Copper coupling matrices

The purple color of the Cua center originates from two S-to-Cu CT transitions of approximately equal intensity. In a dinuclear center like Cua, the valence d orbitals split into pairs with phases that are bonding f) and antibonding (f ) with respect to metal-metal interactions (see Figure 7). The energy separation of the two orbitals, related to the electronic coupling matrix element between the two copper ions see Section 8.4.8.3.3), is determined by... 

The cytochrome c oxidase protein is thought to consist of two heme iron centers (heme a with two axial histidines and heme 03 with one axial histidine (analogous to myoglobin)) and two copper centers (Cua with two histidine, two cysteine, and one water/tyrosine ligand in its oxidized state and Cub with three histidine, one methionine, and one H2O/HO" ligands). The CuA/heme a pair constitute two coupled, one-electron redox couples (low potential, 0.4V) that facilitate (a) electron transfer from cytochrome c(Fe ) at the matrix side of the inner mitochondrial membrane as well as (b) proton transfer from the mitochondrial matrix across the inner membrane to the cytosol. At the cytosol side of the inner mitochondrial membrane, the CuB/heme a- pair constitute the binding site for O2 as well as the conduit for its high-potential four-electron, four-proton reduction to two H2O molecules. 

For a two-pulse (90° - t - 180°), or primary echo experiment, the integrated intensity of the spin echo, which occurs at time t after the 180° pulse, is measured as a fimction of increasing t from the probe s dead-time ( 100 ns) to a time where the echo amplitude has decayed to a few percent of its initial amplitude (2-8 ps for most powder samples). A two-pulse ESE decay envelope for the type-1 Cu(II) site of a multi-copper oxidase, Fet3p, is shown in Figure 1(a). The data show an overall decay characterized by a phase memory time, Tm or T, of < 1.0 ps. Superimposed on this decay are echo modulations that arise ft om hyperfine coupling to the N nuclei of two histidyl imidazole ligands and the protons of the snrronnding matrix. 

The TPQ reduction potential shows a linear variation with pH of -60 mV/pH, indicating a 2e. 211 ET process, in contrast to the 2e, 3H ET observed with model compounds." Presumably the protein matrix or nearby Cu(II) stabilizes the deprotonated topaquinol. No copper electrochemistry was observed. It is not clear whether the copper potential is anomalously low or if it could not be measured due to weak electronic coupling with the phenyl-alkynyl bridge. 

The cyclic oxidation and reduction of the iron and copper in the oxygen reduction center of cytochrome c oxidase, together with the uptake of four protons from the matrix space, are coupled to the transfer of the four electrons to oxygen and the formation of water. Proposed Intermediates in oxygen reduction Include the peroxide anion Oz ) and probably the hydroxyl radical (OH-) as well as unusual complexes of iron and oxygen atoms. These intermediates would be harmful to the cell if they escaped from the reaction center, but they do so only rarely. 

Figure 6. Sets of pathway tubes for the ET coupling from Cu to Ru(bpy)2(im) in HIS 126-modified azurin. The shaded curves (dashed lines are H-bonds) indicate the cores of the tubes that together are responsible for effectively all the electronic coupling of the protein matrix. Nos. 122 and 124 are like 126, but with a subset of the tubes shown here. The copper ligands are 46, 117, and 112, and the coupling is dominated by the 112 fi-strand.

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