Fast electron transfer

The proper quantumdynamical treatment of fast electronic transfer reactions and reactions involving electronically excited states is very complex, not only because the Born-Oppenheimer approximation brakes down but... 

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

Recall that Nemstian behavior of diffusing species yields a r1 /2 dependence, hi practice, the ideal behavior is approached for relatively slow scan rates, and for an adsorbed layer that shows no intermolecular interactions and fast electron transfers. 

For reversible systems (with fast electron-transfer kinetics), the shape of the polarographic wave can be described by the Heyrovsky—Ilkovic equation ... 

Substantial loss in sensitivity is expected for analytes with slow electron-transfer kinetics. This may be advantageous for measurements of species with fast electron-transfer kinetics in the presence of a species (e.g., dissolved oxygen) that is irreversible. (For the same reason, the technique is very useful for the study of electron processes.) Theoretical discussions on AC voltammetry are available in the literature (16-18). 

Thus, 9,10-diphenylanthracene ( p = — 1.83 V vs. SCE) is reduced at too positive a potential and hence its rate of reaction with the sulphonyl moieties is too low. On the other hand, pyrene (Ep = — 2.04 V) has a too negative reduction potential and exchanges electrons rapidly both with allylic and unactivated benzenesulphonyl moieties. Finally, anthracene Ev = —1.92 V) appears to be a suitable choice, as illustrated in Figure 3 (curves a-d). Using increasing concentrations of the disulphone 17b, the second reduction peak of XRY behaves normally and gives no indication of a fast electron transfer from A. 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

In the ci positional state, fast electron transfer from the Rieske protein to cytochrome Ci will he facilitated hy the close interaction and by the hydrogen bond between His 161 of the Rieske protein and a propionate group of heme Ci, but the Rieske cluster is far away from the quinone binding site. 

In the b positional state, The Rieske cluster can interact with quinone bound in the reaction pocket, but the distance to heme Ci is too large (>30 A) to allow fast electron transfer. 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

In bulk solution dynamics of fast chemical reactions, such as electron transfer, have been shown to depend on the dynamical properties of the solvent [2,3]. Specifically, the rate at which the solvent can relax is directly correlated with the fast electron transfer dynamics. As such, there has been considerable attention paid to the dynamics of polar solvation in a wide range of systems [2,4-6]. The focus of this chapter is the dynamics of polar solvation at liquid interfaces. 

F. Lisdat, B. Ge, and F.W. Scheller, Oligonucleotide-modified electrodes for fast electron transfer to cytochrome c. Electrochem. Commun. 1, 65-68 (1999). 

Hippier, M., F. Drepper, J. Farah and J. D. Rochaix (1997) Fast electron transfer from cytochrome c6 and plastocyanin to photosystem I of Chlamydomonas reinhardtii requires PsaF. Biochemistry, 36 6343-6349... 

We consider the transfer of an ion or proton from the solution to the surface of a metal electrode often this is accompanied by a simultaneous discharge of the transferring particle, such as by a fast electron transfer. The particle on the surface may be an adsorbate as in the reaction ... 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

For fast electron transfer kinetics, the surface concentrations of O and R are at dynamic equilibrium and assumed to obey the Nernst law... 

CYCLIC VOLTAMMETRY OF FAST ELECTRON TRANSFERS. NERNSTIAN WAVES... 

Provided that the formal potential is known, the peak potential offers easy access to the standard rate of electron transfer by application of the equations above. In the case of fast electron transfers, the maximal rate constants that can be accessed depend on the maximal scan rates available ... 

These electron transfer reactions are very fast, among the fastest known. This is the reason that impedance methods were used originally to determine the standard rate constant,13,61 at a time when the instrumentation available for these methods was allowing shorter measurement times (high frequencies) to be reached than large-amplitude methods such as cyclic voltammetry. The latter techniques have later been improved so as to reach the same range of fast electron transfer kinetics.22,63... 

For very fast chemical reactions and/or moderately fast electron transfers, the latter become the rate-determining steps. On the cathodic side, the current is controlled by forward electron transfer A —> B. On the anodic side, the current is controlled by forward electron transfer D —> C. This applies whether the rate law for electron transfer is of the Butler-Volmer type or of any other type (e.g., a MHL law). 

TABLE 2.1. Characteristics of the Irreversible Cyclic Voltammetric Responses (Pure Kinetic Conditions) for the Main Mechanisms That Involve the Coupling of a Fast Electron Transfer and a Homogeneous Rate-Determining Follow-up Reaction... 

Such highly ionized species have been detected for Cl-37 produced by the EC decay of Ar-37 in gaseous phase ((>). In solids, however, such anomalous states are not realized or their life time is much shorter than the half-life of the Mossbauer level (Fe-57 98 ns and Sn-119 17-8 ns) because of fast electron transfer, and usually species in ordinary valence states (2+, 3+ for Fe-57 and 2+, 4+ for Sn-119) are observed in emission Mossbauer spectra (7,8). The distribution of Fe-57 and Sn-119 between the two valence states depends on the physical and chemical environments of the decaying atom in a very complicated way, and detection of the counterparts of the redox reaction is generally very difficult. The recoil energy associated with the EC decays of Co-57 and Sb-119 is estimated to be insufficient to induce displacement of the atom in solids. 

In order to further probe the above route for electron transfer, there is a need to study derivatives modified at Tyr83. One such derivative in which the phenolic group of Tyr83 has been N02-modified has so far been prepared [139], On pulse radiolysis reduction with Cd to give the radical Tyr83NOj, fast electron transfer through to the Cu(II) is observed, k>10 s [140]. 

On the extreme right-hand side of the diagram, the follow-up reaction has become so fast that it prevents the back electron transfer. Kinetic control is then by the forward electron transfer and the half-wave potential is then, once more, given by (53). It becomes more and more positive of the standard potential as the electron-transfer step (46) becomes faster and faster. Situations are thus met in which the overall process is kinetically controlled by an endergonic electron transfer due to the presence of a fast follow-up reaction. For such fast electron transfers, the reaction would have been controlled by diffusion in the absence of the follow-up reaction (upper left-hand part of Fig. 5). 

This sensitivity to slow electron transfer kinetics could, however, prove to be an advantage in sensor applications where a mediator, with fast electron transfer kinetics, is used to shuttle electrons to a redox enzyme [82]. Chemical species that are electroactive in the same potential region as the mediator can act as interferants at such sensors. If such an interfering electroactive species shows slow electron transfer kinetics, it might be possible to eliminate this interference at the NEE. This is because at the NEE, the redox wave for the kinetically slow interferant might be unobservable in the region where the kinetically fast mediator is electroactive. We are currently exploring this possibility. 

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