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A dimer (/ˈdaɪmər/) (di-, "two" + -mer, "parts") is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. Examples of dimers include inorganic and biochemical dimers.

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 dimerisation is often called dissociation. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs, after Niels Bjerrum.

Noncovalent dimers

Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers, e.g. the water dimer.

Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but they (dimers) do form the excimers Ar₂*, Kr₂* and Xe₂* under high pressure and electrical stimulation.

Covalent dimers

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 dimerise to give the dimer "A−A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene:

${\displaystyle {\ce {2 C(NR2)2 -> (R2N)2C=C(NR2)2}}}$

Carbenes are highly reactive and readily form bonds.

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:

${\displaystyle {\ce {C10H12 -> 2 C5H6}}}$

Many nonmetallic elements occur as dimers: hydrogen, nitrogen, oxygen, the halogens, i.e. fluorine, chlorine, bromine and iodine. Noble gases can form dimers linked by van der Waals bonds, for example dihelium or diargon. Mercury occurs as a mercury(I) cation (Hg2+2), formally a dimeric ion. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include dilithium (Li₂), disodium (Na₂), dipotassium (K₂), dirubidium (Rb₂) and dicaesium (Cs₂).

Such elemental dimers are homonuclear diatomic molecules.

Many small organic molecules, most notably formaldehyde, easily form dimers. The dimer of formaldehyde (CH₂O) is dioxetane (C₂H₄O₂).

Borane (BH₃) occurs as the dimer diborane (B₂H₆), due to the high Lewis acidity of the boron center.

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:

${\displaystyle {\ce {2 C6H12O6 -> C12H22O11 + H2O}}}$

Here, the resulting dimer has a stoichiometry different from the initial pair of monomers.

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.

Biochemical dimers

Pyrimidine dimers are formed by a photochemical reaction from pyrimidine DNA bases. This cross-linking causes DNA mutations which can be carcinogenic, causing skin cancers.

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. These dimers are not only restricted to homodimers, but also heterodimers with related members of the GPCR family. While not all, some GPCRs require dimerization to function, such as GABA_B-receptor, emphasizing the importance of dimers in biological systems.

Receptor Tyrosine Kinase

Much like G protein-coupled receptor, receptor tyrosine kinases (RTK) are also essential proteins in mammals that take the form of dimers to perform their function in signal transduction, affecting a number of different cellular processes. RTKs typically exist as monomers, but undergo a conformational change upon ligand binding, allowing them to dimerize with nearby RTKs. The dimerization activates the cytoplasmic kinase domains that are responsible for further signal transduction.

See also

• Monomer
• Trimer
• Polymer
• Protein dimer
• Oligomer

References

• "IUPAC "Gold Book" definition". doi:10.1351/goldbook.D01744. S2CID 242984652. Retrieved 2009-04-30. {{cite journal}}: Cite journal requires |journal= (help)

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• Chemical compounds
• Dimers (chemistry)