Which amino acid can protonate & deprotonate at neutral pH: (A) Histidine, (B) Leucine, (C) Hemocysteine, (D) Arginine
Answer is A. Histidine is the only amino acid that has a functional group (imidazole) that titrates in the physiological pH range.
Depending upon localized pH of its environment, histidine can serve both proton donor (deprotonation) and acceptor (protonation).