Electrons oxidation-reduction reactions

The overall reaction catalyzed by PS I is one electron oxidation-reduction reaction. 

NADH dehydrogenase and succinate dehydrogenase also contain Fe atoms that are bound by the S atoms of cysteine residues of the protein, in association with additional, inorganic sulfide atoms. Structures of these complexes are shown in figure 10.19. Succinate dehydrogenase has three iron-sulfur centers, one with a [2Fe-2S] cluster, one with [4Fe-4S], and one with a cluster containing 3 Fe atoms and 3 (or possibly 4) sulfides. Iron-sulfur centers undergo one-electron oxidation-reduction reactions. 

Complex IV Cytochrome Oxidase. Cytochrome oxidase contains two atoms of Cu in addition to the hemes of cytochromes a and a3. The Cu atoms undergo one-electron oxidation-reduction reactions between the cuprous (Cu+) and cupric (Cu2+) states. One of the Cu atoms (CuB) is close to the Fe of cytochrome a3 (fig. 14.12). The other (CuA) is associated with cytochrome a, but not so intimately. Oxidation of cytochrome c takes place on the side of the membrane facing the intermembrane space, whereas the reduction of 02 by cytochrome a3 and CuB occurs on the matrix side. 

Cyclobutanone is a versatile starting material used for numerous synthetic and theoretical studies in the chemistry of small rings. The preparation of this compound by the cooxidation process illustrates the synthetic utilization of three-electron oxidation-reduction reactions. 

In addition to providing an orbital route for electron transfer, the formation of a stable bridge species is analogous to the formation of the precursor complex required during an inner sphere process12). The overall result is that for a bridged complex, both metals can participate in electron transfer with an external reactant with no additional barriers due to intradimer electron transfer. The advantages for two electron oxidation-reduction reactions are dear. 

It is postulated that the polymers contain the structural units 20-23. PolyaniUne can be interconverted chemically or electrochemically between four forms by proton (acid-base) or proton and electron (oxidation-reduction) reactions. Form D is the only conducting version (Eq. (8))... 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

The battery acts as an electron pump, pushing electrons into the cathode, C, and removing diem from the anode, A. To maintain electrical neutrality, some process within the cell must consume electrons at C and liberate them at A. This process is an oxidation-reduction reaction when carried out in an electrolytic cell, it is called electrolysis. At the cathode, an ion or molecule undergoes reduction by accepting electrons. At the anode, electrons are produced by the oxidation of an ion or molecule. 

Thus, Experiment 7 involved the same oxidation-reduction reaction but the electron transfer must have occurred locally between individual copper atoms (in the metal) and individual silver ions (in the solution near the metal surface). This local transfer replaces the wire middleman in the cell, which carries electrons from one beaker (where they are released by copper) to the other (where they are accepted by silver ions). 

Is this an oxidation-reduction reaction Historically, it surely is, for the term oxidation originally referred specifically to reactions with oxygen. Yet our electron-transfer view of oxidation-reduction reactions provides no help in deciding so. Where in reaction (76) is there any evidence of electrons being gained or lost In such a doubtful case, our oxidation number scheme provides an answer. Applying the same assumptions used in treating the HSOf-HSOi"... 

The reactions employed in titrimetric analysis fall into four main classes. The first three of these involve no change in oxidation state as they are dependent upon the combination of ions. But the fourth class, oxidation-reduction reactions, involves a change of oxidation state or, expressed another way, a transfer of electrons. 

We can now apply our knowledge of partial ionic equations to the subject of equivalents. The standard oxidation-reduction process is H H+ + e, where e represents an electron per atom, or the Avogadro number of electrons per mole. If we know the change in the number of electrons per ion in any oxidation-reduction reaction, the equivalent may be calculated. The equivalent of an oxidant or a reductant is the mole divided by the number of electrons which 1 mole of the substance gains or loses in the reaction, e.g. ... 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

Reduced nicotinamide-adenine dinucleotide (NADH) plays a vital role in the reduction of oxygen in the respiratory chain [139]. The biological activity of NADH and oxidized nicotinamideadenine dinucleotide (NAD ) is based on the ability of the nicotinamide group to undergo reversible oxidation-reduction reactions, where a hydride equivalent transfers between a pyridine nucleus in the coenzymes and a substrate (Scheme 29a). The prototype of the reaction is formulated by a simple process where a hydride equivalent transfers from an allylic position to an unsaturated bond (Scheme 29b). No bonds form between the n bonds where electrons delocalize or where the frontier orbitals localize. The simplified formula can be compared with the ene reaction of propene (Scheme 29c), where a bond forms between the n bonds. 

Such a result may have interesting biological implications, for it represents one-electron transfer processes in discrete stages, which is presumably the way oxidation-reduction reactions occur in nature. 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

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