Question

The two amino acids having R groups with a negative net charge at pH 7.0 are

• A. Aspartate and glutamate
• B. Arginine and histidine
• C. Cysteine and methionine
• D. Proline and valine

Hint:

Only \textbf{aspartate (Asp)} and \textbf{glutamate (Glu)} have negatively charged side chains at physiological pH.

Answer 1

The Correct Option is A

Step 1: At physiological pH ($\approx 7.0$), amino acids may carry positive, negative, or neutral charges depending on their side chains (R groups).
Step 2: Aspartate and glutamate contain a carboxyl (-COO$^-$) group in their R chains.
Step 3: At pH 7.0, these carboxyl groups are deprotonated, giving the R groups a negative charge.
Step 4: The other options are incorrect because:

Arginine and histidine have positively charged or basic side chains.
Cysteine and methionine are neutral.
Proline and valine are non-polar and neutral.