Dinuclear copper centres

Electrons from cytochrome c are donated to the dinuclear copper centre Cua, and then transferred consecutively one at a time to haem a, and from there to the dinuclear haem-copper (haem u -Cua) catalytic centre. A tyrosine residue, Y(I-288), which is covalently cross-linked to one of the Cub ligands (His 240), is also part of the active site. The structure of the four-subunit CcO from R. spheroides is presented in Figure 13.9(a), while a more detailed view of the redox-active cofactors and amino acid residues involved in the proton transfer pathways is given in Figure 13.9(b) (Brzezinski Johansson, 2010). 

Copper enzymes are involved in reactions with a large number of other, mostly inorganic substrates. In addition to its role in oxygen and superoxide activation described above, copper is also involved in enzymes that activate methane, nitrite and nitrous oxide. The structure of particulate methane mono-oxygenase from the methanotrophic bacteria Methylococcus capsulatus has been determined at a resolution of 2.8 A. It is a trimer with an a3P33 polypeptide arrangement. Two metal centres, modelled as mononuclear and dinuclear copper, are located in the soluble part of each P-subunit, which resembles CcOx subunit II. A third metal centre, occupied by Zn in the crystal, is located within the membrane. 

Substituents on the hydroquinone with lone pairs enhance the binding (Table 5) and allow bidentate co-ordination of the hydroquinone. It is proposed that the phenolate oxygen is co-ordinated by type 2 copper whereas the lone-pair substituent is associated with the type 3 dinuclear reduction centre. Binding is also affected by bulky substituents and by the distance between the two co-ordinating groups. Electron transfer is controlled by protein activation rather than by the ease of activation of the hydroquinones. 

As mentioned before, a worthwhile model must possess a good accessibility of both the Cu(I) and Cu(II) oxidation states, and retain in solution not only the bimetallic unit but also any bridging group. To detect whether a proposed model of a Type III copper centre satisfies these requirements many studies on the cyclic voltammetry and/or the differential pulse polarography have been carried out and reported [157-210]. In some cases authors started with a di-copper(II) complex, in some other with a di-copper(I) complex and even eventually with a mixed-valence Cu(II)-Cu(I) dinuclear complex. 

Another proposed mechanism of the polymerization is a two-electron transfer mechanism, which involvs phenolate-bridged dinuclear copper(II) complex as starting species. The complex generated phenoxonium cations and phenolate anion through a double one-electron transfer from a phenolate to both copper centres (step v) and form the quinone-ketal intermediate via nucleophilic attack (step vi). This reaction pathway is supported by theoretical calculations of atomic charges of monomeric and dimeric species of 2,6-DMP where phenoxonium cations are proposed as key intermediates. Ab Initio calculations on 2,6-DMP and 4-(2,6-Dimethylphenoxy)-2,6-dimethylphenol provided evidence of the phenoxonium cation in the copper-catalyzed oxidative coupling reaction which proposed that the selective C-O coupling was achieved via the nucleophilic attack of a phenolate on the para-carbon of a phenoxonium cation (25). Based on the experimental evidence currently reported, both... 

Zhang G, Proni G, Zhao S, et al. Chiral tetranuclear and dinuclear copper(II) complexes for TEMPO-mediated aerobic oxidation of alcohols are font metal centres better than two Dalton Trans. 2014 43 12313-12320. 

The results of the kinetic studies provided an impetus to the search for evidence of an intermediate that involved two copper(I) atoms. Despite being entropically disfavoured, those results clearly suggest that two copper centres are involved in the catalytic cycle. Dinuclear and tetranuclear copper(I) complexes are known [95] and could be involved in CuAAC reactions. Computational smdies suggested ways of alleviating the ring strain that is evident in the copper(III) metallacycle shown in Scheme 8. 

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