2-electron pathway

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

The bifurcation of the electron pathways is aided by the mobility of the catalytic domain of the Rieske protein. Three positional states of the catalytic domain of the Rieske protein have been observed in different crystal forms of the 6ci complex (Fig. 8b see Section III,B,5) (41, 42). In each single positional state, the Rieske protein is unable to perform all electron transfer reactions occuring during turnover ... 

In the Fepr protein the two clusters are some 12-13 A apart and probably within electron transfer range. However, as shown in Fig. 15, there are no obvious electron pathways involving the polypeptide chain. A tyrosine residue, Tyr 493, lies approximately midway be-... 

The reduction of Oo Is considered to proceed by two parallel pathways, namely, (A) a 2-electron process which generates peroxide and (B) a 4-electron pathway In which the presence of HO2" In the... 

Using the same complex or some related ones, other groups71-73 have studied the mechanism of this electrocatalysis in a strictly nonaqueous solvent (MeCN) in detail. In this solvent, it has been shown that CO is also produced by electrolysis at a fixed potential either at —1.5V or —1.8V vs. SCE following respectively a one- or a two-electron pathway (Scheme 5). 

Scheme 3. Flavins are capable to undergo both 1 e and 2 e input/output redoxreactions as indicated. Since cytochrome b is a typical 1 e redox reagent, Hemmerich and Schmidt86) suggest a radical mechanism of the sensory transduction (assuming that the cyt b photoreduction is photo-tropically relevant). The nature and fate of the flavin-photosubstrate XH remains obsure. For the case X = cyt b the scheme represents a reversed respiration electron pathway...

A crude scheme of the electron pathway for the primary bluelight reaction was recently suggested by Hemmerich and Schmidt (87 cf. 4.1) ... 

Troisi A, Beebe JM, Picraux LB, Zee RDv, Stewart DR, Ratner MA, Kushmerick JG (2007) Tracing electronic pathways in molecules by using inelastic tunneling spectroscopy. Proc Natl Acad Sci USA 104(36)44255-14259... 

B. Diverse One-Electron Pathways for Reduction of Aqueous Chlorine Species... 

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]. 

FIGURE 4.51 Mechanism for cytochrome P450-catalyzed N-dealkylation via an initial single electron pathway, SET, or via the hydrogen atom abstraction pathway, HAT. 

In the absence of ultrasound, the results show a substantial amount (49 %) of the dimer bicyclohexyl from the one-electron pathway, together with cyclohexylmethyl-ether, cyclohexanol and other products from the two-electron pathway (approx. 30%). The methyl cyclohexanoate ester (17%) can be thought to arise from the acid catalysed chemical esterification of the starting material with the solvent methanol. (As a result of the high current densities needed, (parasitic) discharge of the solvent methanol produces a large quantity of protons around the anode as a competitive reaction [54].)... 

Column IV shows the product ratios in the presence of ultrasound and in the presence of pyridine. Overall there is the same trend with sonication, namely a slight shift from one-electron towards two-electron pathway, though here there is a higher yield of benzaldehyde derived by-products (approx. 16%). 

Work at Coventry has shown the same switch from a two-electron to a one-electron pathway is occuring at higher insonation frequency in that the electroreduction of benzaldehyde at 800 kHz gives almost entirely the pinacol. A major benefit of the use of the higher frequency however is that there is little weight loss of the lead cathode compared to lower frequency (20 kHz) where it is substantially abraded away. 

Immediately after the isolation of macroscopic quantities of Cgo solid [298], highly conducting [299] and superconducting [141] behaviors were verified for the K-doped compounds prepared by a vapor-solid reaction (Haddon, Hebard, et al.). Crystallographic study based on the powder X-ray diffraction profile revealed that the composition of the superconducting phase is KsCeo and the diffraction pattern can be indexed to be a face-centered cubic (fee) structure with a three-dimensional electronic pathway [300]. The lattice parameter (a = 14.24 A) is apparently expanded relative to the undoped cubic Ceo = 14.17 A). The superconductivity has been observed for many A3C60 (A alkali metal), e.g., RbsCeo (Tc = 29 K... 

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