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Electron paramagnetic resonance (EPR) spectroscopy has been proven to be an important technique for studying paramagnetic systems. Probably, the most accessible EPR parameter and the one that provides a significant amount of information about molecular structure and spin density is the hyperfine coupling constant (HFCC). Hence, accurate quantum-chemical modeling of HFCCs is frequently essential to the adequate interpretation of EPR spectra. It requires the precise spin density, which is the difference between the densities of α- and β-electrons, and thus, its quality is expected to reflect the quality of the total electron density. The question of which approximate exchange-correlation density functional yields sufficiently accurate HFCCs, and thus, the spin density remains open. To assess the performance of well-established density functionals for calculating HFCCs, we used a series of 26 small paramagnetic species and compared the obtained results to the CCSD reference values. The performance of DFT was also tested on EPR-studied o-semiquinone radical interacting with water molecules and Mg2+ cation. The HFCCs were additionally calculated by the DLPNO-CCSD method, and this wave function-based technique was found superior to all functionals we tested. Although some functionals were found, on average, to be fairly efficient, we found that the most accurate functional is system-dependent, and therefore, the DLPNO-CCSD method should be preferred for theoretical investigations of the HFCCs and spin density. [ABSTRACT FROM AUTHOR]

Flavodoxins, electron transfer proteins essential for diverse metabolisms in microbes from the domain Bacteria, are extensively characterized. Remarkably, although genomic annotations of flavodoxins are widespread in microbes from the domain Archaea, none have been isolated and characterized. Herein is described the structural, biochemical, and physiological characterization of an unusual flavodoxin (FldA) from Methanosarcina acetivorans, an acetate-utilizing methane-producing microbe of the domain Archaea. In contrast to all flavodoxins, FldA is homodimeric, markedly less acidic, and stabilizes an anionic semiquinone. The crystal structure reveals an flavin mononucleotide (FMN) binding site unique from all other flavodoxins that provides a rationale for stabilization of the anionic semiquinone and a remarkably low reduction potentials for both the oxidized/semiquinone (-301 mV) and semiquinone/hydroquinone couples (-464 mV). FldA is up-regulated in acetate-grown versus methanol-grown cells and shown here to substitute for ferredoxin in mediating the transfer of low potential electrons from the carbonyl of acetate to the membrane-bound electron transport chain that generates ion gradients driving ATP synthesis. FldA offers potential advantages over ferredoxin by (i) sparing iron for abundant iron-sulfur proteins essential for acetotrophic growth and (ii) resilience to oxidative damage. [ABSTRACT FROM AUTHOR]

There is enough proof to believe that free-radical intermediates of anthracycline based anticancer agents are involved in different stages of drug action. Subtle therapeutic differences observed in the actions of different anthracyclines largely influence the mechanism of action of the drugs that distinguish one member from another. Redox properties control biological responses related either to the one-electron quinone/semiquinone couple or the two-electron quinone/quinone-dianion couple. Comproportionation also leads to generation of semiquinone. Hence, whatever the form of reduction of the quinone moiety, a substantial amount of semiquinone is eventually formed in the system. Immediately after formation, there is competition between natural radical-decay pathways and one-electron transfer reactions that generate the superoxide-radical anion. Prototropic properties control rate of radical decay while redox properties control rate of electron transfer to molecular oxygen. In aerated medium, semiquinone-radical anion and superoxide-radical anion co-exist while in de-aerated medium semiquinone-radical anion predominates. All the radicals are damaging to the biological system. Through this study, attempt was made to detect changes induced by the radicals on pyrimidine based nucleic acid bases and calf thymus DNA in aerated and de-aerated (Ar saturated) medium to know the mechanism by which Emodin, its CuII/MnII complexes might affect DNA. Semiquinone-radical anion was generated electrochemically maintaining a glassy carbon electrode at the first reduction potential of each compound. Since the chosen compound (Emodin), its complexes are analogues of anthracyclines, findings on them can be extrapolated to understand the differences in anticancer activity or of adverse drug reactions reported in an innumerable number of clinical studies related to anthracyclines where the difference in structure of different members is due to differences in the relative positioning of hydroxy groups on the hydroxy-9, 10-anthraquinone moiety of anthracyclines. The study helps to realize action of compounds of this class as anticancer agents. Image 108205 • Action of anthracyclines is due to one-electron quinone/semiquinone or two-electron quinone/dianion. • Semiquinone-radical anion was generated electrochemically using glassy carbon electrode. • Study detects changes caused by radicals on pyrimidine nucleic acid bases and calf thymus DNA. • Semiquinone on Emodin in de-aerated medium was most effective on pyrimidine nucleic acid bases. • The CuII complex of Emodin was most effective in aerated medium attributed to Fenton reactions. [ABSTRACT FROM AUTHOR]

