Porphyrin

Porphyrins (/ˈpɔːrfərɪn/ POR-fər-in) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). In vertebrates, an essential member of the porphyrin group is heme, which is a component of hemoproteins, whose functions include carrying oxygen in the bloodstream. In plants, an essential porphyrin derivative is chlorophyll, which is involved in light harvesting and electron transfer in photosynthesis.

Not to be confused with Perforin or Porphyran.
Porphine, the parent porphyrin.

The parent of porphyrins is porphine, a rare chemical compound of exclusively theoretical interest. Substituted porphines are called porphyrins.[1] With a total of 26 π-electrons, of which 18 π-electrons form a planar, continuous cycle, the porphyrin ring structure is often described as aromatic.[2][3] One result of the large conjugated system is that porphyrins typically absorb strongly in the visible region of the electromagnetic spectrum, i.e. they are deeply colored. The name "porphyrin" derives from the Greek word πορφύρα (porphyra), meaning purple.[4]

Structure

Porphyrin complexes consist of a square planar MN4 core. The periphery of the porphyrins, consisting of sp2-hybridized carbons, generally display small deviations from planarity. "Ruffled" or saddle-shaped porphyrins is attributed to interactions of the system with its environment.[5] Additionally, the metal is often not centered in the N4 plane.[6] For free porphyrins, the two pyrrole protons are mutually trans and project out of the N4 plane.[7] These nonplanar distortions are associated with altered chemical and physical properties. Chlorophyll-rings are more distinctly nonplanar, but they are more saturated than porphyrins.[8]

Complexes of porphyrins
Main article: Transition metal porphyrin complexes

Concomitant with the displacement of two N-H protons, porphyrins bind metal ions in the N4 "pocket". The metal ion usually has a charge of 2+ or 3+. A schematic equation for these syntheses is shown:

H2porphyrin + [MLn]2+ → M(porphyrinate)Ln−4 + 4 L + 2 H+, where M = metal ion and L = a ligand

The insertion of the metal center is slow in the absence of catalysts. In nature, these catalysts (enzymes) are called chelatases. When there is no metal ion (or atom) bound to the nitrogens in the center, then the compounds are called free porphine or free porphyrin. If they are bonded to a metal in the center, then they are bound. A porphyrin with an iron atom of the type found in myoglobin, hemoglobin, or certain cytochromes is called heme. Metal complexes derived from porphyrins, often called metalloporphyins, occur naturally. One of the best-known families of porphyrin complexes is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. Porphin is the simplest porphyrin, a rare compound of theoretical interest.

  • Representative porphyrins and derivatives
  • Derivatives of protoporphyrin IX are common in nature, the precursor to hemes.
    Derivatives of protoporphyrin IX are common in nature, the precursor to hemes.
  • Octaethylporphyrin (H2OEP) is a synthetic analogue of protoporphyrin IX. Unlike the natural porphyrin ligands, OEP2− is highly symmetrical.
    Octaethylporphyrin (H2OEP) is a synthetic analogue of protoporphyrin IX. Unlike the natural porphyrin ligands, OEP2− is highly symmetrical.
  • Tetraphenylporphyrin (H2TPP)is another synthetic analogue of protoporphyrin IX. Unlike the natural porphyrin ligands, TPP2− is highly symmetrical. Another difference is that its methyne centers are occupied by phenyl groups.
    Tetraphenylporphyrin (H2TPP)is another synthetic analogue of protoporphyrin IX. Unlike the natural porphyrin ligands, TPP2− is highly symmetrical. Another difference is that its methyne centers are occupied by phenyl groups.
  • Simplified view of heme, a complex of a protoporphyrin IX.
    Simplified view of heme, a complex of a protoporphyrin IX.

Ancient porphyrins

A geoporphyrin, also known as a petroporphyrin, is a porphyrin of geologic origin.[9] They can occur in crude oil, oil shale, coal, or sedimentary rocks.[9][10] Abelsonite is possibly the only geoporphyrin mineral, as it is rare for porphyrins to occur in isolation and form crystals.[11]

The field of organic geochemistry had its origins in the isolation of porphyrins from petroleum. This finding helped establish the biological origins of petroleum. Petroleum is sometimes "fingerprinted" by analysis of trace amounts of nickel and vanadyl porphyrins.

