The nitrogen atoms of the imidazole ring of histidine are denoted by
pros ('
near', abbreviated
π) and
tele ('
far', abbreviated
τ)
to show their position relative to the
side chain. This recommendation arose from the fact that two different systems of numbering
the atoms in the imidazole ring of histidine had both been used for a considerable
time (biochemists generally numbering as 1 the nitrogen atom adjacent to the
side chain, and organic chemists designating it as 3). The carbon atom between the two ring
nitrogen atoms is numbered 2 (as in imidazole), and the carbon atom next to the
τ nitrogen is numbered 5. The carbon atoms of the aliphatic chain are
designated α and β.
This numbering should also be used for the decarboxylation product histamine and for
substituted histidine.
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