HEMOGLOBIN

The hemoglobin protein fits into our study of aromatic compounds in at least two ways:

  • heme is a nearly planar tetapyrrole porphyrin that can be considered as an aromatic [18]-annulene if four of the pi electrons that are not part of the macrocycle are disregarded.

  • the histidine amino acid residue in the globin protein that holds the heme in place is an imidazole whose two nitrogen atoms have their non-bonded electrons in different orbitals and thus resemble the non-bonded electrons of pyrrole and pyridine

The histidine nitrogen that coordinates with the Fe(2+) atom of heme is the one that is more basic, i.e. which does not have its non-bonded electrons as part of the delocalized pi system. The histidine that is close to the O2 coordinated to the Fe atom is oriented with its partially positive hydrogen close to the electronegative oxygen atom of O2.