 Histidine

Histidine (symbol His or H) is an α-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.