Porphyrins (/ˈpɔːrfərɪns/POR-fər-ins) are a group ofheterocyclic,macrocyclic,organic compounds, composed of four modifiedpyrrole subunits interconnected at theirα carbon atoms viamethine bridges (=CH−). Invertebrates, an essential member of the porphyrin group isheme, which is a component ofhemoproteins, whose functions include carryingoxygen in thebloodstream. Inplants, an essential porphyrin derivative ischlorophyll, which is involved inlight harvesting andelectron transfer inphotosynthesis.

The parent of porphyrins isporphine, 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 asaromatic.^[2][3] One result of the largeconjugated 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 fromGreekπορφύρα (porphyra) 'purple'.^[4]

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]

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

H₂porphyrin + [ML_n]²⁺ → M(porphyrinate)L_n−4 + 4 L + 2 H⁺

• Representative porphyrins and derivatives
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  Derivatives ofprotoporphyrin IX are common in nature, the precursor tohemes.
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  Octaethylporphyrin (H₂OEP) is a synthetic analogue of protoporphyrin IX. Unlike the natural porphyrin ligands, OEP²⁻ is highly symmetrical.
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  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.
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  Simplified view ofheme, a complex of a protoporphyrin IX.
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  A macrocycle of 40 porphyrin molecules, model
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  A macrocycle of 40 porphyrin molecules,STM image

A geoporphyrin, also known as a petroporphyrin, is a porphyrin of geologic origin.^[9] They can occur incrude 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 oforganic geochemistry had its origins in the isolation of porphyrins from petroleum.^{[citation needed]} This finding helped establish the biological origins of petroleum. Petroleum is sometimes "fingerprinted" by analysis of trace amounts of nickel andvanadyl porphyrins.^{[citation needed]}

In non-photosyntheticeukaryotes such as animals, insects, fungi, andprotozoa, as well as the α-proteobacteria group of bacteria, thecommitted step for porphyrinbiosynthesis is the formation ofδ-aminolevulinic acid (δ-ALA, 5-ALA or dALA) by the reaction of theamino acidglycine withsuccinyl-CoA from thecitric acid cycle. Inplants,algae,bacteria (except for the α-proteobacteria group) andarchaea, it is produced fromglutamic acid via glutamyl-tRNA andglutamate-1-semialdehyde. The enzymes involved in this pathway areglutamyl-tRNA synthetase,glutamyl-tRNA reductase, andglutamate-1-semialdehyde 2,1-aminomutase. This pathway is known as the C5 or Beale pathway.

Two molecules of dALA are then combined byporphobilinogen synthase to giveporphobilinogen (PBG), which contains a pyrrole ring. Four PBGs are then combined throughdeamination intohydroxymethyl bilane (HMB), which ishydrolysed to form the circular tetrapyrroleuroporphyrinogen 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-productprotoporphyrin 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 theOMIM database. Theporphyria associated with the deficiency of each enzyme is also shown:

A common synthesis for porphyrins is theRothemund 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 acondensation andoxidation process starting with pyrrole and analdehyde.

Porphyrins have been evaluated in the context ofphotodynamic therapy (PDT) since they strongly absorb light, which is then converted to heat in the illuminated areas.^[15] This technique has been applied inmacular degeneration usingverteporfin.^[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]

Porphyrin-based compounds are of interest as possible components ofmolecular electronics and photonics.^[19] Synthetic porphyrin dyes have been incorporated in prototypedye-sensitized solar cells.^[20][21]

Porphyrins have been investigated as possible anti-inflammatory agents^[22] and evaluated on their anti-cancer and anti-oxidant activity.^[23] Several porphyrin-peptide conjugates were found to have antiviral activity against HIVin vitro.^[24]

Heme biosynthesis is used asbiomarker in environmental toxicology studies. While excess production of porphyrins indicateorganochlorine exposure,lead inhibitsALA dehydratase enzyme.^[25]

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

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

Abenzoporphyrin is a porphyrin with a benzene ring fused to one of the pyrrole units. e.g.verteporfin is a benzoporphyrin derivative.^[26]

The first synthetic porphyrinisomer was reported by Emanual Vogel and coworkers in 1986.^[27] This isomer [18]porphyrin-(2.0.2.0) is named asporphycene, and the central N₄ Cavity forms arectangle shape as shown in figure.^[28] Porphycenes showed interestingphotophysical behavior and found versatile compound towards thephotodynamic therapy.^[29] This inspired Vogel andSessler to took up the challenge of preparing [18]porphyrin-(2.1.0.1) and named it ascorrphycene orporphycerin.^[30] 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) orisoporphycene.^[31] The Japanese scientist Furuta^[32] and Polish scientist Latos-Grażyński^[33] almost simultaneously reported theN-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.

• A porphyrin-related disease:porphyria
• Porphyrin coordinated to iron:heme
• A heme-containing group of enzymes:Cytochrome P450
• Porphyrin coordinated to magnesium:chlorophyll
• The one-carbon-shorter analogues:corroles, includingvitamin B₁₂, which is coordinated to a cobalt
• Corphins, the highly reduced porphyrin coordinated to nickel that binds theCofactor F430 active site inmethyl coenzyme M reductase (MCR)
• Nitrogen-substituted porphyrins:phthalocyanine,tetrapyrazinoporphyrazine

• Journal of Porphyrins and Phthalocyanines
• Handbook of Porphyrin Science
• Porphynet – an informative site about porphyrins and related structures