Biosynthesis

In non-photosynthetic eukaryotes such as animals, insects, fungi, and protozoa, as well as the α-proteobacteria group of bacteria, the committed step for porphyrin biosynthesis is the formation of δ-aminolevulinic acid (δ-ALA, 5-ALA or dALA) by the reaction of the amino acid glycine with succinyl-CoA from the citric acid cycle. In plants, algae, bacteria (except for the α-proteobacteria group) and archaea, it is produced from glutamic acid via glutamyl-tRNA and glutamate-1-semialdehyde. The enzymes involved in this pathway are glutamyl-tRNA synthetase, glutamyl-tRNA reductase, and glutamate-1-semialdehyde 2,1-aminomutase. This pathway is known as the C5 or Beale pathway.

Two molecules of dALA are then combined by porphobilinogen synthase to give porphobilinogen (PBG), which contains a 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.

The following scheme summarizes the biosynthesis of porphyrins, with references by EC number and the OMIM database. The porphyria associated with the deficiency of each enzyme is also shown:

Heme B biosynthesis pathway and its modulators. Major enzyme deficiences are also shown.
Enzyme Location Substrate Product Chromosome EC OMIM Disorder
ALA synthase Mitochondrion Glycine, succinyl CoA δ-Aminolevulinic acid 3p21.1 2.3.1.37 125290 X-linked dominant protoporphyria, X-linked sideroblastic anemia
ALA dehydratase Cytosol δ-Aminolevulinic acid Porphobilinogen 9q34 4.2.1.24 125270 aminolevulinic acid dehydratase deficiency porphyria
PBG deaminase Cytosol Porphobilinogen Hydroxymethyl bilane 11q23.3 2.5.1.61 176000 acute intermittent porphyria
Uroporphyrinogen III synthase Cytosol Hydroxymethyl bilane Uroporphyrinogen III 10q25.2-q26.3 4.2.1.75 606938 congenital erythropoietic porphyria
Uroporphyrinogen III decarboxylase Cytosol Uroporphyrinogen III Coproporphyrinogen III 1p34 4.1.1.37 176100 porphyria cutanea tarda, hepatoerythropoietic porphyria
Coproporphyrinogen III oxidase Mitochondrion Coproporphyrinogen III Protoporphyrinogen IX 3q12 1.3.3.3 121300 hereditary coproporphyria
Protoporphyrinogen oxidase Mitochondrion Protoporphyrinogen IX Protoporphyrin IX 1q22 1.3.3.4 600923 variegate porphyria
Ferrochelatase Mitochondrion Protoporphyrin IX Heme 18q21.3 4.99.1.1 177000 erythropoietic protoporphyria

Laboratory synthesis
Main article: Rothemund reaction
Brilliant crystals of meso-tetratolylporphyrin, prepared from 4-methylbenzaldehyde and pyrrole in refluxing propionic acid

A common synthesis for porphyrins is the Rothemund reaction, first reported in 1936,[12][13] which is also the basis for more recent methods described by Adler and Longo.[14] The general scheme is a condensation and oxidation process starting with pyrrole and an aldehyde.

Applications

Photodynamic therapy

Porphyrins have been evaluated in the context of photodynamic therapy (PDT) since they strongly absorb light, which is then converted to heat in the illuminated areas.[15] This technique has been applied in macular degeneration using verteporfin.[16]

PDT is considered a noninvasive cancer treatment, involving the interaction between light of a determined frequency, a photo-sensitizer, and oxygen. This interaction produces the formation of a highly reactive oxygen species (ROS), usually singlet oxygen, as well as superoxide anion, free hydroxyl radical, or hydrogen peroxide.[17] These high reactive oxygen species react with susceptible cellular organic biomolecules such as; lipids, aromatic amino acids, and nucleic acid heterocyclic bases, to produce oxidative radicals that damage the cell, possibly inducing apoptosis or even necrosis.[18]

Toxicology

Heme biosynthesis is used as biomarker in environmental toxicology studies. While excess production of porphyrins indicate organochlorine exposure, lead inhibits ALA dehydratase enzyme.[19]

Biological applications

Porphyrins have been investigated as possible anti-inflammatory agents[20] and evaluated on their anti-cancer and anti-oxidant activity.[21] Several porphyrin-peptide conjugates were found to have antiviral activity against HIV in vitro.[22]

Synthetic applications

Complexes of cobalt(II) porphyrins have been extensively utilized as catalysts in organic synthesis. Due to their distinctive biomimetic radical mechanisms that involve metal-stabilized radical intermediates, the Co(II)–porphyrin-based catalysis system addresses some long-standing challenges in organic transformations.[23][24]

