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Dimer

Dimers of carboxylic acids are often found in vapour phase.

A dimer is a chemical or biological entity consisting of two subunits called monomers, which are held together by either intramolecular forces (covalent bonds) or weaker intermolecular forces.

The dimer of cyclopentadiene although this might not be readily apparent on initial inspection

An example of a molecular dimer (i.e. held together by intramolecular forces) is dicyclopentadiene, wherein two cyclopentadiene molecules have reacted to give the product.

Molecular dimers are often formed by the reaction of two identical compounds e.g.: 2A ? A-A.

In this example, monomer "A" is said to dimerise to give the dimer "A-A". Diaminocarbenes are another example which dimerise, to give tetraaminoethylenes.

An example of an intermolecular or physical dimer is acetic acid wherein hydrogen bonds hold the two molecules together. The water dimer is another such dimer.

The term homodimer is used when the two molecules are identical (e.g. A-A) and heterodimer when they are not (e.g. A-B).

The reverse of dimerisation is often called disassociation.

In biochemistry and molecular biology, dimers of macromolecules like proteins and nucleic acids are often observed. The dimerization of identical subunits is called homodimerization; the dimerization of different subunits or unrelated monomers is called heterodimerization. Most dimers in biochemistry are not connected by covalent bonds with the exception of disulfide bridges. An example of this would be the enzyme reverse transcriptase, which is made of two different amino acid chains.

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