Question

Which of the following gives the correct representation of zwitter ion of the given amino acid ?

• A. Structure 1
• B. Structure 2
• C. Structure 3
• D. Structure 4

Hint:

A zwitterion must simultaneously possess:
- A protonated ammonium group (\(-\text{NH}_3^+\)).
- A deprotonated carboxylate group (\(-\text{COO}^-\)).
Its net molecular electrical charge is zero at the isoelectric point.

Answer 1

The Correct Option is B

Concept:
Amino acids are dipolar molecules containing both an acidic carboxyl group (\(-\text{COOH}\)) and a basic amino group (\(-\text{NH}_2\)).
In neutral aqueous solution, an internal acid-base proton transfer occurs from the carboxyl group to the amino group, generating a dipolar ion known as a zwitterion.

Step 1: Identifying the Substrate Amino Acid:
The amino acid shown is alanine:
\[ \text{H}_2\text{N}-\text{CH}(\text{CH}_3)-\text{COOH} \] It consists of a central \(\alpha\)-carbon bonded to a methyl side chain, a hydrogen atom, an amino group, and a carboxyl group.

Step 2: Intramolecular Proton Transfer Mechanism:
The carboxyl group has a low \(pK_a\) (\(\approx 2.3\)) and acts as a proton donor, while the basic amino group has a higher \(pK_a\) (\(\approx 9.7\)) and acts as a proton acceptor.
The acidic proton of the \(-\text{COOH}\) group transfers to the lone pair of the \(-\text{NH}_2\) group:
\[ \text{H}_2\text{N}-\text{CH}(\text{CH}_3)-\text{COOH} \rightleftharpoons \text{H}_3\text{N}^+-\text{CH}(\text{CH}_3)-\text{COO}^- \]

Step 3: Characterizing the Zwitterion Structure:
In the resulting zwitterion:
- The amino group carries a positive charge: \(-\text{NH}_3^+\),
- The carboxylate group carries a negative charge: \(-\text{COO}^-\).
The molecule contains equal numbers of positive and negative charges, making it electrically neutral overall.
- Form (A) represents the un-ionized covalent structure.
- Form (C) represents the anionic form predominant in strongly alkaline medium.
- Form (D) represents the cationic form predominant in strongly acidic medium.
- Form (B) correctly represents the dipolar zwitterion.
Final Answer:
The correct representation of the zwitterion is \(\text{H}_3\text{N}^+-\text{CH}(\text{CH}_3)-\text{COO}^-\), corresponding to option (B).