4(5)-Alkylimidazoles containing the amidine moiety(-NH-CH=N-) in the ring were frequently used as model compounds to explain tautomeric preferences in histamine. This approach is not quite correct because histamine contains additionally the amino group in the side chain. The amino group can interact with the amidine moiety and change the composition of tauto-meric mixture. The prototropic tautomerism in 4(5)-alkylimidazoles is influenced only by electronic effects of the alkyl group, and tautomeric preferences seem to be slightly dependent on solvent polarity. The same situation is found for neutral histamine. However, for the histamine monocation, a change of tautomeric preferences takes place when proceeding from apolar (or gas phase) to polar solvents. This variation may explain very complex physiological properties of histamine and its interactions with different specific receptors (H1, H2, H3 and H4).
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