Coupled oxidation reaction series

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... 

An alternative oxidation mechanism (Shock 1988, 1989) would be for the reaction in Eq. (23) to proceed directly without involving CO2 or H2O intermediates as required in the coupled oxidation reactions discussed above. The trimolecular interaction of acetic acid molecules is statistically unlikely instead, the reaction in Eq. (23) would have to occur as a series of reaction steps taking place on a surface or as a chain reaction in solution. Reaction involving cleavage of the carboxyl carbon-alkyl carbon bond in aliphatic monocarboxylic acids is likely to require a catalyst for the same structural reasons as discussed in the context of decarboxylation. Although it is possible that propionate absorbed on a surface could more readily lose an alkyl group to form acetate, it is unlikely that the reverse reaction could occur more rapidly than decarboxylation for the following reason. This... 

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

The reaction proceeds with a selectivity of 100%. In the case of [Cu(dppz)]4, the oxidative coupling of a series of different para-substituted aromatic amines was investigated in order to study the influence of the para substitution on the activity of the catalytic system (137). 

Unlike the oxidation of glucose by oxygen (as in a fire), most biological oxidations do not involve direct transfer of electrons from a substrate directly to oxygen. Instead, a series of coupled oxidation-reduction reactions occurs, with the electrons passed to intermediate electron carriers such as NAD+ before they are finally transferred to oxygen. 

The coupling reaction involves the activation of the A,iV-diisopropyl phosphoramidite by an acidic activator. In the DNA series, the most commonly used activator is l-H-tetrazole = 4.8). The coupling reaction is typically performed in a dissociating polar nonprotic solvent such as acetonitrile to facilitate the nucleophilic displacement of the tetrazolide moiety. Following coupling, oxidation and capping steps proceed to oxidize the re-... 

The oxidation reactions involved are catalyzed by a series of nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD) dependent dehydrogenases in the highly conserved metabolic pathways of glycolysis, fatty acid oxidation and the tricarboxylic acid cycle, the latter two of which are localized to the mitochondrion, as is the bulk of coupled ATP synthesis. Reoxidation of the reduced cofactors (NADH and FADH2) requires molecular oxygen and is carried out by protein complexes integral to the inner mitochondrial membrane, collectively known as the respiratory, electron transport, or cytochrome, chain. Ubiquinone (UQ), and the small soluble protein cytochrome c, act as carriers of electrons between the complexes (Fig. 13.1.1). 

Pu is a very reactive metal. The potential for the couple Pu = Pu + e is 2.03 volts, which places it between scanditim (Sc) and thorium (Th) in the EMF series of elements, Pu oxidizes more readily than does U, and resembles cerium (Ce) in its reactions in air. Superficial oxidation of a freshly prepared surface occurs in a few hours in normal air. The oxide is more or less adherent, and in several days the oxidation reaction accelerates until finally the oxidation to PuOg is complete. However, the oxide coating protects the underlying metal in dry air, and the oxidation proceeds more slowly. Pu metal is attacked at elevated temperatures by most gases Hg, N2. halogens. SOg, etc. Pu metal dissolves easily and rapidly in moderately concentrated HCl and other halogen acids. 

As we have noted, potential step methods are particularly attractive for the determination of chemical rate constants in electrochemical mechanisms because the potential can be stepped to a potential at which the forward electron transfer is fast and irreversible, so that the current response depends only on the rates and mechanism of coupled chemical reactions. A complete quantitative evaluation of the mechanism was achieved by combining the potential step results with a series of simulations. The chemical reaction rate constants were determined by single-step experiments (the oxidation of NO2). Early in the step, the single-step response is determined by the equilibrium concentration of NO2. At later times, the response reflects the rate of conversion of N2O4 to NOj. Simulated potential step response curves could be compared to experimental data to extract the Ke, and k, and k, (see Figure 3-1). 

Nature makes use of the benzoquinone-hydroquinone redox couple in reversible oxidation reactions. These processes are part of the comphcated cascade by which oxygen is used in biochemical degradations. An important series of compounds used for this purpose are the ubiquinones (a name coined to indicate their ubiquitous presence in nature), also collectively called coenzyme Q (CoQ, or simply Q). The ubiquinones are substimted p-benzoquinone derivatives bearing a side chain made up of 2-methylbutadiene units (isoprene Sections 4-7 and 14-10). An enzyme system that utilizes NADH (Real Life 8-1 and 25-2) converts CoQ into its reduced form (QH2). 

The electron-transport system is a series of coupled oxidation-reduction (also called redox) reactions which transfer electrons to molecular oxygen. Carrier 1 (Figure 13.2) in its oxidized form may accept electrons which reduce it. In the reduced state, it may donate the electrons to the oxidized form of carrier 2. In the process of the transfer, carrier 1 becomes reoxidized as carrier 2 becomes reduced. Similarly, reduced carrier 2 may donate electrons to carrier 3 and so on. In each reaction, the electron donor can only release the electrons if there is a suitable acceptor. The electron donor is termed the reductant since it reduces the acceptor and the electron acceptor is termed the oxidant since it oxidizes the donor. In the electron-transport system, each electron carrier oscillates between oxidized and reduced forms which constitute a redox couple. 

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