Structure of porphine, the simplest porphyrin.
Space-filling model of porphyrin
A porphyrin is a heterocyclic macrocycle derived from four pyrrole-like subunits interconnected via their α carbon atoms via methine
bridges (=CH-). The macrocycle, therefore, is a highly conjugated system, and is
consequently 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 N4 cavity formed by
the loss of two protons. Most metals can be inserted. A schematic equation for these syntheses is shown:
- H2porphyrin + [MLn]2+ → M(porphyrinate)Ln-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 O2-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.[1][2] His
techniques underpin more modern syntheses such as those described by Alder and Longo.[3] The synthesis of simple porphyrins such as meso-tetraphenylporphyrin is
also commonly done in university teaching labs.[4]
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, e.g. the iron-(III) chloride complex (TPPFeCl) catalyse a variety of reactions in
organic chemistry, but none are 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.
Supramolecular chemistry
An example of a porphyrins involved in
host-guest chemistry reported by Sanders and
coworkers in Angew. Chem., Int. Ed. Engl. 1995, 34, 1096-1099.
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.[5] A guest-free base porphyrin is bound to the center by coordination with its four pyridine
sustituents.
See also
References
- ^ P. Rothemund (1936). "A New Porphyrin
Synthesis. The Synthesis of Porphin". J. Am. Chem. Soc.
58 (4): 625-627. DOI:10.1021/ja01295a027.
- ^ P. Rothemund (1935). "Formation of
Porphyrins from Pyrrole and Aldehydes". J. Am. Chem.
Soc. 57 (10): 2010-2011. DOI:10.1021/ja01313a510.
- ^ A. D. Adler, F. R. Longo, J. D. Finarelli,
J. Goldmacher, J. Assour and L. Korsakoff (1967). "A simplified synthesis for meso-tetraphenylporphine". J. Org. Chem. 32 (2): 476-476. DOI:10.1021/jo01288a053.
- ^ Falvo, RaeAnne E.; Mink, Larry M.; Marsh,
Diane F.. "Microscale Synthesis and 1H NMR Analysis of Tetraphenylporphyrins". J. Chem. Educ. 1999 (76):
237.
- ^ Sanders and coworkers in Angew. Chem., Int. Ed. Engl. 1995, 34,
1096-1099.
External links
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