Which of the following amino acids has a side chain with an ionizable proton and can exist in four different forms, depending on the pH of the solution?

Question

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Amino acid III is histidine, which has a side chain with an ionizable proton and can exist in four different forms depending on the pH of the solution. At a low pH, histidine will be predominantly protonated with a +1 charge; at a neutral pH, it will be predominantly uncharged; at a high pH, it will be predominantly deprotonated with a -1 charge; and at a very high pH, it will be doubly deprotonated with a -2 charge. None of the other amino acids listed have side chains with ionizable protons that can exist in four different forms.

Which of the Following Amino Acids Has a Side Chain