In an inorganic coordination entity, the atoms or groups joined to the central atom.
In biochemistry: if it is possible or convenient to regard
part of a polyatomic molecular entity as
central, then the atoms, groups or molecules bound to that part are called ligands.
Biochemical usage is thus wider, in that the central entity can be polyatomic.
may be a ligand for proteins and for citrate as well as for
It may even be a ligand for a univalent entity such as acetate: in other circumstances,
may be the ligand for
H+, since the
definition makes it clear that the view of which entity is central may change for
convenience. Thus, four calcium ions are ligands for calmodulin, when the protein
is regarded as central: four carboxylate groups of calmodulin ligate (are ligands of) each calcium ion when this ion is regarded as central. It is the
ligand that is said to ligate the central entity, which is said to be ligated. When the hormone binding to a receptor is called a ligand, the receptor is thus regarded as the central entity. Biochemists should bear in mind that the
usage in inorganic chemistry has been that ligands bind only single atoms, so they
should be cautious in fields such as bioinorganic chemistry where confusion may be
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1136