Histidine. Computer model of a molecule of the amino acid histidine (C6.H9.N3.O2). Atoms (solid spheres) are colour-coded: carbon (green), oxygen (red), hydrogen (grey) and nitrogen (blue). It is an essential amino acid (required in the diet), is common in haemoglobin and is essential for growth in infants. It is a precursor to histamine. Here, the zwitterion, an internal salt that forms in the body, is shown. The chemical groups characteristic of an amino acid, the amino (NH2) and the carboxyl (COOH) groups, have reacted together, forming an NH3 group (positively-charged, lower right) and a COO group (negatively-charged, lower right). At upper left is histidine's imidazole ring group.