Model sclerotization studies. 4. Generation of N‐acetylmethionyl catechol adducts during tyrosinase‐catalyzed oxidation of catechols in the presence of N‐acetylmethionine
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Incubation of catechol with mushroom tyrosinase in the presence of N-acetylmethionine resulted in the generation of an adduct. This product was identified to be N-acetylmethionyl catechol, on the basis of spectral characteristics and well-characterized chemical reaction of o-benzoquinone with N-acetylmethionine. Enzyme-catalyzed oxidation of catechol and the subsequent nonenzymatic addition of the resultant quinone to N-acetylmethionine accounted for the observed reaction. That the reaction is not confined to catechol alone, but is of general occurrence, can be demonstrated by the facile generation of similar adducts in incubation mixtures containing N-acetylmethionine, tyrosinase, and different N-acetylmethionines, such as 4-methylcatechol and N-acetyldopamine. Attempts to duplicate the reaction with insect cuticular phenoloxidases were not successful, as the excess N-acetylmethionine used in the reaction inhibited their activity. Nevertheless, occurrence of this nonenzymatic reaction between N-acetylmethionine and mushroom tyrosinase-generated quinones indicates that a similar reaction between enzymatically generated quinones in the cuticle with protein-bound methionine moiety is likely to occur during in vivo quinone tanning as well. Arch. Insect Biochem. Physiol. 38:44–52, 1998. © 1998 Wiley-Liss, Inc.Keywords:
Catechol
Hydroquinone
Catechol oxidase
Moiety
Biocatalysis
Benzoquinone
Additions of strong acids to a solution of quinone in dichloromethane lead to protolytic equilibria which have a marked influence on its electrochemical behaviour. In the presence of free protons, the electroactive species is the monoprotonated form of quinone, QH + , and at higher acid concentrations the species QH 2 2+ is formed with the acidity constant p K (QH + /QH 2 2+ ) = 1.2. The overall equilibrium EC reaction scheme necessarily involves also the protonated forms of hydroquinone, at least the species QH 3 + which is, in contrast to the inactive hydroquinone, electroactive on the dropping mercury electrode in the medium of dichloromethane.
Hydroquinone
Dichloromethane
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The quinone/hydroquinone sesquiterpenes of drimane or rearranged drimane skeletons constitute a wide and diverse group of secondary metabolites of mixed biogenesis. These compounds are mainly of marine origin and their interest is not only for the variety of isolated structure but for the interesting biological activities that they present. In this paper a series of quinone/hydroquinone sesquiterpenes of natural origin that have been reported to date is presented. The structures of these compounds are gathered into eight groups with reference to their biological activities and compounds synthesised. Keywords: Sesquiterpenes quinone/hydroquinone, drimane, rearranged drimane
Hydroquinone
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Abstract The mono(glucosylthio)hydroquinone 2 was prepared by S ‐glycosidation of 2‐mercaptobenzene‐1,4‐diol and by addition of the acetylated 1‐thioglucose 3 to benzo‐1,4‐quinone ( Scheme 1 ). The second, higher yielding procedure was adopted for the preparation of a range of (glucosylthio)hydroquinones. Addition of 3 to 2‐chlorobenzo‐1,4‐quinone, followed by oxidation gave the 1‐thioglucosides 7 and 12 (1.3:1), while addition of HCl to the (glucosylthio)quinone 4 and oxidation gave mainly 12 ( Scheme 1 ). Similarly, the bis(glucosylthio)hydroquinone 33 was obtained from 3 and 4 ( Scheme 4 ), and the (cellobiosylthio)hydroquinone 18 from the thiol 16 and benzo‐1,4‐quinone ( Scheme 2 ). Addition of the 4‐thioglucoside 21 to benzo‐1,4‐quinone (→ 22 ) and to 4 was followed by oxidation to yield the mono(glucosylthio)quinone 23 and the disubstituted quinones 24 and 25 , respectively ( Scheme 3 ). A mixture 24 / 25 was also obtained from the addition of 3 to 23 . The tris(glucosylthio)hydroquinone 36 was obtained by addition/elimination to the dichloroquinone 29 or the dimesylate 31 , which was prepared in a simplified way ( Scheme 4 ). The tetrakis(glucosylthio)hydroquinone 37 was obtained from 3 and chloranil, followed by reduction. The acylated hydroquinones were deprotected (→ 5, 9, 14, 19, 27, 34 , and 38 ), and oxidized to the corresponding quinones ( 6, 10, 15, 20, 28, 35 , and 40 ). The (glucosylthio)quinones 6, 15, 20, 28 , and 35 were tested as