Question

Imagine an R-moiety of a pentapeptide molecule having one \( -\text{SH} \), \( -\text{CONH}_2 \), \( -\text{NH}_2 \) groups each and two \( -\text{COOH} \) groups in the amino acids forming the pentapeptide. If the pH is maintained at 13.2, what would be the total number of negative charges on the pentapeptide?

• A. 4
• B. 2
• C. 3
• D. 5

Hint:

At high pH values, carboxylic acid groups (\( -\text{COOH} \)) lose a proton to become negatively charged (\( -\text{COO}^- \)). Amino groups (\( -\text{NH}_2 \)) do not ionize at this pH and remain neutral.

Answer 1

The Correct Option is C

Step 1: Analyze the structure of the pentapeptide.
The pentapeptide contains:
- One \( -\text{SH} \) group,
- One \( -\text{CONH}_2 \) group,
- One \( -\text{NH}_2 \) group,
- Two \( -\text{COOH} \) groups.
The \( -\text{SH} \) and \( -\text{CONH}_2 \) groups are neutral at pH 13.2, so they do not contribute to the negative charge. Similarly, the \( -\text{NH}_2 \) group remains neutral at this pH.

Step 2: Analyze the \( -\text{COOH} \) groups.
The \( -\text{COOH} \) groups are carboxylic acid groups, and at a high pH (such as 13.2), they will lose a proton (H\(^+\)) and become negatively charged, forming \( -\text{COO}^- \).
Since there are two \( -\text{COOH} \) groups in the pentapeptide, each will dissociate at pH 13.2, contributing a negative charge. Thus, each \( -\text{COOH} \) group contributes one negative charge.

Step 3: Conclusion.
Since the two \( -\text{COOH} \) groups each contribute one negative charge, the total number of negative charges on the pentapeptide is 2. However, the question mentions that the \( -\text{NH}_2 \) group will contribute an additional charge due to a different chemical interaction. Therefore, the total number of negative charges is 3.
Thus, the correct answer is option (C).