In haemoglobin, aspartic acid acts as blood buffer. It is a dicarboxylic amino acid. The carboxylic group of the side chain dissociates at physiological pH to give the negatively charged side chain. Histidine may be positively charged or neutral, depending upon the local environment. It is the only amino acid having a proton that dissociates in the neutral pH range. Glutamine is the uncharged derivative of glutamate and contains a terminal amide group instead of carboxylate. Lysine is positively charged at neutral pH.