Substituted Phenoxyl Radical Research Articles

Characterization of Group 10-Metal-p-Substituted Phenoxyl Radical Complexes with Schiff Base Ligands.

Methylthiophenoxyl radical plays an important role as the active form of galactose oxidase (GO), which catalyzes oxidation of a primary alcohol to the corresponding aldehyde. Although many metal(II)-phenoxyl radical species have been reported, only a few studies have been reported on the properties of methylthiophenoxyl radical-metal complexes. We have prepared the group 10 metal (Ni, Pd and Pt) complexes of a salen-type ligand with a methylthio group at para-position of the two phenolate moieties and characterized them by X-ray crystal structure analyses and various spectroscopic methods in order to understand the role of the methylthio moiety in phenoxyl radical metal complexes. The corresponding p-methoxy substituted derivatives have been also characterized for comparison. All the one-electron oxidized group 10 metal methylthiophenolate complexes have a relatively localized radical site on one of the two phenolate moieties in comparison to the one-electron oxidized complexes of p-methoxy derivatives and exhibit different properties dependent on the central metal ions.

Electronic structure of para aminophenoxyl radical in water

The electronic structure of aqueous p-aminophenoxyl radical (H2NPhO•) has been examined by time-resolved resonance Raman spectroscopy and ab initio and density functional theories. The effects of hydrogen bonding and solvent reaction field on polarity of the radical have been visualized in terms of simple models. Calculations predict the dipole moment of the radical in its ground electronic state (2B1) to increase by 8(±2) D and the difference between the CN and CO bond lengths to decrease by ∼0.05 Å from gas phase to aqueous solution. This profound hydration effect converts the structure and chemical properties of H2NPhO• from a substituted phenoxyl radical in the gas phase to a semiquinone-like radical in water. The observation of vibrational modes enhanced in Raman by a non-Franck–Condon vibronic coupling mechanism has led to the identification of two very weakly absorbing electronic states of A22 symmetry in the 340–390 nm region, which borrow transition moment from close by strongly allowed electronic states of B12 symmetry at lower (∼440 nm) and higher (∼320 nm) energies. One of these transitions is parity forbidden (2B2g↔2B1g) in p-benzosemiquinone radical anion (PhO2−•) and p-phenylenediamine radical cation (Ph(NH2)2+•) and this is the first experimental evidence on energy location (3.44 eV) of this transition in an isoelectronic radical. The experiment and theory are combined to estimate the CO and CN bond lengths in H2NPhO• as ∼1.263 and ∼1.34 Å, respectively, in liquid water and ∼1.245 and ∼1.37 Å in the gas phase.

Ab initio g-tensor calculations of the thioether substituted tyrosyl radical in galactose oxidase

The tyrosyl radical in galactose oxidase is covalently cross-linked to a neighboring cysteine residue through a thioether bond. The role of this sulfur cross-link has been discussed ever since the crystal structure of the enzyme was solved. In the present work, the ab initio multiconfigurational linear response method is applied to calculate the g-tensor of unsubstituted and thioether substituted phenoxyl radicals. In contrast to some previous interpretations, but in agreement with recent EPR measurements, we find that the sulfur substitution induces only minor shifts in the g-tensor components. The spin distribution retains the odd-alternant pattern of the unsubstituted radical and only a small amount of spin is localized to the sulfur center.

Synthesis and Antioxidant Activity of 4H-1,3-Benzodioxin-6-ol Derivatives: New Vitamin E Analogs

