Dimer

In chemistry, a dimer refers to a molecule composed of two identical subunits or monomers linked together.

Chemistry
The molecules in a dimer are connected by covalent bonds or weaker interactions such as hydrogen bonds. An example of the former is dicyclopentadiene, which is a dimer of cyclopentadiene. It can refer to halide chemistry, involving halogen bonding.

A physical dimer is a term that designates the case where intermolecular interaction brings two identical molecules closer together than other molecules. There are no covalent bonds between the physical dimer molecules. Acetic acid is such a case where hydrogen bonds provide the interaction. The water dimer is another dimer of interest, used for modeling hydrogen bonding in water.

The term homodimer is used when the two molecules are identical and heterodimer when they are not.

Biochemistry
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.

Examples

 * Nucleic acids:
 * DNA
 * DNA Polymerase
 * Proteins:
 * Antibodies
 * Protein kinases:
 * Receptor tyrosine kinases
 * Transcription factors
 * Leucine zipper motif proteins
 * Nuclear receptors
 * 14-3-3 proteins
 * G protein-coupled receptors
 * G protein βγ-subunit dimer
 * Kinesin
 * Triosephosphateisomerase (TIM)
 * Alcohol dehydrogenase
 * Factor XI
 * Factor XIII
 * Toll-like receptor
 * Fibrinogen