time‐dependent inactivators of a retaining β‐1,4‐glucosidase from Agrobacterium faecalis ( Abg ), which has a strong exo‐glucosidase action ( Table 1 ). Similarly, compounds 20, 28 , and 35 were tested with a cellulase from Cellulomonas fimi ( Cex ) which degrades cellulose and cellooligosaccharides by hydrolysis of a cellobiose unit from the nonreducing terminus. The most effective inactivators for Abg were 6, 15 , and 35 , which inactivated this enzyme with similar second‐order rate constants. (Glycosylthio)quinone 28 was the worst inactivator and did not show normal saturation behaviour. Inactivation of Cex by the (glycosylthio)quinones was 3–500 times slower than that of Abg . The three inactivators 20, 28 , and 35 had approximately the same efficacy with Cex , suggesting that they bind to this enzyme in a similar mode. Further, the K i values observed are very similar to K m values measured for aryl cellobiosides, implying that they bind at the active site.
Hydroquinone
Chloranil
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Abstract The structures of the 1:1 quinone-hydroquinone complexes of 2-phenyl and of 2-(4′-chloro) phenylbenzoquinone have been studied by X-ray methods. A superficial study would indicate that the quinhydrones are centrosymmetric and belong to the space group P21/c. However, other evidence indicates that the true crystal structure may belong to either the space group P21 or to Pc, or that the crystal may contain regions that would coresspond to each of these two non-centrosymmetric space groups. Some possible consequences of such a structural arrangement are briefly discussed.
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Crystal (programming language)
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Octafluoro[2.2]paracyclophane (AF4) has been oxidized by treatment with HIO 3 in CF 3 CO 2 H to form the corresponding p -quinone along with a unique triketone. This quinone undergoes reduction to the respective hydroquinone as well as a Diels-Alder reaction with 1,3-cyclohexadiene. Its reduction potential was obtained by cyclic voltammetry and is discussed in the context of other quinones.
Hydroquinone
Cyclophane
Oxidation reduction
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Hydroquinone
Catechol
Catechol oxidase
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The kinetics of reduction of ferricytochrome c by hydroquinone have been studied. The reaction does not conform to a simple second-order rate equation and it is demonstrated that the deviations are brought about by the presence of p-quinone, one of the products of the reaction. The accelerating effect of p-quinone is explained tentatively on the basis of an involvement of the semi-quinone. The effects on the reaction of pH, ionic strength, and temperature are reported and used to suggest features of the reaction mechanism.
Hydroquinone
Stepwise reaction
Reaction rate
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Hydroquinone
Benzoquinone
Catechol
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The electrochemical behavior of quinone, hydroquinone, and quinhydrone in pyridine solution at the stationary pyrolytic graphite electrode has been studied by conventional and cyclic voltammetry. In the absence of available protons, quinone is somewhat reversibly reduced in a probable 1e process to a free radical anion; in the presence of protons, the free radical produced on 1e reduction is thought to be oxidized to an N‐dihydroxyphenyl pyridinium ion. The latter is also a likely product from the oxidation of hydroquinone. The quinone‐hydroquinone system itself is irreversible, e.g., quinhydrone behaves as a mixture of quinone and hydroquinone, the potentials of whose waves are 0.5v apart.
Hydroquinone
Pyrolytic carbon
Quinhydrone electrode
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Abstract Polypyridylruthenium(II) complexes with redox‐active, structurally isomeric catechol or hydroquinone units were newly prepared to examine the structure‐stability relationships in the redox‐active sites. A marked difference based on the structure of the active site was confirmed with respect to the formation of the oxidized forms, i. e. the quinone complexes: only the para ‐isomer was completely converted into the corresponding quinone form. The conversion of Ru II to Ru III was not observed. Redox‐mediated interconversion between the hydroquinone and para ‐quinone units is also reported.
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Catechol
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