Pathway of electron transfer

Figure 35 Photo-induced intramolecular electron transfer pathway of complex 80, in the presence of methylviologen (a) The Mn -Ru—MV2+ pathway (b) the Mii - P6S0 --Qa quinone pathway in PSII. (From Ref. 170.)...

Figure 3. The electron-transfer pathway of the NiFe-hydrogenase in D. gigas.

The formation of C02 -CHD has been postulated in order to account for the generation of benzoic acid as a reaction product (see above). We expect that the lifetime of this radical is longer than 50 ps in order to be able to undergo disproportionation to benzoic acid. The observed absorption at A >340 nm may in fact be attributed to the C02 -CHD radical, because of the agreement of its absorption spectrum with that of the structurally related H adduct of benzoic acid showing, both, absorption maxima at around 350 nm. Despite the fact that Ph-CHD phenylcyclohexadienyl radicals also show an absorption with a maximum between 330 and 340 nm, its contribution to the observed spectrum is neglected, because the recombination product, phenylcyclohexadiene, was not found (see above). Consequently, the time-resolved experiments with benzene support both the addition and electron transfer pathways of reaction. 

Scheme I outlines the possible (simplified) overall electron transfer pathways of a a-bonded alkyl or aryl iron porphyrin in non-coordinating media. Spin state changes for (P)Fe(R) and chemical reactions involving [(P)Fe(R)] are not included in Scheme I. The central Fe(II), Fe(III), Fe(IV) and formally Fe(I) atom may lie in or out of the porphyrin plane depending upon the nature of the porphyrin ring and the R group. The movement of the iron atom is also not shown in the scheme.

Schematic diagrams of the electron transfer pathway of (a) common microbes and (b) exoelectrogens.

The next generation of amperomethc enzyme electrodes may weU be based on immobilization techniques that are compatible with microelectronic mass-production processes and are easy to miniaturize (42). Integration of enzymes and mediators simultaneously should improve the electron-transfer pathway from the active site of the enzyme to the electrode. 

Both the Af-fluorosulfonamides and the A -fluoroammonium salts are very effective in the fluormation of enol acetates, enamines, silyl enol ethers, and enolates (Table 2) The reactions are thought to proceed through a mechanism which involves Sf 2 attack on the fluorine atom, but contributions from electron-transfer pathways also exist [65, 68, 73, 75, 76, 79, 80, 81, 82]... 

FIGURE 21.12 The Q cycle in mitochondria. (a) The electron transfer pathway following oxidation of the first UQH9 at the Qy, site near the cytosolic face of the membrane. 

FIGURE 21.17 The electron transfer pathway for cytochrome oxidase. Cytochrome c binds on the cytosolic side, transferring electrons through the copper and heme centers to reduce O9 on the matrix side of the membrane. 

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

Mr 220-250 kDa. Figure 1 shows an overall electron transfer pathway for the nitrogenases where the Fe proteins act as very specific, essential electron donors to the larger proteins. This is not the only role for the Fe proteins (see Section IV,C) and their role in the mechanism is almost certainly more complex than that of a simple electron transfer agent (see below. Section V). Electron transfer from the Fe protein to... 

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

Fig. 11. The metal sites in D. gigas hydrogenase (Hase) (A) and aldehyde oxidore-ductase (AOR) (B). The figure emphasizes the relative positioning of the metal sites Emd their proximity, suggesting an attractive electron transfer pathway. The arrows indicate electron trsmsfer for hydrogen evolution requiring an electron donor (A) Emd aldehyde conversion to carboxylic acid, the electrons being transferred to Em electron acceptor (B).

Another important outcome of the structural analysis is the relative positioning of the metal sites and their distances in order to define plausible electron transfer pathways between electron donors and acceptors. A common pattern starts to emerge (the same applies to cytochrome oxidase (241, 242). Figure 11 gives a pictorial view of the electron transfer pathway ... 

Scheme 36). Interestingly, the higher order cuprate 206 underwent conjugate addition with only moderate selectivity. This is likely due to the intervention of an electron transfer pathway. Competing electron transfer reactions involving a-alkoxymetal reagents of this type have also been reported by Cohen [81]. 

CgHsCl as in the absence of aromatic, suggesting auto-decomposition of the oxidant as the slow step (p. 386). The oxidation of toluene was somewhat faster, implying an additional electron-transfer pathway (c/. the oxidation of aromatic ethers and amines, p. 405). 

Murakami E, U Deppenmeier, SW Ragsdale (2001) Characterization of the intramolecular electron transfer pathway from 2-hydroxyphenazine to the heterodisulfide reductase from Methanosarcina thermophila. J Biol Chem 276 2432-2439. 

Electron transfer from a macrocycle (82), based on cyclen, complex to coordinated riboflavin proceeds via an inner sphere electron transfer pathway. The riboflavin coordinates through the imide and the relevance to the interception of biological electron transfer pathways is discussed.709... 

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