View source for Chemistry:Dimer

{{short description|Oligomer consisting of two monomers joined by bonds of any kind}}
A '''dimer''' ({{IPAc-en|ˈ|d|aɪ|m|ər}}) (''di-'', "two" + ''-mer'', "parts") is an [[Chemistry:Oligomer|oligomer]] consisting of two [[Chemistry:Monomer|monomer]]s joined by bonds that can be either strong or weak, [[Chemistry:Covalent bond|covalent]] or [[Physics:Intermolecular force|intermolecular]].<ref>{{cite web |title=Dimer |url=https://www.chem.ucla.edu/~harding/IGOC/D/dimer.html |website=Illustrated Glossary of Organic Chemistry |publisher=UCLA |access-date=12 May 2022}}</ref> Dimers also have significant implications in polymer chemistry; inorganic chemistry, and biochemistry. 

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 dimerization is often called [[Chemistry:Dissociation|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|pages=194904|doi=10.1063/1.4982885|pmid=28527430|issn=0021-9606|arxiv=1702.04853|bibcode=2017JChPh.146s4904A|s2cid=12227786}}</ref> after Niels Bjerrum.

== Noncovalent dimers ==
[[File:Carboxylic acid dimers.png|right|thumb|Dimers of carboxylic acids are often found in the vapour phase.]]
[[Chemistry:Anhydrous|Anhydrous]] [[Chemistry:Carboxylic acid|carboxylic acid]]s form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen. For example, [[Chemistry:Acetic acid|acetic acid]] forms a dimer in the gas phase, where the monomer units are held together by [[Chemistry:Hydrogen bond|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}}</ref> Under special conditions, most OH-containing molecules form dimers, e.g. the [[Physics:Water dimer|water dimer]].

Excimers and exciplexes are [[Physics:Excited state|excited]] structures with a short lifetime. For example, noble gases do not form stable dimers, but they do form the excimers Ar<sub>2</sub>*, Kr<sub>2</sub>* and Xe<sub>2</sub>* 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}}</ref>

== Covalent dimers ==
[[File: Dicyclopentadiene structure.svg|thumb|right|100px|The dimer of cyclopentadiene, although this might not be readily apparent on initial inspection.]]
[[File:1,2-dioxetane.png|thumb|right|100px|1,2-dioxetane, one of two formaldehyde dimers. As evidenced by this molecule's bonds, covalent dimers are usually not similar in structure to their monomers.]]

Molecular dimers are often formed by the reaction of two identical compounds e.g.: {{chem2|2A -> A\sA}}. In this example, [[Chemistry:Monomer|monomer]] "A" is said to dimerize to give the dimer "{{chem2|A\sA}}". An example is a diaminocarbene, which dimerize to give a [[Chemistry:Tetraaminoethylene|tetraaminoethylene]]:
:<chem>2 C(NR2)2  ->  (R2N)2C=C(NR2)2</chem>
[[Chemistry:Carbene|Carbene]]s are highly reactive and readily form bonds.

[[Chemistry:Dicyclopentadiene|Dicyclopentadiene]] is an asymmetrical dimer of two [[Chemistry:Cyclopentadiene|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: [[Chemistry:Hydrogen|hydrogen]], [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Oxygen|oxygen]], and the [[Chemistry:Halogen|halogen]]s (i.e. [[Chemistry:Fluorine|fluorine]], [[Chemistry:Chlorine|chlorine]], [[Chemistry:Bromine|bromine]] and [[Chemistry:Iodine|iodine]]). Noble gases can form dimers linked by van der Waals bonds, such as dihelium or [[Chemistry:Diargon|diargon]]. [[Chemistry:Mercury (element)|Mercury]] occurs as a mercury(I) cation ({{chem2|Hg2(2+)}}), formally a dimeric ion. Other metals may form a proportion of dimers in their vapour phase. Known metallic dimers include [[Chemistry:Dilithium|dilithium]] ({{chem2|Li2}}), disodium ({{chem2|Na2}}), dipotassium ({{chem2|K2}}), [[Chemistry:Dirubidium|dirubidium]] ({{chem2|Rb2}}) and dicaesium ({{chem2|Cs2}}). Such elemental dimers are [[Chemistry:Homonuclear molecule|homonuclear]] [[Chemistry:Diatomic molecule|diatomic molecule]]s. 

Many small organic molecules, most notably [[Physics:Formaldehyde|formaldehyde]], easily form dimers. The dimer of formaldehyde ({{chem2|CH2O}}) is dioxetane ({{chem2|C2H4O2}}).

[[Chemistry:Borane|Borane]] ({{chem2|BH3}}) occurs as the dimer [[Chemistry:Diborane|diborane]] ({{chem2|B2H6}}), due to the high Lewis acidity of the [[Chemistry:Boron|boron]] center.

