Question

Choose the correct option for structures of A and B, respectively:

• A.
• B.
• C.
• D.

Hint:

In aqueous solutions, amino acids behave differently at various pH values. At low pH, the amino group is protonated and the carboxyl group remains neutral, while at high pH, the amino group is deprotonated and the carboxyl group becomes negatively charged.

Answer 1

The Correct Option is B

The given structure is of an amino acid with both acidic (COOH) and basic (NH₂) groups. The transformation at different pH values is due to the ionization states of these groups.

1. Understanding Zwitterionic Form: At pH 7, amino acids typically exist in a zwitterionic form where the amino group (NH₂) is protonated to NH₃⁺, and the carboxylic group (COOH) loses a proton to become COO^-.
2. Effect of pH:
   • At pH 2: The environment is acidic, so the carboxylic group tends to remain protonated as COOH, while the amino group remains as NH₃⁺. Therefore, the structure will predominantly be the protonated form with COOH and NH₃⁺.
   • At pH 10: This is a basic environment, where the amino group will remain NH₂ and the carboxylic group becomes COO^-. Hence, the structure will be the deprotonated form with NH₂ and COO^-.
3. Correct Structures: The correct structures of A and B are determined by these transformations.

Conclusion: The correct option for the structures of A and B corresponds to the image above, where A is at pH 2 and B is at pH 10.

Answer 2

At pH 2, the amino group (-NH\(_2\)) is protonated to form an ammonium ion (-NH\(_3^+\), and the carboxyl group (-COOH) remains in its protonated form, as carboxyl acids are not deprotonated at this pH.
So, compound A will have the structure: \[ \text{H}_3\text{N} - \text{CH} - \text{COOH} \] At pH 10, the amino group remains in its deprotonated form (-NH\(_2\)) and the carboxyl group is deprotonated to form a carboxylate ion (-COO\(^-\)).
Therefore, compound B will have the structure: \[ \text{H}_2\text{N} - \text{CH} - \text{COO}^- \] Thus, the correct option is option (2).