Everything about Porphyrins totally explained
A
porphyrin is a heterocyclic
macrocycle derived from four
pyrroline subunits interconnected via their α carbon atoms via
methine bridges (=CH-). Porphyrins are aromatic and they obey
Hückel's rule for aromaticity in that they possess 4n+2 pi electrons which are delocalized over the macrocycle. The macrocycle, therefore, is a highly-
conjugated system, and, as a consequence, is deeply coloured - the name
porphyrin comes from a
Greek word for
purple. The macrocycle has 22
pi electrons. The parent porphyrin is
porphine, and substituted porphines are called porphyrins. Many porphyrins occur in nature, such as in green leaves and red
blood cells, and in bio-inspired synthetic catalysts and devices.
Complexes of porphyrins and related molecules
Porphyrins bind
metals to form
complexes. The metal
ion, usually with a charge of 2+ or 3+, resides in the central N
4 cavity formed by the loss of two protons. Most metals can be inserted. A schematic equation for these syntheses is shown:
» H
2porphyrin + [ML
n]
2+ → M(porphyrinate)L
n-4 + 4 L + 2 H
+
A porphyrin in which no metal is inserted in its cavity is sometimes called a
free base. Some iron-containing porphyrins are called
hemes; and heme-containing
proteins, or
hemoproteins, are found extensively in nature.
Hemoglobin and
myoglobin are two O
2-binding proteins that contain iron porphyrins.
Related to porphyrins are several other heterocycles, including
corrins,
chlorins,
bacteriochlorophylls, and
corphins.
Chlorins (2,3-dihydroporphyrin) are more reduced, that contain more hydrogen, than porphyrins, featuring a pyrroline subunit. This structure occurs in
chlorophyll. Replacement of two of the four pyrrolic subunits with pyrrolinic subunits results in either a
bacteriochlorin (as found in some
photosynthetic bacteria) or an isobacteriochlorin, depending on the relative positions of the reduced rings. Some porphyrin derivatives follow
Hückel's rule, but most do not.
Laboratory synthesis
One of the more common syntheses for porphyrins is based on work by Paul Rothemund. His techniques underpin more modern syntheses such as those described by Alder and Longo. The synthesis of simple porphyrins such as
meso-tetraphenylporphyrin is also commonly done in university teaching labs.
In this method, porphyrins are assembled from
pyrrole and substituted
aldehydes. Acidic conditions are essential; formic acid, acetic acid, and propionic acid are typical reaction solvents, or
p-toluenesulfonic acid can be used with a non-acidic solvent. Lewis acids such as
boron trifluoride etherate and
ytterbium triflate have also been known to catalyse porphyrin formation. A large amount of side-product is formed and is removed, usually by chromatography.
Biosynthesis
The "committed step" for porphyrin
biosynthesis is the formation of
D-aminolevulinic acid (dALA) by the reaction of the
amino acid glycine and
succinyl-CoA, from the
citric acid cycle. Two molecules of dALA combine 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:
Applications
Although natural porphyin complexes are essential for life, synthetic porphyrins and their complexes have limited utility. Complexes of meso-tetraphenylporphyrin, for example, the iron-(III) chloride complex (TPPFeCl) catalyse a variety of reactions in
organic chemistry, but none is of practical value. Porphyrin-based compounds are of interest in
molecular electronics and supramolecular building blocks.
Phthalocyanines, which are structurally related to porphyrins, are used in commerce as dyes and catalysts.
Synthetic porphyrin dyes that are incorporated in the design of solar cells are the subject of ongoing research. See
Dye-sensitized solar cells.
In 2008 the UK corporation Destiny Pharma reported successful clinical trials of an intranasally applied porphorin XF-73 against methicillin-resistant
Staphylococcus aureus.
Supramolecular chemistry
Porphyrins are often used to construct structures in
supramolecular chemistry. 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. A guest-free base porphyrin is bound to the center by coordination with its four pyridine sustituents.
Further Information
Get more info on 'Porphyrins'.
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