Side-chain charge at basic pH: Which pair of amino acids carries a net negative charge on the side chain at pH 8.0?

Introduction / Context: Protein charge depends heavily on side-chain ionization. Correctly identifying acidic versus basic residues at a given pH helps predict protein folding, salt bridges, enzyme catalysis, and migration in electrophoresis.

Given Data / Assumptions:

• Side-chain pKa values: Asp/Glu carboxyl groups ≈ 4; Lys ε-NH3+ ≈ 10.5; His imidazole ≈ 6; Asn/Gln amides are nonionizable under normal conditions.
• pH under consideration is 8.0 (mildly basic).

Concept / Approach: For acidic residues (Asp, Glu), at pH well above their pKa the side chains are deprotonated and negatively charged (–COO−). Basic residues (Lys, Arg) tend to be positively charged below their pKa; histidine is partially protonated near pH 6 and largely neutral at pH 8.

Step-by-Step Solution: Compare pH 8 to pKa(Asp/Glu ~4) → deprotonated, −1 charge. Asn/Gln have amide side chains → do not ionize under these conditions. His/Lys at pH 8: His mostly neutral, Lys mostly still protonated (+). Therefore, only Aspartate and Glutamate are negatively charged at pH 8.

Verification / Alternative check: Charge calculations using Henderson–Hasselbalch confirm near-complete deprotonation for Asp/Glu at pH 8.

Why Other Options Are Wrong: Asn/Gln side chains do not carry charge; Leu/Gly are nonpolar/neutral; His/Lys are not negatively charged at pH 8.

Common Pitfalls: Confusing amides (Asn/Gln) with their acidic counterparts (Asp/Glu); assuming histidine is always positively charged.

Final Answer: Aspartate and Glutamate.