Manganese oxidation site

In spite of the high similarity between MnP and VP oxidation sites, crystal structures of wild and recombinant VP, and kinetics of its mutated variants, revealed some significant differences. These result in efficient Mn2+ oxidation in the presence of only two of the three residues forming the binding site [9, 61]. Some MnP residues, which are not conserved in VP, could be responsible for these 

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Ruiz-Duenas FJ, Morales M, Perez-Boada M et al (2007) Manganese oxidation site in Pleurotus eryngii versatile peroxidase A site-directed mutagenesis, kinetic and crystallographic study. Biochemistry 46 66-77... 

Site-binding constants have been determined for only a limited range of simple oxides with only one type of surface site. Multiple-surface site minerals occurring in the deep-well environment such as silicates, aluminosilicates, and complex oxides (such as manganese oxide) will require much more complex TLMs. 

Several reactions between constituents in As-contaminated groundwater and oxic sediments controlled As mobility in the laboratory experiments. Adsorption was the primary mechanism for removing As from solution. The adsorption capacity of the oxic sediments was a function of the concentration and oxidation state of As, and the concentration of other solutes that competed for adsorption sites. Although As(lll) was the dominant oxidation state in contaminated groundwater, data from the laboratory experiments showed that As(lll) was oxidized to As(V) by manganese oxide minerals that are present in the oxic sediment. Phosphate in contaminated groundwater caused a substantial decrease in As(V) adsorption. Silica, bicarbonate and pH caused only a small decrease in As adsorption. 

Reactions between Fe(ll) in contaminated groundwater (5.8 mg/L) and oxic sediment also affected As mobility. Ferrous iron was oxidized by manganese oxides to ferric iron which precipitated as hydrous ferric oxide, creating additional sorption sites. Evidence for this reaction included an increase in ferric oxide concentrations in reacted column sediments and manganese concentrations in leachate that were greater than in the initial eluent. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Tetranuclear Manganese Complexes Modelling the Photosynthetic Water Oxidation Site... 

NMR time scale (ca. 10 s), the lithium spins see an average manganese oxidation state of 3.5 (i.e., Mn 5+ ions), and thus there is only one magnetically inequivalent lithium site (the 8a site). The NMR spectra are clearly sensitive to this hopping mechanism, and NMR spectroscopy may be used to follow the partial charge-ordering process that occurs just below room temperature in the stoichiometric material. 

Solid impurities such as iron oxides, manganese oxide, silicon, and aluminum can become trapped in the steel and form points of weakness in the steel. Weak points can be the site of eventual wear, fracture, or corrosion. 

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