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

One case where this is applicable is with [[Chemistry:Disaccharide|disaccharide]]s. For example, [[Chemistry:Cellobiose|cellobiose]] is a dimer of [[Chemistry:Glucose|glucose]], even though the formation reaction produces [[Chemistry:Water|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 [[Chemistry:Monosaccharide|monosaccharides]] to be considered dimers. An example is [[Chemistry:Sucrose|sucrose]], a dimer of [[Chemistry:Fructose|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 [[Chemistry:Glycylglycine|glycylglycine]], consisting of two [[Chemistry:Glycine|glycine]] molecules joined by a [[Biology:Peptide bond|peptide bond]].  Other examples include [[Chemistry:Aspartame|aspartame]] and carnosine.

== Inorganic dimers ==

=== Group 13 dimers ===

==== Boranes ====
[[File:Borane & Diborane.jpg|thumb|Borane and Diborane]]
[[Chemistry:Diborane|Diborane]] (B<sub>2</sub>H<sub>6</sub>) is a classic example of an inorganic dimer. [[Chemistry:Borane|Borane]] does not exist alone as BH<sub>3</sub>, even though it is often written in that way. B<sub>2</sub>H<sub>6</sub> exists as a structure where two hydrogen atoms bridge the two [[Chemistry:Boron|boron]] atoms. The bridging B-H bonds are lower in [[Chemistry:Bond order|bond order]] than what would be expected for a regular B-H bond (they are lower in bond order than the terminal B-H bonds). This is explained by the bond having three centers but only two electrons, "banana bond", instead of the typical single bond with two centers and two electrons.<ref>{{Cite book |last=Shriver |first=Duward |title=Inorganic Chemistry |publisher=W.H. Freeman and Company |year=2014 |isbn=9781429299060 |edition=6th |pages=306–307 |language=English}}</ref>
==== Aluminium ====
[[File:Trimethylaluminium dimer.png|thumb|Trimethylaluminium dimer]]
[[Chemistry:Organoaluminium chemistry|Organoaluminium]] complexes can exist as either monomers or dimers based on the [[Chemistry:Steric effects|steric bulk]] of the groups attached. For example, [[Chemistry:Methyl group|methyl]] or ethylaluminium exists as a dimer, but when a bulkier group is added, the complex exists as a monomer, such as trimesitylaluminium.<ref name=":0">{{Cite book |last=Shriver |first=Duward |title=Inorganic Chemistry |publisher=W.H. Freeman and Company |year=2014 |isbn=9781429299060 |edition=6th |pages=377–378 |language=English}}</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 [[Biology:Mutation|DNA mutations]], which can be carcinogenic, causing [[Medicine:Skin cancer|skin cancers]].<ref name=":0" /> When [[Biology:Pyrimidine dimer|pyrimidine dimers]] are present, they can block [[Biology:Polymerase|polymerases]], decreasing DNA functionality until it is repaired.<ref name=":0" />

=== Protein dimers ===
[[File:Tubulin_dimer.png|thumb|312x312px|Tubulin dimer]]
[[Biology:Protein dimer|Protein dimers]] arise from the interaction between two [[Biology:Protein|proteins]] which can interact further to form larger and more complex [[Chemistry:Oligomer|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 |url=http://dx.doi.org/10.1016/j.tibs.2004.09.006 |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, [[Biology:Tubulin|tubulin]] is formed by the dimerization of [[Biology:Tubulin|α-tubulin]] and [[Biology:Tubulin|β-tubulin]] and this dimer can then [[Chemistry:Polymerization|polymerize]] further to make [[Biology:Microtubule|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 [[Biology:Protein subunit|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 [[Biology:Receptor (biochemistry)|receptors]] within the human genome, [[Biology:G protein-coupled receptor|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=9780128151075 |s2cid=58577416 }}</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}}</ref> While not all, some GPCRs require dimerization to function, such as [[Biology:GABAB receptor|GABA<sub>B</sub>]]-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}}</ref>[[File:Receptor_Tyrosine_Kinase_Dimerization.jpg|thumb|335x335px|Receptor Tyrosine Kinase Dimerization]]

=== Receptor tyrosine kinase ===
Much like for G protein-coupled receptors, dimerization is essential for [[Biology:Receptor tyrosine kinase|receptor tyrosine kinases]] (RTK) to perform their function in [[Biology:Signal transduction|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 [[Biology:Conformational change|conformational change]] upon [[Biology: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 |pages=a020768 |doi=10.1101/cshperspect.a020768 |issn=1943-0264 |pmid=24691965|pmc=3970421 |doi-access=free }}</ref> The dimerization activates the [[Biology:Cytoplasm|cytoplasmic]] [[Biology:Kinase|kinase]] [[Biology:Protein domain|domains]] that are responsible for further [[Biology:Signal transduction|signal transduction]].<ref name=":2" />


== See also ==
{{Commons|Dimers}}
*[[Chemistry:Monomer|Monomer]]
*Trimer
*[[Chemistry:Polymer|Polymer]]
*[[Biology:Protein dimer|Protein dimer]]
*[[Chemistry:Oligomer|Oligomer]]

== References ==

* {{cite journal | url=http://goldbook.iupac.org/D01744.html | title=IUPAC "Gold Book" definition | doi=10.1351/goldbook.D01744 | s2cid=242984652 | access-date=2009-04-30| doi-access=free }}
<references/>



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

{{Sourceattribution|Dimer (chemistry)|1}}