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Hello! My NSTP books, etc. say histidine is a positively-charged amino acid. But according to the AAMC's solution to this problem, at physiologic pH, histidine is neutral? I know that histidine's side-chain pKa and therefore overall pI make it borderline, but what should we consider histidine to be for the MCAT?
Whether an amino acid is positively, negatively, or neutrally charged depends on its pKa in relation to the pH. In this question, the pH is at 7. Since Histidine's side chain pKa is 6, this means that at pH 7, its amino group is protonated (+1), its carboxyl group is deprotonated (-1), and its side chain is deprotonated (0). Therefore, it's neutral at pH 7. If the solution were to be at pH 5, then histidine would be positively charged since its side chain would still be protonated, giving it a +1 charge.