3225-29-4

Upstream product

• 110-86-1 pyridine 
• 56-23-5 tetrachloromethane 
• 63517-17-9 Oxazine-118 
• 50-99-7 glucose 
• 581-30-6 3H-Phenothiazin-3-one 
• 64-17-5 ethanol 
• 3625-57-8 nile blue A 
• 28286-71-7 3,3'-dihydroxy-1H,1'H-[2,2']biindolyl-5,5'-disulfonic acid 
• 120-80-9 benzene-1,2-diol 
• 605-18-5 3,3'-dioxo-1,3,1',3'-tetrahydro-[2,2']biindolylidene-5-sulfonic acid 
• 483-20-5 indigo carmine 
• 25180-62-5 3,3'-dioxo-1,3,1',3'-tetrahydro-[2,2']biindolylidene-5,7,5',7'-tetrasulfonic acid 
• 68141-98-0 N-(4-Bromophenyl)-1,4-benzoquinone monoimine 
• 2406-04-4 4-(phenylimino)cyclohexa-2,5-dienone 

Downstream Products

• 123-31-9 hydroquinone 
• 106-51-4 p-benzoquinone 
• 106-51-4 1,4-benzoquinone ?-dianion 
• 75877-78-0 (4-chlorophenyl)diazenyl radical 

Relevant academic research and scientific papers

Electron-transfer studies of a peroxide dianion

A peroxide dianion (O22-) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)∪mBDCA-5t-H 6]2-, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)∪mBDCA-5t-H6] 2- with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O22- occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O 22- within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O 2)∪mBDCA-5t-H6]2- as a chemical reagent.

Structure and function of quinones in biological solar energy transduction: A high-frequency d-band EPR spectroscopy study of model benzoquinones

Quinones are utilized as charge-transfer cofactors in a wide variety of reactions that are crucial for photosynthesis and respiration. In photosynthetic protein complexes, both Type I and Type II, including oxygenic and anoxygenic reaction centers contain quinone cofactors that are known to participate in electron- and proton-transfer processes. Type II reaction centers, purple bacterial reaction centers, and photosystem II utilize benzoquinone molecules, ubiquinone, and plastoquinone, respectively, to facilitate proton-coupled electron transfer reactions. Here, we report a systematic study of the principal components of the g-tensor of an extensive library of model benzosemiquinone anion radicals in both protic (2-isopropanol) and aprotic (dimethyl sulfoxide) solvents using high-frequency EPR spectroscopy. A detailed comparison of the experimental g-values of the benzosemiquinone models at D-band EPR frequency allows for the discrimination of substituent effects and solvent hydrogen bonds on the principal components of the g-tensor. Further, we compare the primary plastosemiquinone, QA-, of photosystem II with the substituent and solvent hydrogen bond effects of benzosemiquinone models in vitro. This study significantly extends the experimental basis for elucidating the role of both molecular structure and interactions with environment on the functional tuning of quinone cofactors in biological solar energy transduction.

Investigation of the oxidation of hydroquinone at the liquid/liquid interface

The oxidation of hydroquinone (QH2) was investigated for the first time at liquid/liquid (L/L) interface by scanning electrochemical microscopy (SECM). In this study, electron transfer (ET) from QH2 in aqueous to ferrocene (Fc) in nitrobenzene (NB) was probed. The apparent heterogeneous rate constants for ET reactions were obtained by fitting the experimental approach curves to the theoretical values. The results showed that the rate constants for oxidation reaction of QH2 were sensitive to the changes of the driving force, which increased as the driving force increased. In addition, factors that would affect ET of QH2 were studied. Experimental results indicated ion situation around QH2 molecule could change the magnitude of the rate constants because the capability of oxidation of QH2 would be affected by them.

Quenching of triplet-excited flavins by flavonoids. Structural assessment of antioxidative activity

(Figure Presented) The mechanism of flavin-mediated photooxidation of flavonoids was investigated for aqueous solutions. Interaction of triplet-excited flavin mononucleotide with phenols, as determined by laser flash photolysis, occurred at nearly diffusion-controlled rates (k～1.6x10 9 Lmol-1 s-1 for phenol at pH 7, 293 K), but protection of the phenolic function by methylation inhibited reaction. Still, electron transfer was proposed as the dominating mechanism due to the lack of primary kinetic hydrogen/ deuterium isotope effect and the low activation enthalpy (-1) for photooxidation. Activation entropy worked compensating in a series of phenolic derivatives, supporting a common oxidation mechanism. Anortho-hydroxymethoxy pattern was equally reactive (k～2.3x109Lmol-1 s-1 for guaiacol at pH 7) as compounds with ortho-dihydroxy substitution (k～2.4x109 L mol-1 s-1 for catechol at pH 7), which are generally referred to as good antioxidants. This refutes the common belief that stabilization of incipient phenoxyl radicals through intramolecular hydrogen bonding is the driving force behind the reducing activity of catechol-like compounds. Instead, such bonding improves ionization characteristics of the substrates, hence the differences in reactivity with (photo)oxidation of isolated phenols. Despite the similar reactivity, radicals from ortho-dihydroxy compounds are detected in high steady-state concentrations by electron paramagnetic resonance (EPR) spectroscopy, while those resulting from oxidation of ortho-hydroxymethoxy (or isolated phenolic) patterns were too reactive to be observed. The ability to deprotonate and form the corresponding radical anions at neutral pH was proposed as the decisive factor for stabilization and, consequently, for antioxidative action. Thus, substituting other ionizable functions for the ortho- or para-hydroxyl in phenolic compounds resulted in stable radical anion formation, as demonstrated for para-hydroxybenzoic acid, in contrast to its methyl ester. 2009 American Chemical Society.

