Oxygenation mechanism

Mechanism has been discussed on the rearrangement pathways of a-hydroperoxy peroxyketone. In the three possible processes of oxygen insertion by the rearrangement 

Some of the results obtained by the electronic and EPR spectroscopies are very much suggestive for the reaction of oxygen. 

Numerous halogenated aromatic compounds are released into the environment as solvents, lubricants, insulation and hydraulic fluids, refrigerants, fire retardants, paints, synthetic 

CCD is distinct from 1,2-CTD in its specificity for intradiol vs extradiol cleavage of 3-substituted catechols. Broderick and O Halloran found that less than 0.5% of the 3-methylcatechol product resulted from extradiol cleavage [100]. Bhat et al. observed no 

Bredirick et al adsorbed CCD on a citrate-reduced silver colloid, and found that adsorbed CCD retains 60-85% of its enzymatic activity in the reaction of catechol substrate with O2 to give cisxis-muconate [112]. The first surface-enhanced resonance Raman spectroscopy (SERRS) study has demonstrated that the native conformation of CCD is retained in the adsorbed state. 

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Ingelman-Sundberg, M. and Ekstrom, G. (1982). Aniline is hydroxylated by the cytochrome P-450-dependent hydroxyl radical-mediated oxygenation mechanism. Biochem. Biophys. Res. Commun. 106, 625-631. 

Proponents of each of these mechanisms attack the other with numerous criticisms. Those favoring the mole-oxide intermediate (mechanism A) claim that (1) the singlet oxygen mechanism does not adequately describe the observed kinetics(87) and (2), it is eliminated since a chlorophyll derivative with a triplet energy too low to excite oxygen to its singlet level is still effective in producing product by the steps<90>... 

Those favoring the singlet oxygen mechanism (mechanism B) counter by asking why, if a mole-oxide is formed, must the oxygen be transferred to another anthracene to obtain a stable product <85) It would appear that if the intermediate has the structure shown below, that a single closure would produce the endoperoxide directly ... 

An YZ, Chen CB, Anderson JL, Sigman DS, Foote CS, Rubin Y (1996) Sequence-specific modification of guanosine in DNA by a C60-linked deoxyoligonucleotide evidence for a non-singlet oxygen mechanism. Tetrahedron 52 5179-5189. 

As seen, the Ir(I) complexes show (in addition to two one-electron reductions, of little interest with regard to the oxygenation mechanism) an oxidation process. While for derivatives with less bulky alkyl groups (Et, Pr1) the oxidation involves an irreversible M(I)— M(III) process, for those with the bulky group Bul it involves a reversible M(I)— M(II) process. 

FIGURE 10.29 Typical photoproducts observed (A) in the irradiation (A = 300 nm) in air of naphthalene and 1-methylnaphthalene adsorbed on silica and formed by a Type I electron transfer (superoxide) mechanism (Barbas et al., 1993) and (B) in the irradiation in air of acenaphthylene (A = 350 nm) adsorbed on silica formed by a Type II singlet oxygen mechanism (Barbas et al., 1994) (adapted from Dabestani, 1997). 

It is evident that, for the reaction sequences proposed, the quantitative analysis of product B as a function of added DABCO cannot be used as a decisive argument in favour of or against a singlet oxygen mechanism. Moreover, the increase of the rate of phenothiazinone formation with the concentration of protic solvent where singlet oxygen is quenched efficiently supports the given interpretation. 

Much effort has been devoted to the elucidation of the oxygenation mechanism (see reviews [8,9]). Extensive studies by Barton and coworkers led to the assumption that Gif chemistry was not a radical process. This assumption was based on two observations. First, in radical chemistry, the order of reactivity is C -H > Csec-H 3> Cprim-H, whereas in Gif systems, a significant preference for oxidation in the secondary position of the hydrocarbon was observed [10, 11] (Csec-H > C -H 3> Cprim-H). A second hint was the low kinetic isotope effect (about 2 for the ketone). Both findings were combined to the mechanism depicted in Scheme 3.2. 

Hickerson RP, Prat F, Muller CE, Foote CS, Burrows CJ (1999) Sequence and stacking dependence of 8-oxoguanine oxidation camparison of one-electron vs singlet oxygen mechanism. J Am Chem Soc 121 9423-9428... 

A series of hindered, a-branched tetraalkylolefms, such as biadamantylidene, have been converted to dioxetanes via an alternative oxygenation mechanism, involving a cation radical catalyzed chain (CRCC). This reaction can be initiated by chemical,... 

Doerge DR, Cooray NM, Brewster ME (1991) Peroxidase-catalyzed S-oxygenation mechanism of oxygen transfer for lactoperoxidase. Biochemistry 30 8960-8964... 

This section provides an overview of several potential mechanisms involved in the development of asbestos-induced health effects (direct interaction with macromolecules, active oxygen mechanisms, and other cell-mediated mechanisms). An expert panel convened by lARC concluded in 1996, Overall, the available evidence in favor or against any of these mechanisms leading to the development of lung cancer and mesothelioma in either animals or humans is evaluated as weak (lARC Expert Panel 1996). Pulmonary inflammatory factors (a subset of other cell-mediated mechanisms) were considered by the lARC panel as having the most support among the potential mechanisms involved. For additional information on the molecular mechanisms of asbestos-induced pulmonary disease discussed below, including potential interactions between a number of the mechanisms, see recent reviews (Kamp and Weitzman 1999 Kinnula 1999 Lee and Testa 1999 Murthy and Testa 1999 Robledo and Mossman 1999). 

The mechanism of oxidation in all these systems containing hydrogen or water vapour will consist of the addition of reaction (xxiii), and to a lesser extent reactions (Ivii), (Ixxiii) and (Lxxv), to the hydrogen—oxygen mechanism. The experimental findings are in accord with the theoretical eqn. (111). 

Asbestos produces its toxic effects by direct contact with lung tissue or by stimulating an acute or chronic inflammatory reaction in the tissue (via active oxygen mechanism or other cell-mediated mechanisms). The important determinants of asbestos toxicity are fiber size, fiber durability, and iron content. 

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