dimer

A dimer is a molecule or molecular complex composed of two subunits (monomers) that are held together by chemical bonds or by non-covalent forces. The two subunits may be identical, forming a homodimer, or different, forming a heterodimer. Dimers range from covalently linked species to non-covalently bound assemblies and can involve proteins, nucleic acids, or small organic molecules.

In chemistry, dimerization refers to the association of two molecules of the same species. Dimers may form through disulfide bonds between cysteine residues, radical coupling, or non-covalent interactions such as hydrogen bonds, van der Waals forces, or π–π stacking. When dimers and monomers coexist in solution, their balance is described by a dissociation constant (Kd).

In biology, dimerization is a common mechanism for regulating activity. Many transcription factors form dimers to bind DNA, and receptor proteins dimerize in response to ligand binding to propagate signals. Enzymes may require dimerization for catalytic activity or allosteric regulation; some metabolic enzymes function as dimers or as part of higher-order oligomers.

In broader contexts, dimers can be transient or stable, and they appear in materials science, gas-phase chemistry, and crystallography as two-unit assemblies. The term emphasizes the twofold symmetry and the two-subunit composition at its core.