Vitiligo (VL) is a chronic autoimmune pigmentation disorder characterized by destruction of melanocytes. The condition is associated with several other autoimmune diseases, but autoimmune thyroid diseases, especially Hashimoto's thyroiditis (HT), is the most prevalent organ-specific autoimmune disease with a co-morbidity up to 34%. Among the many hypotheses that have been proposed for the pathogenesis of both diseases, autoimmunity and oxidative stress-mediated toxicity in melanocytes or thyrocytes, respectively, have been the most widely accepted - with autoimmunity being the presumed consequence of oxidative stress-mediated toxicity. However, the predominant etiologic basis for impairment of redox balance has rarely been studied. The two autoimmune diseases are not only linked by a concordance of clinical presentations and an autoimmune/oxidative stress-mediated toxicity pathogenesis but also by an apparent biochemical commonality. The target molecules produced in the thyroid and skin, i.e., thyroxine and melanin, respectively, are derived from the same primordial parent molecule, tyrosine. On the basis of these similarities between Hashimoto's thyroiditis and vitiligo, specifically with respect to the activation of oxidative stress, we propose a novel hypothesis accounting for the destruction of melanocytes or thyrocytes in VL and AT. We suggest a new therapeutic regimen of quinone derivatives to combat ROS-induced autoimmunity resulting from this common biochemical etiologic error. [ABSTRACT FROM AUTHOR]

Abstract Synthesis, characterization and anticancer activity of reduced form of n -alkylamino analogs of Vitamin K3 (1Na to 8Na) are undertaken. Molecular structure and molecular association of oxidized form of 2-ethylamino-3-methyl-1,4-naphthoquinone (2) and 2-hexylamino-3-methyl-1,4-naphthoquinone (6) were studied by single crystal X-ray analysis. 2 crystallize in monoclinic C2/c and 6 in P2 1 space group. The reduced form of eight homologated analogs of 2-(n -alkylamino)-3-methyl-1,4-naphthoquinone (1Na to 8Na) were synthesized using sodium metal as reductant at 0 °C in methanol. The formations of naphthosemiquinone radical in 1 to 6 have been confirmed from their EPR spectra. Polycrystalline powder X-band EPR spectra of 1 to 6 shows signals ∼2.0020 ± 0.0026 at 133 K. Anticancer activity of 2-(n -alkylamino)-3-methyl-1,4-naphthoquinone (1Na to 8Na) and one electron reduced forms have been evaluated against breast cancer (HeLa) cell line, 1 and 1Na showed promising anticancer activity. Graphical abstract Image 1 Highlights • Reduced form of aminonaphthoquinones of vitamin K3 synthesized chemically. • Radical signatures are confirmed by EPR studies. • Redox behavior is studied by cyclic voltammetry. • Compounds were optimized to three different redox forms by DFT. • Anticancer activity evaluated for oxidized and reduced form of the compounds. [ABSTRACT FROM AUTHOR]

