A porphyrin is a heterocyclic macrocycle derived from four pyrroline subunits interconnected via their α carbon atoms via methine bridges (=CH-). Porphyrins are aromatic and they obey Hückel's rule for aromaticity in that they possess 4n+2 pi electrons which are delocalized over the macrocycle. The macrocycle, therefore, is a highly-conjugated system, and, as a consequence, is deeply coloured - the name porphyrin comes from a Greek word for purple. The macrocycle has 22 pi electrons. The parent porphyrin is porphine, and substituted porphines are called porphyrins. Many porphyrins occur in nature, such as in green leaves and red blood cells, and in bio-inspired synthetic catalysts and devices.
Related to porphyrins are several other heterocycles, including corrins, chlorins, bacteriochlorophylls, and corphins. Chlorins (2,3-dihydroporphyrin) are more reduced, that contain more hydrogen, than porphyrins, featuring a pyrroline subunit. This structure occurs in chlorophyll. Replacement of two of the four pyrrolic subunits with pyrrolinic subunits results in either a bacteriochlorin (as found in some photosynthetic bacteria) or an isobacteriochlorin, depending on the relative positions of the reduced rings. Some porphyrin derivatives follow Hückel's rule, but most do not.
One of the more common syntheses for porphyrins is based on work by Paul Rothemund. His techniques underpin more modern syntheses such as those described by Alder and Longo. The synthesis of simple porphyrins such as meso-tetraphenylporphyrin is also commonly done in university teaching labs.
In this method, porphyrins are assembled from pyrrole and substituted aldehydes. Acidic conditions are essential; formic acid, acetic acid, and propionic acid are typical reaction solvents, or p-toluenesulfonic acid can be used with a non-acidic solvent. Lewis acids such as boron trifluoride etherate and ytterbium triflate have also been known to catalyse porphyrin formation. A large amount of side-product is formed and is removed, usually by chromatography.
|ALA synthase||Glycine, succinyl CoA||D-Aminolevulinic acid||3p21.1||18.104.22.168||125290||none|
|ALA dehydratase||D-Aminolevulinic acid||Porphobilinogen||9q34||22.214.171.124||125270||ALA-Dehydratase deficiency|
|PBG deaminase||Porphobilinogen||Hydroxymethyl bilane||11q23.3||126.96.36.199||176000||acute intermittent porphyria|
|Uroporphyrinogen III synthase||Hydroxymethyl bilane||Uroporphyrinogen III||10q25.2-q26.3||188.8.131.52||606938||congenital erythropoietic porphyria|
|Uroporphyrinogen III decarboxylase||Uroporphyrinogen III||Coproporphyrinogen III||1q34||184.108.40.206||176100||porphyria cutanea tarda|
|Coproporphyrinogen III oxidase||Coproporphyrinogen III||Protoporphyrinogen IX||3q12||220.127.116.11||121300||coproporphyria|
|Protoporphyrinogen oxidase||Protoporphyrinogen IX||Protoporphyrin IX||1q22||18.104.22.168||600923||variegate porphyria|
|Ferrochelatase||Protoporphyrin IX||Heme||18q21.3||22.214.171.124||177000||erythropoietic protoporphyria|
In 2008 the UK corporation Destiny Pharma reported successful clinical trials of an intranasally applied porphorin XF-73 against methicillin-resistant Staphylococcus aureus.
Porphyrins are often used to construct structures in supramolecular chemistry. These systems take advantage of the Lewis acidity of the metal, typically zinc. An example of a host-guest complex that was constructed from a macrocycle composed of four porphyrins. A guest-free base porphyrin is bound to the center by coordination with its four pyridine sustituents.