Coupled oxidation system

Oxidation of Mercaptans by Ov. Mercaptans are autoxidized in the presence of 02 in alkaline medium. In general, the oxidation is slow in the absence of catalyst because of unfavorable spin state symmetries that result from differences in the electronic configuration of the reactants (54). However, the reaction proceeds rapidly in the presence of traces of metal ions or transition metal phthalocyanines (55—58). The catalyst tends to alter the electronic structure of either the reductant and/or 02 so as to surmount the activation energy barrier imposed on the reaction by spin-state symmetry restriction. The coupled oxidation system in the presence of catalyst can be represented by ... 

The first well-defined step in tryptophan catabolism is the splitting of the indole ring with the formation of formylkynurenine. This is accomplished by a labile enzyme system present only in liver which is composed of a tryptophan peroxidase and an oxidase which produces hydrogen peroxide as a product of its activity. This coupled oxidation system induces the reaction given in the following equation ... 

Drillon M, Darriet J (1992) Progress in Polymetallic Exchange-Coupled Systems, some Examples in Inorganic Chemistry. 79 55-100 Duffy JA (1977) Optical Electronegativity and Nephelauxetic Effect in Oxide Systems. 32 147-166... 

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

Hotchandani S, Kamat P (1992) Charge-transfer processes in coupled semiconductor systems. Photochemistry and photoelectrochemistry of the colloidal cadmium sulfide-zinc oxide system. J Phys Chem 96 6834—6839... 

The bioluminescent determinations of ethanol, sorbitol, L-lactate and oxaloacetate have been performed with coupled enzymatic systems involving the specific suitable enzymes (Figure 5). The ethanol, sorbitol and lactate assays involved the enzymatic oxidation of these substrates with the concomitant reduction of NAD+ in NADH, which is in turn reoxidized by the bioluminescence bacterial system. Thus, the assay of these compounds could be performed in a one-step procedure, in the presence of NAD+ in excess. Conversely, the oxaloacetate measurement involved the simultaneous consumption of NADH by malate dehydrogenase and bacterial oxidoreductase and was therefore conducted in two steps. 

Methods for determination of thiol drugs (i.e., captopril [21-25], penicillamine [26-28], hydrochlorothiazide [24, 25, 29, 30], and tiopronin [31, 32]) have been developed. These methods are based on CL from a cerium (IV) oxidation system sensitized by adequate fluorophores such as quinine and rhodamine B. By using HPLC-coupled CL-flow-injection analysis method, tiopronin and its metabolite 2-mercaptopropionic acid in human urine were sensitively determined with the detection limits of 0.8 and 1 pM, respectively [32],... 

The very active unstable tin(III) ion is supposed to play an important role in this chain mechanism of tin(II) oxidation. Cyclohexane, introduced in the system Sn(II) + dioxygen, is oxidized to cyclohexanol as the result of the coupled oxidation of tin and RH. Hydroxyl radicals, which are very strong hydrogen atom acceptors, attack cyclohexane (RH) with the formation of cyclohexyl radicals that participate in the following transformations ... 

Stimulated by nature and in particular by the idea of modelling biotic coupled reaction systems such as ion transport and oxidative phosphorylation, recent attention has focused on a new generation of abiotic host... 

Recently a new hybrid power source has been reported that couples oxidation at a dye-photosensitized nanocrystalline semiconducting SnOz photoanode with the enzyme-catalyzed reduction of O2. Although miniaturization has not yet been reported for this new hybrid, the developments already achieved to miniaturize biofuel cells coupled to those being developed for charge-insertion oxides should be technically transferable to this system. 

Biocatalytic Systems Involving Coupled Oxidizing Enzymes... 

SCHEME 177. Coupled catalytic system for olefin dihydroxylation with H2O2 as terminal oxidant... 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

The cross-coupling with acylperoxyl radicals was shown to lead to high-valent metal species and reactive organic intermediates (144). The Craq002+/CMe3C(0)00 reaction appears to be the sole example of such chemistry reported so far. Extension to other metals and types of radicals is essential before one can even begin to understand whether such reactions take place in catalytic oxidation systems and/or in aerobic organisms, and whether or how to exploit or suppress them. 

The factors controlling the mechanical behavior of polymer-coupling agent-metal oxide systems have been discussed in terms of the weakest link in a chain concept. Determination of the locus of failure and thus the weak link is not usually reliable by visual inspection, and surface roughness can cause misleading spectroscopic results if failure is near an interface. 

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