Paired oxidation-reduction reactions

Introducing excessive amounts of standards into the samples, which disturbs the equilibria in the material as a result of physicochemical modification of the nature of the matrix Introducing an internal standard that interacts with the investigated compounds (via complexing, formation of ion pairs, oxidation-reduction reaction, addition or hydrolysis reactions) Conducting extraction in uncontrolled pH conditions, or under elevated pressure and temperature, triggering processes of hydrolysis or alkylation... 

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. 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

Redox pairs Oxidation (loss of electrons) of one compound is always accompanied by reduction (gain of electrons) of a second substance. For example, Figure 6.11 shows the oxidation of NADH to NAD+ accompanied by the reduction of FAD to FADH2. Such oxidation-reduction reactions can be written as the sum of two halfreactions an isolated oxidation reaction and a separate reduction reaction (see Figure 6.11). NAD+ and NADH form a redox pair, as do FAD and FADH2. Redox pairs differ in their tendency to lose electrons. This tendency is a characteristic of a particular redox pair, and can be quantitatively specified by a constant, E (the standard reduction potential), with units in volts. 

As we learned in Chapter 9, chemicals that react with one another are called reactants. In the process of reacting, the reactants form new chemicals known as products. In most acid-base reactions, a proton is transferred from one reactant to the lone pair of another reactant. In this chapter we look at a class of reactions in which an electron or a series of electrons are transferred from one reactant to another.These types of reactions are called oxidation-reduction reactions. 

We live under a blanket of the powerful oxidant 02. By cell respiration oxygen is reduced to H20, which is a very poor reductant. Toward the other end of the scale of oxidizing strength lies the very weak oxidant H+, which some bacteria are able to convert to the strong reductant H2. The 02 -H20 and H+ - H2 couples define two biologically important oxidation-reduction (redox) systems. Lying between these two systems are a host of other pairs of metabolically important substances engaged in oxidation-reduction reactions within cells. 

Before we review the methods used to determine surface acidity, we wish to define the type of acidity that should be measured. An acid is an electron-pair acceptor. In our opinion, the term acid should be limited to this definition rather than broadening the term to include oxidizing agents as well. We agree with Flockhart and Pink (10) who suggest a clear distinction be made between Lewis acid-Lewis base reactions (which involve coordinate bond formation) and oxidation-reduction reactions (which involve complete transfer of one or more electrons). 

An important aspect of enzymatic oxidation-reduction reactions involves the transfer of hydrogen atoms. This transfer is mediated by coenzymes (substances that act together with enzymes) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These two species pick up H atoms to produce NADH and NADPH, respectively, both of which can function as hydrogen atom donors. Another pair of species involved in oxidation-reduction processes by hydrogen atom transfer consists of flavin adenine triphosphate (FAD) and its hydrogenated form FADH2. The structural formulas of NAD and its cationic form, NAD+, are shown in Figure 4.7. 

The vast majority of flavoenzymes catalyze oxidation-reduction reactions in which one substrate becomes oxidized and a second substrate becomes reduced and the isoalloxazine ring of the flavin prosthetic group (Figure 1) serves as a temporary repository for the substrate-derived electrons. The catalytic reaction can be broken conveniently into two steps, a reductive half reaction (from the viewpoint of the flavin) and an oxidative half reaction. The flavin ring has great utility as a redox cofactor since it has the ability to exist as a stable semiquinone radical. Thus, a flavoenzyme can oxidize an organic substrate such as lactate by removal of two electrons and transfer them as a pair to a 2-electron acceptor such as molecular oxygen, or individually to a 1-electron acceptor such as a cytochrome. 

The reaction in a galvanic cell is always an oxidation-reduction reaction that can be broken down into half-reactions. It would be convenient to assign a potential to each half-reaction so that when we construct a cell from a given pair of half-reactions, we can obtain the cell potential by summing the halfcell potentials. For example, the observed potential for the cell shown in Fig. 11.5(a) is 0.76 volt, and the cell reaction5 is... 

Throughout this chapter, you have read about oxidation-reduction reactions. You know that redox reactions involve the loss and gain of electrons. Thus, the pairing or complementary nature of redox reactions is probably apparent to you. So, let s consider the two halves of redox reactions. 

What happens to the electrons that are lost by the zinc atom Electrons do not wander around by themselves they must be transferred to another atom or ion. This is why oxidation reactions never occur alone. They are always paired with reduction reactions. A reduction reaction is one in which an element gains one or more electrons. The element that picks up the electrons and becomes more negatively charged during the reaction is said to be reduced. Its oxidation number decreases, or is reduced. Because oxidation and reduction reactions occur together, each is referred to as a half-reaction. 

FMN, also known as riboflavin phosphate, is a flavin containing electron carrier in the cell (Figure 14.7). It participates in oxidation/ reduction reactions and, like FAD, differs from the nicotinamide coenzymes (NAD+ and NADP ) in being able to accept electrons either singly or in pairs (Figure 14.8). NAD+ and NADP+ can only accept electrons in pairs. 

Oxidation-reduction reactions always involve a pair of chemicals an electron donor, which is oxidized in the reactions, and an electron acceptor, which is reduced in the reaction. In fuel metabolism, the fuel donates electrons, and is oxidized, and NAD and FAD accept electrons, and are reduced. 

Oxidation-Reduction Reactions — Chemical reactions in which reactants undergo PAIRED CHANGES in their oxidation states. (They may be caiied Redox and Eiectron Transfer Reactions.)... 

Lead appears to be able to interact with complex small biomolecules as well, such as flavins for example, bis(lO-methylisoalloxazine) perchlorate tetrahy-drate (223). IsoaUoxazine is a planar three-ringed heterocychc amino cofactor associated with riboflavin and is active in oxidation-reduction reactions with metals such as Mo and Fe. Lead binds to bis(lO-methylisoalloxazine) in a 1 1 metal-ligand complex, with two additional waters bound resulting in a four coordinate molecule with a total of four oxygen donors. An active lone pair results in a distorted square-pyramidal structure. As is the case for citrate, extensive hydrogen bonding was observed in the crystal lattice. 

Oxidation-reduction reactions or redox reactions are defined as a family of reactions in which electrons are transferred between species. The species that receives electrons is reduced and that donates electrons is oxidized. Similar to acid-base reactions, redox reactions are always a matched pair of half-reactions. An oxidation reaction cannot occur without a reduction reaction occurring simultaneously. 

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