Porphyrinogen

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Hexahydroporphine, the core of porphyrinogens. Hexahydroporphine.svg
Hexahydroporphine, the core of porphyrinogens.

In biochemistry, a porphyrinogen is a member of a class of naturally occurring compounds with a tetrapyrrole core, a macrocycle of four pyrrole rings connected by four methylene bridges. [1] They can be viewed as derived from the parent compound hexahydroporphine by the substitution of various functional groups for hydrogen atoms in the outermost (20-carbon) ring.

Contents

Porphyrinogens are intermediates in the biosynthesis of porphyrins, cofactors with a porphine core which are found in many enzymes and proteins including myoglobin, hemoglobin, cytochromes, and chlorophylls. [2]

Porphyrins differ from porphyrinogens by having the four pyrrole rings linked by methine bridges =CH− instead of methylene bridges −CH2, and by lacking the hydrogen atom in two of the four amine −NH− groups, turning them into imines =N−. In the biosynthesis of porphyrins, the parent porphyrinogen is dehydrogenated by protoporphyrinogen oxidase.

Because of their limited delocalization, porphyrinogens are colorless. Loss of all four central hydrogen atoms in the core yields a tetravalent anion that can act as a ligand to metal cations, creating a coordination compound. [3] Subsequent biosynthetic intermediates en route to porphyrins are deeply colored and often phytotoxic.

Natural porphyrinogens

Porphyrogens that occur in living organisms usually have sidechains replacing some or all of the hydrogen atoms in two outermost carbon atoms of each pyrrole ring (as opposed to the hydrogen atoms in the methylene bridges).

Non-natural porphyrinogens

A variety of synthetic porphyrinogens have been produced and studied in laboratories. These often have side groups that do not occur in nature, and possibly at the carbons in the methylene bridges (meso positions) instead of the pyrrole rings. The Meso-substituted porphyrinogens are intermediates in the so-called Lindsey synthesis of meso-substituted porphyrins. Oxidation turns the central hexahydroporphine core into a porphine core, yielding the desired porphyrin. [4]

Under acid catalysis, pyrrole and ketones R−(C=O)−R' or aldehydes R−(C=O)−H condense to give many oligomers, including the cyclic ones [−(CRR')−(C4H2NH)−]n\. The desired porphyrinogens (n = 4) can then be separated. [4] Meso-substituted porphyrinogens with eight non-hydrogen side chains are also called calix[4]pyrroles. These products resist dehydrogenation of the outer ring better than the natural porphyrinogens. [3]

For example, condensation with benzaldehyde C6H5−(C=O)−H yields meso-tetraphenylporphyrinogen, which can be oxidized to meso-tetraphenylporphyrin. [4] Condensation with acetone H3C−(C=O)−CH3 yields meso-octamethyporphyrinogen, parent of meso-octamethylporphyrin. [3]

Alternatively, pyrrole with sidechains substituted at carbons 3 and 4 (those not adjacent to the nitrogen) can be condensed with formaldehyde H−(C=O)−H to give porphyrinogens that more closely resemble the natural ones. For example, with 3,4-diethylpyrrole one obtains octaethylporphyrinogen, parent of octaethylporphyrin.[ citation needed ]

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<span class="mw-page-title-main">Protoporphyrinogen IX</span> Chemical compound

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<span class="mw-page-title-main">Hydroxymethylbilane</span> Intermediate in the synthesis of porphyrins

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<span class="mw-page-title-main">Parent structure</span> Chemical structure from which derivatives can be visualized

In chemistry, a parent structure is the structure of an unadorned ion or molecule from which derivatives can be visualized. Parent structures underpin systematic nomenclature and facilitate classification. Fundamental parent structures have one or no functional groups and often have various types of symmetry. Benzene is a chemical itself consisting of a hexagonal ring of carbon atoms with a hydrogen atom attached to each, and is the parent of many derivatives that have substituent atoms or groups replacing one or more of the hydrogens. Some parents are rare or nonexistent themselves, as in the case of porphine, though many simple and complex derivatives are known.

<span class="mw-page-title-main">2,5-Bis(hydroxymethyl)pyrrole</span> Chemical compound

2,5-Bis(hydroxymethyl)pyrrole is an organic chemical compound with formula C6H9O2N, or (HOCH2)2(C4H3N). Its molecule can be described as that of pyrrole C4H5N with hydroxymethyl groups HO−CH2 replacing the two hydrogen atoms adjacent to the nitrogen atom.

<span class="mw-page-title-main">Hexahydroporphine</span> Chemical compound

Hexahydroporphine is an organic chemical compound with formula C20H20N4. The molecule consists of four pyrrole rings connected by methylene bridges −CH2 into a larger (non-aromatic) macrocycle ring, which makes it one of the simplest tetrapyrroles, and the simplest "true" one. As indicated by the name, it may be viewed as derived from porphine by the addition of six hydrogen atoms: four on the methine bridges, and two on the nitrogen atoms.

<i>meso</i>-Octamethylporphyrinogen Chemical compound

meso-Octamethylporphyrinogen, usually referred to simply as octamethylporphyrinogen, is an organic compound with the formula (Me2C-C4H2NH)4 (Me = CH3. It is one of the simplest porphyrinogens, a family of compounds that occur as intermediates in the biosynthesis of hemes and chlorophylls. In contrast to those rings, porphyrinogens are colorless since they lack extended conjugation. The prefix meso-octamethyl indicates that the eight methyl groups are located on the carbon centers that interconnect the four pyrrole rings. Also unlike porphyrins, the porphyrinogens are highly ruffled.

<span class="mw-page-title-main">Phosphorus porphyrin</span> Organophosphorus compound

Phosphorus-centered porphyrins are conjugated polycyclic ring systems consisting of either four pyrroles with inward-facing nitrogens and a phosphorus atom at their core or porphyrins with one of the four pyrroles substituted for a phosphole. Unmodified porphyrins are composed of pyrroles and linked by unsaturated hydrocarbon bridges often acting as multidentate ligands centered around a transition metal like Cu II, Zn II, Co II, Fe III. Being highly conjugated molecules with many accessible energy levels, porphyrins are used in biological systems to perform light-energy conversion and modified synthetically to perform similar functions as a photoswitch or catalytic electron carriers. Phosphorus III and V ions are much smaller than the typical metal centers and bestow distinct photochemical properties unto the porphyrin. Similar compounds with other pnictogen cores or different polycyclic rings coordinated to phosphorus result in other changes to the porphyrin’s chemistry.

References

  1. porphyrinogens - IUPAC Gold Book
  2. Paul R. Ortiz de Montellano (2008). "Hemes in Biology". Wiley Encyclopedia of Chemical Biology. John Wiley & Sons. doi:10.1002/9780470048672.wecb221. ISBN   978-0470048672.
  3. 1 2 3 Sessler, Jonathan L.; Anzenbacher, Pavel Jr.; Miyaji, Hidekazu; Jursikova, Karolina; Bleasdale, Ellen R.; Gale, Philip A. (2000). "Modified Calix[4]pyrroles". Industrial & Engineering Chemistry Research. 39 (10): 3471–3478. doi:10.1021/ie000102y.
  4. 1 2 3 Lindsey, Jonathan S. (2000). "Synthesis of meso-substituted porphyrins". In Kadish, Karl M.; Smith, Kevin M.; Guilard, Roger (eds.). Porphyrin Handbook. Vol. 1. pp. 45–118. ISBN   0-12-393200-9.