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{{short description|Chemical process of joining two molecular entities by bonds of any kind}}
{{Redirect|Dimer (chemistry)|other uses|Dimer (disambiguation)}}
{{refimprove|date=April 2009}}

In [[chemistry]], '''dimerization''' is the process of joining two identical or similar [[Molecular entity|molecular entities]] by [[Chemical bond|bonds]]. The resulting bonds can be either strong or weak. Many symmetrical [[chemical species]] are described as '''dimers''', even when the [[monomer]] is unknown or highly unstable.<ref>{{cite web |title=Dimerization |url=https://goldbook.iupac.org/terms/view/D01744}}</ref>

The term ''homodimer'' is used when the two subunits are identical (e.g. A–A) and ''heterodimer'' when they are not (e.g. A–B). The reverse of dimerization is often called [[Dissociation (chemistry)|dissociation]]. When two oppositely-charged [[ion]]s associate into dimers, they are referred to as ''Bjerrum pairs'',<ref>{{Cite journal|last1=Adar|first1=Ram M.|last2=Markovich|first2=Tomer|last3=Andelman|first3=David|date=2017-05-17|title=Bjerrum pairs in ionic solutions: A Poisson-Boltzmann approach|journal=The Journal of Chemical Physics|volume=146|issue=19|page=194904|doi=10.1063/1.4982885|pmid=28527430|issn=0021-9606|arxiv=1702.04853|bibcode=2017JChPh.146s4904A|s2cid=12227786}}</ref> after Danish chemist [[Niels Bjerrum]].

== Noncovalent dimers ==
[[File:Carboxylic acid dimers.svg|thumb|class=skin-invert-image|Dimers of [[carboxylic acid]]s are often found in the vapour phase.]]
[[Anhydrous]] [[carboxylic acid]]s 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 by [[hydrogen bond]]s.<ref>{{Cite journal |last1=Karle |first1=J. |last2=Brockway |first2=L. O. |date=1944 |title=An Electron Diffraction Investigation of the Monomers and Dimers of Formic, Acetic and Trifluoroacetic Acids and the Dimer of Deuterium Acetate 1 |url=https://pubs.acs.org/doi/abs/10.1021/ja01232a022 |journal=Journal of the American Chemical Society |language=en |volume=66 |issue=4 |pages=574–584 |doi=10.1021/ja01232a022 |issn=0002-7863|url-access=subscription }}</ref> Many OH-containing molecules form dimers, e.g. the [[water dimer]].

Dimers that form based on weak [[Electrostatic Interaction|electrostatic interaction]] and/or [[van der Waals interactions]] have a short lifetime, but can be stabilized through special laboratory setups such as [[Matrix isolation|matrix-isolation]]. A prominent example is the [[carbon dioxide]] dimer,<ref>{{Cite journal |last=Fredin |first=Leif |last2=Nelander |first2=Bengt |last3=Ribbegård |first3=Göran |date=1974-12-01 |title=On the dimerization of carbon dioxide in nitrogen and argon matrices |url=https://linkinghub.elsevier.com/retrieve/pii/0022285274900770 |journal=Journal of Molecular Spectroscopy |volume=53 |issue=3 |pages=410–416 |doi=10.1016/0022-2852(74)90077-0 |issn=0022-2852|url-access=subscription }}</ref> which is likely to be relevant to Venus atmosphere.<ref>{{Cite journal |last=Dinu |first=Dennis F. |last2=Bartl |first2=Pit |last3=Quoika |first3=Patrick K. |last4=Podewitz |first4=Maren |last5=Liedl |first5=Klaus R. |last6=Grothe |first6=Hinrich |last7=Loerting |first7=Thomas |date=2022-05-19 |title=Increase of Radiative Forcing through Midinfrared Absorption by Stable CO2 Dimers? |url=https://pubs.acs.org/doi/10.1021/acs.jpca.2c00857 |journal=The Journal of Physical Chemistry A |volume=126 |issue=19 |pages=2966–2975 |doi=10.1021/acs.jpca.2c00857 |issn=1089-5639 |pmc=9125687 |pmid=35533210}}</ref>

