Macrocycle

The term macromolecule by definition implies "large molecule". In the context of biochemistry, the term may be applied to the four conventional biopolymers (nucleotides, proteins, carbohydrates, and lipids), as well as non-polymeric molecules with large molecular mass such as macrocycles.A macrocycle is, as defined by IUPAC, "a cyclic macromolecule or a macromolecular cyclic portion of a molecule.In the chemical literature, organic chemists may consider any molecule containing a ring of more than nine, or any arbitrarily large number of atoms (3 of them "donor atom") to be macrocyclic.Coordination chemists study macrocycle with three or more potential donor atoms in rings of greater than nine atoms as these compounds often have strong and specific binding with metals.This property of coordinating macrocyclic molecules is the macrocycle effect. It is in essence a specific case of the chelation effect: complexes of bidentate and polydentate ligands are more stable than those with unidentate ligands of similar strength (or similar donor atoms). A macrocycle has donor atoms arranged in more fixed positions and thus there is less of an entropic effect in the binding energy of macrocycles than monodentate or bidentate ligands with an equal number of donor atoms. Thus the macrocycle effect states that complexes of macrocyclic ligands are more stable than those with linear polydentate ligands of similar strength (or similar donor atoms). The same can be said for multicyclic macrocycles, or cryptates, being stronger complexing agents (a cryptate effect).