Remember that a buffer is most effective when its pKa is within the pH range of the surrounding medium. Histidine is the only amino acid with good buffering capacity at physiologic pH. The imidazole side chain of histidine has a pKa around 6.0 and can reversibly donate and accept protons at physiologic pH. Arginine and lysine are basic amino acids with pKa's of 12.5 and 10.5, respectively; at physiologic pH both will behave as bases and accept protons. Aspaic acid and glutamic acid are acidic amino acids with pKa's of approximately 4; at physiologic pH they will behave as acids and donate protons. Ref: Rodwell V.W. (2011). Chapter 30. Conversion of Amino Acids to Specialized Products. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.