Oxidative couplings

Fig. 1. After dia2o coupling, oxidation leads tria2ene ring formation. Alkaline reduction and coupling lead to (9).

A High-Energy Chemical Intermediate Coupling Oxidation and Phosphorylation Proved Elusive... 

Eschenmoser reagent 784 Eschenmoser coupling -.oxidative 102 Eschenmoser sulfide contraction 102, 117ff 122, 474, 478 -.alkylative 119 -.oxidative 119 Eschenmoser-Claisen rearrangement 605 ff., 617 f.. estrone 153 ff. 

Transfer of hydrogen from one substrate to another in a coupled oxidation-reduction reaction (Figure 11-3). These dehydrogenases are specific for their substrates but often utilize common coenzymes or hydrogen carriers, eg, NAD". Since the reactions are re-... 

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

Redox couple Supporting electrolyte Concentration of redox couple (oxid red) D0 x 106 (cm2/s) a (meas.) k°a (cm/s) Zero-point method ... 

Carrots (Daucus carota) are excellent sources of (3-carotene and vitamin A, although they have been reported to exert low antioxidant activity compared to some other vegetables (Al-Saikhan and others 1995 Cao and others 1996 Ramarathnam and others 1997 Vinson and others 1998 Beom and others 1998). However, boiling carrots for 30 min significantly improved their antioxidant activity toward coupled oxidation of (3-carotene and linolenic acid (Gazzani and others 1998). 

This assay, developed by Taga and others (1984), is based on the coupled oxidation of (3-carotene and linoleic acid. The method estimates the relative ability of antioxidant compounds to scavenge the radical of linoleic acid peroxide (LOO ) that oxidizes (3-carotene in the emulsion phase. 

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 ... 

Although permanganate ions are not generally used to effect oxidative coupling of phenols, it has been shown that, in the presence of a catalyst in an organic solvent, 2-methoxyphenols are coupled oxidatively under very mild conditions to produce the dimeric products (>50%) [48], Unsaturated substituents are not oxidized under the mild conditions. 

ACO requires ascorbate as a cosubstrate and catalyzes the coupled oxidation of ACC and ascorbate to ethylene, cyanhydric acid, carbon dioxide, and dehydroascorbate, using a single nonheme ferrous ion and dioxygen. The active site contains a single Fe(2+) ion linked to the side chains of two histidines and one aspartate. Both ACC and dioxygen are coordinated to the Fe(2+) ion, generating an Fe(3- -)-ACC-superoxo... 

Cytochrome c and ubiquinol oxidases are part of an enzyme superfamily coupling oxidation of ferrocytochrome c (in eukaryotes) and ubiquinol (in prokaryotes) to the 4 e /4 reduction of molecular oxygen to H2O. After this introduction, we will concentrate on the cytochrome c oxidase enzyme. The two enzymes, cytochrome c oxidase (CcO) and ubiquinol oxidase, are usually defined by two criteria (1) The largest protein subunit (subunit I) possesses a high degree of primary sequence similarity across many species (2) members possess a unique bimetallic center composed of a high-spin Fe(II)/(III) heme in close proximity to a copper ion. Cytochrome c oxidase (CcO) is the terminal... 

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. 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

In the presence of ascorbate and oxygen, oxyMb and other heme proteins undergo a series of reactions that resemble the catalytic cycle of HO, albeit with less efficiency 278-281). Although the spectroscopic similarities of Mb and corresponding derivatives of HO are remarkable 264, 267, 272), the mechanism of the coupled oxidation reaction... 

The reaction catalyzed by HO and involved in coupled oxidation of Mb results in release of 1 mol of CO during conversion of a-meso-hydroxyheme to verdoheme. In a number of Mb variants, the formation... 

Prior to the purification of heme oxygenase in sufficient quantities to carry out in vitro studies, the coupled oxidation of myoglobin was... 

Although the coupled oxidation reaction closely resembles the enzymatic reaction, it may not be identical. One clear difference is that heme free in solution is oxidized at all four meso positions 40), heme in myoglobin only at the a-meso position 6, 46) and heme in hemoglobin at the a- and -positions 6, 46). Prior to the observation that hemin in... 

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