Hybrid Density Functional Study of the <i>p</i>-Benzosemiquinone Anion Radical:  The Influence of Hydrogen Bonding on Geometry and Hyperfine Couplings

O’Malley, Patrick J.

doi:10.1021/jp971238l

Cited by 81 publications

(90 citation statements)

References 17 publications

(25 reference statements)

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“…The calculated H-bond length R O …H in PCM solvent for models 8 and 10 are 1.74 and 1.85Å, respectively, and agree well with results reported for similar systems by O'Malley [35] (1.79Å for a p-benzoquinone system with four water molecules) and by Sinnecker et al [36] (1.74Å for the case of a p-benzoquinone system with up to 20 water molecules) (see Table 2). For model 8, almost linear H-bonds are found because the calculated angle OHO value is close to 180.0°, whereas, model 10 deviates by about 20°from a linear arrangement.…”

Section:  H-bonding Geometriessupporting

confidence: 88%

An EPR and DFT study on the primary radical formed in hydroxylation reactions of 2,6-dimethoxy-1,4-benzoquinone

et al. 2016

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The quinone compound 2,6-dimethoxy-1,4-benzoquinone is hydroxylated in alkaline aqueous solution with pH above 12. Electron paramagnetic resonance experiments showed that two transient radicals are formed in this reaction. The radical appearing first is assigned to a one electron reduced 2,6-dimethoxy-1,4-benzoquinone, receiving the electron from an intermediate anionic hydroxylated species. For this primary radical, all proton couplings were determined (quinoid ring protons: 1.453 G, methyl protons: 0.795 G). The density functional theory method was applied to obtain electronic and structural information of the primary radical and a solution structure is suggested. For approaching the experimental hyperfine couplings in theoretical models, it was necessary to consider effects of external polarisation arising from water molecules near one carbonyl group, and the orientation of methoxy groups towards the quinone ring. With this approach, the secondary radical formed in the hydroxylation reaction, and the transient radicals found for other biologically important quinones (including coenzymes Q) and their hydroxylated species may become accessible.

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Section:  H-bonding Geometriessupporting

confidence: 88%

An EPR and DFT study on the primary radical formed in hydroxylation reactions of 2,6-dimethoxy-1,4-benzoquinone

et al. 2016

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“…The protons of hydrogen bonds are characterized by almost purely anisotropic hyperfine couplings, with values for T between Ϫ2.6 and Ϫ2.9 MHz and with values for a between 0 and 0.3 MHz (20 -22). The experimentally observed values are supported by density functional theory calculations (23,24); and 2) when bulky substituents are present on the ring system, hydrogen bond formation is apparently forced out of the ring plane and can occur either above or below the plane.…”

Section: Discussionmentioning

confidence: 62%

Hydrogen Bonds Involved in Binding the Qi-site Semiquinone in the bc1 Complex, Identified through Deuterium Exchange Using Pulsed EPR

Dikanov

Samoilova

Kolling

et al. 2004

Journal of Biological Chemistry

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The quinone 1 processing sites of photosynthetic and respiratory electron transfer chains have spawned an extensive literature dealing with mechanism in the context of structure. This has been dominated by the two-electron gates of bacterial reaction centers (reviewed in Refs. 1 and 2), which until recently have provided the only high resolution structures. Discussion has centered on the features of structure that allow the sites to mediate between the one-electron chemistry of photochemical reactions (or of cytochrome or iron-sulfur center redox reactions) and the two-electron chemistry of quinones. Since reduction or oxidation of the quinone systems involves uptake or release of protons, the sites also have to include the features needed to deal with these processes. Over the last few years, structures at high resolution have become available for several other proteins with quinone processing sites. Among these, the bc 1 complex has provoked the most interest because of the dynamic features associated with the Q o -site, through which ubihydroquinone (quinol, QH 2 ) is oxidized (reviewed in Refs. 3-8). However, recent structures at high resolution have suggested that the second quinone processing site of the bc 1 complex, the Q i -site, through which ubiquinone (quinone, Q) is reduced through a two-electron gate, might also have interesting dynamic features, albeit on a smaller scale (9 -11). Of the three high resolution structures available in which a quinone is modeled at the site, all show marked differences in the pattern of ligation by protein side chains, in the orientation of the quinone, and in the involvement of bridging waters. Earlier studies of functional aspects show the same general two-electron gate behavior at the Q i -site as that at the Q B -site of bacterial reaction centers but with heme b H of the cytochrome (cyt) b subunit acting as an electron donor (12). In both systems, a relatively stable semiquinone (SQ) intermediate stores one electron from the donor chain and is reduced to quinol by a second electron. The involvement of the SQ necessitates that the sites accommodate binding of all three components of the quinone redox system. The three species (Q, SQ, and QH 2 ) require different properties of the H-bond partners, which therefore have to respond during different phases of the catalytic cycle. The structures have shown that the thermodynamic

