Porphyrins
The porphyrins and their closely related molecules are essential to all plants and animals. An iron porphyrin, called heme, is the portion of hemoglobin that carries oxygen from the lungs to our body's cells. The heme of another protein, myoglobin, carries oxygen within muscle cells. Other iron porphyrins in the cytochromes transport electrons in metabolic processes. A closely related type of molecule is chlorophyll, a key component of the photosynthetic process.
A porphyrin is a macrocyclic (large cycle) molecule that is made up of four smaller pyrrolenine rings. Each pyrrolenine ring is made up of a nitrogen atom and four carbon atoms. In the porphyrin ring, they are linked to each other by a carbon atom that bridges from the carbon atom nearest the nitrogen atom of one ring to the carbon atom nearest the nitrogen atom of the next ring (called the alpha carbon). The porphyrin ring is unsaturated, having alternating carbon-carbon single bonds and double bonds throughout the ring. The class of porphyrin molecules is very large because the ring can be substituted at many positions. In biochemically derived porphyrins, the hydrogen atoms on the carbon atoms of each nitrogen-containing ring that are farthest from the nitrogen atom (called the beta carbon) are replaced by carbon-containing groups such as a methyl group (-CH3), and ethyl group (-CH2CH3), a carboxymethyl group (-CH2COOH), or one of a large variety of other groups. The substituents of porphyrins profoundly affect their solubilities and their interactions with other molecules. They also change the ease with which the porphyrin ring can be oxidized or reduced. Substituents can greatly affect the oxidation-reduction properties of the metalloporphyrin complex.
The most common types of synthetic porphyrins have substituents on the carbon atoms bridging the five-membered nitrogen-containing rings. These synthetic porphyrins are often much less susceptible to photodegradation and other decomposition reactions than the biochemically derived porphyrins and have proven to be very useful in studying properties and reactions of the porphyrin ring system. They are also being studied intensely as candidates for building molecular scale computing devices and switches among other applications. The phthalocyanine ring system, which is used industrially in dyes and other applications, is closely related to the porphyrin ring system, having a benzene ring attached to the beta carbon positions of each of a porphyrin ring's four nitrogen-containing rings.
The metal complexes of porphyrins in living systems carry out many critical functions: hemoglobin and myoglobin transport oxygen, the cytochromes transport electrons in the metabolism of foods by which oxygen reacts with carbon- and hydrogen-containing molecules to produce carbon dioxide, water and energy, catalases catalyze the decomposition of potentially harmful hydrogen peroxide, peroxidases convert alkyl hydroperoxides (R-OOH) to alcohols (R-OH), and the cytochrome-P 450 enzymes in the liver detoxify organic substances by the conversion of -CH groups to more water soluble -COH groups. In these processes, the unusually extensive unsaturated network of the porphyrin ring plays an important role. The highly unsaturated ring tightly binds the iron atom, preventing it from being removed by oxygen to form highly stable iron oxides. It also allows electronic charge that develops at the iron atom to be spread throughout the ring. This allows high formal charges (Fe(IV), for example, that is postulated to develop in cytochrome P-450 reactions) to be stabilized.
Another consequence of the extensive unsaturated ring of porphyrins is its very great chemical stability. When carbon-carbon double and single bonds alternate appropriately, they average. The energy of the molecule is lower, that is the molecule is thereby more stable, because electrons are spread over a greater area, decreasing repulsions. The amount that the energy is lowered by this mechanism is called resonance energy. The resonance energy of the porphyrin ring is about 1,000 kilojoules/mol, many times that of a benzene ring and equivalent to the energy needed to break several chemical bonds. This energy lends so much stability to the porphyrin structure that porphyrin ring compounds are the only complex molecular structures other than hydrocarbons of all the compounds originally present in plants and animals to remain intact in oil and oil shale.
Porphyrins, metalloporphyrins, and related macrocyclic species such as the chlorophylls are noted for their very intense coloration (chlorophyll comes from the words for color and plant and cytochrome comes from the words color and cell). A 10 micromolar solution of any of these molecules typically absorbs over 90% of the light at some part of the visible spectrum but allows approximately 90% of light in other regions in the visible spectrum to be transmitted or reflected. Differences in the regions of light absorption for the different species leads to the color differences. The metalloporphyrin protoporphyrin IX iron(II) in hemoglobin gives blood its intense red color (that turns to brown as the Fe(II) atom is oxidized to Fe(III) when hemoglobin decomposes in air) and the magnesium complex of a related macrocylic ring system in chlorophyll gives leaves their bright green coloration. The intense coloration of the porphyrin ring system and the related ring system of the chlorophylls (there are plant and bacterial chlorophylls that differ structurally) is a property of the very extensive electron delocalization, resulting from the extensive single bond-double bond alternation. This property also leads to the intense luminescence and electron transfer capability of porphyrins and many metalloporphyins.
Not only are the natural porphyrins essential to life, but synthetic porphyrins and related ring systems have found uses as cancer treatment agents, diagnostic reagents, analytical reagents and dyes. There is a great deal of research underway worldwide to synthesize and understand the chemistry of new porphyrins with unusual and useful properties.
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