Potential applications

Biomimetic catalysis

Although not commercialized, metalloporphyrin complexes are widely studied as catalysts for the oxidation of organic compounds. Particularly popular for such laboratory research are complexes of meso-tetraphenylporphyrin and octaethylporphyrin. Complexes with Mn, Fe, and Co catalyze a variety of reactions of potential interest in organic synthesis. Some complexes emulate the action of various heme enzymes such as cytochrome P450, lignin peroxidase.[25][26] Metalloporphyrins are also studied as catalysts for water splitting, with the purpose of generating molecular hydrogen and oxygen for fuel cells.[27]

Molecular electronics and sensors

Porphyrin-based compounds are of interest as possible components of molecular electronics and photonics.[28] Synthetic porphyrin dyes have been incorporated in prototype dye-sensitized solar cells.[29][30]

Metalloporphyrins have been investigated as sensors.[31]

Phthalocyanines, which are structurally related to porphyrins, are used in commerce as dyes and catalysts, but porphyrins are not.

Supramolecular chemistry
On a gold surface porphyrin derivative molecules (a) form chains and clusters (b). Each cluster in (c,d) contains 4 or 5 molecules in the core and 8 or 10 molecules in the outer shells (STM images).[32]
An example of porphyrins involved in host–guest chemistry. Here, a four-porphyrin–zinc complex hosts a porphyrin guest.[33]

Porphyrins are often used to construct structures in supramolecular chemistry.[34] 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.[33] A guest-free base porphyrin is bound to the center by coordination with its four-pyridine substituents.

Theoretical interest in aromaticity

Porphyrinoid macrocycles can show variable aromaticity.[35] An Hückel aromatic porphyrin is porphycene.[36] antiaromatic, Mobius aromatic, and non aromatic porphyrinoid macrocycles are known.[37]

  • Lewis structure for meso-tetraphenylporphyrin
    Lewis structure for meso-tetraphenylporphyrin
  • UV–vis readout for meso-tetraphenylporphyrin
    UV–vis readout for meso-tetraphenylporphyrin
  • Light-activated porphyrin. Monatomic oxygen. Cellular aging
    Light-activated porphyrin. Monatomic oxygen. Cellular aging

In nature

Several heterocycles related to porphyrins are found in nature, almost always bound to metal ions. These include

Caption text
N4-macrocycleCofactor namemetalcomment
chlorinchlorophyllmagnesiumseveral versions of chlorophyll exist (sidechain; exception being chlorophyll c)
bacteriochlorinbacteriochlorophyll (in part)magnesiumseveral versions of bacteriochlorophyll exist (sidechain; some use a usual chlorin ring)
sirohydrochlorin (an isobacteriochlorin)sirohemeironImportant cofactor in sulfur assimilation
biosynthetic intermediate en route to cofactor F430 and B12
corrinvitamin B12cobaltseveral variants of B12 exist (sidechain)
corphinCofactor F430nickelhighly reduced macrocycle

Synthetic

A benzoporphyrin is a porphyrin with a benzene ring fused to one of the pyrrole units. e.g. verteporfin is a benzoporphyrin derivative.[38]

Non-natural porphyrin isomers
Porphycene, first porphyrin isomer, synthesised from bipyrrole dialdehyde through McMurry coupling reaction

The first synthetic porphyrin isomer was reported by Emanual Vogel and coworkers in 1986.[39] This isomer [18]porphyrin-(2.0.2.0) is named as porphycene, and the central N4 Cavity forms a rectangle shape as shown in figure.[40] Porphycenes showed interesting photophysical behavior and found versatile compound towards the photodynamic therapy.[41] This inspired Vogel and Sessler to took up the challenge of preparing [18]porphyrin-(2.1.0.1) and named it as corrphycene or porphycerin.[42] The third porphyrin that is [18]porphyrin-(2.1.1.0), was reported by Callot and Vogel-Sessler. Vogel and coworkers reported successful isolation of [18]porphyrin-(3.0.1.0) or isoporphycene.[43] The Japanese scientist Furuta[44] and Polish scientist Latos-Grażyński[45] almost simultaneously reported the N-confused porphyrins. The inversion of one of the pyrrolic subunits in the macrocyclic ring resulted in one of the nitrogen atoms facing outwards from the core of the macrocycle.

Various reported Isomers of porphyrin

See also

References
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