EPR spectroscopic investigation of radical-induced degradation of partially fluorinated aromatic model compounds for fuel cell membranes

EPR spectroscopic investigations of reactions between monomeric model compounds representing typical structural moieties of poly(aryl) ionomers and photochemically generated hydroxyl radicals are reported. Deoxygenated solutions of the model compounds (in

Oxidative and photochemical stability of ionomers for fuel-cell membranes

To predict hydroxyl-radical-initiated degradation of new proton-conducting polymer membranes based on sulfonated polyetherketones (PEK) and polysulfones (PSU), three nonfluorinated aromatics are chosen as model compounds for EPR experiments, aiming at the identification of products of HO.-radical reactions with these monomers. Photolysis of H2O2 was chosen as the source of HO. radicals. To distinguish HO .-radical attack from direct photolysis of the monomers, experiments were carried out in the presence and absence of H2O2. A detailed investigation of the pH dependence was performed for 4,4′-sulfonylbis[phenol] (SBP), bisphenol A (-4,4′- isopropylidenebis[phenol]; BPA), and [1,1′-biphenyl]-4,4′-diol (BPD). At pH > pKA of HO. and H2O 2, reactions between the model compounds and O2 . or 1O2 are the most probable ways to the phenoxy and 'semiquinone' radicals observed in this pH range in our EPR spectra. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings. Investigations at low pH are particularly relevant for understanding degradation in polymer-electrolyte fuel cells (PEFCs). However, the chemistry depends strongly on pH, a fact that is highly significant in view of possible pH inhomogeneities in fuel cells at high currents. It is shown that also direct photolysis of the monomers leads to 'semiquinone'-type radicals. For SBP and BPA, this involves cleavage of a C-C bond.

Involvement of semiquinone radicals in the in vitro cytotoxicity of cigarette mainstream smoke

Free radicals in cigarette smoke have attracted a great deal of attention because they are hypothesized to be responsible in part for several of the pathologies related to smoking. Hydroquinone, catechol, and their methyl-substituted derivatives are abundant in the particulate phase of cigarette smoke, and they are known precursors of semiquinone radicals. In this study, the in vitro cytotoxicity of these dihydroxybenzenes was determined using the neutral red uptake (NRU) assay, and their radical-forming capacity was determined by electron paramagnetic resonance (EPR). All of the dihydroxybenzenes studied were found to generate appreciable amounts of semiquinone radicals when dissolved in the cell culture medium employed in the NRU assay. Hydroquinone exhibited by far the highest capacity to form semiquinone radicals at physiological pH, even though it is not the most cytotoxic dihydroxybenzene. Methyl-substituted dihydroxybenzenes were found to be more cytotoxic than either hydroquinone or catechol. The formation of semiquinone radicals via auto-oxidation of the dihydroxybenzenes was found to be dependent on the reduction potential of the corresponding quinone/semiquinone radical redox couple. The capacity to generate semiquinone radicals was found to be insufficient to explain the variance in the cytotoxicity among the dihydroxybenzenes in our study; consequently, other mechanisms of toxicity must also be involved. The observed interactions between 2,6-dimethylhydroquinone and hydroquinone in the cytotoxicity assay and EPR analysis suggest that care needs to be taken when the bioactivity of cigarette smoke constituents is evaluated, i.e., the effect of the cigarette smoke complex matrix on the activity of the single constituent studied must be taken into consideration.

