Metal chelate ligands

Figure 8 Swelling of hybrid hydrogels in response to competing metal-chelating ligand. The hydrogels were prepared from HPMA-DAMA copolymers, Ni(II), and TEK42 coiled coil. The gels were swollen in PBS (pH 7.4) at 25°C. The arrow indicates the addition of 100 mM buffered imidazole.

Fig. 27. Principle of the template synthesis of a catenane. The U fragment is a metal-chelating ligand and the black dot is a transition-metal ion with a pseudo-tetrahedral coordination geometry...

Metal chelate affinity chromatography is a kind of separation method which has, as a ligand, a metal ion. Some proteins and peptides are purified on the basis of affinity for metal ions immobilized by chelation on the adsorbents. Histidine and cysteine form complexes with the chelated metals around neutral pH. Biological proteins include many histidines as well as recombinant proteins as polyhistidine fusions for instance, His-tag proteins have a specific metal chelate affinity. The adsorbent is prepared by coupling a metal chelate ligand with an iminodiacetic acid group, which forms a chelate with divalent metal ions such as Zn2+, Cu2+, Cd2+, Hg2+, Co2+, Ni2+, Fe2+, etc. 

Certainly the use of cation exchange resins with weakly acidic eluting buffers (i.e. the standard amino-acid analyser) may detract from any distinction between free and metal-chelated ligands. 

Because in most cases no clear lUPAC nomenclature exists for metal-containing macromolecules or macromolecular metal complexes, it is not possible to obtain by a Chemical Abstract literature search a detailed information on them. One has to look for each individual metal, metal ion, metal complex, metal chelate, ligand or also polymer. For type I usually rational nomenclature is used (for example cobalt(II) complex with/ of poly(4-vinylpyridine) or 2,9,16,23-tetrakis(4-hydroxyphenyl)phthalocyanine zinc(II)... 

One of the most successful and well-known technologies in affinity purification is using metal chelate ligands for protein separation, particularly for proteins genetically modified with a polyhistidine affinity tag (His tag). 

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