metal cations. One of the applications of imidazole is in the purification of His-tagged proteins in immobilised metal affinity chromatography (IMAC). Imidazole is used to elute tagged proteins bound to nickel ions attached to the surface of beads in the chromatography column. An excess of imidazole is passed through the column, which displaces the His-tag from nickel coordination, freeing the His-tagged proteins. Use in biological research Imidazole is a suitable buffer for pH 6.2-7.8,. Pure imidazole has essentially no absorbance at protein relevant wavelenths (280 nm), however lower purities of imidazole can give notable absorbance at 280 nm. Imidazole can interfere with the Lowry protein assay. Salts of imidazole Salts of imidazole where the imidazole ring is the cation are known as imidazolium salts (for example, imidazolium chloride or nitrate). These salts are formed from the protonation or substitution at nitrogen of imidazole. These salts have been used as ionic liquids and precursors to stable carbenes. Salts where a deprotonated imidazole is an anion are also well known; these salts are known as imidazolates (for example, sodium imidazolate, NaC3H3N2). Related heterocycles Benzimidazole, an analog with a fused benzene ring Dihydroimidazole or imidazoline, an analog where the 4,5-double bond is saturated Pyrrole, an analog with only one nitrogen atom in position 1 Oxazole, an analog with the nitrogen atom in position 1 replaced by oxygen Thiazole, an analog with the nitrogen atom in position 1 replaced by sulfur Pyrazole, an analog with two adjacent nitrogen atoms Triazoles, analogs with three nitrogen atoms Safety Imidazole has low acute toxicity as indicated by the of 970 mg/kg (Rat, oral). See also 1-Methylimidazole 4-Methylimidazole References Imidazoles Alkaloids Simple aromatic rings Aromatic bases