Porphyrins are a group of
organic compounds, many naturally occurring. One of the best-known porphyrins is
heme, the pigment in red
blood cells; heme is a
cofactor of the protein
hemoglobin. Porphyrins are
heterocyclic macrocycles composed of four modified
pyrrole subunits interconnected at their α carbon atoms via
methine bridges (=CH-). Porphyrins are
aromatic. That is, they obey
Hückel's rule for aromaticity, possessing 4n+2 π electrons (n=4 for the shortest cyclic path) delocalized over the macrocycle. Thus porphyrin macrocycles are highly
conjugated systems. As a consequence, they typically have very intense absorption bands in the visible region and may be deeply colored; the name
porphyrin comes from a
Greek word for
purple. The macrocycle has 26
pi electrons in total. The parent porphyrin is
porphine, and substituted porphines are called porphyrins. The specific porphyrin in
heme B (see Figure) is called
protoporphyrin IX and has 4 methyl, two vinyl, and two propionic acid substituents at the indicated positions.