Associative electron transfer

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

An alternative theory associates electron transfer with transfer of a state of aromaticity from molecule to molecule within the stack (77AG(E)519). Efficient charge transport was identified with conversion of a neutral, antiaromatic system to a charged, aromatic radical by electron transfer. This interpretation has been eroded by the synthesis of conductors from aromatic systems such as perylene hexafluoroarsenate (81MI11301) or polypyrrole tetrafluoroborate (80CC397, 81MI11300) where an electron is transferred from a neutral, aromatic molecule to a non-aromatic charged radical. 

Somewhat more complex are electron transfer reactions that are coupled to bond cleavage or bond formation (dissociative/associative electron transfer). Some of these appear to be of the outer-sphere type. In Table 9.9 are shown a few examples of such reactions where the radicals undergo reductive cleavage and the rates have been measured in both directions. The roster of such reactions where the rates have been measured in only one direction is vastly greater. Radicals known to undergo reductive cleavage include I2, Br2, Cl2, and (SCN)2. Oxidative cleavage can also occur, as discussed below. 

The above definition stems from the amount of energy lowering during an interaction of an electrophile with a perfect nucleophile and the associated electron transfer from the latter to the former. 

As noted earlier in Sect 8.5.1, SIFT-MS instruments can operate at different conditions of temperature and pressure. For example, most of the Syft Technologies Ltd Voice200 SIFT-MS instruments operate at a flow tube temperature of 110 °C and a carrier gas pressnre of 0.6 Torr. In some reactions where association reactions compete with electron transfer, the product ion peak ratios of association/electron transfer are sensitive to temperature and pressure. For example, the reaction of the terpenoid carvone, with NO+ has product ions of reaction C,(,H,40 and... 

P450camin these films [45], At pH greater than 6, E° shifted by about-59 mV/pH unit, suggesting proton-associated electron transfer. 

NaCioHg or photolysis of [ (nacnac )FeH 2] under an N2 atmosphere yield [ (nacnac )Fe 2(li-T T -N2)] (251, Vnn= 1778 cm ). Despite this moderate activation of the N2 unit, no direct N2 functionalization was observed instead N2 loss occurs with associated electron transfer back to the Fe atom, and coordination of an additional ligand (commonly a t-acceptor) or reaction with an oxidant.Furthermore, binding of olefins, alkynes, and phosphines is thermodynamically more favorable than that of N2. ° Compound 251 may therefore be considered as an interesting Fe(l) synthon for the [(nacnac )Fe] fragment. Further reduction of the 251 with K or Na additionally weakens the N—N bond to form the (N2) bridged species K2[ (nacnac )... 

The most serious difficulty lies in the low current density observed for the cobalt complexes. The reasons for this are not clear. It is not a mass transfer problem since the oxygenated complexes give reasonable current densities and therefore the reason must lie in the fundamental kinetics of the electron transfer itself. Fortunately much work has been carried out by Weaver in the area of the "simple" electron transfer reaction and by Saveant in the area of the associated electron transfer... 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

Figure C3.2.1. A slice tlirough tlie intersecting potential energy curves associated witli tlie K-l-Br2 electron transfer reaction. At tlie crossing point between tlie curves (Afy, electron transfer occurs, tlius Tiarjiooning tlie species,...

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

The properties of electron transfer proteins that are discussed here specifically affect the electron transfer reaction and not the association or binding of the reactants. A brief overview of these properties is given here more detailed discussions may be found elsewhere (e.g.. Ref. 1). The process of electron transfer is a very simple chemical reaction, i.e., the transfer of an electron from the donor redox site to the acceptor redox site. 

The first type of interaction, associated with the overlap of wavefunctions localized at different centers in the initial and final states, determines the electron-transfer rate constant. The other two are crucial for vibronic relaxation of excited electronic states. The rate constant in the first order of the perturbation theory in the unaccounted interaction is described by the statistically averaged Fermi golden-rule formula... 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

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