Redox active metal ions

Belouzov-Zhabotinsky reaction [12, 13] This chemical reaction is a classical example of non-equilibrium thermodynamics, forming a nonlinear chemical oscillator [14]. Redox-active metal ions with more than one stable oxidation state (e.g., cerium, ruthenium) are reduced by an organic acid (e.g., malonic acid) and re-oxidized by bromate forming temporal or spatial patterns of metal ion concentration in either oxidation state. This is a self-organized structure, because the reaction is not dominated by equilibrium thermodynamic behavior. The reaction is far from equilibrium and remains so for a significant length of time. Finally,... 

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

Metal hexacyanoferrates possessing only one kind of redox-active metal ions Most of the metal hexacyanoferrates show only... 

Metal hexacyanoferrates possessing two kinds of redox-active metal ions Here we consider those hexacyanoferrates that... 

In the most general situation, a redox-active metal ion is translocated from a given site to another site of the same molecular system, following a chemical (a redox reaction) or an electrochemical input. The redox-driven reversible translocation of a metal ion in a two-component molecular system is schematically sketched in Fig. 2.2. 

Moreover, under certain conditions these phenolic compounds could also act as pro-oxidants. In the presence of redox-active metal ions such as Cu or Fe, phenolic compounds react with O2 to generate phenoxyl radicals. Under normal growth conditions phenoxyl radicals can be rapidly deactivated by polymerization or enzymatic reduction. However, if the phenoxyl radical concentrations are too high and/or the lifetime is increased, they could initiate DNA damage or lipid peroxidation and exhibit cytotoxicities. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin have been reported to induce... 

Evidence is now accumulating to show that reactions involving metals might be the common denominator underlying AD and PD. In these disorders, an abnormal reaction between a protein and a redox-active metal ion (copper or iron) promotes the formation of ROS. It is especially intriguing how the antioxidant Cu/Zn-SOD activity can convert into a pro-oxidant activity, a theme echoed in the recent proposal that Ap and PrP, the proteins respectively involved in AD and prion diseases, possess similar redox properties [Bush, 2002],... 

The superoxide oxide radical interacts with nitric oxide to produce peroxynitrite at a rate which three times faster than the rate at which superoxide dismutase utilizes superoxide (Beckman, 1994). Peroxynitrite is capable of diffusing to distant places in neural cells where it induces lipid peroxidation and may be involved in synaptosomal and myelin damage (Van der Veen and Roberts, 1999). After protonation and decomposition, peroxynitrite produces more hydroxyl radicals. This mechanism of hydroxyl radical generation is not dependent on redox active metal ions and may be involved in initiating lipid and protein peroxidation in vivo (Warner et al., 2004). 

What distinguishes multicopper oxidases from other copper proteins is that they contain one each of these three types of copper site (Solomon and Lowery, 1993 Solomon et al., 1996). Not only does this make them excellent models for all copper proteins, but because they have four redox-active metal ions, they also serve as paradigms for other enzymes that couple a one-electron reductant to a four-electron oxidant, most notably cytochrome c oxidase. Indeed, the three copper sites (and four copper atoms) in the multicopper oxidases play essentially equivalent roles in comparison to the two heme groups and two copper atoms in cytochrome c oxidase. 

B. Bimetallic Catalysis with Redox Active Metal Ions / 129... 

Redox-active metal ions may assist in the nitrosylation of organic substrates such as alcohols, amines, and thiols ... 

To date, there is no generally accepted theory that accounts for the development of AD pathology. The multifactorial basis of the disease makes such a theory unlikely to be possible in the foreseeable future. In addition to the NFTs and amyloid-hypothesis of the disease, oxidative stress, systemic levels of redox active metal ions, cardiovascular disease, the apoEe4 allele and type 2 diabetes are all clear risk factors for development of AD. However, recent research supports the notion that the Ap buildup may be a key event in AD and that other manifestations of the disease, like NFT formation, result from an imbalance between AP production and AP clearance (17). 

Mineralization in apoferritin involves a prior oxidation of Fe(II) as it enters channels in the protein shell. The ferroxidase center seems to be composed of Glu (Gin) and His residues situated between four helices (P. M. Harrison, personal communication). There is scope for exploring the design of agents that could block the entry of iron into the core of the protein or hasten its passage out. It is possible that non-redox-active metal ions such as Ga(III), In(III), and Al(III) can act in this way. The nature of the Fe(II) complex in the cytoplasm, which acts as a donor to ferritin, is not clear yet, but perhaps it could be Fe(II) glutathione. 

Redox-active metal ions (Fe, Cu), present within specific brain regions, can generate oxidative stress by production of reactive oxygen and nitrogen species (ROS, RNS). 

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