<unk>  base, obtained as a mineral oil mull, melts at 83–84 °C. Hydrochloride and phosphorus salts form white hygroscopic crystals and are easily dissolved in water or ethanol, but not in ether. In aqueous solution, the imidazole ring of<unk>  exists in two tautomeric forms, identified by which of the two nitrogen atoms is protonated. The nitrogen farther away from the side chain is the 'tele' nitrogen and is denoted by a lowercase tau sign and the nitrogen closer to the side chain is the 'pros' nitrogen and is denoted by the pi sign. The tele tautomer, Nτ-H-histamine, is preferred in solution as compared to the pros tautomer, Nπ-H-histamine. The tele tautomer (Nτ-H-histamine), on the left is more stable than the pros tautomer (Nπ-H-histamine) on the right.<unk> <unk> s two basic centres, namely the aliphatic amino group and whichever nitrogen atom of the imidazole ring does not already<unk> ve a proton. Under physiological conditions, the aliphatic amino group <unk> ving a pKa around 9.4) will be protonated, whereas the second nitrogen of the imidazole ring (pKa ≈ 5.8) will not be protonated. Thus,<unk>  is normally protonated to a singly charged cation. Since human blood is slightly basic (with a normal pH range of 7.35 to 7.45) therefore the predominant form of<unk>  present in human blood is monoprotic at the aliphatic nitrogen.<unk>  is a monoamine neurotransmitter.
Histamine
Leukotriene
Serotonin
Nitric oxide
Antileukotriene
Defensin
Acetylcholine
Interleukin