Electron cycles

Figure 7.1 Schematic illustration of the electron cycles that ultimately cause a sensation of pain in the teeth in people who have metallic fillings and who have inadvertently eaten a piece of aluminium ( silver ) foil, e.g. while eating sweets...

Figure 7.2 Illustration of the electron cycles that allow for the trouble-free cleaning of silver we immerse the tarnished silver in an electrolyte, such as vinegar, and touch the silver with aluminium foil...

Similarly the redox transformations Mn(III,IV) oxides / Mn2+ causes rapid electron cycling at suitable redox intensities. Two differences of Mn and Fe in their redox chemistries are relevant ... 

The same group reported additions of tetrahydrofuran or dioxolane 243 to alkynes 242b catalyzed by 10 mol% CuBr and using ferf-butyl hydroperoxide as the oxidant [337]. The reaction does not proceed in the absence of tert-butyl hydroperoxide however, CuBr was not essential as a catalyst, since the additions also proceeded in its absence, although reduced yields of products 244b were obtained. In the catalyzed process a two-electron cycle was proposed to operate since the reaction was not inhibited by BHT, while the uncatalyzed was as expected inhibited. It remains thus unclear what the role of CuBr is and how it is regenerated under the oxidative conditions (see above). 

In this chapter I will propose a kinetic estimate for the thermodynamics of reactions like Eq (lb). The solid phases listed in Table 1 may act as a reductant or an oxidant. One of the prominent geochemical electron donors is pyrite. From an estimate of global pyrite weathering of 36 Tgy"1 (Garrels et al., 1973) we may deduce an average electron flux on the land surface in the order of 0.02 mol m 2 y1. At redox boundaries in salt marshes and in lake sediments microbial sulfate reduction will intensify this electron cycling. Luther (Chapter 6, this volume) discusses the details of sulfide redox mechanisms. 

Measurements of cytochrome b-559 in chloroplasts have shown that the cytochrome can exist at two midpoint redox potentials (4) and it has been suggested that cyt b55g is involved in an electron cycle (5). Therefore the effect of pH on cyt b059 in PS2 particles has been investigated. 

An electron cycle, activated by a decrease in pH, would prevent the primary electron acceptor, Q, from becoming over-reduced and therefore prevent photo inhibit ion (11) At even lower another... 

The effect of this mild water stress on PSII quantum yield is consistent with activation of an energy-dissipating alternative pathway of electron flow in PSII, e.g. a PSII electron cycle or H2O2 production at the water-splitting site. 

We conclude that in vitro we observe two mechanisms of change one related to qNP, and the second independent of changes in fluorescence yield. The second mechanism could be an electron cycle (or possibly a back reaction) within the PS2 core. It is not clear under vdiat conditions such a cycle might operate in vivo However, it could explain the variation in the relationships reported between and qNP in vivo [1,2,9,10,11,12]... 

THE EFFECT OF ELECTRON CYCLING AROUND PSII MATHEMATICAL MODELLING. 

The Effect of Electron Cycling Around PS II on Fluorescence Induction Mathematical... 

In our previous work [88] we studied systems containing odd r-electron cycles. In the study, in addition to classical fulvalenes, we also considered systems where the cycles are connected by frani-butadiene and / ara-quinodimethane bonding fragments. In this section, only classical members of the fulvalene family, where each two cycles are connected with a double bond, are described. For convenience we use the notation shown in Fig. 3.16. 

Figure 2.4 illustrates the overall reaction mechanism by which two-electrons from NADFH are transferred to the one-electron acceptor, ferric F450. Two electrons from NADFH must enter the enzyme as a hydride ion to the FAD, followed by intramolecular electron transfer to FMN. The FMN semiquinone is extremely stable, indicating that it is the hydroquinone FMN that transfers electrons to electron acceptors and that the fully oxidized enzyme form does not accumulate. The FOR flavins cycle in a 1-3-2-1 electron cycle (upper half circle in Fig. 2.4a). The air-stable form, FMN /FAD can be formed from the fully oxidized form during the priming reaction (Fig. 2.4b). At high concentrations of NADFH, the intermediate FMNH2/FAD is reduced to a four-electron reduced form [33, 34], Since the air-stable semiquinone form is found predominantly in hver microsomes [26], the 1-3-2-1 cycle is likely the major mechanism in vivo. Although the low reduction potential of FAD, near...

Polymer-coated electrodes exhibit various functions for which electron transfer is an essential step. Polymer-coated electrodes can control electron processes according to their design, as will be described in this review article. Since electron cycles and their control are essential and most important processes in nature, they are briefly discussed in Sect. 2. The processes concerned are pumping, transport, storage and utilization of electrons. 

As described in Sect. 2, electron pumping powered by solar radiation is one of the important electron processes found in the electron cycles of nature. Artificial electron pumping is achieved by two main types of processes photophysical and photochemical ones. In this section electron pumping by photophysical processes is described. 

The preparation, mechanisms and possible applications of polymer-coated electrodes have been described in this review. Processes based on the transportation, storage, activation, pumping, and utilization of electrons (Tables 1, 2, 6) which form important electron cycles in nature (Fig. 1) have been explained. 

For neutral 10-electron systems, the story is more complex. Naphthalene is often viewed as a 10-electron system, but really it is better viewed as two six-electron cycles that share an edge. Note that six-electron cycles are especially stable even within the context of aromatic systems. For more complex ring systems, it is generally observed that the larger the number of benzene-like rings, the more stable the structure. 

Benzene (Section 13.1) The archetypal aromatic compound a planar, regular hexagon of sp hybridized carbons. The six 2p orbitals overlap to form a six-electron cycle above and below the plane of the ring. The molecular orbital system has three fully occupied bonding molecular orbitals and three unoccupied antibonding orbitals. 

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