Oxygen pathway

Biodegradation of aliphatic compounds generally occurs by one of the three pathways as shown in Fig. 18. The most common is a direct enzymatic incorporation of molecular oxygen (pathway 1). All three pathways result in the... 

The photosensitized oxidation of DPB in vesicles in the first mode gave 1 and 2 as the unique products (Fig. 22). We believe that these products are derived from the singlet-oxygen pathway. In contrast, the photosensitized oxidation of DPB in vesicles in the second mode only produced the electron-transfer-mediated products 1,2,3, and 5 (Fig. 22). No singlet-oxygen products were detected. These observations demonstrate, once again, that one can control the selectivity in pho-... 

However, as indicated in the text of this chapter, there are many other ways of generating such radical cations, occasionally leading, under oxygen atmosphere, to reaction products similar to those obtained in the photoinduced oxygenation pathway. Although, in many instances, the different modes of evolution of radical cation intermediates, dictated by reaction conditions (solvent polarity, oxygenated or inert atmosphere, the presence or absence of other intermediates, nucleophiles,... 

In order to understand the oxygen pathways during the reaction, SSITKA experiments involving 2 136) have also proven useful. Some results... 

FIGURE 4.15 Solvent-dependent oxygenation pathways for a mononuclear copper complex with a sterically hindered ketimine ligand.54... 

Several 1,2-dioxolanes [7, 75-76] and polycyclic peroxides [7, 77[ have been obtained in modest yields from unsaturated hydroperoxides by peroxyl radical cyclization-oxygenation pathways under initiation with DBPO or DTBN (cf. Scheme 43) [76a, 77[. In general, there is a strong bias for exo cyclization [7a, 75b], although an exception has been noted [78]. Moreover, when there is a choice, the... 

Metastable Ru(III) and oxoRu(V) species have been proposed as key intermediates in the catalytic cycle of rapid oxygenation which can be viewed as a part of the general scheme of diverse oxidative chemistry of ruthenium porphyrins (Scheme 1.16). The aerobic oxygenation pathway, involving the even oxidation states, is shown in the left half of Scheme 1.16. The new fast catal)dic process on the right half of the figure depicts the chemical interconnectivity between the fast and slow catalytic regimes. 

An attempt to induce a metal-free, radical-based oxygenation of quercetin with superoxide instead also resulted in an electrophilic oxygenation pathway via the initial protonation and disproportionation of superoxide. However, aerobic fiavonol oxidation can be induced by the free-radical initiators... 

The mechanism of photooxidation of petroleum probably includes significant contributions from both radical and singlet oxygen pathways. Petroleum contains polycyclic aromatic hydrocarbons and other compounds with known photosensitizing activity Lichtenthaler et al. (1989) demonstrated quantum yields for O2 formation ranging from 0.5 to 0.8 for a number of crude oils. The photooxidation of a 2 fuel oil was inhibited by /3-carotene, an excellent quencher of O2 and free radicals, at concentrations > lO" M (Larson and Hunt, 1978). A mechanism incorporating... 

The initial step in both carboxylation and oxygenation pathways as catalyzed by Rubisco is abstraction of the C3 proton of RuBP to form an enediol intermediate this reaction can be monitored by transfer of tritium from solvent (T2O) into RuBP or by loss of tritium from [3- H]ribulose-P2. 

Cai et al. suggested that carbon oxidation proceeded in two pathways an oxygen pathway that leads to mass loss due to the formation of gaseous products, and a water pathway that leads to mass gains [40]. They concluded that the surface loss of carbon from corrosion accounted for the electrochemical active surface loss of the electrode because the size of the Pt particles supported on the carbon did not change much under their study. 

Figure 13.5. Pathways for the oxidative modification of low density lipoprotein. The lipoxygenase pathway produces hydroperoxides that catalyse the oxidation of LDL. The active oxygen pathway involves the direct oxidation of LDL with reactive oxygen species such as superoxide. The oxidized fatty acids (FA-OOH) of phosphatidylcholine (PC) in LDL are preferentially hydrolysed by phospholipase A and decomposed into aldehydes that interact with ApoB of LDL, leading to the formation of foam cells via macrophage and scavenger receptors.

In the autoxidation of CLA, which yields identified products likely produced by a singlet oxygen pathway, i.e., furan fatty acids, the identified products are not exceptionally stable relative to the reactants (12). The reason that this reaction proceeds is then more likely due to the presence of initiators. Few reports that identify the products of CLA autoxidation are available in the literature (8-13). The results of an experiment performed in our laboratory are presented below to illustrate the effect of initiators. 

Cobalamin is biosynthesized via two distinct routes, an aerobe and an anaerobic pathway, that differ in the timing of cobalt ion insertion into precorrin macrocycle. The oxygen pathway, which was delineated in Pseudomonas denitrificans, incorporates molecular oxygen in the precorrin-3 molecule and cobalt is inserted to the macrocycle late in the pathway. In the anaerobic pathway found in Salmonella typhimurium, the cobalt ion is chelated into precorrin-2 in the absence of oxygen. The biosynthetic pathways of cobalamin are reviewed in detail by Roper et al. (2000). 

Mauti, R. and Mims, C. (1993). Oxygen Pathways in Methane Selective Oxidation over SUica-snpported Molybdena, Catal. Lett., 21, pp. 201-207. 

The difference in reaction products is explained in the reaction scheme shown in Fig. 3.33 which gives an insight into the different reaction pathways. The thiyl radical, derived from cysteine by abstraction of an H-atom, is added to the epoxyallyllic radical only in the absence of oxygen (pathway 2 in Fig. 3.33). In the presence... 

Dinosaurs no longer roam the Earth, but today millions of hving beings are exposed to hypoxic conditions, either natural or pathological. Oxygen pathways and sensing are controlled by cellular and molecular processes, but also by. .. yes... genetic determinants. 

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