Hydroquinone anion

The redox states of the flavin cofactor in a purified flavoenzyme can be conveniently studied by optical spectroscopy (see also Elavoprotein Protocols article). Oxidized (yellow) flavin has characteristic absorption maxima around 375 and 450 nm (Fig. lb and Ic). The anionic (red) and neutral (blue) semiquinone show typical absorption maxima around 370 nm and 580 nm, respectively (Fig. lb and Ic). During two-electron reduction to the (anionic) hydroquinone state, the flavin turns pale, and the absorption at 450 nm almost completely disappears (Fig. lb and Ic). The optical properties of the flavin can be influenced through the binding of ligands (substrates, coenzymes, inhibitors) or the interaction with certain amino acid residues. In many cases, these interactions result in so-called charge-transfer complexes that give the protein a peculiar color. 

Figure 1 (a) Redox states of the flavin cofactor. Flavoenzymes generally stabilize the anionic hydroquinone state (p/Ca free reduced flavin = 6.7). 

The two-electron reduced, or hydroquinone, isoalloxazine is pale yellow. It is an electron-rich heterocycle and, when planar, is antiaromatic according to Hiickel s rule. The ring system of the hydroquinone in some small-molecule structures and some protein structures is bent by as much as 30° along the N5—NIO axis, presumably to relieve the antiaromaticity. However, the majority of hydroquinones bound to proteins do not deviate from planarity much more than oxidized isoalloxazines. This is likely to be influenced by protonation of Nl, whose pA a is 6.7 in aqueous solution. Quantum calculations show that neutral hydroquinone adopts butterfly conformation but the anion is planar. A survey of crystal structures agrees with this correlation — anionic hydroquinones are generally nearly planar, while the few instances of the butterfly conformation belonged to neutral hydroquinones. Hydroquinones react as single-electron donors, as hydride donors, or as nucleophiles at N5 or C4a. 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

The strong Bronstedt acid nature of some hexacoordinated phosphorus derivatives, [7",H ] (Et20)4 in particular, was recently used within the context of an industrial application [36]. The conjugated acid of tris(oxalato)phosphate anion 7 was found to effectively catalyze the ring-forming reaction of trimethyl-hydroquinone 63 with isophytol 64 to give (all rac)-a-tocopherol 65 (ethylene-carbonate/heptane 1 1,100 °C, 90%, Scheme 19). This process is particularly... 

The global fitting studies revealed that the hydroquinone species shown in Scheme 7.5 affords the quinone methide at rates of 1-2 x 10 3 min-1 that are independent of both pH (from 7 to 9.5) and the concentration of buffers used to hold pH. We interpreted this observation as protonation of the C-5 center followed by the slow loss on the nitrogen-leaving group. The anionic C-5 center of the electron-rich hydroquinone ring would be very basic resulting in complete protonation near neutrality. 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

Structural characterisation of 58 and 59 have demonstrated that the PF, anion is located in the cleft between the two dialkylammonium cations forming hydrogen-bonds with the benzylic hydrogen atoms of one of the cations and with one of the hydrogen atoms of a hydroquinone ring. In contrast, a polymeric as-... 

The effect of acetate, citrate, and thiourea on Pb underpotential deposition on Ag(lll) has been studied. The effects of the anionic character of the additives were discussed in relation to changes in voltammograms. The two-dimensional phase transformation was also discussed. The influence of various additives such as dimethylfluoride (DMF) and pyridine on cyclic voltammograms of Cu underpotential deposition on Pt(lll) was observed. Cu underpotential deposition on Pt(lll) was studied in the presence of crystal violet, coumarin, and hydroquinone. ... 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

In order to model the oxygenation of vitamin K in its hydroquinone form, a naph-thohydroquinone derivative with a 1-hydroxy group and 4-ethyl ether was prepared and its alkoxide subjected to oxidation with molecular oxygen. Products consistent with two possible mechanisms were isolated, the epoxy-quinone which must derive from a peroxy anion intermediate at the 4-position, and a 2-hydroxy product which... 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

The absorption spectral properties of the neutral and anionic forms are quite different as shown in Fig. 1. Due to the rapid dismutation of flavin radicals to form an equilibrium mixture with the hydroquinone and oxidized forms of the flavin, special procedures must be employed to measure the spectral properties of free flavin radicals. Nearly quantitative amounts of anion radical can be formed in aprotic solvents under basic conditions Alkylation of the N(5) position of the flavin hydroquinone followed by oxidation results in nearly quantitative formation of the... 

The above considerations provide a rationale for the redox properties of flavodoxin which function between the flavin hydroquinone and neutral semiquinone redox forms. Further studies are required to determine whether similar properties exist in flavoproteins in which both redox couples (PF/PFl- and PFI/PFIH2) are operative and in situations where the anionic semiquinone rather than the neutral form is functional. 

Substances undergoing redox reactions (such as quinone-hydroquinone, sulphide-disulphide, metal complexes, redox couples) may serve as electron carriers and allow the coupling of oxidation-reduction processes across membranes (see, for instance, [6.44-6.46]) to cation or anion transport. 

The solid state structure of the pyridinium-chloride pseudorotaxane Figure 8) reveals the interpenetrative nature of the components and provides evidence for anion complexation by the macrocycle s isophthalamide motif, n-n donor-acceptor interactions between the electron rich hydroquinone units of the macrocycle and... 

Specifically adsorbed cations and anions may lower reductive dissolution rates by blocking oxide surface sites or by effecting release of Mn(II) into solution. Stone and Morgan (1984a) found that PO4- considerably inhibited the reductive dissolution of Mn(III/IV) oxides by hydroquinone. For example, addition of 10-2 M PO4- at pH 7.68 resulted in the dissolution rate being only 25% of the rate in the absence of PO4-. The dissolution rate was affected more by PO than by Ca2+. 

HjCat represents catechol or hydroquinone, Q quinone, SQ semiquinone anion, HCat- catechol monoanion, and Cat2 catechol dianion DTBQ represents 3,5-di-rerr-butyl-o-quinone, o-Q o-benzoquinone, p-Q p-benzoquinone, TCQ tetrachloro-o-benzoquinone, and TFQ tetrafluoro-o-benzoquinone. 

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