Inchemistry, dimerization is the process of joining two identical or similar molecular entitiesbybonds. 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.
Anhydrouscarboxylic 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 excimersAr2*, Kr2* and Xe2* under high pressure and electrical stimulation.[4]
The dimerization of cyclopentadiene gives dicyclopentadiene, although this might not be readily apparent on initial inspection. This dimerization is reversible1,2-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:
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:
One case where this is applicable is with disaccharides. For example, cellobiose is a dimer of glucose, even though the formation reaction produces water:
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.
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]
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 GABAB-receptor, emphasizing the importance of dimers in biological systems.[11]
