Porphyrins are heterocyclic organic macrocycles composed of four modified pyrrole rings connected by their α-carbon atoms via methine [-CH=] linkages. Porphyrins are aromatic and thus obey Hückel’s 4n + 2 π electron rule. Porphyrins are highly conjugated and have very intense absorption in the visible region and may be deeply colored. The name porphyrin derives from the Greek word for purple. Porphyrins are the conjugate acids of ligands that form complexes by binding metal ions, which are usually in the +2 or +3 oxidation state. Some porphyrins, such as heme, coordinate iron; other porphyrins chelate cobalt, magnesium, manganese, or nickel. Porphyrin-like compounds have been reported in meteorites, and a prebiotic synthesis has been proposed starting from pyrroles and aldehydes. Acidic conditions are required for this synthesis.
F. M. Johnson proposed in the 1970s that the porphyrins were the carriers of the diffuse interstellar bands. This proposal has not been...