C6H9N3O2 • HCl • H2O
M.W.: 209.63 g/mole

Physical State : Solid (Crystalline solid)
Appearance :  Colourless crystalline
Color : White
Purity :  98.5% - 101.5%
Odour : Weak characteristic odour
Solubility : 0.5 M in H2O at 20 °C, clear, colorless
Partially soluble in cold water ;
Insoluble in methanol, diethyl ether.

The essential amino acid L-histidine is one of the three amino acids with a basic side chain, and is very hydrophilic in character.  It contains an imidazole group in the side chain. Because the pKa of the imidazole group is close to physiological pH, the imidazole moiety can be either uncharged or positively charged, depending on the local environment.

This property makes histidine residues of exceptional interest in the active sites of many proteins, such as hemoglobin and myoglobin, where the imidazole ring can readily alternate between the charged (imidazolium) and uncharged (imidazole) states to participate in bond formation and breakage. 

Store at Room Temperature.