Benzoquinone, photoreduction

Thus, the concentration necessary for 50% displacement roughly corresponds to the PIjq value. It is possible, therefore, to assay the pl value of a new compound just by examination of its displacement behaviour. It is no longer necessary to determine the pi value by testing the inhibition of a light-driven photoreduction. Another very potent inhibitor of photosynthetic electron transport, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-l,4-benzoquinone) (13), almost completely fails to displace metribuzin from the membrane (Figure 3). This is due to the fact that DBMIB has a completely different site of action as compared to the photosystem II herbicides, i. e. it inhibits plastohydroquinone oxidation by acting at the cytochrome b /f-complex (13). 6... 

Irradiation from the side with a high-intensity uv source may also introduce some problems. The volume of the solution within the receiver coil may be heated by the accompanying light, and the polarized products may diffuse faster away from the receiver coil zone. This effect can be illustrated in Fig. 7, which shows the polarization of the recombined product p-benzo-quinone in the photoreduction of benzoquinone obtained under two different experimental conditions. In the upper curve the polarization observed was greatly reduced and showed only a "diminished absorption" when the sample was allowed to heat up by light... 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

The photoreduction of quinones has also been studied in micellar environment 44-46) Thg relative quantum yields for disappearance of benzoquinone is various surfactants have been found to be influenced by the nature of the micelle. For example, the rate of reaction (Scheme XV) is accelerated in anionic surfactants44),... 

Me R = Br, R = H) by various donors in viscous soJventa are decreased by a factor of <2 in a 0.34 T magnetic field and pressure is reported to accelerate the photoreduction of p-benzoquinone in SOS and CTAB micelles in Aerosol OT reversed micelles, the reaction is retarded.The yield of anthraquinone-2-sulphonate radical anion (AQS) generated by irradiation of AQS in aqueous acetonitrile containing propan-2-oJ has been found to depend on the water concentration and to follow Perrin s equation.Complexation of aryl alkyl ketones with /3-cyclodextrin leads to changes in the ratio of the products of elimination and cyclisation which result from reaction of the... 

The photoreduction efficiency of carbonyl compounds such as 9,10-anthraquinone, 1,4-anthraquinone, 6,13-pentacenequinone, and 2-phenyl-9.10-anthraquinone, in the presence of anthracene, pyrene, naphthalene, biphenyl, 1,4-benzoquinone, and 1,4-naphthoqulnone has been shown to depend on the relative positions and nature of the electronic levels of the substrate and acceptor. Photoreduction of aromatic ketones to alcohols or pinacols can be catalysed by CdS powders using triethylamine as sacrificial donor in MeCN, and morphology is reported by the same authors to affect the two-electron transfer photoreductions of aromatic ketones on CdS induced by visible llghti under analogous conditions olefins behave similarly. Aldehydes of the type RCHO (R -jB -tolyl, g-anlsyl, hexyl) have been photoreduced to the corresp>onding... 

Photohydroxylation of p-benzoquinone to give the 2-hydroxy derivative was inhibited by a- and P-CD in favor of photoreduction to hydroquinone [312]. 

Photolytic decomposition of peroxides is not v y efBcient in crosslinking. An enhancement effect on the extent of photocrosslinking of polyolefins in the presence of peroxides is displayed by aromatic hydrocarbons such as naphthalene. These transfer the exdtation energy absorbed to a peroxide. This procedure, however, does not represent an important improvement when compared with that refored to earlier, namely the photoreduction of pdyethylene with aromatic ketones and quinones [84. From aromatic ketones and quinones, particulariy bena>phenone [32], chlorinated benzophenones, benzoyl-l-( dohexanol [82], a, -dimethot - hen acdr henone, 2,4,6-trimethyl benzoyl phenyl phosphinic ethyl ester [85], anthrone [86], anthraqui-none [87], naphthoquinone, benzoquinone, and their d vatives have all been examined. 

Lahtinen, R., D.J. Fermin, K. Kontturi, and H.H. Girault (2000). Artificial photosynthesis at hquid/liquid interfaces photoreduction of benzoquinone by water soluble porphyrin species. J. Electroanal. Chem. 483, 81-87. 

The activity of O2 evolution was measured in the MES medium (1 ml) containing TNM-treated PSII membranes (10 ix g Chi), 0.3 mM phenyl- -benzoquinone and 1 mM K3pe(CN)0 using a Clark-type O2 electrode (Hansatech Co., Ltd., England). DCIP photoreduction was measured in the MES medium (2 ml) containing Tris-treated or TNM-Tris-treated PSII membranes (50 // g Chi), 0.5 mM DPC and 40 // M DCIP. 

Oxygen evolution was measured in the presence of K3pe(CN) (2 mM) and phenyl-p-benzoquinone (Ph-p-BQ, 200 /iM) at a Chl-concentration of 25 /ig/ml using a Clark type electrode. The rate of DCIP photoreduction with diphenylcarbazide (DPC) as a donor was spectrophotometrically monitored at 590 nm. The reaction mixture contained 5 /xM Chi, 40 /xM DCIP and 600 /xM DPC. 

The concentration dependence for ij as discussed above was confirmed for these photocurrent transient measurements, and a Langmuir adsorption isotherm according to Eq. 3a was successfully used to fit the data for the photoreduction of benzoquinone at the surface of PcZn [44] and for the photooxidation of Ce " at the surface of FisPcZn [129]. In these studies quite remarkable differences were seen in the dependence or independence of charging and discharging currents on the... 

Miscellaneous. The photoreduction of benzoquinone, deuterobenzo-quinone and benzohydroquinone has been investigated in ethylene glycol by laser flash TR EPR by Jager et By hydrogen transfer from the solvent, semiquinone radical and the 1,2-dihydroxyethyl radical both emissive polarized by the triplet mechanism are generated. No alkyl radicals were observed in solutions of benzoquinone with excess benzohydroquinone. Analysis of the polarization patterns confirmed that the reaction of benzoquinone triplet with the benzohydroquinone yields two semiquinone radicals. 

In vivo electron transport assays demonstrate that the site of Ca /Na action is Photosystem II. Ascorbate/DAD to methyl viologen rates (measured as O2 uptake) in the presence of DCMU are not affected by depletion of Ca " and Na" " while water to benzoquinone rates (measured as 2 evolution) in the presence of DBMIB show a decline concommitent with the loss of complete photosynthetic electron transport. Jji vitro examination of partial reactions within PS II reveals that the site of the cation effect excludes the water-splitting enzyme and occurs prior to the site of DCMU inhibition diphenylcarbazide-mediated silicomolybdate photoreduction after heat inactivation of the water-splitting enzyme shows diminished rates in membranes prepared from cells partially depleted of calcium and sodium. 

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