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Chemical process of joining two molecular entities by bonds of any kind

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. 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, after Danish chemist Niels Bjerrum.

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. 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.

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

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

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.

Pyrimidine dimers (also known as thymine dimers) are formed by a photochemical reaction from pyrimidine DNA bases when exposed to ultraviolet light. For example, tubulin is formed by the dimerization of α tubulin and β tubulin and this dimer can then polymerize further to make microtubules. 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. GPCR dimers include both homodimers and heterodimers formed from related members of the GPCR family. While not all, some GPCRs require dimerization to function, such as GABAB receptor, emphasizing the importance of dimers in biological systems.

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. 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.