First electron transfer

There are three types of electron transfers, firstly the generation of an electron electrochemically, by y-irradiation, or by photolytic dissociation, secondly the transfer of an electron from an inorganic or organic compound, referred to as a nucleophilic homolytic leaving group (Zollinger, 1973 a), and thirdly a transfer from a transition metal or transition metal ion complex. In this section we will discuss the fundamental aspects of these three types. In the following sections and in Chapter 10, specific examples and synthetic applications will be summarized. 

With the compilation of data in Table 1 available, it is possible to begin to assess the factors at the molecular level which determine rates of electron transfer, first outer-sphere because of its relative simplicity and then inner-sphere. Because of the absence of bond-making or bond-breaking steps, outer-sphere electron transfer is far easier to treat theoretically than inner-sphere. 

Perhaps two key points are emerging from the frenzied activity in protein electron transfer. First, it is clear that rapid electron transfer can occur over long distances in proteins. Furthermore, the rates of these reactions are generally consistent with expectations based on simple small molecule reactions. In particular it appears that proteins, like other polymers, may provide relatively high reorganization energies for electron transfer. 

Our strategy here was to develop Pt colloids of minimum particle size which would render the water reduction by the electron relay R peirticularly effective. The goal was to obteiin hydrogen evolution rates in the microsecond time domain which would compete with beick electron transfer. First a centrifuged Pt sol protected by polyvinyl alcohol weis employed which gave clear and almost colorless aqueous suspensions even at hi Pt concentrations ( 28). Flash photolysis technique was applied for the first time to study the dynamics of intervention of the Pt particles in the water reduction process. Methylviologen (MV ) wets reduced... 

In order to regenerate the covalently bound redox mediator, it is required for the modified enzyme to get very close to the electrode surface, as the charge is intermediately located in the enzyme-bound redox spades from which the electrons have to be transferred to the electrode surface. To solve this problem, long and flexible spacer chains have been integrated either to the electrode surface seaweed mechanism) [128], a suitable matrix, or to the enzyme itself (whip mechanism) [120] (Fig. 8). These sensor configurations assure productive electron transfer, first between the redox mediator and the enzyme and finally between the redox mediator and the electrode surface, owing to the quasi-diffusional movement of the spacer-bound redox relays. 

Double potential steps are usefiil to investigate the kinetics of homogeneous chemical reactions following electron transfer. In this case, after the first step—raising to a potential where the reduction of O to occurs under diffrision control—the potential is stepped back after a period i, to a value where tlie reduction of O is mass-transport controlled. The two transients can then be compared and tlie kinetic infomiation obtained by lookmg at the ratio of... 

Williams R M, Zwier J M and Verhoeven J W 1995 Photoinduced intramolecular electron transfer in a bridged Cgg (acceptor)-aniline (donor) system. Photophysical properties of the first active fullerene diad J. Am. Chem. See. 117 4093-9... 

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... 

Decomposition of diphenoylperoxide [6109-04-2] (40) in the presence of a fluorescer such as perylene in methylene chloride at 24°C produces chemiluminescence matching the fluorescence spectmm of the fluorescer with perylene was reported to be 10 5% (135). The reaction follows pseudo-first-order kinetics with the observed rate constant increasing with fluorescer concentration according to = k [flr]. Thus the fluorescer acts as a catalyst for peroxide decomposition, with catalytic decomposition competing with spontaneous thermal decomposition. An electron-transfer mechanism has been proposed (135). 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

H2 or O2 from water in the presence of a sacrificial reductant or oxidant employ a mthenium complex, typically [Ru(bipy)2], as the photon absorber (96,97). A series of mixed binuclear mthenium complexes having a variety of bridging ligands have been the subject of numerous studies into the nature of bimolecular electron-transfer reactions and have been extensively reviewed (99—102). The first example of this system, reported in 1969 (103), is the Creutz-Taube complex [35599-57-6] [Ru2(pyz)(NH3. 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Metal oxide electrodes have been coated with a monolayer of this same diaminosilane (Table 3, No. 5) by contacting the electrodes with a benzene solution of the silane at room temperature (30). Electroactive moieties attached to such silane-treated electrodes undergo electron-transfer reactions with the underlying metal oxide (31). Dye molecules attached to sdylated electrodes absorb light coincident with the absorption spectmm of the dye, which is a first step toward simple production of photoelectrochemical devices (32) (see Photovoltaic cells). 

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... 

While being very similar in the general description, the RLT and electron-transfer processes differ in the vibration types they involve. In the first case, those are the high-frequency intramolecular modes, while in the second case the major role is played by the continuous spectrum of polarization phonons in condensed 3D media [Dogonadze and Kuznetsov 1975]. The localization effects mentioned in the previous section, connected with the low-frequency part of the phonon spectrum, still do not show up in electron-transfer reactions because of the asymmetry of the potential. 

Equation (5-69) is an important result. It was first obtained by Marcus " in the context of electron-transfer reactions. Marcus derivation is completely different from the one given here. In electron transfer from one molecule (or ion) to another, no bonds are broken or formed, so the transition state theory does not seem to be applicable. Marcus assumed negligible orbital overlap in the electron-transfer transition state, but he later obtained the same equation for group transfer reactions requiring significant overlap. Many applications have been made to proton transfers and nucleophilic displacements. ... 

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. 

The mitochondrial complex that carries out ATP synthesis is called ATP synthase or sometimes FjFo-ATPase (for the reverse reaction it catalyzes). ATP synthase was observed in early electron micrographs of submitochondrial particles (prepared by sonication of inner membrane preparations) as round, 8.5-nm-diameter projections or particles on the inner membrane (Figure 21.23). In micrographs of native mitochondria, the projections appear on the matrixfacing surface of the inner membrane. Mild agitation removes the particles from isolated membrane preparations, and the isolated spherical particles catalyze ATP hydrolysis, the reverse reaction of the ATP synthase. Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP. In one of the first reconstitution experiments with membrane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis. 

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. 

Inner-sphere. Here, the two reactants first form a bridged complex (precursor)- intramolecular electron transfer then yields the successor which in turn dissociates to give the products. The first demonstration of this was provided by H. Taube. He examined the oxidation of ICrfHoOijj by lCoCl(NHr)< and postulated that it occurs as follows ... 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

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