[[Excimers]] and [[exciplex]]es are [[Excited state|excited]] structures with a short lifetime. For example, [[noble gases]] do not form stable dimers, but they do form the [[excimers]] Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation.<ref>{{Cite journal |last=Birks |first=J B |date=1975-08-01 |title=Excimers |url=https://iopscience.iop.org/article/10.1088/0034-4885/38/8/001 |journal=Reports on Progress in Physics |volume=38 |issue=8 |pages=903–974 |doi=10.1088/0034-4885/38/8/001 |s2cid=240065177 |issn=0034-4885|url-access=subscription }}</ref>

== Covalent dimers ==
[[File: Dicyclopentadiene structure.svg|thumb|right|100px|class=skin-invert-image|The dimerization of [[cyclopentadiene]] gives dicyclopentadiene, although this might not be readily apparent on initial inspection. This dimerization is reversible.]]

[[Molecular]] dimers are often formed by the reaction of two identical compounds e.g.: {{chem2|2A -> A\sA}}. In this example, [[monomer]] "A" is said to dimerize to give the dimer "{{chem2|A\sA}}".

[[Dicyclopentadiene]] is an asymmetrical dimer of two [[cyclopentadiene]] molecules that have reacted in a [[Diels-Alder reaction]] to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers:
:<chem>C10H12 -> 2 C5H6</chem>

Many nonmetallic elements occur as dimers: [[hydrogen]], [[nitrogen]], [[oxygen]], and the [[halogen]]s [[fluorine]], [[chlorine]], [[bromine]] and [[iodine]]. Some metals form a proportion of dimers in their vapour phase: [[dilithium]] ({{chem2|Li2}}), [[disodium]] ({{chem2|Na2}}), [[dipotassium]] ({{chem2|K2}}), [[dirubidium]] ({{chem2|Rb2}}) and [[dicaesium]] ({{chem2|Cs2}}). Such elemental dimers are [[homonuclear molecule|homonuclear]] [[diatomic molecule]]s.

==Polymer chemistry==
In the context of [[polymer]]s, "dimer" also refers to the [[degree of polymerization]] 2, regardless of the stoichiometry or [[condensation reaction]]s.

One case where this is applicable is with [[disaccharide]]s. For example, [[cellobiose]] is a dimer of [[glucose]], even though the formation reaction produces [[water]]:
: <chem>2 C6H12O6 -> C12H22O11 + H2O</chem>
Here, the resulting dimer has a stoichiometry different from the initial pair of monomers.

Disaccharides need not be composed of the same [[monosaccharide]]s to be considered dimers. An example is [[sucrose]], a dimer of [[fructose]] and glucose, which follows the same reaction equation as presented above.

Amino acids can also form dimers, which are called [[dipeptide]]s. An example is [[glycylglycine]], consisting of two [[glycine]] molecules joined by a [[peptide bond]]. Other examples include [[aspartame]] and [[carnosine]].

== Inorganic and organometallic dimers ==
Many molecules and ions are described as dimers, even when the monomer is elusive.

=== Boranes ===
[[File:Borane & Diborane.jpg|thumb|class=skin-invert-image|Borane and diborane]]
[[Diborane]] (B2H6) is an dimer of [[borane]], which is elusive and rarely observed. Almost all compounds of the type R2BH exist as dimers.<ref>{{Cite book |last=Shriver |first=Duward |title=Inorganic Chemistry |publisher=W.H. Freeman and Company |year=2014 |isbn=978-1-4292-9906-0 |edition=6th |pages=306–307 |language=English}}</ref>

===Organoaluminium compounds===
[[File:Trimethylaluminium dimer.png|thumb|class=skin-invert-image|Trimethylaluminium dimer]]
[[Organoaluminium chemistry|Trialkylaluminium compounds]] can exist as either monomers or dimers, depending on the [[Steric effects|steric bulk]] of the groups attached. For example, [[trimethylaluminium]] exists as a dimer, but trimesitylaluminium adopts a monomeric structure.<ref name=":0">{{Cite book |last=Shriver |first=Duward |title=Inorganic Chemistry |publisher=W.H. Freeman and Company |year=2014 |isbn=978-1-4292-9906-0 |edition=6th |pages=377–378 |language=English}}</ref>

