Amino acid side-chain charge at physiological pH
At pH 7.4, which amino acid side chain carries a net positive charge?
Answer C: explanation for every choice
A
Aspartate's side chain has a pKa of about 3.9, so at pH 7.4 it is deprotonated and negative.
B
Histidine's side chain has a pKa of about 6.0, so at pH 7.4 only a small fraction is protonated. It is mostly neutral.
C
Correct. Lysine's side chain has a pKa of about 10.5, so at pH 7.4 it is protonated and positive.
D
Glutamine has a polar but uncharged amide side chain.
TakeawaypH below a group's pKa → protonated. Lysine and arginine are positive at physiological pH. Histidine is mostly neutral.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb8.