Porphyrin

Complexes of porphyrins

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, where M = metal ion and L = a ligand:

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

Ancient porphyrins

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

The field of organic geochemistry had its origins in the isolation of porphyrins from petroleum. These findings helped establish the biological origins of petroleum. Petroleum is sometimes "fingerprinted" by analysis of trace amounts of nickel and vanadyl porphyrins. Metalloporphyrins in general are highly stable organic compounds, and the detailed structures of the extracted derivatives made clear that they originated from chlorophyll.

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:

Laboratory synthesis

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

Potential 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. This technique has been applied in macular degeneration using verteporfin.

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. 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.

Molecular electronics and sensors

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

Biological applications

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

Toxicology

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

Photochemistry

Free-base porphyrins, including naturally occurring protoporphyrin IX (PPIX), function as viable photoredox catalysts under red-light irradiation. Red light provides deeper tissue and media penetration, making this approach suitable for aqueous biomolecule functionalization.

Related species

In nature

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

Synthetic

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

Non-natural porphyrin isomers

The first synthetic porphyrin isomer was reported by Emanual Vogel and coworkers in 1986. 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. Porphycenes showed interesting photophysical behavior and found versatile compound towards the photodynamic therapy. This result was followed by the preparation of [18]porphyrin-(2.1.0.1), named it as corrphycene or porphycerin. Other non-natural porphyrins include [18]porphyrin-(2.1.1.0) and [18]porphyrin-(3.0.1.0) or isoporphycene. The N-confused porphyrins feature one of the pyrrolic subunits with the nitrogen atoms facing outwards from the core of the macrocycle.