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A porphyrin is a heterocyclic macrocycle derived from four pyrrole-like subunits interconnected via their α carbon atoms via methine bridges (=CH-). The macrocycle, therefore, is highly conjugated, and consequently deeply coloured, hence the name porphyrin, from a Greek word for purple. The macrocycle has 26 pi electrons. The parent porphyrin in porphine, and substituted porphines are called porphyins. Many porphyrins occur in nature and in bio-inspired synthetic catalysts and devices.
Related to porphyrins are corrins, chlorins (2,3-dihydroporphyrin) and bacteriochlorophylls (2,3,12,13-tetrahydroporphyrin). Some follow Hückel's rule, having aromatic properties.
Porphyrins combine readily with metals, coordinating with them in the central cavity. Iron- (heme), magnesium- (chlorophyll), zinc-, copper-, nickel-, and cobalt- containing porphyrins are known, and many other metals can be inserted. A porphyrin in which no metal is inserted in its cavity is called a free base.
Some iron-containing porphyrins are called hemes; and heme-containing proteins, or hemoproteins, are found extensively in biochemistry, e.g., hemoglobin. Hemoglobin iron is the actual transfer site for oxygen and can be preferentially bound up with carbon monoxide, thus poisoning by asphyxiation can occur. Some shellfish with green-colored blood have a copper-centered porphyrin.
If one of the four pyrrole subunits is reduced to pyrroline, a chlorin is produced, the ring structure found in chlorophyll. If two of the four pyrrole subunits are reduced, then either a bacteriochlorin (as found in some photosynthetic bacteria) or an isobacteriochlorin is formed, depending on the relative positions of the reduced pyrroles.
The committed step for porphyrin synthesis is the formation of D-Aminolevulinic acid from glycine (an abundant amino acid) and succinyl-CoA (from the citric acid cycle). Two dALA molecules are combined into porphobilinogen (PBG), which contains the pyrrole ring. Four PBGs are then combined through deamination into hydroxymethyl bilane (HMB), which is hydrolysed to form the circular tetrapyrrole uroporphyrinogen III. This molecule undergoes a number of further modifications. Intermediates are used in different species to form particular substances, but, in humans, the main end-product protoporphyrin IX is combined with iron to form heme. Bile pigments are the breakdown products of heme.
This is a schematic representation of porphyrin biosynthesis, with references by EC number and the OMIM database. The porphyria associated with the deficiency of each enzyme is also shown:
One of the more common syntheses for porphyrins is based on work by Paul Rothemund.[1][2] His techniques have formed the basis for more modern syntheses such as by Alder and Longo.[3] The synthesis of simple porphyrins such as meso-tetraphenylporphyrin is also commonly done in university teaching labs.[4]
In this method, porphyrins are assembled from pyrrole and substituted aldehydes. Acidic conditions are essential; formic acid, acetic acid, proprionic acid are used as solvents, or p-toluenesulfonic acid can be used with a non-acidic solvent. A large amount of side product is formed and is removed, usually by chromatography.
Although natural porphyin complexes are essential for life, synthetic porphyrins and their complexes have limited utility. Complexes of meso-tetraphenylporphyrin, e.g. the iron-(III) chloride complex (TPPFeCl) catalyse certain reactions in organic chemistry. Porphyrin-based compounds are of interest in molecular electronics and supramolecular building blocks. Phthalocyanines, which are derivatives of porphyrins, are more widely used in commerce as dyes and catalysts. Highly efficient solar cells using synthetic porphyrin dyes are a field of ongoing research. See Dye-sensitized solar cells.
Porphyrins are often used to construct structures in supramolecular chemistry. These systems take advantage of the binding with the coordinated metal. Although they are also useful since they provide a 90 degree angle between substituents. An example of a host-guest complex that was constructed from a macrocycle composed of four porphyrins.[5] A guest free base porphyrin is bound to the center by coordination with its four pyridine sustituents.