Redox equivalents

There are many constituent elementary reactions. The ones written here are balanced for redox equivalents but not for H+/H20. In the rate laws, [H+] dependences are omitted as well, even though most of the reactions show them. In effect, these are the rates at constant [H+]. 

The thioester world postulated by de Duve should in fact be called the sulphur-iron world , since iron ions are essential for the redox processes occurring in such a thioester world, de Duve (1991, 1996) asks a question which is vital for the whole of prebiotic chemistry where did the redox equivalents necessary for the construction of biomolecules on the primeval Earth come from This question becomes largely irrelevant if the strongly reducing atmosphere postulated by Miller/Urey and... 

Oparin is accepted. However, since most experts do not now accept this hypothesis, the question remains valid. The term redox equivalent can be equated with the presence or availability of electrons. So where do the required electrons come from ... 

Complex 9 (Scheme 43.3) can be reduced by different redox equivalents to the active rhodium(I) species 10 namely, by electrons, formate [37, 38], and hydrogen. This hydrido complex then transfers the hydride ion onto the nicotinamide. In electrochemical applications, TOFs in the range of 5 to 11 h-1 have been reported [31, 39]. It is noteworthy that this complex accepts NAD+ and NADP+ as substrates with the same efficiency and almost exclusively produces the 1,4-reduced cofactor (selectivity >99%). 

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

With biocatalysis becoming increasingly accepted in synthetic organic chemistry on both the laboratory and industrial scale, there is a huge need for new complexes that can utilize electrons or hydrogen as redox equivalents in cofactor reduction. These redox equivalents are very inexpensive, readily available, and produce no side products, which in turn significantly facilitates the downstream processing of products. 

The transfer of redox equivalents can be achieved by an electrocatalyst (mediator) or a modified electrode. Indirect electrolysis can lead to a better selectivity due to the specific interaction of the mediator with the substrate. However, low turnovers and the need to separate the mediator from the product are possible disadvantages, as mentioned above. The nickel hydroxide electrode [195,196] is fairly free from these disadvantages. The following mechanism for the oxidation at the nickel hydroxide electrode has been proposed in the literature [195]. 

The most recent model of this membrane, the fluid mosaic model [13] is pictured in cartoon fashion in Fig. 2. In this model, the transduction proteins (complexes I-IV) are randomly dispersed in the membrane and redox equivalents are delivered from one complex to another via the mobile electron carriers cytochrome c and ubiquinone. It is necessary that cytochrome c be able to move relatively facilely from one complex to another. Thus the binding constants cannot be too high without making the associated OS rates too slow. Conversely, to prevent unproductive short circuits via cytochrome c from complex I directly to IV, there must exist molecular recognition which favors selective binding of cytochrome c to hcj and cytochrome oxidase (and perhaps disfavors binding to complex 1 or II). 

Several catalases, including the type B catalase-peroxidases, seem to show true substrate saturation at much lower levels of peroxide than originally observed for the mammalian enzyme (in the range of a few millimolar). This means that the limiting maximal turnover is less and the lifetime of the putative Michaelis-Menten intermediate (with the redox equivalent of two molecules of peroxide bound) is much longer. The extended scheme for catalase in Fig. 2B shows that relationships between free enzyme and compound I, and the presumed rate-limiting ternary complex with least stability or fastest decay in eukaryotic enzymes of type A and greatest stability or slowest decay in prokaryotic type B enzymes. 

In a first reactor, where benzoylformate decarboxylase (BFD) is retained, benz-aldehyde and acetaldehyde are coupled to yield (S)-hydroxy-l-phenylpropanone. This hydroxy ketone is then reduced to the corresponding diol in a second reactor by an alcohol dehydrogenase (ADH). Regeneration of the necessary cofactor is achieved by formate dehydrogenase (FDH) or by other methods. To avoid additional consumption of redox equivalents by unselective reduction of residual starting material from the first reactor, the volatile aldehydes are removed via an inline stripping module between the two membrane reactors. In this setup the diol was produced with high optical purity (ee, de > 90%) at an overall space-time yield of 32 g L d . ... 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Another important classification of electroorganic reactions is obtained by dividing them into those in which the substrate undergoes direct electron transfer with the electrode (direct electrolysis) and those in which an additional compound (redox catalyst, mediator) transfers the redox equivalents between the substrate and the electrode (indirect electrolysis). 

The type of electron exchange, which can take place either by direct heterogeneous electron transfer between the electrode and the substrate or by using an indirect process, in which a redox catalyst mediates the exchange of redox equivalents between the electrode and the substrate... 

Band gap photochemical excitation of a semiconductor particle promotes an electron from the valence band to the conduction band, thus forming an electron-hole pair. Under illumination, the bands shift from their dark equilibrium positions to ones closer to the flat band condition, Scheme 9. Here the chemical potential of the electrons becomes different from that of the holes and a photovoltage develops. The concentration of free carriers, and hence of the number of available redox equivalents, will depend linearly on the incident light intensity. The free energy of these charge carriers will be related to... 

Four modes of degradation of 2 were discovered without the net consumption or production of redox equivalents. These four modes produce the final metabolites ethyl 3-ethoxy-4-chloro-2-butenoate, ethyl 3,3-bisethoxy-4-chlorobutanoate, oxol-2,4-dione, and 2,4-bischloromethyl-3,5-bisethoxycarbonyl-4-methyl pyridine. As a consequence, these modes do not depend on or disturb the glycolytic activity of the cell. 

The detailed pathways of redox equivalents to and from the active centers... 

The wealth of seemingly random flavoproteins can be brought into a certain order by distinguishing them according to the number of redox equivalents operating at once in their input and output reactions (Table... 

Table I. Prototypes of Flavoproteins and Number of Redox Equivalents Transferred in a Single Input and Output Step0...

Redox-Equivalents Input/Output Flavoprotein Function Prototype... 

In these systems, binary carrier contacts, whether of measurable lifetime or of shorter duration, implicate, therefore, two classes— interflavin and flavin—heme contacts and flavin—iron (sulfur) contacts. Figure 3 makes it clear that, even irrespective of whether one or two redox equivalents are being transferred, the geometry of these contacts (roughly spoken irora) must be asked for, and if it is o-, the site of contact must be discussed, whether inner sphere or outer sphere. 

Of course, [ADPC] and [NADH]/A, do not vary independently in vivo, as in the preceding model analysis. Neither does cytoplasmic PI concentration stay fixed for varying work rates in cells. The integrated system behavior can only be captured by simultaneously simulating the generation of redox equivalents by the TCA cycle (as... 

In this case cheap sources of redox equivalents, such as carbohydrates, can be used since the microorganism possesses all the enzymes and cofactors which are required for the metabolism. In Fig. 3.41, an example of the production of a pharmaceutically relevant prochiral ketone is given using fermentation technology [117]. 

Reprogramming the central metabolism, if necessary, to supply the required redox equivalents and metabolic energy, usually in the form of ATP. 

As shown in Fig. 8.10, energy and redox equivalents are required to drive the transformation of glucose into 1,3PD. These are provided by converting some of the glucose all the way into C02, which restricts the stoichiometric yield on glucose to 1.4 mol mol-1 maximum. A stoichiometric yield of 1.18 mol mol-1 of 1,3PD (50% by weight) has been obtained in practice [64], which corresponds with 85% of the theoretical yield, at a product concentration of 135 g L 1. 

Control experiments showing conservation of redox equivalents and not the origin of aldehyde [Schonbaum (101a)]. 

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