Ligand-associated metal binding site

The metals are generally found either bound directly to proteins or in cofactors such as porphyrins or cobalamins, or in clusters that are in turn bound by the protein the ligands tire usually O, N, S, or C. Proteins with which transition metals and zinc are most commonly associated catalyze the intramolecular or intermolecular rearrangement of electrons. Although the redox properties of the metals are important in many of the reactions, in others the metal appears to contribute to the structure of the active state, e.g., zinc in the Cu-Zn dismutases and some of the iron in the photosynthetic reaction center. Sometimes equivalent reactions are catalyzed by proteins with different metal centers the metal binding sites and proteins have evolved separately for each type of metal center. 

With the larger Ms(calix) crown (3.127),65 two dynamic processes are observed. The intermolecular association/disassociation equilibrium in which the complex equilibrates with the uncomplexed ligand and free metal cation is slow on the NMR time scale. The complex also exhibits a faster intramolecular vibration of the metal cation from one binding site to another (Scheme 3.26). Compound 3.127 is also interesting because the mechanism of this fast, intramolecular cation shuttling process may have important implications on the analogous movement of metal cations through transmembrane ion channels (Section 2.2). 

In very general terms, two factors make a site particularly good for binding N2 (1) the metal must be electron-rich and be associated with a relatively low formal oxidation state and with electron-releasing ligands, and (2) the site must be polarizable, again associated with the same features as in (1), and coordinated by soft donor atoms such as sulfur [89]. 

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