Any of a group of organic pigments characterized by the possession of a cyclic group of four linked nitrogen-containing rings (a tetrapyrrole nucleus), the nitrogen atoms of which are often coordinated to metal ions. Porphyrins differ in the nature of their side-chain groups. They include the chlorophylls, which contain magnesium; and haem, which contains iron and forms the prosthetic group of haemoglobin, myoglobin, and the cytochromes. Porphyrins are also used in host-guest chemistry. See supramolecular chemistry.