Dimerization

{{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 entities by 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. When two oppositely-charged ions 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 acids are often found in the vapour phase.]] Anhydrous carboxylic 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 by hydrogen bonds.<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 and/or van der Waals interactions have a short lifetime, but can be stabilized through special laboratory setups such as 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 exciplexes are 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 halogens 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 diatomic molecules.

==Polymer chemistry== In the context of polymers, "dimer" also refers to the degree of polymerization 2, regardless of the stoichiometry or condensation reactions.

One case where this is applicable is with disaccharides. 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 monosaccharides 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 dipeptides. 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 === 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=== thumb|class=skin-invert-image|Trimethylaluminium dimer Trialkylaluminium compounds can exist as either monomers or dimers, depending on the 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 bases when exposed to ultraviolet light.<ref name=":0"/> This cross-linking causes DNA mutations, which can be carcinogenic, causing skin cancers.<ref name=":0" /> When pyrimidine dimers are present, they can block polymerases, decreasing DNA functionality until it is repaired.<ref name=":0" />

=== Protein dimers === thumb|312x312px|class=skin-invert-image|Tubulin dimer Protein dimers arise from the interaction between two proteins which can interact further to form larger and more complex oligomers.<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 α-tubulin and β-tubulin and this dimer can then polymerize further to make microtubules.<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 subunits, 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 receptors within the human genome, G protein-coupled receptors (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 GABA_B-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>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 kinases (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 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 cytoplasmic kinase domains that are responsible for further signal transduction.<ref name=":2" />

== See also == {{Commons|Dimers}} *Monomer *Oligomer *Polymer *Protein dimer *Trimer

== 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)