===Organochromium compounds===
Cyclopentadienylchromium tricarbonyl dimer exists in measureable equilibrium quantities with the monometallic radical {{chem2|(C5H5)Cr(CO)3}}.<ref>{{cite journal | doi = 10.1021/ja00810a019 | issue = 5| pages = 749–754| title = Unusual structural and magnetic resonance properties of dicyclopentadienylhexacarbonyldichromium| journal = Journal of the American Chemical Society| volume = 96| year = 1974| last1 = Adams| first1 = Richard D.| last2 = Collins| first2 = Douglas E.| last3 = Cotton| first3 = F. Albert}}</ref>

== Biochemical dimers ==

=== Pyrimidine dimers ===
[[Pyrimidine dimers]] (also known as thymine dimers) are formed by a [[photochemical reaction]] from pyrimidine [[DNA base]]s when exposed to ultraviolet light.<ref name=":0"/> This cross-linking causes [[Mutation|DNA mutations]], which can be [[carcinogenic]], causing [[skin cancer]]s.<ref name=":0" /> When [[pyrimidine dimer]]s are present, they can block [[polymerase]]s, decreasing DNA functionality until it is repaired.<ref name=":0" />

=== Protein dimers ===
[[File:Tubulin_dimer.png|thumb|312x312px|class=skin-invert-image|Tubulin dimer]]
[[Protein dimer]]s arise from the interaction between two [[protein]]s which can interact further to form larger and more complex [[oligomer]]s.<ref name=":1">{{Cite journal |last1=Marianayagam |first1=Neelan J. |last2=Sunde |first2=Margaret |last3=Matthews |first3=Jacqueline M. |date=2004 |title=The power of two: protein dimerization in biology |journal=Trends in Biochemical Sciences |volume=29 |issue=11 |pages=618–625 |doi=10.1016/j.tibs.2004.09.006 |pmid=15501681 |issn=0968-0004}}</ref> For example, [[tubulin]] is formed by the dimerization of [[Tubulin|α-tubulin]] and [[Tubulin|β-tubulin]] and this dimer can then [[Polymerization|polymerize]] further to make [[microtubule]]s.<ref>{{Cite journal |last=Cooper |first=Geoffrey M. |date=2000 |title=Microtubules |url=https://www.ncbi.nlm.nih.gov/books/NBK9932/ |journal=The Cell: A Molecular Approach. 2nd Edition |language=en}}</ref> For symmetric proteins, the larger protein complex can be broken down into smaller identical [[protein subunit]]s, which then dimerize to decrease the genetic code required to make the functional protein.<ref name=":1" />