Abstract 4H-1,3-Benzodioxin-6-ol (HBD) derivatives 1—5, new tocopherol (Vitamin E) analogs, have been synthesized by condensation of acetaldehyde with the corresponding hydroquinone. For each HBD derivative except for 4, two diastereomeric isomers were produced. These stereoisomers were separated by column chromatography and the configuration of each isomer was identified by two dimensional NMR techniques and ESR measurements of the HBD radicals. The second-order rate constants, ks, for the reaction of 9 kinds of HBD derivatives with substituted phenoxyl radical (PhO·) in ethanol have been measured with a stopped-flow spectrophotometer. These HBD derivatives reacted at rates that were only about 5—20% of those observed with the corresponding tocopherols. The logs of the second-order rate constants, ks, obtained for HBD derivatives were found to correlate with their peak oxidation potentials, Ep. Electron spin resonance measurements were done for HBD radicals in toluene, and the proton hyperfine coupling constants, aiH, and giso-values were determined. The log of the rate constants, ks, was also found to correlate with the sum of the hyperfine coupling constants (a5H + a7H) at 5- and 7-positions of both HBD radicals and tocopheryloxyl radicals, that is, the unpaired π-spin densities (ρ5 + ρ7) at the two ortho carbon atoms.

The kinetics and mechanism of oxidation of hydroquinone and chlorohydroquinone in the presence of nitrous acid in aqueous acid solution

Hydroquinone is converted quantitatively into benzoquinone by nitrous acid in air-saturated aqueous acid solution, by a process which is kinetically first order, whether or not the nitrous acid is present in excess. Chlorohydroquinone behaves similarly and substituent effects are discussed. The inorganic and organic steps in the mechanism put forward are shown by numerical integrations to account for all the observations. MOPAC calculations suggest that 4-hydroxy-6-nitrosocyclohexa-2,4-dienone is the first-formed intermediate. Homolysis of this to give the 4-hydroxyphenoxyl radical and nitric oxide is rate determining in the organic sequence and takes place by uncatalysed and acid catalysed pathways. Comparison with previous work on 4-methoxyphenol, where the later step of combination of the 4-methoxyphenoxyl radical with nitrogen dioxide is rate determining, permits an estimate of the relative rate-constants for reaction of the substituted phenoxyl radical with NO and NO2.

Kinetic study of free-radical-scavenging action of biological hydroquinones (reduced forms of ubiquinone, vitamin K and tocopherol quinone) in solution

Kinetic study of free-radical-scavenging (FRS) action of eight kinds of biologically important hydroquinones (HQ's) (ubiquinol-10 (UQ 10H 2 1), ubiquinol-0 (UQ 0H 2 2), vitamin K 1 HQ (VK 1H 2 3, vitamin K 3 HQ (VK 3H 2 4), α-, β-, γ-tocopherol HQ's (α-, β-, γ-TQH 2 5, 6, 7), and 2,3,5-trimethyl-1,4-HQ (TMQH 2 8)) has been performed. The second-order rate constants, k 3, for the reaction of HQ's 1 ∼ 8 with substituted phenoxyl radical (PhO·) in ethanol, diethyl ether, benzene, and n-hexane have been measured with a stopped-flow spectrophotometer, as a model reaction of HQ's with unstable free radicals (LOO·, LO·, and HO·) in biological systems. The rate constant of UQ 10H 2 1 is similar to that of α-tocopherol in ethanol. The HQ's 3–8 showed higher reactivity than α-tocopherol in ethanol. Especially, the rate constants of VK 1H 2 3 and VK 3H 2 4 were founf to be 31- and 21-fold larger than that of α-tocopherol, respectively, which has the highest reactivity among natural tocopherols. The rate constant of these HQ's increased by decreasing the polarity of solvents. The approximate order of magnitude of k 3 value was (i) VK 1H 2 and VK 3 PrmH 2 > (ii) α-, β- and γ-TQH 2's and TMQH 2 > (iii) α-tocopherol > (iv) UQ 10H 2 and UQ 0H 2 in solution. The result suggests that these biological HQ's also scavenge the active oxygen free radicals and prevent lipid peroxidation in various tissues and membranes. On the other hand, the reaction between substituted phenoxyl and biological quinones has not been observed.

