Question: what are ionizable amino acids?
Answer: Seven amino acid side chains contain groups that ionize between pH 1 and 14. For Asp, Glu, Tyr, and Cys, the ionizable groups are uncharged below their pK and negatively charged above their pK. For His, Lys, and Arg, the ionizable groups are positively charged below their pK and uncharged above their pK.