=== G protein-coupled receptors ===
As the largest and most diverse family of [[Receptor (biochemistry)|receptors]] within the human genome, [[G protein-coupled receptor]]s (GPCR) have been studied extensively, with recent studies supporting their ability to form dimers.<ref>{{Citation |last1=Faron-Górecka |first1=Agata |title=Chapter 10 - Understanding GPCR dimerization |date=2019-01-01 |url=https://www.sciencedirect.com/science/article/pii/S0091679X18301080 |journal=Methods in Cell Biology |volume=149 |pages=155–178 |editor-last=Shukla |editor-first=Arun K. |series=G Protein-Coupled Receptors, Part B |publisher=Academic Press |language=en |doi=10.1016/bs.mcb.2018.08.005 |access-date=2022-10-27 |last2=Szlachta |first2=Marta |last3=Kolasa |first3=Magdalena |last4=Solich |first4=Joanna |last5=Górecki |first5=Andrzej |last6=Kuśmider |first6=Maciej |last7=Żurawek |first7=Dariusz |last8=Dziedzicka-Wasylewska |first8=Marta|pmid=30616817 |isbn=978-0-12-815107-5 |s2cid=58577416 |url-access=subscription }}</ref> GPCR dimers include both homodimers and heterodimers formed from related members of the GPCR family.<ref>{{Cite journal |last1=Rios |first1=C. D. |last2=Jordan |first2=B. A. |last3=Gomes |first3=I. |last4=Devi |first4=L. A. |date=2001-11-01 |title=G-protein-coupled receptor dimerization: modulation of receptor function |url=https://www.sciencedirect.com/science/article/pii/S0163725801001607 |journal=Pharmacology & Therapeutics |language=en |volume=92 |issue=2 |pages=71–87 |doi=10.1016/S0163-7258(01)00160-7 |pmid=11916530 |issn=0163-7258|url-access=subscription }}</ref> While not all, some GPCRs require dimerization to function, such as [[GABAB receptor|GABAB]]-receptor, emphasizing the importance of dimers in biological systems.<ref>{{Cite journal |last=Lohse |first=Martin J |date=2010-02-01 |title=Dimerization in GPCR mobility and signaling |url=https://www.sciencedirect.com/science/article/pii/S1471489209001672 |journal=Current Opinion in Pharmacology |series=GPCR |language=en |volume=10 |issue=1 |pages=53–58 |doi=10.1016/j.coph.2009.10.007 |pmid=19910252 |issn=1471-4892|url-access=subscription }}</ref>[[File:Receptor_Tyrosine_Kinase_Dimerization.jpg|thumb|335x335px|class=skin-invert-image|Receptor tyrosine kinase dimerization]]

=== Receptor tyrosine kinase ===
Much like for G protein-coupled receptors, dimerization is essential for [[receptor tyrosine kinase]]s (RTK) to perform their function in [[signal transduction]], affecting many different cellular processes.<ref name=":2">{{Cite journal |last=Hubbard |first=Stevan R |date=1999-04-01 |title=Structural analysis of receptor tyrosine kinases |journal=Progress in Biophysics and Molecular Biology |language=en |volume=71 |issue=3 |pages=343–358 |doi=10.1016/S0079-6107(98)00047-9 |pmid=10354703 |issn=0079-6107|doi-access=free }}</ref> RTKs typically exist as monomers, but undergo a [[conformational change]] upon [[Ligand (biochemistry)|ligand]] binding, allowing them to dimerize with nearby RTKs.<ref>{{Cite journal |last1=Lemmon |first1=Mark A. |last2=Schlessinger |first2=Joseph |date=2010-06-25 |title=Cell Signaling by Receptor Tyrosine Kinases |journal=Cell |language=English |volume=141 |issue=7 |pages=1117–1134 |doi=10.1016/j.cell.2010.06.011 |issn=0092-8674 |pmc=2914105 |pmid=20602996}}</ref><ref>{{Cite journal |last1=Lemmon |first1=Mark A. |last2=Schlessinger |first2=Joseph |last3=Ferguson |first3=Kathryn M. |date=2014-04-01 |title=The EGFR Family: Not So Prototypical Receptor Tyrosine Kinases |journal=Cold Spring Harbor Perspectives in Biology |language=en |volume=6 |issue=4 |article-number=a020768 |doi=10.1101/cshperspect.a020768 |issn=1943-0264 |pmid=24691965|pmc=3970421 |doi-access=free }}</ref> The dimerization activates the [[cytoplasm]]ic [[kinase]] [[Protein domain|domains]] that are responsible for further [[signal transduction]].<ref name=":2" />

== See also ==
{{Commons|Dimers}}
*[[Monomer]]
*[[trimer (chemistry)|Trimer]]
*[[Polymer]]
*[[Protein dimer]]
*[[Oligomer]]

== References ==

* {{cite web | url=http://goldbook.iupac.org/D01744.html | title=IUPAC "Gold Book" definition | doi=10.1351/goldbook.D01744 | s2cid=242984652 | access-date=2024-07-11| doi-access=free }}
<references/>

[[Category:Chemical compounds]]
[[Category:Dimers (chemistry)| ]]

Dimerization - Revision history

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