ligand

In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a Lewis base). Fewer examples exist where a molecule can be described as a ligand that does not accept electrons from a Lewis base (hence, the ligand acts as a Lewis acid).
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Noun
1. an atom or molecule or radical or ion that forms a complex around a central atom
(hypernym) substance, matter

 Say it 
1. In inorganic chemistry, a molecule or ion that binds to a metal cation to form a complex. 2. In biochemistry, a molecule that binds to a receptor , having a biological effect.

a molecule that binds to a specific site on a protein.

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