Quinones redox potential

Whether quinones act as retarders, inhibitors, or even as comonomers depends on the quinone redox potential. Quinones of high redox potential, such as 2,5,7,10-tetrachlorodiphenoquinone, copolymerize with styrene to high-molecular-weight products ... 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Table 3. Spectral (Uv) Data and Redox Potentials for Selected Quinones ...

The electron on the bj heme facing the cytosolic side of the membrane is now passed to the bfj evcie on the matrix side of the membrane. This electron transfer occurs against a membrane potential of 0.15 V and is driven by the loss of redox potential as the electron moves from bj = — O.IOOV) to bn = +0.050V). The electron is then passed from bn to a molecule of UQ at a second quinone-binding site, Q , converting this UQ to UQ . The result-... 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

Ishikita, H. Morra, G. Knapp, E.W., Redox potential of quinones in photosynthetic reaction centers from Rhodobacter sphaeroides dependence on protonation of Glu-L212 and Asp-L213, Biochemistry 2003, 42, 3882-3892... 

Redox mediators, such as flavins or quinones, are usually involved in the azo bond reduction. Therefore, the azo bond cleavage is a chemical, unspecific reaction that can occur inside or outside the cell, relying on the redox potential of the redox mediators and of the azo compounds. Also the reduction of the redox mediators can be both a chemical and an enzymatic process. As a consequence, it is an evidence that environmental conditions can affect the azo dyes degradation process extent both directly, depending on the reductive or oxidative status of the environment, and indirectly, influencing the microbial metabolism. 

Most quinone reductions go through an intermediate radical or semiquinone stage, usually revealed by a one-electron step in the redox potential.100 The radical formed by the reduction of compound VI is especially stable, probably because of the additional involvement of the benzoyl group.101 The ordinary semiquinones are more stable in basic solution since some of the resonance structures of the neutral radical involve separation of charges. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

Smertenko et al. (2000) have proposed a special index for this purpose based on differences in redox potentials between a quinone and oxygen. This index takes into account the peak merging for electrochemical reductions of the quinone and oxygen and, according to first estimations, works well. 

Redox potentials for i-2 were determined in butyronitrile containing O.IM tetra-n-butylammonium perchlorate using a Pt disc electrode at 21. These potentials were measured relative to a saturated calomel electrode using ac voltammetry.(lQ) Both the one electron oxidations and reductions of i-2 exhibited good reversibility. The half-wave potentials for the one-electron oxidation and reduction of i-2, ZnTPP, and two model quinones are given in Table I. 

The one electron redox potentials of 1-2 along with those of the appropriate reference compounds are presented in Table I. The redox potentials of both the porphyrins and the quinones are not altered by linking the two molecules. These potentials were used to obtain the exothermicity of the charge separation, and that of the charge recombination, -AG from... 

Benzoquinones are conveniently prepared in solution by the anodic oxidation of catechols. 1,2-Quinones are unstable in solution but they have a sufficient lifetime for the redox process to be reversible at a rotating disc electrode. Reaction involves two electrons and two protons and the half-wave potential varies with pH at 25 °C according to Equation 6.1. Some redox potentials for catechols and hy-droquinones are given m Table 6.6. 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

An ab initio study using the 6-31G basis set was used to investigate the vibrational and electronic structures of a benzodithiophene and benzodifuran quinone as potential redox switches. The calculated and experimental data were... 

A similar mechanism could operate in the reduction of oxygen on chelate catalysts, as in the organic cathodes with air regeneration described by Alt, Binder, Kohling and Sandstede 13-40>. These cathodes contain a reversible insoluble quinone/hydroquinone system. The quinone, which is electrochemically reducible, can be obtained either by electrochemical oxidation or by purely chemical oxidation with H2O2 or oxygen (air). A cathodic current is observed in these systems only at potentials below the redox potential, and unusually hard current/ voltage characteristic curves are obtained. 

The equilibria governing semi-quinone formation from quinones are similar to those for the flavin semiquinones which were discussed in Section B,6. Two consecutive one-electron redox steps can be defined. Their redox potentials will vary with pH because of a pfCa for the semiquinone in the pH 4.5 -6.5 region. For ubiquinone this pKa is about 4.9 in water and 6.45 in methanol. A pKa of over 13 in the... 

The key to the construction of the system is the choice of the quinone redox couple in the oil phase and the oil itself. The quinone compound must be reduced by Fe(II) ions, and the reduced form must be oxidized by bromine. These requirements indicate that the redox potential must be in the range between 0.77 and 1.07 V vs. NHE. After investigating of many redox compounds, we found that 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) dissolved in n-butyronitrile may be a good candidate for the system. DDQ has a largely positive redox potential because of its strong electron withdrawing substituents. 

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