Copper complexes Coupled electron proton transfer

Bis-//-oxodicopper(in)-phenolate intermediate (6) can be observed at -120 C in the rapid oxidation of 2,4-di-t-butylphenolate by [Cu202(A,A/ -di-t-butyldiethyl-enediamine)2] to a mixture of catechol and quinone. A hybrid DFT study, based on the calculated free-energy profile, suggests that the first step is the 0-0 bond cleavage in the peroxo complex which subsequently coordinates to one of the copper ions in the bis-//-oxodi-Cu(in) complex to yield the phenolate intermediate (6). The rate-limiting decay of (6) involves C-O bond formation, followed by coupled internal proton and electron transfer, and electron transfer coupled to proton transfer from an external donor. 

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. 

Structure and Mechanism. Cytochrome-c oxidase catalyzes the four-electron reduction of molecular oxygen to water and couples these redox processes to proton transfer across the mitochondrial membrane.As depicted in Fig. 24, the enzyme is structurally complex and contains four metal centers which are redox active, two copper ions and two heme a groups. One copper ion, Cug, and one of the heme a groups, cytochrome <13, (cyt a ), form a binuclear center which binds dioxygen. Electrons are ... 

Complex IV, or cytochrome c oxidase, was the first of the mitochondrial electron transport complexes to have its molecular stmcture and the internal path of electron transfer revealed by X-ray crystallography. The catalytic core of the complex consists of two subunits. Subunit II contains a binuclear copper center (Cua) that is directly responsible for the oxidation of cytochrome c. From there electrons are passed to haem a and then to the adjacent binuclear center that consists of haem 03 and another copper ion (Cub), which are all held within subunit I (Fig. 13.1.4). Oxygen is bound and reduced between Cub and the iron of haem 03, and access paths for protons from the inside of the membrane and for oxygen from within the membrane have been defined from several crystal stmctures available for bovine and bacterial enzymes. In addition to the protons taken up for the reduction of oxygen, translocation of further protons across the membrane is coupled to electron transfer by a mechanism that is not yet understood (reviewed in Refs. [71, 72]). 

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

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