And one-electron reduction potentials

Table 14.4 Names, Abbreviations, and One-Electron Reduction Potentials [Eh (ArN02) Eq. 14-32] of a Series of Substituted Nitrobenzenes...

Radiation chemical methods and pulse radiolysis technique in particular, have been proved to be extremely useful in the selective generation of ROS and RNS and the direct monitoring of their reactions. Using these methods, a number of natural and synthetic products have been evaluated as new antioxidant molecules. Estimation of rate constants and one-electron reduction potentials in most of the promising cases confirmed the role of electron transfer... 

Figure 9. Relationship between relative reaction rates and one-electron reduction potentials for the reaction of a series of substituted nitrobenzenes with HLAW (see Table 7).

A good linear correlation was obtained between reactivity (expressed by log k oivi) and one-electron reduction potential of the compounds (Figure 7). In fact, the slopes of the linear regression line is 1.0 (Eq. 3-8), indicating that the actual transfer of the electron from the reduced NOM moieties to the NACs is rate limiting. A very similar result was found for the reduction of NACs in H2S solutions containing NOM derived from exudates of bacteria (17). 

Taking into account the results obtained by polarography as well as controlled potential electrolysis, the reaction which proceeded in Range A and gave the polarographic wave was estimated to be composed of two-electron oxidation of NADH and one-electron reduction of CQ at the W/DCE interface. The oxidation of NADH is accompanied by the dissociation of one H in W. 

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. 

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). 

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... 

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...

Peroxynitrous acid is a powerful oxidizing agent with estimated one- and two-electron reduction potentials of ° (ONOOH, H+/"N02, HjO) = 1.6-1.7 V and ° (ONOOH, H /N02 , H2O) = 1.3-1.4 V, respectively . In addition, it was reported that, upon protonation, ONOO can undergo decomposition via homolytic 0—0 cleavage to generate nitrogen dioxide radical ("NO2) and hydroxyl radical ( OH) in approximately 30% yields... 

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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