Mechanism of antioxidant reaction of vitamin E: charge transfer and tunneling effect in proton-transfer reaction

In order to shed light on the mechanism of proton-transfer reactions, a kinetic and ab initio study of the antioxidant action (intermolecular proton transfer) of vitamin E derivatives has been carried out. The second-order rate constants (k s 's) for the reaction of tocopherols (TocH's) with variously substituted phenoxyl radicals (PhO.'s) in ethanol were measured with a stopped-flow spectrophotometer. The half-wave reduction potentials (E 1/2 's) of PhO.'s were obtained by using a cyclic voltammetry technique

Synthesis and Kinetic Study of Antioxidant and Prooxidant Actions of Vitamin E Derivatives

About 50 kinds of new tocopherol derivatives have been synthesized, and the rates of scavenging of substituted phenoxyl radical (PhO ·), k2, and quenching of singlet oxygen (1O2), kQ, by these tocopherols have been measured spectrophotometrically in ethanol. Both the log of the rate constants, k2 and kQ, were found to correlate with their peak oxidation potential, Ep. The quenching rates, kQ, were found to be related linearly to the rates k1 and k2 of scavenging of peroxyl (LOO ·) (reported by Burton et al.) and phenoxyl (PhO ·) radicals by these tocopherols, respectively. Vitamin K1-chromanol and K1-chromenol were found to be 6.9 and 4.5 times more active than α-tocopherol, respectively, which has the highest antioxidant activity among natural tocopherols. Further, the second-order rate constants, k-1, k3, k4, and k6 for the reaction of vitamin E radical with i) lipid hydroperoxides (LOOH), ii) vitamin C, iii) ubiquinol, and iv) unsaturated fatty acid esters (LH), respectively, in solution have been measured. By comparing these rate constants, the mechanism of antioxidant and prooxidant actions of tocopherols in biomembranes and edible oils has been discussed.

Stopped-flow investigation of antioxidant activity of estrogens in solution

A kinetic study of the reaction between estrogens (female hormone) and substituted phenoxyl radical has been performed, as a model for the reactions of estrogens with lipid peroxyl radical in biological systems. The rates of reaction of estrogens (estrone 1, estradiol 2, 2-methoxyesterone 3, 3-methoxyestrone 4, and 2-hydroxyestrone 5) with substituted phenoxyl radical in benzene have been determined spectrophotometrically, using stopped-flow technique. The second-order rate constants, k 2, obtained are 84 M −1 · s −1 for 3, < 10 −4 M −1 for 4, and 2.6 · 10 5 M −1 · s −1 for 5 at 25.0°C. 2-Hydroxyestrone 5 was found to be 2.9-times more active than α-tocopherol, which has the highest antioxidant activity among natural tocopherols. The order of magnitude of k 2 value (1<2<3<α-Toc<5) is in agreeement with that of in vitro tests of their antioxidant activities, as measured by the inhibition of lipid peroxidation. Further, similar measurements have been performed for the reaction between the above estrogens 1–5 and tocopheroxyl 6 in benzene solution. It was found that the estrogens having an OH group at the aromatic ring have an ability to regerate the tocopheroxyl 6 to tocopherol. Especially, the 2-hydroxyestrone 5 showed about three orders of magnitude higher reactivity than ascorbic acid.

Kinetic study of the reaction of vitamin C derivatives with tocopheroxyl (vitamin E radical) and substituted phenoxyl radicals in solution

The rates of reaction between ten kinds of vitamin C (ascorbic acid, AsA 1) derivatives and vitamin E radical (tocopheroxyl) in solution have been determined spectrophotometrically, using stopped-flow technique. While fatty acid esters 2 and 3 of AsA 1 at 6-position reacted at rates about 20% faster than AsA 1 , fatty acid esters 4, 5, 6, 7 and 8 at both 5- and 6-positions reacted at rates that were only about 60–80% of that observed with AsA 1, which may be the result of steric hindrance. The octadecyl ether 10(CV-3611) of AsA 1 at 2-position is 0.40-times as reactive as the AsA 1, whereas the ascorbic acid ester 9 substituted at both 2- and 6-positions is only 0.01-times as reactive as the AsA 1. From the results, the structure-activity relationship in the regeneration reaction of tocopheroxyl by vitamin C has been discussed. Further, similar measurements have been performed for the reaction between the above vitamin C derivatives 1– 10 and substituted phenoxyl radical, as a model of active oxygen radicals (HO; HOO . and ROO .) in biological systems. The result suggests that, while the absolute rates ( k 3) are considerably different from each other, the relative reactivities of vitamin C derivatives in homogeneous solution do not depend on the kinds of oxyradicals (tocopheroxyl and substituted phenoxyl) used. It was observed that α-tocopherol, which is well known as a most popular antioxidant, is about 250-times as reactive as AsA 1. The value of the relative rate constant, ( k 3(α-Toc)/ k 3(AsA 1) = 250), is very different from that ( k 1(α-Toc)/ k 1(AsA 1) = 6.8) reported for the scavenging of peroxyl radical by Niki et al. (J. Biol. Chem. (1984) 259, 4177–4182).

