Oxidation-reduction reactions transfer mechanisms

The SODs are ubiquitous metallo-enzymes in oxygen-tolerant organisms and protect the organism against the toxic effects of 02 by efficiently catalyzing its dismuta-tion into O2 and H2O2 via a cyclic oxidation-reduction electron transfer mechanism as shown in Scheme 1, with Cu, Zn-SOD with an example. Two steps have been proposed in the dismutation reaction, in which the oxidized form of the metal central ions (i.e. Cu (II), Fe(III), Mn(III) for Cu, Zn-SOD, Fe-SOD and Mn-SOD, respectively) were first reduced to the reduced form (i.e. Cu (I), Fe(II), Mn(ll) for Cu, Zn-SOD, Fe-SOD and Mn-SOD, respectively) with the formation of O2 followed by a subsequent equivalent oxidation of the reduced form of the metal ions into their oxidized form by 02 with the release of H2O2 (Scheme 6.1) [98]. 

Theoretically, according to the mechanism of biological azo dye reduction, the processes of biological decolorization are oxidation-reduction reactions, in which transfer of electrons match with the proton flow by the help of coenzymes, such as NADPH/NADP+ and NADH/NAD+. The oxidation-reduction potentials of the couples of NADPH/NADP+ and NADH/NAD+ are -324 and -320 mV, respectively [25, 46]. The least AGo value of the conversion NADPH/NADP+ and NADH/NAD+ is 44 kJ [47]. Therefore, —93 mV, which is obtained from (1), could be considered as a rough limited ORP value for ordinary primary electron donors of the third mechanism of biological azo dye reduction. This was demonstrated by the results of many researches (Table 1). Hence, the observed failure of cyanocobala-min [30] and ethyl viologen [48] to act as a mediator is most probably due to their too low Ed values 530 and —480 mV, respectively. 

Reaction Mechanism series of reactions that shows how reactants are converted into products in a chemical reaction Redox Reaction reaction involving the transfer of electrons, an oxidation-reduction reaction... 

The mechanism of an oxidation-reduction reaction can be simple, as illustrated by the ferrocene-ferricenium self-exchange in equation (1) where only electron transfer need occur.1 In other cases the mechanistic demands imposed by the net reaction are far greater. An example is shown in reaction (2) where, in the net sense, two protons and two electrons must be transferred from isopropanol, which is the reducing agent, to the RuIV oxidizing agent.2... 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

Marcus R. A. (I960), Theory of oxidation-reduction reactions involving electron transfer. Part 4. A statistical-mechanical basis for treating contributions from solvent, ligands and inert salt , Faraday Discuss. Chem. Soc. 29, 21-31. 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

Measurements of the rates of oxidation-reduction reactions began in the late 1940s. A great deal of the early experimental work was carried out by inorganic chemists, and by the 1970s the reactivity patterns of many complexes had been uncovered." Chemists studying the mechanisms of metalloprotein electron-transfer reactions frequently seek parallels with the redox behavior of less-complicated inorganic complexes. 

Before we try to understand the mechanism of oxidative phosphorylation, let s first look at the molecules that carry out this complex process. Embedded within the mitochondrial inner membrane are electron transport systems. These are made up of a series of electron carriers, including coenzymes and cytochromes. All these molecules are located within the membrane in an arrangement that allows them to pass electrons from one to the next. This array of electron carriers is called the respiratory electron transport system (Figure 22.7). As you would expect in such sequential oxidation-reduction reactions, the electrons lose some energy with each transfer. Some of this energy is used to make ATP. 

As a second example of how one interprets kinetic data in terms of mechanism, some simple electron-transfer (oxidation-reduction) reactions in solution are discussed. A complete review of this subject is not intended instead, we consider mainly the reduction of Co(NH3)5X by Cr " (aq) and Cr(bipy)3. Here X " can be any of a number of ligands, such as H2O, Cr, Br, OH , etc., and bipy is a,a -bipyridine. At a constant pH, the rate law for the oxidation-reduction reaction is first order in each of the reactants. The second-order rate constant is often dependent on pH because the reactants can exist in different ionized forms which react at different rates. We... 

Faradaic reaction mechanism in which there is transfer of electrons between the electrode and electrolyte, which results in oxidation reduction reactions of the chemical species in the electrolyte. Faradaic reactions are further subdivided into electrochemical reversible faradaic reactions and Surface redox non-reversible faradaic reactions. In reversible process the products do not diffuse far away from... 

The nature and properties of metal complexes have been the subject of important research for many years and continue to intrigue some of the world s best chemists. One of the early Nobel prizes was awarded to Alfred Werner in 1913 for developing the basic concepts of coordination chemistry. The 1983 Nobel prize in chemistry was awarded to Henry Taube of Stanford University for his pioneering research on the mechanisms of inorganic oxidation-reduction reactions. He related rates of both substitution and redox reactions of metal complexes to the electronic structures of the metals, and made extensive experimental studies to test and support these relationships. His contributions are the basis for several sections in Chapter 6 and his concept of inner- and outer-sphere electron transfer is used by scientists worldwide. 

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