One-electron transfer reactions

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

Cluster 1 is a conventional [4Fe-4S] cubane cluster bound near the N-terminus of the molecule as shown in Fig. 13. Within the cluster the Fe-S bonds range from 2.26 to 2.39 A. The cluster is linked to the protein by four cysteine residues with Fe-S distances ranging from 2.21 to 2.35 A, but the distribution of the cysteine residues along the polypeptide chain contrasts markedly with that found, for example, in the ferredoxins as indicated in Section II,B,4 [also see, for example, 41) and references therein]. In the Fepr protein all four cysteine residues (Cys 3, 6, 15, and 21) originate from the N-terminus of the molecule, and the fold of the polypeptide chain in this region is such that it wraps itself tightly around the cluster, yet keeps it near the surface of the molecule. In such a position the cluster is ideally placed to participate in one-electron transfer reactions with other molecules. 

EPR techniques were used to show (Polyakov et al. 2001a) that one-electron transfer reactions occur between carotenoids and the quinones, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), and tetrachlorobenzoquinone (CA). A charge-transfer complex (CTC) is formed with a -values of 2.0066 and exists in equilibrium with an ion-radical pair (Car Q ). Increasing the temperature from 77 K gave rise to a new five-line signal with g=2.0052 and hyperfine couplings of 0.6 G due to the DDQ radical anions. At room temperature a stable radical with y=2.0049 was detected, its... 

MS6C6R.6], much interest has been generated in these complexes, particularly with respect to attempts to elucidate their electronic structures.4,5 The most noteworthy feature of the chemistry of these complexes is that many with the same M and R may be interrelated by relatively facile one-electron-transfer reactions which may be effected chemically or electrochemically. Complexes with varying over-all charges z may then be formed which constitute members of electron-transfer series.5,8,9,16 Such series with two or three members have been obtained. Tables I and II list representative complexes or series of complexes which either have been isolated or whose existence has been demonstrated by electrochemical measurements. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

In contrast to superoxide, which participates in one-electron transfer reactions as a reductant, nitric oxide is apparently able to oxidize various transition metal-containing proteins and enzymes. The study of NO reaction with hemoglobin has been started many years ago when... 

If XO is an undoubted historical pioneer among free radical-producing enzymes, whose capacity to catalyze one-electron transfer reactions opened a new era in biological free radical studies, NADPH oxidase is undoubtedly the most important superoxide producer. This enzyme possesses numerous functions from the initiation of phagocytosis to cell signaling, and it is not surprising that its properties have been considered in many reviews during last 20 years [56-58]. 

First, SENAs should be rather readily oxidized in one-electron transfer reactions. For example, MeCH=N(0)0SiMe2Bu gives an oxidation peak in the... 

For enzymatic reductions with NAD(P)H-dependent enzymes, the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, in all cases leads to varying amounts of enzymatically inactive NAD-dimers generated due to the one-electron transfer reaction. One rather complex attempt to circumvent this problem is the combination of the NAD+ reduction by electrogenerated and regenerated potassium amalgam with the electrochemical reoxidation of the enzymatically inactive species, mainly NAD dimers, back to NAD+ [51]. If one-electron... 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

A simple reaction of the former molecule with H2 would reduce its two clusters again. The final level of reduction of the Ee-S clusters would then only depend on the effective redox potential in the system. Under 1 per cent H2 at pH 6, where the redox potential is about —295 mV, part of the Fe-S clusters are oxidized. At this pH they only reduce under f 00 per cent H2. At pH 8 or higher, however, the redox potential of both fOO per cent H2 and 1 per cent H2 is low enough to keep all clusters fully reduced. So via intermolecular, one-electron transfer reactions the Fe-S clusters can presumably follow the current potential imposed upon the system by H2 even in the absence of redox mediators. Of course, the presence of such mediators facilitates electron (re)distribution. 

Oxaziranes are in a real sense active oxygen compounds and exhibit many reactions grossly analogous to those of organic peroxides. Thus they undergo one electron transfer reaction with ferrous salts and on pyrolysis they are converted to amides. Oxaziranes are also useful synthetic intermediates since in appropriate cases they may be isomerized to aromatic nitrones which are a convenient source of N-alkylhydroxylamines. The reaction of oxaziranes with peracids also provides a source of nitrosoal-kanes and is in many instances the method of choice for preparation of these compounds. ... 

The pronounced tendency of radicals to engage in one-electron transfer reactions is well documented [3]. This reaction channel is favored because it provides the simplest way for radicals to lose their radical nature, i.e. to b ome species with an even number of electrons (closed-shell molecules). The direction of the electron flow between the radical X and the molecule Y depends on the oxidizing or reducing power of X and on the ability of Y to either donate or accept an electron the final result of the interaction between X and Y is then the either one-electron-reduced or -oxidized former radical (X" or X ) or the open-shell molecle (Y or Y"" ), (cf. Eq. 1) ... 

