Copper—imidazole coordination

Fig 3, Representation of copper-imidazole coordination showing the important multiple-scattering pathways. 

The copper-binding sites in copper-containing proteins are characterized by three distinct classes. In Type-1, or blue copper centers, the copper is coordinated to at least two imidazole nitrogens from two histidyl residues and one sulfur from a cysteinyl residue. Type-1 coppers have small copper hyperfine couplings and a strong visible absorption in the Cu(ll) state. Type-2 or non-blue copper... 

The structure of the parent material in this study—due to its uniqueness with respect to its inequivalent chloride ions both as coordinated and lattice chlorides—represents yet still another example of a solid state copper imidazole material. Others include copper(II) perchlorate salts complexed with amines [10] and copper(I) imidazole complexes based on a cavitand ligand design [11], Copper(I) imidazole complexes have been... 

One recombinant FetSp mutant is unique among multicopper oxidase species and has been particularly informative about the structure of the type 3 binuclear cluster in these species. This is the T1D/T2D double mutant that contains only this type 3 site (Blackburn et al., 2000). EXAFS analysis of this protein contains contributions from electron ejection and scattering from only the type 3 copper atoms and thus provides direct structural information about this cluster. The K-edge XAS spectrum for this mutant in its oxidized and reduced states is shown in Fig. 21. The oxidized sample has a nearly featureless edge with a midpoint energy of 8990 eV typical of tetragonally distorted type 2 Cu(ll) centers, i.e., those with predominantly histidine imidazole coordination. The reduced type 3 cluster exhibited a pronounced shoulder at 8984 eV just below the... 

The type 3 binuclear Cu(II) pair in an MCO is characteristic of this type of copper site in biology. First, both copper atoms are ligated by three protein side chains each all six are His residues. Five of the imidazoles coordinate the Cu(II) atoms via the Ne2 nitrogens while the sixth His does so via N51. This coordination pattern is a conserved feature of the MCO structure.Second, the two Cu(n)... 

Initially, copper(II) coordinates to the octapeptide angiotensin II (Asp Arg Val Tyr Ile-His-Pro-Phe) at the imidazole nitrogen. At neutral pH the main copper complex is CuL, which is a 3N complex (Figme 2). The copper(II) is bonded via the imidazole nitrogen of the histidine residue, and two deprotonated peptide nitrogens. As the pH is raised above 8.0, the coordination site changes to the N-terminus... 

Fig. 5. The active-site of bovine erythrocyte Cu Zn superoxide dismutase. View from the solvent. The copper is coordinated by His 44,46,61 and 118. The Zn is buried. The geometry of its ligands is tetrahedral. The imidazolate ring of His 61 is the brid ng ligand between Cu and Zn. (With permission from Ref. )...

The last example to be mentioned deals with the application of coordination compounds attached to polymers and their use as immobilized catalysts. This technique has been used for a long time in organometallic catalysis. Similar reactions with biomimetic catalysts, as with Cu(II) oxidases, are less well known, and a review for polymeric copper imidazole complexes used in oxidative phenol coupling is available. 

The coordination chemistry of such ligands, containing three of each type of donor, with Zn(II) and Cu(I) was explored. While addition of one equivalent of Zn(II) was shown to form the imidazole-coordinated species exclusively (and then a di-Zn(II) species upon addition of a second equivalent), it was found that, when compared to copper, Zn(II) was largely ambivalent in its coordination site, with Cu(II) preferentially coordinated to the imidazole donors, and Cu(l) preferring the triazole donors (Scheme 21). Thus, a 1-electron oxidation of the Zn(ll)/Cu(I) species led to an electrochemically triggered double translocation of the metal ions, with the Cu and Zn ions exchanging coordination sites [218]. 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

FIGURE 21.18 (a) The Cn site of cytochrome oxidase. Copper ligands inclnde two histidine imidazole groups and two cysteine side chains from the protein, (b) The coordination of histidine imidazole ligands to the iron atom in the heme a center of cytochrome oxidase. 

An imidazole/pyridyl ligand (4) coordinates to a copper(II) center and on deprotonation the imidazole reacts with bis(hexafluoroacetylacetonato)zinc to form a trimeric Cu2Zn species bridged by imidazoles. The zinc ion is in an octahedral coordination environment with Zn-N distances of 2.029(2) A.116... 

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