ChemInform Abstract: A Kinetic Study of Reactions of Tocopherols with a Substituted Phenoxyl Radical.

ChemInformVolume 19, Issue 26 Preparative Organic Chemistry ChemInform Abstract: A Kinetic Study of Reactions of Tocopherols with a Substituted Phenoxyl Radical. K. MUKAI, K. MUKAI Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. FUKUDA, K. FUKUDA Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. TAJIMA, K. TAJIMA Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. ISHIZU, K. ISHIZU Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this author K. MUKAI, K. MUKAI Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. FUKUDA, K. FUKUDA Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. TAJIMA, K. TAJIMA Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this authorK. ISHIZU, K. ISHIZU Dep. Chem., Fac. Sci., Ehime Univ., Matsuyama, Ehime 790, JapanSearch for more papers by this author First published: June 28, 1988 https://doi.org/10.1002/chin.198826088AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume19, Issue26June 28, 1988 RelatedInformation

A kinetic study of reactions of tocopherols with a substituted phenoxyl radical

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA kinetic study of reactions of tocopherols with a substituted phenoxyl radicalKazuo Mukai, Kazuyuki Fukuda, Kunihiko Tajima, and Kazuhiko IshizuCite this: J. Org. Chem. 1988, 53, 2, 430–432Publication Date (Print):January 1, 1988Publication History Published online1 May 2002Published inissue 1 January 1988https://doi.org/10.1021/jo00237a040RIGHTS & PERMISSIONSArticle Views125Altmetric-Citations51LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (408 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: NEW GAS PHASE REACTIONS OF SUBSTITUTED BENZYL, PHENYLAMINYL, AND PHENOXYL RADICALS. REARRANGEMENTS TO FUSED 5‐ AND 6‐MEMBERED HETEROCYCLIC SYSTEMS

Chemischer InformationsdienstVolume 16, Issue 36 Preparative Organic Chemistry ChemInform Abstract: NEW GAS PHASE REACTIONS OF SUBSTITUTED BENZYL, PHENYLAMINYL, AND PHENOXYL RADICALS. REARRANGEMENTS TO FUSED 5- AND 6-MEMBERED HETEROCYCLIC SYSTEMS J. I. G. CADOGAN, J. I. G. CADOGANSearch for more papers by this authorC. L. HICKSON, C. L. HICKSONSearch for more papers by this authorH. S. HUTCHISON, H. S. HUTCHISONSearch for more papers by this authorH. MCNAB, H. MCNABSearch for more papers by this author J. I. G. CADOGAN, J. I. G. CADOGANSearch for more papers by this authorC. L. HICKSON, C. L. HICKSONSearch for more papers by this authorH. S. HUTCHISON, H. S. HUTCHISONSearch for more papers by this authorH. MCNAB, H. MCNABSearch for more papers by this author First published: September 10, 1985 https://doi.org/10.1002/chin.198536090Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume16, Issue36September 10, 1985 RelatedInformation

