Explain why, when the guanidino group of arginine is protonated, the double-bonded nitrogen is the nitrogen that accepts the proton.

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The guanidino group of arginine consists of three nitrogen atoms bonded to a central carbon atom. Among these, one nitrogen is double-bonded to the carbon, while the other two are single-bonded.

The double-bonded nitrogen has a lone pair of electrons that is more readily available for protonation compared to the single-bonded nitrogens. This is because the double bond creates a partial positive charge on the carbon, making the lone pair on the double-bonded nitrogen more nucleophilic.

The single-bonded nitrogens are involved in resonance stabilization with the central carbon atom. This delocalization of electrons reduces the availability of their lone pairs for protonation.

When the guanidino group is protonated, the proton preferentially binds to the double-bonded nitrogen because it is the most nucleophilic site due to the reasons mentioned above.

This selective protonation maintains the resonance stabilization of the guanidino group, which is crucial for the stability of the molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Guanidino Group Structure

The guanidino group is a functional group found in arginine, characterized by a central carbon atom bonded to three nitrogen atoms. Two of these nitrogens are double-bonded to the carbon, while the third nitrogen is single-bonded and can carry a positive charge. Understanding this structure is crucial for analyzing how protonation affects the group.

Protonation and Basicity

Protonation refers to the addition of a proton (H+) to a molecule, which typically occurs at basic sites. In the case of the guanidino group, the nitrogen that accepts the proton is the one that is less sterically hindered and has a lone pair of electrons available for bonding. This concept is essential for understanding the reactivity of amino acids in biological systems.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, leading to a delocalization of electrons. In the guanidino group, protonation of the double-bonded nitrogen enhances resonance, allowing the positive charge to be distributed across the molecule. This stabilization makes the protonation of this nitrogen more favorable compared to others.