Electron transfer kinetics clusters

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

The solvated C02 anion radical has been observed both in the gas phase and in condensed matter (Holroyd et al. 1997) and has been well characterized by ESR spectroscopy (Knight et al. 1996). Being solvated, C02 anion radicals form complexes that yield quasi-free electrons upon photoexcitation. Gas-phase studies (Saeki and others 1999) and ab initio calculations (Tsukuda et al. 1999) indicate that static ion-dipole interactions stabilize the [(C02)n m(R0H)m] type of small clusters. In supercritical carbon dioxide, monomers and dimers of water, acetonitrile, and alcohols also form metastable complexes (Shkrob Sauer 2001a,b). Such complexation should be taken into account in studies of electron-transfer kinetics in reactions with the participation of C02. ... 

The redox potential of short-lived metal clusters may be evaluated by the study of the electron transfer kinetics involving a donor-acceptor couple of known redox potential and used as a monitor (5, 6). The metal atoms and the electron donor are generated in the aqueous solution through a short electron pulse. During the coalescence of the clusters, their redox potential increases. 

The preceding discussion illustrates the versatility of electroactive SAMs for addressing fundamental issues in electron-transfer kinetics. So far, an extended kinetic analysis with comparison to theory has been applied only to simple one-electron-transfer redox molecules. Clearly, there is an opportunity to study more complex redox systems. For example, a triruthenium cluster bound to a SAM exhibits multiple oxidation states and the reversible binding and release of CO [240]. Another area deserving more quantitative work is electrocatalysis in which the attached redox molecule mediates the electron transfer between a solution redox molecule and the electrode. 

Gallagher and co-workers have characterized the reductase component by EPR and fluorescence spectroscopy. They showed that it contained one FAD and a [IFe-lS] " cluster. The FAD could be reduced in a two-step reaction to the fully reduced flavin. The optical spectrum of the semiquinone species was typical of a neutral flavin radical. The [2Fe-2S] + cluster could also be reduced by one electron equivalent to [2Fe-2S] +. Both paramagnetic species could be detected by EPR. It could also be shown by a combination of mid-point potential measurements and electron-transfer kinetics that this component could supply the energy required for the epoxygenation reaction. 

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

If the rate constants for parallel reactions are to be resolved, then analysis of the products is essential (Sec. 1.4.2). This is vital for understanding, for example, the various modes of deactivation of the excited state (Sec. 1.4.2), Only careful analysis of the products of the reactions of Co(NH3)jH20 + with SCN, at various times after initiation, has allowed the full characterization of the reaction (1.95) and the detection of linkage isomers. Kinetic analysis by a number of groups failed to show other than a single second-order reaction.As a third instance, the oxidation of 8-Fe ferredoxin with Fe(CN)g produces a 3Fe-cluster, thus casting some doubt on the reaction being a simple electron transfer. 

For clusters of higher nuclearity too, the kinetic method for determining the redox potential °(M]] /M ) is based on electron transfer, for example, from mild reductants of known potential which are used as reference systems, towards charged clusters M](. [31] Note that the redox potential differs from the microelectrode potential M /M ) by the... 

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer and, on the other hand, the radiation-induced reduction of both metal ions, which depends on the dose rate (Table 5). The pulse radiolysis study of a mixed system [66] (Fig. 7) suggested that a very fast and total reduction by the means of a powerful and sudden irradiation delivered for instance by an electron beam (EB) should prevent the intermetal electron transfer and produce alloyed clusters. Indeed, such a decisive effect of the dose rate has been demonstrated [102]. However, the competition imposed by the metal displacement is more or less serious, because, depending on the couple of metals, the process may not occur [53], or, on the contrary, may last only hours, minutes, or even seconds [102]. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

This bacterial enzyme, which is isolated from a methylotrophic bacterium, is of interest to flavin enzymologists in two respects. The first is the presence of an unusual covalently bound FMN moiety which has been identified as 6-S-cysteinyl FMN >and is present in a single molar stoichiometry with an Fe4/S4 cluster on the enzyme. In addition, it is the only known metalloflavoenzyme where the rate of molecular electron transfer between the two above redox centers is slow enough to be followed by kinetic techniques in the msec time range... 

The kinetics of oxidation and reduction of [4Fe-4S] proteins by transition metal complexes and by other electron-transfer proteins have been studied. These reactions do not correlate with their redox potentials.782 The charge on the cluster is distributed on the surface of HiPIP through the hydrogen bond network, and so affects the electrostatic interaction between protein and redox agents such as ferricyanide, Co111 and Mnin complexes.782 783 In some cases, limiting kinetics were observed, showing the presence of association prior to electron transfer.783... 

The rate coefficient of a reactive process is a transport coefficient of interest in chemical physics. It has been shown from linear response theory that this coefficient can be obtained from the reactive flux correlation function of the system of interest. This quantity has been computed extensively in the literature for systems such as proton and electron transfer in solvents as well as clusters [29,32,33,56,71-76], where the use of the QCL formalism has allowed one to consider quantum phenomena such as the kinetic isotope effect in proton transfer [31], Here, we will consider the problem of formulating an expression for a reactive rate coefficient in the framework of the QCL theory. Results from a model calculation will be presented including a comparison to the approximate methods described in Sec. 4. 

If these shift data really do represent the onset of an intermolecular electron transfer reaction in DABCO, ABCO, HMT clustered with amine, either, and aromatic solvents, one ought to be able to observe the reaction kinetics or dynamics. Consider the specific instance of DABCO. The singlet Rydberg state lifetime for DABCO (and all the other Rydberg molecules studied for this determination (Shang et al. 1993c, 1994a) is ca. 2 ps for the isolated molecule and ca. 1.2 ps for the nonpolar rare gas, hydrocarbon, and fluorocarbon solvents. This... 

Melkozernov et al., 2001). According to kinetic investigation (Guergova-Kuras et al., 2001), electron transfer in PS I involves both brunches with different rate constants of 35xl06 s 1 (brunch B) and 4.4xl06 s 1 (brunch A) for the ET from each phylloquinone to the iron-sulfur cluster Fx. 

Electrons zip along an intramolecular wire between the eatalytie Cluster C and Cluster B at a rate of 3200s at 55 C under optimal eondi-tions. The mechanism of electron transfer from CO to Cluster C to Cluster B is relatively well understood, since it has been studied by a variety of rapid kinetics and electrochemical methods. 

---