porphyrin
Any of a class of biologically important heterocyclic compounds of a characteristic chemical structure that includes four pyrrole groups (five-membered organic rings each containing a nitrogen atom) linked by additional carbon atoms to form a large flat ring. As biological pigments, they and closely related molecules are responsible for many of the vivid colours in living organisms, where they often occur combined with metal ions and various substituents as coordination complexes (see compound). These include the magnesium-containing chlorophylls and the iron-containing heme group, a constituent (along with protein) of, e.g., hemoglobin, the cytochromes, and the enzyme catalase. In medicine, porphyrins are used in conjunction with light, often a laser beam, to induce reactions in the body against cancer and other diseases.
porphyrin [′pȯr·fə·rən]
Porphyrin
One of a class of cyclic compounds in which the parent macrocycle consists of four pyrrole-type units linked together by single carbon bridges. Several porphyrins with selected peripheral substitution and metal coordination carry out vital biochemical processes in living organisms. Chlorins, bacteriochlorins, and corrins are related tetrapyrrolic macrocycles that are also observed in biologically important compounds.
The complexity of porphyrin nomenclature parallels the complex structures of the naturally occurring derivatives. Hans Fischer used a simple numbering system for the porphyrin nucleus and a set of common names to identify the different porphyrins and their isomers. A systematic naming based on the 1–24 numbering system for the porphyrin nucleus was later developed by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Biochemistry (IUB), and this system has gained general acceptance. The need for common names is clear after examination of the systematic names; for example, protoporphyrin IX has the systematic name 2,7,12,18-tetramethyl-3,8-divinyl-13,17-dipropanoic acid.
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The aromatic character (hence stability) of porphyrins has been confirmed by measurements of their heats of combustion. In addition, x-ray crystallographic studies have established planarity of the porphyrin macrocycle which is a basic requirement for aromatic character.
Most metals and metalloids have been inserted into the central hole of the porphyrin macrocycle. The resulting metalloporphyrins are usually very stable and can bind a variety of small molecules (known as ligands) to the central metal atom. Heme, the iron complex of protoporphyrin IX, is the prosthetic group of a number of major proteins and enzymes that carry out diverse biological functions. These include binding, transport, and storage of oxygen (hemoglobin and myoglobin), electron-transfer processes (cytochromes), activation and transfer of oxygen to substrates (cytochromes P450), and managing and using hydrogen peroxide (peroxidases and catalases). See Cytochrome, Hemoglobin
Chlorophylls and bacteriochlorophylls are magnesium complexes of porphyrin derivatives known as chlorins and bacteriochlorins, respectively. They are the pigments responsible for photosynthesis. Several chlorophylls have been identified, the most common being chlorophyll a, which is found in all oxygen-evolving photosynthetic plants. Bacteriochlorophyll a is found in many photosynthetic bacteria. See Chlorophyll, Photosynthesis
Porphyrins and metalloporphyrins exhibit many potentially important medicinal and industrial properties. Metalloporphyrins are being examined as potential catalysts for a variety of processes, including catalytic oxidations. They are also being examined as possible blood substitutes and as electrocatalysts for fuel cells and for the electrochemical generation of hydrogen peroxide. The unique optical properties of porphyrins make them likely candidates for photovoltaic devices and in photocopying and other optical devices. A major area where porphyrins are showing significant potential is in the treatment of a wide range of diseases, including cancer, using photodynamic therapy.
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