Kinetic isotope effect for hydrogen abstraction by .OH radicals from normal and carbon-deuterated ethyl alcohol and methylamine in aqueous solutions

Selective H/D kinetic isotope effects (KIEs) have been determined for the various routes by which .OH radicals react with ethanol (CH3CH2OH and CD3CD2OH) and methylamine (CH3NH2 and CD3NH2) in H2 Osolutions. The KIEs have been evaluated from overall rate constants and the yields in which the individual primary radicals were generated in the .OH reaction with these compounds. The analytical method applied for the yield determinations was redox titration with suitable scavengers, namely, methyl viologen and Fe(CN)63- for the reducing radicals (CH3C.HOH/CD3C.DOH and .CH2NH2/.CD2NH2 ) and I- and hydroquinone for the oxidizing radicals (CH3CH2O./CD3CD2O. and .NHCH3/.NHCD3). The numerical results obtained also include, besides yields relative to total available .OH, absolute rate constants for most of these scavenging reactions. For the alcohol, the major process (almost 90%) is H/D abstraction from the Ca bond with KIE = 1.96. For methylamine, abstraction from C?± H/D occurs with only 37% (H) and 26% (D) but at a similar KIE = 1.86. The remainder, denoting the major process in this case, accounts for the formation of aminyl radicals. The secondary KIEs for N-H cleavage, and O-H cleavage in the case of the alcohol, are close to unity, reflecting the expected negligible influence of C-H/D substitution in the attached alkyl groups. Abstraction of ?2-C-attached H/D in ethanol shows, also in qualitative agreement with expectation, a larger KIE = 3.4 than that for ?±-C H/D. An interestingly high KIE a?? 50 was obtained for the 1,2 hydrogen shift that converts the CH3CH2O. and CD3CD2O. oxyl radicals into the corresponding ?±-hydroxyethyl radicals CH3C.HOH and CD3C.DOH. All of the results are discussed in light of existing literature data on relevant H/D isotope effects, the influence of solvent relative to gas phase, the selectivity of the .OH attack, and other mechanistic considerations. Specifically, the mechanism by which the .OH reacts with the amine seems likely to involve a transient, caged aminium/hydroxide ion pair.

Intramolecular hydrogen bonding in hydroxylated semiquinones inhibits semiquinone-Mg2+ complex formation

Complex formation between the semiquinones of 5,8-dihydroxynaphtho-1,4-quinone, NZQ-, 1,4-dihydroxy-anthracene-9,10-dione, QNZ-, benzo-1,4-quinone, BQ-, and phenanthraquinone, PHQ-, and Mg2+ was studied utilizing EPR spectroscopy. Weighted average EPR spectra between those corresponding to the uncomplexed and complexed semiquinones were observed for NZQ-, QNZ- and BQ- while these species were observed simultaneously for PHQ-. The semiquinones NZQ-, QNZ- and BQ- behave as weak Mg2+ chelators while PHQ- chelates this cation much more strongly (binding constant = (1.1 ± 0.5) × 103 dm3 mol-1). The weak binding of Mg2+ by NZQ- and QNZ- is in contrast with the large complex formation constants between the parent quinones NZQ and QNZ and different metal cations. This apparent paradox is explained by the strong intramolecular hydrogen bonding existing in NZQ- and QNZ-.

Reaction of OH radicals with benzoquinone in aqueous solutions. A pulse radiolysis study

Hydroxyl radicals have been generated by pulse radiolysis in N2O-saturated aqueous solutions. Their addition to 1,4-benzoquinone BQ (k3 = 6.6 × 109 dm3 mol-1 s-1 by competition with thiocyanate) in neutral solution leads to a build-up of optical absorption that shows different rates at wavelengths at around 330 and at >400 nm. At 330 nm the rate of build-up is proportional to the benzoquinone concentration, and its rate constant agrees with the value (k3) obtained by competition. At the longer wavelengths, it becomes independent of benzoquinone concentration beyond 4 × 10-4 mol dm-3 (k6 = 6.9 × 105 s-1). Kinetic analysis in the ns time-range show that the primarily-generated benzoquinone-OH-adduct radical 1 undergoes rapid (k4 = 2.5 × 106 s-1) keto-enol tautomerization yielding the 2,4-dihydroxyphenoxyl radical 2. To gain support for this proposed reaction, radical 2 [pKa(2) ≈ 4.9 ± 0.2] has been independently generated by one-electron oxidation of 1,2,4-trihydroxybenzene using ·OH in acidic or N3· in neutral and basic solution. Its absorption characteristics compare favourably with those observed in the benzoquinone system. On the basis of spectrophotometric and conductometric data it is proposed that in neutral solution the radical anion 2a is rapidly oxidized by benzoquinone itself (k15 ≥ 2 × 109 dm3 mol-1) into the end product 2-hydroxy-1,4-benzoquinone anion 4a [pKa(4) = 4.1 ± 0.1; λmax(4) = 380 nm; λmax(4a) = 482 nm] and the semibenzoquinone radical anion 3a. The latter decays bimolecularly into benzoquinone and hydroquinone (2k16 = 3.1 × 108 dm3 mol-1 s-1). In acidic solution the rate of oxidation of 2 by benzoquinone is considerably slower (k13 ≤ 2.4 × 107 dm3 mol-1 s-1). The assignment of the final product to 2-hydroxy-1,4-benzoquinone 4 has been confirmed by the absorption characteristics and pKa value of the authentic material obtained by the two-electron electrochemical oxidation of 1,2,4-trihydroxybenzene.