Oxidation equivalent

Oxygen Evolution Requires the Accumulation of Four Oxidizing Equivalents in PSII... 

The oxidation behavior of 3-oxa-chromanols was mainly studied by means of the 2,4-dimethyl-substituted compound 2,4,5,7,8-pentamethylM /-benzo[ 1,3]dioxin-6-ol (59) applied as mixture of isomers 27a it showed an extreme dependence on the amount of coreacting water present. In aqueous media, 59 was oxidized by one oxidation equivalent to 2,5-dihydroxy-3,4,6-trimethyl-acetophenone (61) via 2-(l-hydroxyethyl)-3,5,6-trimethylbenzo-l,4-quinone (60) that could be isolated at low temperatures (Fig. 6.41). This detour explained why the seemingly quite inert benzyl ether position was oxidized while the labile hydroquinone structure remained intact. Two oxidation equivalents gave directly the corresponding para-quinone 62. Upon oxidation, C-2 of the 3-oxa-chroman system carrying the methyl substituent was always lost in the form of acetaldehyde. 

Oxidant (Equivalent Concentrations) % Bromate After 60 Days Storage... 

This of course is a grossly oversimplified picture, as we shall see later, but it serves as a starting point for our discussion. In actuality, four reducing equivalents and four oxidizing equivalents are required to convert one molecule of C02 to carbohydrate and to liberate one molecule of Oa from water. 

Piperidine derivatives 161 and 164 could be cyclized to hexahydropyridooxadiazines 162, 165 via a dehydrogenation process by six oxidation equivalents of Hg(n)-EDTA. However, in both reactions, side products were also formed. From 161, piperidone derivative 163 was obtained, whereas starting from the amide 164, pyridopyrimidine 166 was isolated via cyclization by the amide nitrogen instead of oxime oxygen (Scheme 21) <1999ZNB632>. 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

Since S03/H2S04 is clearly not the most desirable system for industrial applications, a formidable challenge is to find an oxidant that oxidizes Pt(II) much faster than S03 does, operates in an environmentally friendly solvent, and can be (like SVI/SIV) reoxidized by oxygen from air. Ideally, the reduced oxidant would get reoxidized in a continuous process, such that the oxidant acts as a redox mediator. In addition, the redox behavior has to be tuned such that the platinum(II) alkyl intermediate would be oxidized but the platinum(II) catalyst would not be completely oxidized. Such a system that efficiently transfers oxidation equivalents from oxygen to Pt(II) would be highly desirable. A redox mediator system based on heteropolyacids has been reported for the Pt-catalyzed oxidation of C-H bonds by 02, using Na8HPMo6V6O40... 

The price of the oxidation equivalent varies with the reagent [288]. Oxygen in air is the cheapest, followed by chlorine, electricity, hydrogen peroxide and finally ozone. Oxydation with oxygen at low temperature is only feasible biologically. Chlorine often forms very stable, highly toxic chlorinated compounds, which often limits its use. 

Three aspects are interesting. First, the reaction produces quite concentrated solutions of H202—no disproportionation has been observed. Second, the catalytic reaction is pH neutral. Third, in the catalytic cycles, Cu(I) is formally not involved only the oxidation equivalents stored in the two phenoxyl groups are used. 

Figure 6.2 Electrostatic adsorption mechanism of Brunelle [1] (a) surface polarization as a function of pH (b) measurement of PZC of some oxides (equivalent to isoelectric point) by electrophoresis.

If cyclohexanecarboxaldehyde is incubated with CYP2B4, NADPH, and cytochrome P450 reductase, the aldehyde-cyclohexyl ring carbon-carbon bond is cleaved generating cyclohexene and formic acid (150) (Fig. 4.81). The reaction is supported if hydrogen peroxide replaces NADPH and cytochrome P450 reductase but is not supported if other oxidants at the same oxidation equivalent as peroxide but bypass the peroxy form of P450 such as iodosobenzene, m-chloroperbenzoic acid, or cumyl hydroperoxide are used. These... 

In the absence of chemical reactions coupled with the electron transfer steps, the injection, or removal, of a second electron into, or from, a molecule (Scheme 1.2) is usually more difficult, thermodynamically speaking, than the first (E > f°r reductions, < E° for oxidations). Equivalently, the... 

Long-range ET rates have been measured in c/ccp complexes [73, 74] the reactions between cyt c and Feccp [ES is the oxidation product of Fe(II)ccp and peroxide it has two oxidizing equivalents, namely, Fe(IV)0 and a protein-based organic radical cation] are given in Eq. (3) ... 

The first step of the reaction path involves the addition of H2O2 to the Fe " resting state to form an iron-oxo derivative known as Compound I, which is formally two oxidation equivalents above the Fe state (Fig. 2). The well studied Compound I contains a Fe" = 0 structure and a n cation radical. In the second step. Compound I is reduced to Compound II with a Fe =0 structure. The reduction of the n cation radical by a phenol or enol is accompanied by an electron transfer to Compound I and a proton transfer to a distal basic group (B), probably His 42 (Fig. 3, step 1). The native state is regenerated on one-electron reduction of Compound II by a phenol or an enol. In this process, electron and proton transfers occur to the ferryl group with simultaneous reduction of Fe" to Fe (Fig. 3, steps 2-3) and formation of water as the leaving group (Fig. 3, step 4). 

In subsequent work, Hamberg has reported that anerobic oxidation of lineolic acid by cumene hydroperoxide catalyzed hy sperm whale Mb results in formation of five products, the two major products being ll(i ,S)-hydroylinoleic acid (29% 5deld) and ( )cis-9,10-epoxy-(12Z)-octadecenoic acid (16%) (235). In this work, it was proposed that the second oxidizing equivalent required for substrate hydroxylation was probably provided by a protein-centered radical. 

Fig. 14. A mechanism to explain heme modification in the P. vitcde catalase and possibly E. coli HPII. For simplicity, the phenyl ring of T3rr415 is not shown, and only ring III of the heme and the heme iron are shown. Compound I is an oxyferryl species formed, along with water, in the reaction of one H2O2 with the heme. The iron is in a formal Fe oxidation state, but one oxidation equivalent is delocalized on the heme to create the 0x0-Fe" -heme cation, shown as the starting species, compound I. A water on the proximal side of the heme is added to the heme cation species of compound 1 shown in A to generate a radical ion in B. The electron flow toward the oxo-iron would generate the cation shown in (C), leading to the spirolactone product shown in D. In E, an alternate mechanism for the His-Tyr bond formation in HPII is presented that could occur independently of the heme modification reaction. Reprinted with permission of Cambridge University Press from Bravo et al. (93).

IV, F and Fig. 7). In catalatic mode, HPI exhibits no significant spectral change, snggesting that compoimd I has an oxidation equivalent in the form of a protein radical rather than a porphyrin radical. On the other hand, the W105F variant of HPI, which operates only in peroxidatic mode, has a porphyrin radical clearly evident in the absorbance and EPR spectra 101). 

The two oxidizing equivalents in compoimd I are next utilized to oxidize two substrate molecules. In the second step in the scheme, the first reducing substrate (S) delivers one electron to compoimd I, which reduces the porphyrin n cation radical, thereby generating compound II. A second substrate molecule reduces compoimd II back to the resting state. 

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