Copper ligand substitution

When copper is bound to one sulfur atom of a cysteine and two nitrogens of two histidines in an essentially tetrahedrally distorted - trigonal ligand environment (type I copper proteins), the excited levels are low in energy, and the values are reduced to about 5 x 10 ° s (29). Examples are blue copper proteins, like ceruloplasmin and azurin, and copper(II) substituted liver alcohol dehydrogenase (30-32). 

DNA cleavage by, 43 158-159 reactions, copper proteins, 39 25 Oxo-trichloroselenates(IV), 35 270-271 Oxo-type molybdenum enzyme, see Molybdenum enzymes, pterin-containing Oxovandium (IV), solvent exchange and ligand substitution, 42 47-49 Oxyanions, Groups VIB and VIIB, redox reactions, kinetics and mechanism, 40 269-274... 

The reactions of copper(II) complexes to be considered are of two types (i) ligand substitution reactions and (ii) redox reactions, i.e. electron transfer involving a change in oxidation state, copper(I)/(II)/(III). 

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

In contrast with 17, complex 21 (Scheme 1.18) did not undergo ligand substitution with the copper(I) bis(biquinoline) complex, possibly as a result of the different steric properties of the two complexes. The imine exchange reaction with phenylenediammonium worked well, creating the possibility of a new kind of domino or cascade reaction (Scheme 1.18). The intermediate product 18 (from... 

For a study on variation of product stereochemistry with ligand substitution in copper catalyzed reactions see Evans, D. A. Johnson, J. S. Burgey, C. S. Campos, K. R. Tetrahedron Lett. 1999, 40, 2879-2882. 

The process we are seeing in the copper(II)-ammonia solution is a ligand substitution process, where one ligand is replacing another. In general, we can represent this, for reaction in aqueous solution with a neutral monodentate ligand at this stage, by Equation (5.1) ... 

Copper(II)-substituted zinc proteins are generally inactive with respect to the natural and most artificial substrates (Table 2.4). In model compounds cop-per(II) is often principally four-coordinate, with at most two more ligands present at metal-ligand distances that are longer than normal coordination bonds. As a consequence, tbe ability of zinc to switch between four- and five-coordinate species without any appreciable barrier and with usual metal-donor distances is not mimicked by copper. Furthermore, binding at the four principal coordination positions is generally stronger for copper than for zinc. It follows that substrates may have slow detachment kinetics. These properties are unfavorable for catalysis. 

A second question relates to mechanisms of biological interconversions of metallobleomycins. Blm and its Fe-, Cu-, and Zn-complexes are similarly cytotoxic to cells exposed under conditions that minimize extracellular cross reaction with other metal ions, implying that each can be converted into FeBlm. Yet, Cu binds to Blm with higher afifinity than does Fe, and cellular Zn may well be available for reaction with Blm. CuBlm has also been detected in urine after administration of Blm. Opposite conclusions have been reached on whether copper can be removed from Blm by reductive ligand substitution using thiol reagents. 

Bis(thiosemicarbazones) have been made that display powerful activity against established solid tumours in rodents, and that are dependent upon the presence of copper in the host. The N2S2 metal coordination site in these ligands reacts with Cu to form highly stable complexes. Thus, ligand substitution reactions are unfavorable. As the number of alkyl groups increases, the redox potential declines as much as 100-200 mV, until it is out of range for... 

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