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“…The stoichiometry of the hydrogen-bonded complexes with MP (1:n) is considered to be n = 1 -4 for AQ, AQ • -, and AQ 2-. 28 The association processes are assumed to be carried out in consecutive stages according to the reactions shown in Scheme 2. A digital simulation was performed with equal diffusion coefficients for the AQ(MP)n, AQ • -(MP)n and AQ 2-(MP)n hydrogen-bonded complexes, regardless of the charge of AQ.…”

Section: Electrochemistry Of Aq In the Presence Of Moderately Interacmentioning

confidence: 99%

“…The predominant species are AQ 2-(CH3OH)2, AQ 2-(CH3OH)4, and AQ 2-(CH3OH)6, which is generally accepted for hydrogen-bonding systems of the quinone dianions in view of the strength and directionality of the hydrogen-bonds of the dianion. 11,28 This diagram can easily give the main simple pathway from the complex redox mechanisms, as follows:…”

mentioning

confidence: 99%

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

et al. 2012

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The electrochemical reduction of 9,10-anthraquinone (AQ) was investigated in CH3CN in both the absence and presence of the hydrogen-bond and proton donating additives, CH3OH, CH(CF3)2OH, phenol, 4-methoxyphenol, 4-cyanophenol, 2,4,6-trichlorophenol, and benzoic acid (BA). Three clearly different types of electrochemical behavior were observed with increasing concentrations of the additives, and were simulated to analyze the reaction mechanisms. Type I was observed for weakly interacting additives, such as CH3OH, characterized by positive shifts of the two well-separated reduction waves, corresponding to the formation of AQ • -and AQ 2-, with no loss of reversibility. The second wave shifted more strongly, and finally merged with the first. These behaviors are explained by the association of AQ 2-with the additives via strong hydrogen-bonding. Type II is attributed to a reduction mechanism involving quantitative formation of strong hydrogen-bonded complexes of AQ 2-with additives, such as CH(CF3)2OH, phenol and 4-methoxyphenol, showing a reversible or quasireversible two-electron reduction wave with increasing concentrations of the additives. The behavior of Type III, observed in the presence of strongly interacting additives, such as 2,4,6-trichlorophenol and BA, is characterized by a voltammogram composed of the 2-electorn cathodic and the broad anodic waves without keeping reversibility, facilitated by proton transfer in the hydrogen-bonded complexes, AQ • --BA and AQ 2--BA. The effects of hydrogen-bonding and protonation on the electrochemistry of AQ have been systematically demonstrated in terms of the potentials and reaction pathways of the various species, which appear in quinone-hydroquinone systems.

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Hybrid Density Functional Study of the p-Benzosemiquinone Anion Radical: The Influence of Hydrogen Bonding on Geometry and Hyperfine Couplings

Cited by 81 publications

References 17 publications

An EPR and DFT study on the primary radical formed in hydroxylation reactions of 2,6-dimethoxy-1,4-benzoquinone

An EPR and DFT study on the primary radical formed in hydroxylation reactions of 2,6-dimethoxy-1,4-benzoquinone

Hydrogen Bonds Involved in Binding the Qi-site Semiquinone in the bc1 Complex, Identified through Deuterium Exchange Using Pulsed EPR

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

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