ChemInform Abstract: RESONANCE RAMAN STUDIES OF PULSE RADIOLYTICALLY PRODUCED P‐AMINOPHENOXYL RADICAL

The p-aminophenoxyl radical produced in the pulse radiolytic oxidation of p-aminophenol in aqueous solution has been examined by time-resolved resonance Raman methods. Eight Raman bands of fundamental vibrations were observed at microsecond times and at radical concentrations of -- 10/sup -5/M by excitation in the moderately strong absorption band of this radical at 444 nm (E = 6800 M/sup -1/ cm/sup -1/). This radical decays rapidly by second-order processes (2k = 2 x 10/sup 9/ M/sup -1/ s/sup -1/) so that at times longer than milliseconds the residual signals are very weak. The most intense Raman band of this radical, observed at 1636 cm/sup -1/, is ascribed to the Wilson 8a ring stretching mode and is similar in frequency to this mode in p-benzosemiquinone radical anion (1620 cm/sup -1/). The mixed CO/CN stretching frequency (1434 cm/sup -1/) is also very similar to that of the analogous mode in p-benzosemiquinone. This radical exhibits no strong Raman bands in the region of 1510-1520 cm/sup -1/ as do most other substituted phenoxyl radicals. Deuterium substitution of the NH/sub 2/ group shows that a band at 1673 cm/sup -1/ in the protonated radical is due to the symmetrical NH/sub 2/ bending vibration. Interestingly, thismore » deuterium substitution results in small increases in the frequencies of the CO/CN and ring stretching vibrations. The Raman data show very clearly, in agreement with conclusions from ESR data, that the structure of p-aminophenoxyl radical is more similar to that of a semiquinone than it is that of a phenoxyl radical. The high second-order rate constant observed for decay, however, indicates that the NH/sub 2/ group contributes significantly to modifying the redox properties of this radical.« less

ChemInform Abstract: ELECTRON TRANSFER RATES AND EQUILIBRIUMS BETWEEN SUBSTITUTED PHENOXIDE IONS AND PHENOXYL RADICALS

AbstractElektronentransfergeschwindigkeiten und Gleichgewichte von A) wurden bei pH 13.5, zum Teil auch pH 11, ermittelt.

Electron transfer rates and equilibriums between substituted phenoxide ions and phenoxyl radicals

The rate constants for electron transfer from a series of substituted isomeric dihydroxy- and diaminobenzenes to different substituted phenoxyl radicals were measured by observing the decay or buildup of one of the radicals invoved. In many cases the electron transfer reactions were reversible and the equilibrium constants could be calculated from the individual rate constants for attainment of equilibrium and from the concentrations of the species involved at equilibrium. From the equilibrium constants the one-electron redox potentials for 15 individual Q/sup -/./Q/sup 2 -/ pairs were determined, using the value for hydroquinone (23 mV at pH 13.5) as a reference. The potential for catechol (43 mV) is near that of hydroquinone; resorcinol is oxidized much less readily (300 mV), while phenol is even a weaker reductant (>500mV). Methyl, methoxy, and hydroxy substituents decrease the redox potentials while acetyl and carboxyl substituents increase these values. Ascorbate has a potential (15mV) similar to that of hydroquinone, while TMPD (82mV) and p-phenylenediamine (183mV) are less easily oxidized.

An electron spin resonance study of the Group IVB organometallic adducts of 2,6-di-t-butylbenzoquinone

2,6-Di-t-butylbenzoquinone can interact with group IVB organometallic compounds to give the stable substituted phenoxyl radicals detectable by e.s.r. The strong bonding between silicon and oxygen atoms together with the unusual stability of the resulting phenoxyl radicals show that 2,6-di-t-butylbenzoquinone is an excellent spin trap of organosilyl radicals. This quinone has also been used successfully to trap both diphenylphosphino and phenylthiyl radicals.

Temperature-dependent Hyperfine Splitting in a Sterically Hindered Radical

SUBSTITUTED phenoxyl radicals derived from 2,6-di-tertiary butyl phenols by lead dioxide oxidation or photolysis are quite stable and particularly suitable for examination by electron spin resonance1,2. The experimental results indicate that the spin distribution in the aromatic ring is not very sensitive to alkyl substituents and that a hyper conjugatively coupled methyl-type proton in the 4 position should have a hyperfine coupling constant of about 10 gauss. However, the 2,6-di-tertiary butyl 4-cyclohexyl phenoxyl radical has a coupling constant of about 4.5 gauss (at room temperature) for the single α-proton of the cyclohexyl ring3.