Quinone Molecule oxidation

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

The ot-tocopheryl dimer continues to possess antioxidant activity. Also, two toco-pheryl semiquinone radicals can form one tocopheryl quinone molecule and regenerate one tocopherol molecule. The decomposition products of tocopherols (during thermal oxidation) can slowly oxidize and release tocopherol that can act as an antioxidant (100). 

Introduction of oxygen into the molecule Oxidation with bromine, cerium(IV), H.O., HNO., NaNO.. Menadiol to the quinone [239], lidocaine to its N-oxide [207], chloropromazine to its sulphoxide [212]. 

By building the quinone molecule into a macrocycle, a more efficient palladium-catalyzed aerobic 1,4-oxidation was developed [63], Thus, with catalytic amounts of 40 and Pd(OAc)2, 1,3-cyclohexadiene was oxidized to 1,4-diacetoxycyclohex-2-ene at a rate which was more than twice that with the system having quinone and porphyrin as separate molecules. The trans selectivity with tetrakis(hydroquinone)poiphyrin 40, however, was moderate (trans/cis = 70 30). 

The existence of Qa was initially inferred from chlorophyll-a fluorescence measurements. In 1963 when Duysens and Sweers found that the fluorescence yield of dark-adapted chloroplasts increases with time of illumination. These workers explained the phenomenon by suggesting that when the PS-II electron acceptor is present in the oxidized state, it can quench fluorescence, whereas it does not quench fluorescence once it is reduced, i.e., in its reduced state it inhibits the normal utilization of absorbed light energy to promote electron transport. Therefore, the electron acceptor was called Q, taken from the expression quencher of fluorescence. When it was subsequently established that the stable primary electron acceptor is a quinone molecule, the symbol Q became even more appropriate. 

Bidirectional PCET is also featured on the reduction side of the photosynthetic apparatus. In the bacterial photosynthetic reaction center, two sequential photo-induced ET reactions from the P680 excited state to a quinone molecule (Qg) are coupled to the uptake of two protons to form the hydroquinone [213-215]. This diffuses into the inter-membrane quinone pool and is re-oxidized at the Qq binding site of the cytochrome bcj and coupled to translocation of the protons across the membrane, thereby driving ATP production. These PCET reactions are best described by a Type D mechanism because the PCET of Qg appears to involve specifically engineered PT coordinates among amino acid residues [215]. In this case PT ultimately takes place to and from the bulk solvent. Coupling remains tight in... 

The mechanism of catecholase activity (outer circle) starts from the oxy and met states. A diphenol substrate binds to the met state (for example), followed by the oxidation of the substrate to the first quinone and the formation of the reduced state of the enzyme. Binding of dioxygen leads to the oxy state, which is subsequently attacked by the second diphenol molecule. Oxidation to the second quinone forms the met state again and closes the catalytic cycle. 

Oxidation of hydroquinone (1,4-benzenediol) produces a compound known as p-benzo-quinone. The oxidation can be brought about by mild oxidizing agents, and, overall, the oxidation amounts to the removal of a pair of electrons (2 e ) and two protons from hydroquinone. (Another way of visuahzing the oxidation is as the loss of a hydrogen molecule, H H, making it a dehydrogenation.)... 

An example of photo-induced oxidative addition is that of trans-[IrCKCOXPPhslal with 9,10-phenanthraquinone the iridium(i) complex effectively acts as a trap for excited quinone molecules. ... 

The properties of quinones are to a certain extent similar to those of carbonyl compoimds. They form oximes, thiosemicarbazones, and phenylhydrazones. The rate of the reaction of quinones with hydroxylamine varies, and a number of quinones do not react with it at all. Substituents already present in the quinone molecule play a predominant role (16). In acid media hydroxylamine can have an oxidative influence, parallel to oximation, for example in the case of quinol, which is converted to quinone dioxime (17) free hydroxylamine can, however, have a reductive influence it reduces quinone to hydroquinone (18). 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

The ionized developers are then capable of diffusing. Transfer of an electron reduces the silver and generates the semiquinone ion radical of the auxiUary developer (eq. 10). In turn, a dye developer molecule of the adjacent layer transfers an electron to the semiquinone, returning the auxiUary developer to its original state and leaving the dye developer in the semiquinone state (eq. 11). Further oxidation of the semiquinone leads to the quinone state of the dye developer. 

The ready reversibility of this reaction is essential to the role that quinones play in cellular respiration, the process by which an organism uses molecular- oxygen to convert its food to carbon dioxide, water, and energy. Electrons are not transfened directly from the substrate molecule to oxygen but instead are transfened by way of an electron transport chain involving a succession of oxidation-reduction reactions. A key component of this electron transport chain is the substance known as ubiquinone, or coenzyme Q ... 

Br20 a dark-brown solid moderately stable at —60° (mp —17.5° with decomposition), prepared by reaction of Bt2 vapour on HgO (cf. CI2O p. 846) or better, by low-temperature vacuum decomposition of BrOa. The molecule has C2v symmetry in both the solid and vapour phase with Br-O 185 1pm and angle BrOBr 112 2° as determined by EXAFS (extended X-ray absorption fine structure). It oxidizes I2 to I2O5, benzene to 1,4-quinone, and yields OBr in alkaline solution. 

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