Quinones distribution

The product distribution appears to depend on the radiation used for quinone excitation, the structure of the quinone, and the quinone—alkene ratio. In the example cited, l,4-ben2oquinone gives only the spirooxetane, whereas chlorarul gives both products in amounts related to the ratio of starting materials... 

Electrophilic addition to quinones, eg, the reaction of 2-chloro-l,4-ben2oquinones with dia2onium salts, represents a marked contrast with acetoxylation in product distribution (58). Phenyldia2onium chloride (Ar = C H ) yields 8% 2,3-substitution [80632-59-3] 75% 2,5-substitution [39171-11-4] and 17% 2,6-substitution [80632-60-6]. Fory)-chlorophenyldia2onium chloride, the pattern is 28% 2,3-substitution [80632-61-7], 35%... 

Despite the importance of the oxidative polymerization of 5,6-dihydroxyin-dole, in the biosynthesis of pigments, little experimental data are known on the oxidation chemistry of the oligomers of 1. For such reasons, three major dimers of 1, such as 2-4 (Scheme 2.9), have been computationally investigated at PBEO/ 6-31+G(d,p) level of theory both in gas and in aqueous solution (by PCM solvation model) to clarify the quinone methide/o-quinone tautomeric distribution. 

The tautomeric product distribution has been a prerequisite for a further investigation aimed at predicting absorption properties of the transient semiquinones and quinones generated by pulse radiolytic oxidation of 2-4. The simulation of electronic absorption spectra has been computed using the TD-DFTapproach both in vacuum and in aqueous solution, using the large 6-311 + +G(2d,2p) basis set.19... 

Fig. 12.7a,e) it is important to realize that a protonated quinone methide QM1H + is actually a benzylic carbocation (Fig. 12.7a). Water will also add to the quinone methide under fairly neutral conditions.86-88 The isomer distribution of the resulting compounds, PI can be determined directly from or 13C (or 2D 13C/1H correlation)... 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Of considerable interest was the detection of 2,5-dimethylbenzo-quinone (61), 2,5-dimethyl-l,4-benzenediol (63), acetoin (62), and butane-dione (60) in mixtures form hydrothermolyzed cellulose. The results from experiments performed at 300° with [l- C]acetoin suggested that two molecules of biacetyl combined to form 61 and 63, as equal distribution was observed in four positions. This implied that a symmetrical pre-... 

However, reactions of PAHs in ambient air to form more polar species (e.g., nitro-PAHs, ketones, quinones, lactones, and dicarboxylic acids) greatly enhance their solubilities in aqueous systems. This has major implications when one considers the distribution of PAHs, and their atmospherically formed PAC derivatives, through the air, water, and soil environments. These increases in solubility upon reaction are important not only from an environmental chemistry perspective but also in terms of possible impacts on public health and ecosystems, e.g., in both the exposure and the health effect... 

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

The adsorption of electron acceptors (quinone, chloranil) from the gas phase does not substantially influence the photo-emf of PAC but decreases the dark conductivity and the photoconductivity. The same compounds, however, adsorbed on certain polyacetylenides from solution, increase the photo emf without causing any appreciable change in the spectral distribution. Mercury vapor depresses reversibly the dark conductivity and photoconductivity [276-278]. 

At first it appeared that PQQ had a broad distribution in enzymes, including eukaryotic amine oxidases. However, it was discovered, after considerable effort, that there are additional quinone cofactors that function in oxidation of amines. These are derivatives of tyrosyl groups of specific enzyme proteins. Together with enzymes containing bound PQQ they are often called quinoproteins.11, 1... 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

A scries of quinones which are widely distributed in animals, plants, and microorganisms, these quinones have been shown to function in biological electron transport systems which are responsible for energy conversion with living cells. The nature and significance of coenzyme Q was first... 

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