Quinone moiety

The anthracyclines represent a broad family of antibiotics that exhibit activity in numerous tumors. The first anthracyclines, doxorubicin (DOX) and dau-notubicin (DNR), were isolated from Streptomyces var peucetius they were shown to be composed of a tetracyclic ring system with adjacent quinone-hydro-quinone moieties, a short side chain with a carbonyl group, and an aminosugar bound to the C-7 of the four-ring system. DOX and DNR only differed in the side chain terminus (-CH2OH in DOX vs. -CH3 in DNR). Second generation anthracyclines, like epitubicin (EPI) and idatubicin (IDA), were obtained after minor chemical modifications of DOX or DNR, respectively (Fig- 1). 

Scott DT, DM McKnight, EL Blunt-Harris, SE Kolesar, DR Lovley (1998) Quinone moieties act as electron acceptors in the reduction of humic substances by humics-reducing microorganisms. Environ Sci Tech-nol 32 2984-2989. 

Redox behavior of anthraquinone is shown in Scheme 4. The quinone moiety may be reduced to the hydroquinone form and converted to a leuco salt under alkali conditions. In general, the leuco salt has a strong affinity for cellulose and is soluble in water. The hydroquinone form is insoluble in water and has low affinity to cellulose. The preferred dyeing procedure depends on the structure and properties of the vat dye. The variables that are used to control the process include, e.g., strength and amount of alkali, reduction temperature, and the presence of salts. During the process of reduction, some side reactions, such as overreduction, hydrolysis,... 

This reagent combination reduces a tertiary alcohol in the presence of a quinone moiety (Eq. 19).162 Tertiary alcohols are also reduced with the reagent combinations Et3SiH/MeS03H14° and Et3SiH/AlCl3/HCl.136... 

A linear hydrogen-bonded structure VIII has also been proposed, but accounts less satisfactorily for the color. The surprising thing about these complexes is the lack of any measurable exchange of the hydroxyl hydrogen atoms from the hydroquinone moiety to the quinone moiety... 

The hydroxyl group placed para to the anthracene core is protonated in acidic solutions (i.e. when the hydrangea sap is slightly acidic). The proton is abstracted in alkaline sap, causing molecular rearrangement to form the quinone moiety. 

Phenolphthalein is colourless and clear in acidic solutions, but imparts an intense puce pink colour in alkaline solutions of higher pH, with k(max> = 552 nm. The coloured form of phenolphthalein contains a quinone moiety in fact, any chromophore based on a quinone has a red colour. But if a solution is prepared at pH 7 (e.g. as determined with a pH meter), we find the phenolphthalein indicator is still colourless, and the pink colour only appears when the pH reaches 8.2. Therefore, we have a problem the indicator has not detected neutrality, since it changes colour at too... 

Compounds [54] and [55] have been shown to complex group 1 and 2 metal cations and also ammonium and alkylammonium cations by nmr and UV/Vis spectroscopies and also by a number of solid-state X-ray crystallographically determined structures. The quinone moieties in these molecules constitute not only the coordination site but also the redox-active centre. The complexation... 

Natural photosynthesis applies electron transfer systems, where a relay of electron-transfer reactions evolves among chlorophyll and quinone moieties embed-... 

The tricyclic carbazolequinones represent an important class of carbazole alkaloids with a quinone moiety in the A-ring of the carbazole nucleus. This family of... 

The characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

The structure of doxorubicin includes a quinone moiety therefore, it can easily accept an electron and undergo redox cycling (Fig. 7.47). Because it accumulates in the mitochondria, it can accept electrons from the electron transport chain and divert them away from complex I. It becomes reduced to the semiquinone radical in the process. This will then reduce oxygen to superoxide and return to the quinone form (Fig. 7.47). This could lead to oxidation of GSH and mtDNA. The subsequent damage may lead to the opening of the mitochondrial permeability transition pore. Consequently, mitochondrial ATP production will be compromised, and ATP levels will decline. 

This anthracydine type anticancer drug has a quinone moiety so can easily accept an electron and undergo redox cycling. As a result, it interferes with the mitochondrial electron transport chain, damages mitochondrial DNA, and leads to ATP depletion. The result is a dose-dependent cardiomyopathy. 

Stepwise chemical reduction of humic acid caused a variation in spin content as shown in Figure 7. The initial rise in radical content is attributed to anion radical formation caused by sodium the following decrease in spin content with further addition of sodium is probably caused by the reduction of these anion radicals. The subsequent increase in radical content could be caused by the one-step reduction of the remaining quinone moieties. 

Quinones are among the best-behaved organic compounds to undergo redox reactions in aqueous solutions. There are a reasonably large number of synthetic and natural products containing the quinone moiety, and many are excellent candidates for selective determination by LCEC. Unfortunately, some of the most important of these compounds are extremely hydrophobic due to the presence of long hydrocarbon side chains, and are therefore quite difficult to study by reversed-phase LC. 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Trisubstituted (3-lactams have been reported to be obtained through oxidative removal of the /V-alkyl group from more complex (3-lactams by treatment with cerium ammonium nitrate [260], Anologous methodology was employed for a general synthesis of cis- and trans-(3-lactams bearing a quinone moiety at the N-l, the C-3, or the C-4 position (I, II and III, respectively, Fig. 8), [261]. 

A polyrotaxane 29 possesses two electron-donating sites (TTF and hydro-quinone moieties) as stations in the polymer backbone, hence, the incorporated cyclic acceptor 28 moves by external stimuli and possibly two translational isomers (29a and b) would exist (Scheme 10) [106, 107]. The ratio between two isomers was reported to be very solvent dependent (Table 2), with a preference however for the hydroquinone moiety. In the CV measurement, it was also observed that the cyclic acceptor 28 moved from TTF to hydroquinone moiety along the chain of 29 upon oxidation of the TTF unit. 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

Another example of a photoredox molecular switch is based on a ferrocene-ruthenium trisbipyridyl conjugate, in which the luminescent form 4 switches to the non-luminescent form 5 upon electrochemical oxidation (Figure 2/bottom)171. Biological systems exploit the interplay of redox and molecular recognition to regulate a wide variety of processes and transformations. In an attempt to mimic such redox systems, Deans et al. have reported a three-component, two-pole molecular switch, in which noncovalent molecular recognition can be controlled electrochemically. x Willner et al. have reported on their research activities in developing novel means to achieve reversible photostimulation of the activities of biomaterials (see Chapter 6).[91 Recently, we have shown that it is possible to switch the luminescence in benzodi-furan quinone 6 electrochemically. 101 The reduction in THF of the quinone moiety... 

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