A macrocycle is, as defined by IUPAC, "a cyclic macromolecule or a macromolecular cyclic portion of a molecule." In the chemical literature, organic chemists may consider any molecule containing a ring of twelve or more atoms to be a macrocycle. In general, coordination chemists define a macrocycle more narrowly as a cyclic molecule with three or more potential donor atoms that can coordinate to a metal center. A well-known example is the group of drugs known as macrolides.
The macrocyclic effect was discovered in 1969. Coordination chemists study macrocycles with three or more potential donor atoms in rings of greater than nine atoms, as these compounds often have strong and specific binding with metals. This property of coordinating macrocyclic molecules is the macrocycle effect. It is a special example of the chelate effect.
In general, macrocycles are synthesized from smaller, usually linear, molecules.
Macrocycles have been in use for several decades as synthetic dyes. Phthalocyanine is a porphyrin analogue, which is arguably the most useful, in uses as dyes and pigments since their discovery in 1928, due to their dark-blue colour. However, there are many other uses for them. Their name comes from their synthetic precursor, phthalodinitrile.
- Heme, the active site in the hemoglobin (the protein in blood that transports oxygen), is a porphyrin containing iron.
- Chlorophyll, the green photosynthetic pigment found in plants, contains a chlorin ring.
- Vitamin B12 contains a corrin ring.
Related molecular categories
- Ligand: an atom, ion, or functional group that is bonded to one or more central atoms or ions.
- Chelate: a multidentate ligand, containing more than one donor atom.
- Cryptand: a macrocycle with multiple loops (e.g., bicyclic).
- Rotaxane: macrocycle(s) stuck on a stick, generally freely
- Catenane: interlocked molecular rings (like a chain).
- Molecular knot: a molecule in the shape of a knot such as a trefoil knot.
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- ^ D. K. Cabbines and D. W. Margerum (1969). "Macrocyclic effect on stability of copper(II) tetramine complexes". J. Am. Chem. Soc. 91: 6540-6541. http://dx.doi.org/10.1021/ja01051a091
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