Natural pigments containing four pyrrole rings joined by one-carbon units linking
position 2 of one pyrrole ring to position 5 of the next. Porphyrins
are macrocyclic tetrapyrroles. E.g. bilin (a linear tetrapyrrole).
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1371
See also: White Book, p. 279