NADH-dependent reduced ferredoxin:NADP+ oxidoreductase (Nfn) is an electron-bifurcating enzyme first discovered in the strict anaerobes Clostridium kluyveri and Moorella thermoacetica. In vivo , Nfn catalyzes the endergonic reduction of NADP+ with NADH coupled to the exergonic reduction of NADP+ with reduced ferredoxin. Most Nfn homologs consist of two subunits, although in certain species Nfn homologs are fused. In contrast to other electron-bifurcating enzymes, Nfn possess a simpler structure. Therefore, Nfn becomes a perfect model to determine the mechanism of flavin-based electron bifurcation, which is a novel energy coupling mode distributed among anaerobic bacteria and archaea. The crystal structures of Nfn from Thermotoga maritima and Pyrococcus furiosus are known, and studies have shown that the FAD molecule of the NfnB (b-FAD) is the site of electron bifurcation, and other cofactors, including a [2Fe2S] cluster, two [4Fe4S] clusters, and the FAD molecule on the NfnA subunit, contribute to electron transfer. Further, the short-lived anionic flavin semiquinone (ASQ) state of b-FAD is essential for electron bifurcation. Nfn homologs are widely distributed among microbes, including bacteria, archaea, and probably eukaryotes, most of which are anaerobes despite that certain species are facultative microbes and even aerobes. Moreover, potential evidence shows that lateral gene transfer may occur in the evolution of this enzyme. Nfn homologs present four different structural patterns, including the well-characterized NfnAB and three different kinds of fused Nfn homologs whose detailed properties have not been characterized. These findings indicate that gene fusion/fission and gene rearrangement may contribute to the evolution of this enzyme. Under physiological conditions, Nfn catalyzes the reduction of NADP+ with NADH and reduced ferredoxin, which is then used in certain NADPH-dependent reactions. Deletion of nfn in several microbes causes low growth and redox unbalance and may influence the distribution of fermentation products. It's also noteworthy that different Nfn homologs perform different functions according to its circumstance. Physiological functions of Nfn indicate that it can be a potential tool in the metabolic engineering of industrial microorganisms, which can regulate the redox potential in vivo. [ABSTRACT FROM AUTHOR]

Quinalizarin, an analogue of anthracycline anticancer agents, is an anticancer agent itself. A CuII complex was prepared and characterized by elemental analysis, UV-Vis & IR spectroscopy, mass spectrometry, EPR and DFT. The intention behind the preparation of the complex was to increase cellular uptake, compare its binding with DNA against that of quinalizarin, modulation of semiquinone formation, realization of human DNA topoisomerase I & human DNA topoisomerase II inhibition and observation of anticancer activity. While the first two attributes of complex formation lead to increased efficacy, decrease in semiquinone generation could results in a compromise with efficacy. Inhibition of human DNA topoisomerase makes up this envisaged compromise in free radical activity since the complex shows remarkable ability to disrupt activities of human DNA topoisomerase I and II. The complex unlike quinalizarin, does not catalyze flow of electrons from NADH to O 2 to the extent known for quinalizarin. Hence, decrease in semiquinone or superoxide radical anion could make modified quinalizarin [as CuII complex] less efficient in free radical pathway. However, it would be less cardiotoxic and that would be advantageous to qualify it as a better anticancer agent. Although binding to calf thymus DNA was comparable to quinalizarin, it was weaker than anthracyclines. Low cost of quinalizarin could justify consideration as a substitute for anthracyclines but the study revealed IC 50 of quinalizarin/CuII-quinalizarin was much higher than anthracyclines or their complexes. Even then, there is a possibility that CuII-quinalizarin could be an improved and less costly form of quinalizarin as anticancer agent. Synopsis: A CuII complex of quinalizarin was prepared with the intention of increasing cellular uptake, study binding with DNA, modulate formation of semiquinone and check for inhibition of human DNA topoisomerase I and II to correlate observed anticancer activity. Low cost of quinalizarin justifies its consideration as a substitute for anthracyclines. [Display omitted] • Quinalizarin (THAQ), an analogue of anthracyclines, is active pharmacologically. • Complex formation modulates generation of free radicals that affect its mechanism of action. • The complex is an efficient inhibitor of topoisomerase enzymes, crucial for drug action. • Interaction of [CuII(THAQ) 2 (H 2 O) 2 ] with topoisomerase was realized by molecular docking. • THAQ, the acetylated form, shows more efficacy on ALL MOLT 4 cells than [CuII(THAQ) 2 (H 2 O) 2 ]. [ABSTRACT FROM AUTHOR]

