User:MD092022/Dimer (chemistry)

Lead
A dimer (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.

Boranes
Diborane (B2H6) is a classic example of an inorganic dimer. Borane does not exist alone as BH3, even though it is often written in that way. B2H6 exists as a structure where two hydrogen atoms bridge the two boron atoms. The bridging B-H bonds are lower in 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.

Aluminium
Organoaluminium complexes can exist as either monomers or dimers based on the steric bulk of the groups attached. For example, methyl or ethylaluminium exists as a dimer, but when a bulkier group is added, the complex exists as a monomer, such as trimesitylaluminium.

Pyrimidine Dimers
Pyrimidine dimers (also known as thymine dimers) are formed by a photochemical reaction from pyrimidine DNA bases when DNA is exposed to ultraviolet light. This cross-linking causes DNA mutations which can be carcinogenic, causing skin cancers. When pyrimidine dimers are present they can block polymerases which decreases DNA functionality until it is repaired.

Protein Dimers
Protein dimers arise from the interaction of two proteins which can then interact further to make larger more complex oligomer. 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 in order to decrease the amount of genetic code required to make the functional protein.

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 GABAB-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.