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Inchemistry,dimerization is the process of joining two identical or similarmolecular entities bybonds. The resulting bonds can be either strong or weak. Many symmetricalchemical species are described asdimers, even when themonomer is unknown or highly unstable.[1]
The termhomodimer is used when the two subunits are identical (e.g. A–A) andheterodimer when they are not (e.g. A–B). The reverse of dimerization is often calleddissociation. When two oppositely-chargedions associate into dimers, they are referred to asBjerrum pairs,[2] after Danish chemistNiels Bjerrum.
Anhydrouscarboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen. For example,acetic acid forms a dimer in the gas phase, where the monomer units are held together byhydrogen bonds.[3] Many OH-containing molecules form dimers, e.g. thewater dimer.
Dimers that form based on weakelectrostatic interaction and/orvan der Waals interactions have a short lifetime, but can be stabilized through special laboratory setups such asmatrix-isolation. A prominent example is thecarbon dioxide dimer,[4] which is likely to be relevant to Venus atmosphere.[5]
Excimers andexciplexes areexcited structures with a short lifetime. For example,noble gases do not form stable dimers, but they do form theexcimers Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation.[6]
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 dimerize to give the dimer "A−A".
Dicyclopentadiene is an asymmetrical dimer of twocyclopentadiene molecules that have reacted in aDiels-Alder reaction to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers:
Many nonmetallic elements occur as dimers:hydrogen,nitrogen,oxygen, and thehalogensfluorine,chlorine,bromine andiodine. Some metals form a proportion of dimers in their vapour phase:dilithium (Li2),disodium (Na2),dipotassium (K2),dirubidium (Rb2) anddicaesium (Cs2). Such elemental dimers arehomonucleardiatomic molecules.
In the context ofpolymers, "dimer" also refers to thedegree of polymerization 2, regardless of the stoichiometry orcondensation reactions.
One case where this is applicable is withdisaccharides. For example,cellobiose is a dimer ofglucose, even though the formation reaction produceswater:
Here, the resulting dimer has a stoichiometry different from the initial pair of monomers.
Disaccharides need not be composed of the samemonosaccharides to be considered dimers. An example issucrose, a dimer offructose and glucose, which follows the same reaction equation as presented above.
Amino acids can also form dimers, which are calleddipeptides. An example isglycylglycine, consisting of twoglycine molecules joined by apeptide bond. Other examples includeaspartame andcarnosine.
Many molecules and ions are described as dimers, even when the monomer is elusive.
Diborane (B2H6) is an dimer ofborane, which is elusive and rarely observed. Almost all compounds of the type R2BH exist as dimers.[7]
Trialkylaluminium compounds can exist as either monomers or dimers, depending on thesteric bulk of the groups attached. For example,trimethylaluminium exists as a dimer, but trimesitylaluminium adopts a monomeric structure.[8]
Cyclopentadienylchromium tricarbonyl dimer exists in measureable equilibrium quantities with the monometallic radical(C5H5)Cr(CO)3.[9]
Pyrimidine dimers (also known as thymine dimers) are formed by aphotochemical reaction from pyrimidineDNA bases when exposed to ultraviolet light.[8] This cross-linking causesDNA mutations, which can becarcinogenic, causingskin cancers.[8] Whenpyrimidine dimers are present, they can blockpolymerases, decreasing DNA functionality until it is repaired.[8]
Protein dimers arise from the interaction between twoproteins which can interact further to form larger and more complexoligomers.[10] For example,tubulin is formed by the dimerization ofα-tubulin andβ-tubulin and this dimer can thenpolymerize further to makemicrotubules.[11] For symmetric proteins, the larger protein complex can be broken down into smaller identicalprotein subunits, which then dimerize to decrease the genetic code required to make the functional protein.[10]
As the largest and most diverse family ofreceptors within the human genome,G protein-coupled receptors (GPCR) have been studied extensively, with recent studies supporting their ability to form dimers.[12] GPCR dimers include both homodimers and heterodimers formed from related members of the GPCR family.[13] While not all, some GPCRs require dimerization to function, such asGABAB-receptor, emphasizing the importance of dimers in biological systems.[14]
Much like for G protein-coupled receptors, dimerization is essential forreceptor tyrosine kinases (RTK) to perform their function insignal transduction, affecting many different cellular processes.[15] RTKs typically exist as monomers, but undergo aconformational change uponligand binding, allowing them to dimerize with nearby RTKs.[16][17] The dimerization activates thecytoplasmickinasedomains that are responsible for furthersignal transduction.[15]
