One-electron reduction potentials

The Eo values for 2-substituted 1,4-benzoquinones (sets 45-4 through 45-7, 45-10) show an average value of pr of 59. Thus the resonance effect predominates. For most of these sets, the Op constants are not the best parameters for correlation. By contrast, the electron reduction potentials (set 45-8) show a Pr value of 39, which indicates predominance of the localized effect. The 2,5-disubstituted 1,4-benzoquinones differ distinctly in their behavior from the 2-substituted 1,4-benzoquinones in that they show an average Pr value of 53. The one-electron reduction potentials of these compounds show about the same composition of the electrical effect, with a value of Pr of 50. The only set of Eq values available for the 2,6-disubstituted 1,4-benzoquinones pve a Pr value of 51, comparable to the values observed for the 2,5-disubsti-tuted 1,4-benzoquinones. The 2,3,5,6-tetrasubstituted 1,4-benzoquinones have... 

Burke M, Edge R, Land EJ, McGarvey DJ, and Truscott TG. 2001. One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments. FEBS Letters 500(3) 132-136. Bystritskaya EV and Karpukhin ON. 1975. Effect of the aggregate state of a medium on the quenching of singlet oxygen. Doklady Akademii Nauk SSSR 221 1100-1103. 

These results produce an ordering of the one-electron reduction potentials as shown in Figure 14.9. This order is consistent with results on the reactions of oxygen and porphyrins with carotenoids (McVie at al. 1979, Conn et al. 1992), for example, p-CAR - reacts much more efficiently with oxygen than LYC - and DECA -. Comparative studies have been made in benzene due to the decreased solubility of XANs in hexane and Table 14.8 gives the corresponding bimolecular rate constants for electron transfer. Overall, the one-electron reduction potentials increase in the order ZEA < P-CAR LUT < LYC < APO - CAN < ASTA. 

FIGURE 14.9 Relative ordering of the one-electron reduction potentials (E(CAR/CAR )) of several carotenoids in hexane. 

The studies of this equilibrium as a function of pH enabled the estimation of the absolute one-electron reduction potentials of CAR + in an aqueous micellar environment (Edge et al. 2000, Burke et al. 2001b) (see Table 14.12 for typical results). As can be seen, the potentials of all the dietary carotenoid radical cations are very similar but LYC + has the lowest potential implying that it is the best carotenoid antioxidant against free radicals (of course, this is an oversimplification, see above). 

Edge, R, Land, EJ, McGarvey, D, Mulroy, L, and Truscott, TG, 1998. Relative one-electron reduction potentials of carotenoid radical cations and the interactions of carotenoids with the vitamin E radical cation. J Am Chem Soc 120, 4087 1090. 

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

Competition between dioxygen and quinones depends on the one-electron reduction potentials of quinones [29], and therefore, quinones may inhibit or stimulate superoxide production. 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

There is probably one more mechanism of MPO-mediated lipid peroxidation. Kettle and Candaeis [174] have studied the oxidation of tryptophan by neutrophil MPO. They suggested that tryptophan, which is present in plasma at the similar concentration as tyrosine and has a similar one-electron reduction potential, can contribute to oxidative stress at inflammation sites. It was proposed that the formed tryptophan free radicals may stimulate oxidative stress during inflammation. 

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

However, to be a quantitative assay of superoxide detection, Reaction (1) had to be an exothermic reaction, i.e., the difference between the one-electron reduction potentials of reagents AE° = / °[02 /02] / °[A /A] must be <0. In this case the rate constants of Reaction (1) will be sufficiently high (10s—109 1 mol 1 s ). Among traditionally applied assays, three compounds satisfy this condition cytochrome c, lucigenin, and tetranitromethane (Table 32.1). 

It should be mentioned that Spasojevic et al. [57] recently determined the two-electron reduction potential of lucigenin in water as —0.14 V. As this value is close to the one-electron reduction potential of dioxygen °[02 702] = — 0.16 V, these authors regarded their finding as a support for lucigenin redox cycling. However, it has been demonstrated long ago that two-electron reduction potentials cannot be used for the calculation of equilibrium for one-electron transfer processes [58]. 

The poly(I)-based transistor is the first illustration of a microelectrochemical transistor based on a combination of a conducting and a conventional redox polymer as the active material. The transistor "turns on" at VG corresponding to oxidation of the polythiophene backbone. The resistivity of poly(I) declines by a factor of 105 upon changing VG from 0.4 V to 0.8 V vs. Ag+/Ag. When Vg is moved close to the one-electron reduction potential of V2+/+, the conventional redox conductivity gives a small degree of "turn on". A sharp Iq-Vq characteristic results, with an Ip(peak) at Vq = E° (V2+/+). Though the microelectrochemical devices based on conventional redox conduction have both slow switching speed and a... 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

One-Electron Reduction Potentials (E°) of the Phenoxyl-Phenolate and Phenoxyl-Phenol Couples... 

Indoles can be also be converted into their radical cations by the use of C1C>2 as the oxidant produced by pulse radiolysis. From the reactivity of the resultant cation it was possible to establish the one-electron reduction potential of the indole in question. Typical results from this are illustrated in Table 234. As can be seen, the one-electron reduction potential is influenced by alkyl substitution. 

TABLE 2. One-electron reduction potentials of some indoles ... 

The feasibility of electron transfer oxidation is dictated by the thermodynamic potential , of the substrate RH and requires an anode potential or an oxidant to match the value of El. It is essential to choose an oxidant with an one-electron reduction potential sufficient for the desired oxidation and a two-electron reduction potential insufficient for further oxidation of the radical cation. The suitable oxidant may be a metal ion, a stable radical cation, or a typical PET-acceptor in its excited state. The advantage of electrochemically performed oxidation is obvious. 

In the presence of oxygen, the lifetimes of both radical ion pairs (i.e., ZnP +-C6o and ZnP +-H2P-C6o ) are decreased significantly due to oxygen-catalyzed back-electron transfer (BET) processes between Ceo and ZnP " [76]. The catalytic participation of O2 in an intramolecular BET between Ceo and ZnP + in ZnP-linked Ceo is depicted in Scheme 6 [76]. The intermolecular ET from Ceo to O2 is facilitated by the partial coordination of O2 to ZnP " in the transient state (denoted as in Scheme 6) [76]. Consequently, the one-electron reduction potential of the resulting 02 is shifted toward positive values, namely in favor of the ET event. The strong coordination of O2 to Zn(II) ion has been well established [77]. The complexation is then followed by a rapid intramolecular... 

Table 2 Formation Constants K), Fluorescence Maxima (Xmax), Fluorescence Lifetimes (x), the One-Electron Reduction Potentials (E°ed ) of the Singlet Excited States of Mg(C104)2, Sc(OTf)3 and MesSiOTf Complexes of Aromatic Carbonyl Compounds...

In 1969, Legg and Hercules [56] measured the difference between the one-electron reduction potentials of lucigenin and dioxygen in DMF, ° = °[Luc /Luc]—/ [CV /O2] = 0.6 V. Estimate of this difference in aqueous solution yields AE° = 0.35 V [4]. It means that the equilibrium of Reaction (10) is completely shifted to the right, i.e., the back reaction (Reaction... 

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