Three-stage electron-transfer

In 1967, the first three-stage electron-transfer process examined by pulse radiolysis was reported [67], Such a cascade process is of relevance to electron transport in biological systems. By irradiating an aqueous solution containing acetone (0.82 mol dm ), acetophenone (3.34 mmol dm ), and benzophenone (72 pmol dm ) at pH 13, Adams et al. [67] were able to observe, at 2 ps after the pulse, the spectrum of the acetophenone radical anion (2max 445 nm, max 260 m mol ) [68] originating from the reduction of acetophenone by the (CH3)2CO radical. In the following 50 ps, the acetophenone radical anion reacted with benzophenone k — 7.8 X 10 dm mol" s ) so that the absorption band at 445 nm disappeared... 

The change of electronic conductivity G(r) over diameter of such two-sphere model composition as element in a system of contacting particles is shown in a Figure 10.6b. The transfer of electron across this composition consists of three stages electron tunneling over the interspace — Rq is replaced by the M/SC conductivity across a particle with subsequent electron tunneling over the further interspace R — Rq. The probability of electron tunneling falls down exponentially with increase in distance from the surface of particle. 

The integration of PS I and PS II in chloroplasts occurs as shown in Figure 7.10 [37,39]. The overall reaction of the transfer of electrons starting from water takes place in three stages. 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

The qualitative elements of Marcus theory are readily demonstrated. For example, the process of transferring an electron between two metal ions, Fe2+ and Fe3 +, may be described schematically by Fig. 33 (Eberson, 1982 Albery and Kreevoy, 1978). The reaction may be separated into three discrete stages. In the first stage the solvation shell of both ions distorts so that the energy of the reacting species before electron transfer will be identical to that after electron transfer. For the self-exchange process this of course means that the solvation shell about Fe2+ and Fe3+ in the transition state must be the same if electron transfer is not to affect the energy of the system. In the second phase, at the transition state, the electron is transferred without... 

Electron transfer reactions have been characterized with much more rigor in inorganic chemistry than with organic molecules. Marcus has provided the principal description relating the kinetics and thermodynamics of electron transfer between metal complexes (1). The Marcus theory, a computationally simple approach with good predictive power, is an empirical treatment which uses thermodynamic parameters and spectroscopic measurements to calculate kinetic data. It assumes that bimolecular electron transfer reactions occur in three stages as shown in Scheme 1 (1) formation of the precursor complex, (2) electron transfer, and (3) solvation of the redox pair. 

Let us compare the probabilities of tunnel electron transfer from singly and doubly charged metallic nanoparticles (Z — —l and Z = —2) to an adsorbed molecule. In the general case, tunnel electron transfer occurs in three stages (i) thermal activation of an electron in the metal, (ii) tunneling of the electron through the barrier to a molecular level, and (iii) transformation of the adiabatic potential of the molecule. 

Electron transport has three major stages (1) transfer of electrons from NADH to coenzyme Q, (2) electron transport from coenzyme Q to cytochrome c, and (3) electron transport from cytochrome c to oxygen. These stages are briefly described below. 

Figure 5-1. Schematic representation of the three stages of photosynthesis in chloroplasts (1) The absorption of light can excite photosynthetic pigments, leading to the photochemical events in which electrons are donated by special chlorophylls. (2) The elections are then transferred along a series of molecules, causing the oxidized form of nicotinamide adenine dinucleotide phosphate (NADP+) to become the reduced form (NADPH) ATP formation is coupled to the electron transfer steps. (3) The biochemistry of photosynthesis can proceed in the dark and requires 3 mol of ATP and 2 mol of NADPH per mole of C02 fixed into a carbohydrate, represented in the figure by (CH20).

Marcus [12-14] provided a simple approach allowing the prediction of the kinetics of the process, using thermodynamic parameters and spectroscopic measurements. Marcus theory assumes that bimolecular electron transfer, as shown in Scheme 1, occurs in three stages ... 

The transfer of electrons from NADH to 02 occurs in three stages, each of which involves a large protein complex in the inner mitochondrial membrane. 

