NCIt definition : A group of organic heterocyclic compounds that are comprised of four modified pyrrole
subunits connected via methine bridges and form an aromatic macrocyclic structure,
which has one or more side chains attached. Many porphyrins are naturally occurring
pigments. Porphyrins with metal ions bound to the porphin ring can be cofactors for
biological processes, such as oxygen transport by hemes and photosynthesis by chlorophyll.;