Electron transfer systems

Electron transfer reactions involving alkali metals are heterogeneous, and for many purposes it is desirable to deal with a homogeneous electron transfer system. It was noticed by Scott39 that sodium and other alkali metals react rapidly with aromatic hydrocarbons like diphenyl, naphthalene, anthracene, etc., giving intensely colored complexes of a 1 to 1 ratio of sodium to hydro-... 

The redox potentials of iron-sulfur centers vary considerably, depending on the type of center (242) and its environment in the protein (243), ranging from about -700 mV in the case of some [4Fe-4S] centers (244, 245) to about +450 mV in the case of some [4Fe-4S] centers (246). Iron-sulfur centers therefore act as redox centers in numerous electron transfer systems, including the respiratory and photo synthetic bioenergetic chains as well as a wide variety of redox enzymes. Among the many studies that have dealt with these systems, some have yielded structural information of the kind described in Section III. Most of them were designed, however, to measure the... 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

In a first approach, the spiroketone 3-171a was prepared by treatment of 3-170a with a Zn/Hg/TMSCl/2,6-lutidine electron-transfer system (Scheme 3.45). Besides 52% yield of the desired product 3-171a, 34% of the uncydized compound 3-172 and 9% of starting material were isolated. 

In contrast, the use of SmI2/DMPU as an electron-transfer system led to the smooth production of spiroketone 3-171a in 79% yield, without any side products (Scheme 3.45). It was also possible to cydize the unprotected cydopropyl ketone 3-170b to give the spiroketone 3-171b in 57% yield. 

Figure 10. Vibronic levels of an electron transfer system. Here the initial electronic state is the D A state and the final electronic state is the D+A charge separation state. The vibrational levels within the D A manifold are labeled by v and those within D+A by v. ...

These two processes can be treated on equal footing. A schematic representation of vibronic levels of an electron transfer system is shown in Fig. 10. In other words, we are dealing with two vibronic manifolds, iv for D A and / for D+A or DA. For convenience, hereafter we shall use the following notation ... 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 

Fig.24 Electron transfer system constructed in a biosensor for ethanol...

Marcus, R. A. Schrodinger equation for strongly interacting electron-transfer systems, J.Phys.Chem., 96 (1992), 1753-1757... 

For example, in the electron-transfer system shown in Figure 6.25 the protein-bound groups between which electron transfer occurs are designated A, B and C and the direction of electron transfer is shown as arrows. 

As for the photophysical aspects for the bio-mimic photoinduced electron transfer systems, studies on amides206 and amino acid assemblies207 have recently begun to be popular. 

One of the most popular photo-promoted electron transfer systems is the trisbipyridylruthenium(II)-methyl viologen (MV++) system which, on excitation, produces Ru and reduced methyl viologen (MV+) ... 

The electron transfer system has not been studied in detail in fish, but the metabolism of compounds such as biphenyl (37), benzo(a)pyrene (21) and 2,5-diphenyloxazole (38) by fish liver microsomes has been shown to require oxygen and NADPH generating system. The metabolism of BP (21), 2,5-diphenyloxazole (Ahokas, unpublished observation) and aldrin (27.) by fish liver microsomal enzyme system is inhibited strongly by carbon monoxide. This information and the fact that cytochrome P-1+50, as well as NADPH cytochrome c reductase system are present in fish, suggest strongly that fish have a cytochrome P-1+50 mediated monooxygenase system which is very similar to that described in mammals. 

Let us consider an electron transfer system, whose Hamiltonian may be written (r,Q)=H(r,Q)-t-TN,... 

It is evident that the preceding considerations do not apply to all biological electron transfer systems. Even in the bacterial reaction center, the transfer between the two quinones Qa Qbj which takes place over 18 A [18], is characterized in Rhodobacter sphaeroides by a large entropic contribution, which has been attributed to the high solvent exposure of Qg [126]. By using the activation energy value reported in Ref. [126], two very different X values may be deduced from Eq. (23) = 0.1 eV and Aj = 2.5 eV. The previous considerations... 

A Chapter of this volume is devoted to these techniques, which are merely illustrated in this section by one particular example. The electron transfer system that is the most intensively submitted to genetic manipulations is certainly the physiological complex between yeast cytochrome c and peroxide-oxidized cytochrome c peroxidase, which presents many advantages [143], Among the modifications performed on cytochrome c peroxidase, one may mention the substitution of Trp 191 which interacts directly with His 175 of the heme [144], and of His 181 [145] which was proposed as a bridging unit in a superexchange path involving Phe 87 of cytochrome c [136,146]. On the cytochrome c side, Phe 87 las been substituted [147], as well as other residues expected to play an important role in the stabilization of the noncovalent complex [143]. 

We consider an electron transfer system containing two redox centers D and A, and we are interested by the relation between the driving force AG° relevant to the electron transfer from D to A, and the redox potentials which are effectively measured. Two different situations may be distinguished ... 

Indirect electrochemical reactions usually involve a multi-electron-transfer system that consists of a set of electron transmission units (Fig. 4). Although the overall feature of an electron-transfer process in indirect electrosynthetic reactions is understandable, each step of the electron transmission has not yet been elucidated [10]. 

Let us begin with the one-mode electron-transfer system. Model IVa, which still exhibits relatively simple oscillatory population dynamics [205]. SimUar to what is found in Fig. 5 for the mean-field description, the SH results shown in Fig. 13 are seen to qualitatively reproduce both diabatic and adiabatic populations, at least for short times. A closer inspection shows that the SH results underestimate the back transfer of the adiabatic population at t 50 and 80 fs. This is because the back reaction would require energetically forbidden electronic transitions which are not possible in the SH algorithm. Figure 13 also shows the SH results for the electronic coherence which are found to... 

Figure 6.14 Cyclic voltammogram obtained for a multiple-electron-transfer system, where a thin film of sputtered V2O5 on a platinum working electrode has been immersed in an electrolyte solution of propylene carbonate containing LiCI04 (1.0 mol dm ). From Cogan, S. F., Nguyen, N. M Perrotti, S. J. and Rauh, R. D Electroctromism in sputtered vanadium pentoxide , SPIE, 1016, 57-62 (1989). Reproduced by permission of the International Society for Optical Engineering (SPIE).

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

II. CONTROL OF MULTISTEP PHOTOINDUCED ELECTRON-TRANSFER SYSTEMS... 

Natural photosynthesis applies electron transfer systems, where a relay of electron-transfer reactions evolves among chlorophyll and quinone moieties embed-... 

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