Quinones, detection

In summary, there is abundant evidence that alkaline peroxide reacts with lignin to form hydroquinones, some of which are not oxidized further under the bleaching conditions. It seems likely that the p-quinones detected in bleached TMP have been formed by subsequent oxidation by air. 

The number of quinones normally encountered in routine quahtative organic analysis is very limited the following notes will be found useful for their detection and characterisation. 

For instance morphine (1) can be detected by the formation of various quinones via apomorphine as intermediate [4, 12, 13]. All morphines with an OH group in the 6-position and a double bond (codeine, ethylmorphine etc.) first undergo an acid-catalyzed rearrangement according to the following scheme [12] ... 

Iodine addition to the ter- [13] tiary nitrogen of the opium alkaloids and to the OCH3 group of the brucine with formation of an o-quinone derivative, probably ring opening in the case of phenylbutazone, ketazone and trimethazone detection by IR... 

The availability of aniline has made possible a direct study of the reactions of aniline with humic and fulvic acids (Thom et al. 1996), and the detection of resonances attributed to anilinoquinone, imines, and N-heterocyclic compounds are fully consistent with reactions involving quinone and ketone groups. 

This organism is able to oxidize acetate to CO2 under anaerobic conditions in the presence of Fe(III). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

The oxidation of N ADH has been mediated with chemically modified electrodes whose surface contains synthetic electron transfer mediators. The reduced form of the mediator is detected as it is recycled electrochemically. Systems based on quinones 173-175) dopamine chloranil 3-P-napthoyl-Nile Blue phenazine metho-sulphatemeldola blue and similar phenoxazineshave been described. Conducting salt electrodes consisting of the radical salt of 7,7,8,8-trtra-cyanoquinodimethane and the N-methylphenazium ion have been reported to show catalytic effects The main drawback to this approach is the limited stability... 

A recently characterized class of dehydrogenases are the quinoproteins which contain a pyrroloquinolene quinone prosthetic group and do not require a separate co-factor Electron transfer mediators such as phenazine ethosulphate 2,6-dichloroindophenol and ferricenium ions have been used to recycle the quinoprotein the reduce mediator is detected amperometrically. 

Quinones, lipid-soluble substances involved in electron transport, can also be used as biomarkers. Lipski et al. (155) u.sed quinone analyses, physiological tests, and fatty acid profiles to differentiate Gram-negative non-fermentative bacteria isolated from biofilters. Quinone type was found to be an efficient method to group isolates prior to the analysis of results from the physiological tests. The detection of quinones appears to be restricted to the discrimination of isolated colonies and has limited potential to the analysis of mixed populations. 

Wan s group showed that the observed photodehydration of hydroxybenzyl alcohols can be extended to several other chromophores as well, giving rise to many new types of quinone methides. For example, he has shown that a variety of biphenyl quinone methides can be photogenerated from the appropriate biaryl hydroxybenzyl alcohols.32,33 Isomeric biaryls 27-29 each have the benzylic moiety on the ring that does not contain the phenol, yet all were found to efficiently give rise to the corresponding quinone methides (30-32) (Eqs. [1.4—1.6]). Quinone methides 31 and 32 were detected via LFP and showed absorption maxima of 570 and 525 nm, respectively (in 100% water, Table 1.2). Quinone methide 30 was too short lived to be detected by LFP, but was implicated by formation of product 33 that would arise from electrocyclic ring closure of 30 (Eq. 1.4). 

Through these works, Wan has conclusively demonstrated that the photodehydration of hydroxybenzyl alcohols is a general reaction, and a wide variety of quinone methides can be photogenerated and detected using this method. Quinone methide photogeneration via this method has been shown to have importance in the photochemistry of Vitamin B641,42 and in model lignins 43... 

Photohydration was observed for biphenylalkenes 113 and 114 via biphenyl-quinone methides 115 and 116, respectively (Eqs. 1.31 and I32).31meta-Biphenylquinone methide 116 was detected on LFP of 114 in TFE and showed a strong and sharp absorption band at 425 nm, consistent with spectra of previously... 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

A molecular probe with dual output signals offers two detection modes allowing use of the same probe in different environments. We have demonstrated how an AB2 self-immolative dendron with double quinone methide release mechanism can be applied to create a molecular probe with UV-Vis and fluorescence modes for the detection of a specific catalytic activity.15 The molecular probe is illustrated in Fig. 5.36. The central unit of the probe (the molecular adaptor) is linked to an enzymatic substrate that acts as a trigger and to two different reporter molecules. Cleavage of the enzymatic substrate triggers the release of the two reporters and a consequent activation of their signals. 

The UV-Vis spectral detection of an intermediate in the catalytic reductive alkylation reaction provides only circumstantial evidence of the quinone methide species. If the bioreductive alkylating agent has a 13C label at the methide center, then a 13C-NMR could provide chemical shift evidence of the methide intermediate. Although this concept is simple, the synthesis of such 13C-labeled materials may not be trivial. We carried out the synthesis of the 13C-labeled prekinamycin shown in Scheme 7.5 and prepared its quinone methide by catalytic reduction in an N2 glove box. An enriched 13C-NMR spectrum of this reaction mixture was obtained within 100 min of the catalytic reduction (the time of the peak intermediate concentration in Fig. 7.2). This spectrum clearly shows the chemical shift associated with the quinone methide along with those of decomposition products (Fig. 7.3). 

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