Q. Which of the following amino acids have side chains that are negatively charged under physiological conditions (i.e., near pH 7)?
✅ Correct Answer: (A)
Asp
Explanation:
Aspartic acid (Asp) has a carboxyl group in its side chain that loses a proton at physiological pH (~7), making it negatively charged.
Why others are incorrect:
- (B) Histidine (His): Side chain is positively charged or neutral around pH 7 because its imidazole group has a pKa close to physiological pH but usually not negatively charged.
- (C) Tryptophan (Trp): Nonpolar, uncharged side chain.
- (D) Cysteine (Cys): Has a thiol group, which is mostly neutral at physiological pH (can be negatively charged but only under more alkaline conditions).
Final Answer: (A) Asp