One-electron transfer reactions are typical in living organisms. Ion-radicals are acting participants of metabolism. Of course, such ion-radicals are instantly included in further biotransformations. Therefore, it is reasonable to consider the problem of ion-radical formation together with the data on their behavior in biosystems. Chapter 3 contains a special section covering this topic. However, the issue of competition for an electron during ion-radical formation deserves to be mentioned here. 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

Catalases can also act as peroxidases (catalyzing a peroxidatic reaction) in which electron donors are oxidized via one-one electron transfers (Reactions (4) and (5)). 

Quinone Reduction This is a reversible, one-electron transfer reaction to the semi-quinone radical, followed by a second, reversible electron transfer that results in the formation of hydroquinone, as shown in Fig. 13.2. 

Symbolized by A, the reorganization energy of a one-electron transfer reaction is that energy needed for all structural adjustments, not only in the two reactants but in the neighboring solvent molecules as well, required for the two reactants to assume the correct configuration needed to transfer the sole electron. See Intrinsic Barrier Marcus Equation... 

Taking our cue again from photosynthesis, the primary photochemical reactions should be simple one-electron transfer reactions producing an oxidant and a reductant. That is... 

Fe S-containing enzymes frequently have other redox-active prosthetic groups, notably, flavins FAD or FMN. Likewise, the redox partner for many Fe S proteins is a flavoprotein this provides a convenient mechanism for turning a one-electron transfer reaction into a two-electron donor/acceptor. Hence, the structures elucidating the interactions between Fe S clusters and other cofactors are of considerable interest. At present there are only two examples for which we have crystallographic structures, yet both provide a basis to propose possible mechanisms for electron transfer to Fe S clusters. 

Next we considered the oxide ion electrode, and we were a little fearful of this one. There had been some work done in sulfates, and several hundred degrees warmer than this, in which there was some question as to whether or not the oxygen electrode was really acting reversibly on platinum. Furthermore, studies by Yeager and others have shown that whenever one uses oxygen in aqueous solution at electrodes there is a tendency to equilibrate with peroxide rather than with oxide or hydroxide. We were afraid that we might end up with some one-electron transfer reactions and get peroxide ions and the like but we had to try it because the other electrode just wouldn t work. As it turned out, it worked very nicely. 

Some years later, at the beginning of the 1970s, first ECL system based on the luminescent transition metal complex tris(2,2 -bipyridine)ruthenium(II)-Ru (bipy)32 + -has been reported.11 It was shown that the excited state 3 Ru(bipy)32 + can be generated in aprotic media by annihilation of the reduced Ru(bipy)31 + and oxidized Ru(bipy)33 + ions. Due to many reasons (such as strong luminescence and ability to undergo reversible one-electron transfer reactions), Ru (bipy)32+ later has become the most thoroughly studied ECL active molecule. 

In comparing the general and the simple equations, it is seen that the transfer coefficients play the same role in a multistep, n-electron-transfer reaction as the symmetry factor does in one-step, one-electron transfer reaction, i.e., thea s determine how the input electrical energy (Ft)) affects the reaction rate. Table 15 shows the tabulation of values for y, r, v, y, and n, from which a and a have been evaluated. 

To transport the two electrons from NADPH to the acceptor molecule (A), the one-electron transfer reactions must proceed in two consecutive steps. These two enzymes demonstrate how nature is making use of one and the same redox system to split the incoming electron-pair into single electrons of equipotential energy to reduce a particular acceptor system. 

In spite of this progress, the gaps in our knowledge of the molecular mechanisms of the participation of flavins in one-electron transfer reactions are enormous. Whether the reduction of flavins by obligatory two-electron donors occurs by a concerted two-electron process or by sequential one-electron transfers remains a matter of controversy and is a subject under current active investigation. It is hoped that this review will convince the reader of the usefulness and necessity of redox potential measurements in the understanding of electron transfer reactions in flavoenzymes. These type of measurements have become more numerous in recent years however, more information of this type is needed. We have seen that the apoprotein environment can alter the one-electron potentials of their respective bound flavin coenzymes by several hundred millivolts, yet virtually nothing is known, on a molecular basis, of how this is achieved. 

Conversely to their usual stability and chemical inertness, saturated perfluorocarbons can be susceptible to reductive defluorination in one-electron-transfer reactions. Thus, per-fluorodeeahydronaphthalene is converted by sodium benzenethiolate to octakisfphenylsul-fanyl)naphthalene, attacking first the weaker tertiary C — F bond (see Section 3.5.). Independent of the strong C — C bonds, in hydrocarbon-perfluorocarbon copolymers elimination of hydrogen fluoride takes place above 350 C. 

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