4$ , while N1 is non-basic due to lone pair delocalization in the predominant lactam form. In Arginine, the $sp^2$ hybridized imine nitrogen of the side chain is protonated, yielding a resonance-stabilized guanidinium cation.
B.
In Guanine, N1 is the most basic site and is protonated at $\text{pH} = 7.4$ , driven by the thermodynamic stability of the lactim tautomer. In Arginine, the terminal $sp^3$ hybridized amine nitrogen is protonated due to superior aqueous solvation of the localized charge.
C.
In Guanine, N3 is the most basic site and remains unprotonated at $\text{pH} = 7.4$ , while N1 is non-basic due to strong intramolecular hydrogen bonding. In Arginine, the $sp^2$ hybridized imine nitrogen remains unprotonated because its lone pair is involved in cross-conjugation.
D.
In Guanine, N7 is the most basic site and is fully protonated at $\text{pH} = 7.4$ , while N1 is non-basic because the lactim (enol) form predominates. In Arginine, the terminal $sp^3$ hybridized amine nitrogen is protonated, forming a kinetically controlled intermediate.

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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

Basicity-Solvation Competition|Amino Acid Zwitterion pH Sensitivity|Nitrogenous Base Keto-Enol Tautomerism

Why This Question Is Tricky

Common Mistakes

This question tests the 'Chemical Exception' of visual basicity vs. actual electronic state.

Students often fall into the trap of assuming N1 in Guanine is a basic secondary amine, ignoring the dominant lactam tautomer which makes it an amide.

Furthermore, it tests the 'Hidden Variable' of resonance vs. solvation in Arginine; students frequently misidentify the terminal $-NH_2$ as the basic site due to perceived lower steric hindrance or typical alkyl amine behavior, failing to account for the massive thermodynamic driving force of symmetrical resonance stabilization in the guanidinium ion formed by protonating the $sp^2$ imine nitrogen.

Finally, it requires applying the Henderson-Hasselbalch relationship to determine the dominant species at physiological pH.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Basicity-Solvation Competition
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free