Figure 4-13. The electron transport chain and oxidative phosphorylation. Heavy arrows indicate the flow of electrons. Fe-S = iron-sulfur centers FMN = flavin mononucleotide CoQ = coenzyme Q (ubiquinone) Cyt = cytochrome. nH+ indicates that an undetermined number of protons are pumped from the matrix to the cytosolic side. The numbers at the top of the figure correspond to the three major stages of electron transfer described in the text in V B.

The literature offers many more examples of electron transfer induced rearrangements of compounds containing three-membered rings. In general it is not known whether the rearrangement occurs at the radical anion or dianion stage, as is the case in the above-mentioned examples. 

Bimolecular electron transfer "" occurs in three stages ... 

Electron photoemission from solvated electron solution (in solvents such as hexa-methylphosphotriamide and liquid ammonia the solvated electrons are fairly stable) to vapour phase has been studied by Delahay and co-workers (whose works are reviewed in Ref. ). According to them, this process proceeds in three stages solvated electron photoionization diffusion of generated delocalized electrons to the solution s surface and emission proper, i.e. transition of electrons to the vapour phase where they are transferred from the cathode surface (i.e., from the solution) to the anode by the external electric field. 

Fig. 3 Mitochondrion-dependent signaling at a early apoptotic stage. The three mayor actions of NO in mitochondria are indicated as reversible binding to complex IV, inhibition of electron transfer at complex III, and oxidation of ubiquinol yielding Oy. The fates of Oy, H2O2, and NO are indicated together with their release into cytosol (the former through VDAC). Modulation of JNK activity (stimulation by H2O2 and inhibition by NO) is indicated. JNK phosphorylates Bcl-2 and Bcl-XL...

If gaseous, electrochemicaUy active components of the measuring environment are not dissolved in the electrode, then the electrode process will consist of the following stages (also shown in Figure 1.18). They are adsorption-desorption of electrochem-icaUy active gaseous components on gas-electrolyte (GE) and gas-metal (GM) interfaces, ionization reaction (with electron transfer) on the metal-electrolyte (ME) and gas-electrolyte interfaces, and mass-transfer processes on all boundaries of three phases (gas-metal, gas-electrolyte, and metal-electrolyte). Furthermore, mass transfer of electrons and holes on the surface electrolyte layer may also occur. It is evident that the quantity of the current in the stationary state is equal to the quantity of the nonmetal component adsorbing on the gas-metal and gas-electrolyte surfaces as a result of ionization of this component on the ME and GE surfaces. 

Recently we carried out kinetic studies with Hildenborough and Miyazaki cytochrome c3 using deazariboflavin semiquinone (dRf ), MV +, and propylene diquat (PDQ +), produced by laser flash photolysis, as reductants (37). Initially, all three reactions were accurately second order, consistent with all hemes being reduced with the same rate constant or with a single site reduced, followed by fast intramolecular electron transfer to reduce the remaining three hemes. However, by measuring reduction kinetics with cytochrome c3 poised at different extents of reduction, the kinetics of reduction of individual hemes could be resolved. Thus, reduction of cytochrome c3 in approximately 5% steps and application of the known macroscopic redox potentials (see previous section) enabled calculation of the concentration of each heme (c.) at each stage of reduction. The plot of kohs versus percent reduction can thereby be fitted to solve for the rate constant for each heme (kt) ... 

In this review we have seen examples representing three main types of biomolecules and biomodels proteins, nucleic acids, and lipids. Although it is not always classified as a biomolecule, water represents a fourth major type and it has been center stage in many of the examples we have treated. For each of these classes of biomolecules, DFT has played a major role in our work and in that of other workers. But this is not a one-man show we have also shown how DFT can be combined with molecular dynamics, either in the Bom-Oppenheimer-MD approach or in hybrid QM/MM methodologies. And we have shown examples that go beyond strictly Kohn-Sham DFT in the use of constraints that allow coimections with other theories and concepts, notably the Marcus theory of electron transfer. Finally, we have given a glimpse of some of the tools that can be used to analyze and interpret the DFT-based computations. 

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