In chemistry, dimerization refers to the process of joining two 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.^[1]

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,^[2] after Danish chemist Niels Bjerrum.

Noncovalent dimers

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.^[3] Many 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 do form the excimers Ar₂*, Kr₂* and Xe₂* under high pressure and electrical stimulation.^[4]

Covalent dimers

The dimerization of cyclopentadiene gives dicyclopentadiene, although this might not be readily apparent on initial inspection. This dimerization is reversible

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.: 2A → A−A. In this example, monomer "A" is said to dimerize to give the dimer "A−A". An example is a diaminocarbene, which dimerize 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, and the halogens (i.e. fluorine, chlorine, bromine and iodine). Noble gases can form dimers linked by van der Waals bonds, such as 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 phase. 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.

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 dimers

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

Group 13 dimers

Boranes

Borane and Diborane

Diborane (B₂H₆) is an inorganic dimer of Borane. B₂H₆ exists as a structure where two hydrogen atoms bridge the two boron atoms.^[5]

Aluminium

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.^[6]

Biochemical dimers

Protein dimers

Tubulin dimer

Protein dimers arise from the interaction between two proteins which can interact further to form larger and more complex oligomers.^[7] For example, tubulin is formed by the dimerization of α-tubulin and β-tubulin and this dimer can then polymerize further to make microtubules.^[8] 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.^[7]

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.^[9] GPCR dimers include both homodimers and heterodimers formed from related members of the GPCR family.^[10] While not all, some GPCRs require dimerization to function, such as GABA_B-receptor, emphasizing the importance of dimers in biological systems.^[11]

Receptor Tyrosine Kinase Dimerization

See also

Wikimedia Commons has media related to Dimers.

• 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)