= Imidazole
<Imidazole> is a five-membered aromatic heterocycle with <nitrogen> <atoms> at positions 1 and 3. In a neutral form, the two <nitrogens> have different lone-pair roles: one pair participates in the aromatic system and the other is available for <proton> acceptance or metal binding.
The ring occurs in the <histidine> side chain. Free <imidazole> can compete with histidine contacts during <immobilized metal affinity chromatography>, helping control <protein> retention and elution.
<Organic Chemistry by John McMurry>[Chapter24, Amines and Heterocycles: imidazole and the distinct roles of nitrogen lone pairs]{ref}
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