Electron transfer between radicals

Experimental data (Figure 4.2) for the dissociative electron transfer between radical anions and the carbon-halogen bond in alkyl halides indicates a linear relationship between log(k ) and Ed over a wide range of reaction rates [5, 9]. Very fast reactions become controlled by the rate of diffusion of two species towards each other, when every close encounter gives rise to electron transfer. A parabolic... 

Pulse-radiolysis experiments allow an examination of the first steps in the decay of radical-cations. Solutions of the radical-cation in the region of 10 M are generated. Bimolecular reactions between species at this level of concentration proceed relatively slowly and this simplifies interpretation of the experimental data. Particularly, electron transfer between radical-cations and radical species derived from them is not observed during the experiment. 

Steenken S (1988) Electron transfer between radicals and organic molecules via addition/elimina-tion. An inner-sphere path. In Rice-Evans C, Dormandy T (eds) Free radicals chemistry, pathology and medicine. Richelieu Press, London, pp 53-71 Steenken S (1989) Purine bases, nucleosides and nucleotides Aqueous solution redox chemistry and transformation reactions of their radical cations e and OH adducts. Chem Rev 89 503-520 Steenken S (1992) Electron-transfer-induced acidity/basicity and reactivity changes of purine and pyrimidine bases. Consequences of redox processes for DNA base pairs. Free Radical Res Commun 16 349-379... 

Bruce et al. (1956) have examined the electron transfer rates between AAjA JV -tetramethyl-p-phenylenediamine (TPMD) and Wurster s Blue (WB) which is the one-electron oxidation product of this molecule. In acid solutions of pH 3-2 containing acetic acid and TPDM the proton resonance of all signals is sharp, but on adding a small amount of W.B. the ring and methyl protons of TPDM selectively broaden. The rate constant for electron transfer between radical and molecule is given by... 

Intermolecular single-electron transfers between radicals and ions have been extensively studied, mainly with aromatic radical-ions and ketyls (Kaiser and Kevan, 1968 Szwarc, 1968). The electron exchange between the tri-phenylmethyl radical and its carbanion causes broadening of the esr lines, which allows the relevant bimolecular rate constants to be determined, the activation energies being of the order of one kcal mol . ... 

The kinetics of radical consumption with = 10 s can be studied at a flash duration of 1 ps. This allows the study of the kinetics of the bimolecular reaction with =10 s or = at a length of the reactor of 10 cm / = 0.1. Usually = l/(mol s), and constants up to lo" l/(mol s) are accessible for measurement. The method is widely used for measuring rate constants of atom and radical recombination in solution, reactions of molecules in the excited Uriplet state, electron transfer between radicals, and fast reactions of radicals with molecules (see Chapter 7). 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

However, ECL was not then studied in detail until 1963 [4, 5], At this time ECL from solutions of aromatic hydrocarbons was first recorded, and mechanisms involving electron transfer between electrically generated radical anions and cations were proposed. Between the mid-1960s and late 1980s there was considerable interest in the phenomenon of ECL. More than 60 publications in the literature focused almost solely on the mechanism of ECL reactions, identi-... 

In the oxygen-independent Type III reactions the excited/sensi-tized psoralen donates its excitation energy directly to, or reacts with, the target compound. This occurs if the substrate and the target compound (e.g., DNA) are already in close proximity or intercalated. The reactions will proceed very rapidly via the excited singlet state, and are, typically, cyclization reactions or electron-transfer between the sensitizer and the target. In addition, the psoralen can be ionized, either directly or via the excited state, and react with the target compound in the form of a radical cation. Furocoumarins are also employed in treatment of cutaneous T-cell lymphoma and some infections connected with AIDS, by so-called photopheresis processes [71, 74-76]. In this case, peripheral blood is exposed to, e.g., photoactivated (sensitized) 8-methoxypsoralen (8-MOP) in an extracorporeal flow system. This... 

In this case, direct electron transfer between the catalyst and O2 produces the superoxide radical (or other reduced forms of oxygen) which can be involved in a series of subsequent redox reactions. If these reactions are relatively fast, the rate determining step is Eq. (3) and the overall process can be interpreted in terms of relatively simple rate laws. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

However, in certain cases, the rate of electron uptake by a particular species just happens to be slow. For example, electron transfer between the methyl viologen radical cation (MV ) and hydrogen peroxide has a rate constant of 2.0 (mol dm ) s , while the reaction between MV and just about any other chemical oxidant known is so fast as to be dijfusion-controlled. The reason for this is simply not known at the present time. 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

---