Macrocycle

A macrocycle is, as defined by IUPAC, "a cyclic macromolecule or a macromolecular cyclic portion of a molecule." In the chemical literature, macrocycles varyingly include molecules containing rings of 8 or more atoms,^[1][2] or 12 or more atoms. 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 IUPAC definition notes that a "cyclic macromolecule has no end-groups but may nevertheless be regarded as a chain," and that "macrocycle is sometimes used [in the literature] for molecules of low relative molecular mass that are not considered 'macromolecules.^[3]

Macrocycle effect

Crystal structure of a Zn(II) ion coordinated to cyclen and ethanol reported in Inorg. Chem., 1997, 4579-4584.

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 termed the macrocycle effect, and is a special example of the chelate effect. This so-called macrocyclic effect is thought to be a combination of the entropic effect seen in the chelate effect, together with an additional energetic contribution that comes from the preorganized nature of the ligating groups (that is, no additional strains are introduced to the ligand on coordination).^[4]

Synthesis

In general, macrocycles are synthesized from smaller, usually linear, molecules.

Historical uses

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.

Biological macrocycles

• 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.

References

1. ^ Parker, S.P. (2002). McGraw-Hill Science & Technology Dictionary. McGraw-Hill. ISBN 978-0070423138. 
2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "macrocycle".
3. ^ Milgrom, L.R (1997). The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds. New York: Oxford University Press. ISBN 0-19-855380-3.  (hardbound) ISBN 0-19-855962-3 (pbk.)
4. ^ Cabbines, D. K.; Margerum, D. W. (1969). "Macrocyclic effect on stability of copper(II) tetramine complexes". J. Am. Chem. Soc. 91 (23): 6540–6541. doi:10.1021/ja01051a091. 
5. ^ Melson, G.A., Ed. (1979). Coordination Chemistry of Macrocyclic Compounds. New York: Plenum Press. ISBN 0-306-40140-1. 
6. ^ Jung, J.E.; Seung, S.Y. (2002). Bulletin of the Korean Chemical Society 23 (10): 1483–1486. doi:10.5012/bkcs.2002.23.10.1483.  Missing or empty |title= (help)

See also

• Cyclic compound
• Effective molarity
• Macrocyclic Stereocontrol

Further reading

• Chambron, J-C.; Dietrich-Buchecker, C.; Hemmert, C.; Khemiss, A-K.; Mitchell, D.; Sauvage, J-P.; Weiss, J. (1990). "Interlacing molecular threads on transition metals" (PDF). Pure Appl. Chem. 62 (6): 1027–34. doi:10.1351/pac199062061027.