[31] Inert Co(III) complexes as reagents for nucleotide binding sites

The chapter elaborates on synthesis of Co(III) complexes as regents for nucleotide binding sites. About one-third of all currently purified enzymes require a metal cation for their activity. In some cases, as with nucleotide binding sites, the metal ion is acting as a bridge between the substrate and the enzyme. It would therefore be useful to devise techniques for labeling the metal binding sites of enzymes. Among the transition metal ions, those that form d 3 or d 6 low spin complex ions in octahedral fields are the most inert with regard to substitution of their ligands; they have dissociation times of the order of 107 sec. The cobaltic cation (d 6 low spin) binds its ligand in an octahedral ligand field and the distances of Co (III)-nitrogen cr Co(III)-oxygen resemble those observed for corresponding Mg(II) complexes because of the similarity in ionic radii (about 0.65 A). In addition, Co(III) complexes are frequently stabilized, in spite of their high redox potential, by reason of the low probability of exchange of the ligands by external groups. It is therefore possible to synthesize Co(III) analogs of metal substrate complexes, by oxidation of the corresponding Co(II) complexes with biological ligands, and to try to use them as affinity labels of the active sites of enzymes. The chapter elaborates on synthesis Co(III) complexes.