Cobalt complexes metal binding

There has been some uncertainty concerning the metal content of alkaline phosphatase and the role of zinc in the catalytic process. Early measurements by Plocke et al. (36, 50) showed that there were 2 g-atoms per dimer. The zinc requirement for enzymic activity was demonstrated by the inhibition of the enzyme with metal binding agents in accord with the order of the stability constants of their zinc complexes. It appears that in some cases (EDTA) zinc is removed from the enzyme and in other cases (CN) the ligand adds to the metalloprotein. A zinc-free inactive apoenzyme was formed by dialysis against 1,10-phenanthro-line. Complete activity was restored by zinc only zinc, cobalt, and possibly mercury produce active enzyme. 

Metal binding in procarboxypeptidase A is weaker than in the active enzyme ( 107), Table 7). It was proposed that the bonding involves sulfur and a weaker ligand than N (107). In view of the present concept of the chelating site in carboxypeptidase, further studies of the zymogen are necessary. In that connection, the cobalt complex should be valuable. 

The circular dichroic spectrum of cobalt alkaline phosphatase (Fig. 16) shows more clearly the complexity of the visible absorption. Although it can not be ruled out that the spectrum of this Co (I I) enzyme represents two slightly different Co(II) sites, there are striking similarities with Co(II) carbonic anhydrase, which has only one metal-binding site. At high pH, cobalt carbonic anhydrase and cobalt alkaline phosphatase have several spectral features in common, and both may have a similar kind of irregular coordination. It should be noted, however, that the absorption coefficient for Co(II) alkaline phosphatase per equivalent of activity-linked metal ion is only half of the value for Co(II) carbonic anhydrase. 

The bleomycins (Fig. 12.6) are a family of glycopeptide-derived antibiotics which are used in the treatment of various tumors. They bind iron in the blood and form an active hypervalent oxo-iron species. The two-dimensional structure is well known but no crystal structures of bleomycin or its metal complexes have been reported. The MM2 force field was modified and extended by modeling of the crystal structures of the cobalt complexes of two bleomycin analogues in order to develop a force field for... 

Figure 7-36. The ligand 7.56 contains two didentate metal-binding domains. On reaction with cobalt(n), a triple-helical dinuclear complex [Co2(7.56)3]4+ is formed, in which each six-co-ordinate cobalt(n) centre is co-ordinated to a didentate metal-binding domain from each of three ligand threads.

At the same time, according to x-ray data for zinc chelate 909, the nitrogen atom is turned to the side of the metal. The distance Npy-Zn is 2.80 A, that allows us to consider the possible participation of the examined donor center in binding with the metal, leading to formation of a hexacoordinated structure (two-capped tetrahedron) [243]. In relation with this result, let s pay attention to the data reported in Refs. 244 and 248. The tetrahedral configuration without coordination of the nitrogen atom of pyridine is attributed to the cobalt complex 907 (X = NTs, M — Co), although this N atom is rotated to the side of the metal [244]. The pentacoordinated complex 910 is described in Ref. 248, in which only one pyridine substituent is coordinated (the distance Npy-Co is 2.45 A) ... 

Another example described in Section 9.5.3 reports the synthesis and screening of a synthetic receptor library aimed at transition metal binding. Other similar examples have been reported recently. Burger and Clark Still (211) prepared ionophoric, cyclen-based libraries decorated by amino acid units and screened them for their ability to complex copper and cobalt ions Malin et al. (212) identified novel hexapeptidic technetium-binding sequences from the screening of cellulose-bound libraries and... 

Figure 3 Structure of the Co(ll)-ldeR-DNA complex. (Top) Two IdeR dimers bind to the DNA recognition sequence, and each monomer (gray) binds two cobalt ions (spheres) (pdb 1U8 R). (Bottom) The two metal-binding sites in one monomer are connected by a hydrogen bond (not shown) between H79 and El 05.

The tris(mercaptophenylimidazolyl)borate iron and cobalt complexes [(Tmph)2M] (M = Fe, Co) have been synthesized by reaction of (Tmph)Tl with MI2.156 Structural characterization by X-ray diffraction demonstrates that the potentially tridentate Tmph binds through only two sulfur donors in these sandwich complexes and that the tetrahedral metal centers supplement the bonding by interactions with two BH groups. Comparison of the structures of [(Tmph)2M] with those of related tris(pyrazolyl)borate counterparts indicates that the Tm favors lower primary coordination number in divalent metal complexes. 

Our scheme calls for the preparation of coordinatively unsaturated, or at least substitution labile, cobalt complexes that will bind O2. Reduction of halide precursor TpCo-I with magnesium metal under N2 yielded the corresponding dinitrogen complexes, see eq. 2.[8,9]... 

The presence of a carbon in the core of the ring indicates that these macrocycles will form organometallic interactions upon metal binding. In biological systems, naturally occurring organometallic complexes are rare. The cobalamin cofactor is one of the more important examples, which possesses an axial metal-carbon bond in a cobalt porphyrinoid macrocycle (77). Another common example is seen in carbon-monoxide deactivated ferrous heme (22) ... 

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