Histidine (symbol His or H)[2] is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO− form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also.[3] It is encoded by the codons CAU and CAC. Histidine was first isolated by Albrecht Kossel and Sven Gustaf Hedin in 1896.[4] The name stems from its discovery in tissue, from ἱστός histós "tissue".[2] It is also a precursor to histamine, a vital inflammatory agent in immune responses. The acyl radical is histidyl.