Catalytic reactions of quinol oxidoreductases may lead to the generation of superoxide due to electron leaks from unstable semiquinone intermediates (SQ). For cytochrome bc1, the mechanism of suppression of superoxide generation remains unknown. We analyzed conditions of formation of a spin‐spin–coupled state between SQ and the Rieske cluster (SQ‐FeS) associated with catalysis of the quinol oxidation site of cytochrome bc1. We reveal that mutants that preclude direct interaction between SQ and the Rieske cluster do not form SQ‐FeS and release enhanced superoxide. In the enzymes generating SQ‐FeS, little or no superoxide is detected. We propose that SQ‐FeS suppresses superoxide generation, becoming an element modulating superoxide release under physiologically relevant conditions slowing electron flow through the enzyme. [ABSTRACT FROM AUTHOR]

Catalytic reactions of quinol oxidoreductases may lead to the generation of superoxide due to electron leaks from unstable semiquinone intermediates (SQ). For cytochrome bc1, the mechanism of suppression of superoxide generation remains unknown. We analyzed conditions of formation of a spin‐spin–coupled state between SQ and the Rieske cluster (SQ‐FeS) associated with catalysis of the quinol oxidation site of cytochrome bc1. We reveal that mutants that preclude direct interaction between SQ and the Rieske cluster do not form SQ‐FeS and release enhanced superoxide. In the enzymes generating SQ‐FeS, little or no superoxide is detected. We propose that SQ‐FeS suppresses superoxide generation, becoming an element modulating superoxide release under physiologically relevant conditions slowing electron flow through the enzyme. [ABSTRACT FROM AUTHOR]

Quinone group is an important fraction of humic acid. The pseudo-first-order rate constant ( k obs ) of levofloxacin (LF) degradation by permanganate (MnO 4 − ) significantly increased from 0.010 (without benzoquinone, BQ) to 0.042–0.443 min −1  at [BQ] o :[MnO 4 − ] o (molar ratio) = 0.03–0.25 at pH 7.5, and an acidic pH facilitated LF degradation. In the presence of BQ, MnO 4 − was first reduced to Mn(II). Then, Mn(II) reacted with BQ to produce Mn(III) and semiquinone radical, which was promoted under acidic conditions. With dissolved oxygen available, Mn(III) further oxidized semiquinone radical to produce singlet oxygen ( 1 O 2 ) and superoxide radical (O 2 − ) as well as regenerate BQ. In addition, MnO 4 − could also react with Mn(II) to produce Mn(III), whose complexation with semiquinone radical in turn promoted this reaction. Due to the predominant scavenging of O 2 − by BQ, 1 O 2 and Mn(III) mainly contributed to the accelerated LF degradation, with a notable formation of hydroxyl, ketone and endoperoxide groups in the degradation byproducts. This study helps better understand the role of natural organic matter in the degradation of organic micropollutants by MnO 4 − in water treatment. [ABSTRACT FROM AUTHOR]

Reduction of 1,10-phenanthroline-5,6-dione (pd) with CoCpR2 resulted in the first molecular compounds of the pdṖ− semi-quinone radical anion, [CoCpR2]+[pd]Ṗ− (R = H, (1); R = Me4, (2)). Furthermore compounds 1 and 2 were reacted with [Y(hfac)3(thf)2] (hfac = 1,1,1-5,5,5-hexafluoroacetylacetonate) to synthesise the rare earth-transition metal heterometallic compounds, [CoCpR2]+[Y(hfac)3(N,N′-pd)]Ṗ− (R = H, (3); R = Me4, (4)). [ABSTRACT FROM AUTHOR]

All cells obtain 2'-deoxyribonucleotides for DNA synthesis through the activity of a ribonucleotide reductase (RNR). The class I RNRs found in humans and pathogenic bacteria differ in (i) use of Fe(II), Mn(II), or both for activation of the dinuclear-metallocofactor subunit, β; (ii) reaction of the reduced dimetal center with dioxygen or superoxide for this activation; (iii) requirement (or lack thereof) for a flavoprotein activase, NrdI, to provide the superoxide from O2; and (iv) use of either a stable tyrosyl radical or a high-valent dimetal cluster to initiate each turnover by oxidizing a cysteine residue in the α subunit to a radical (Cys•). The use of manganese by bacterial class I, subclass b-d RNRs, which contrasts with the exclusive use of iron by the eukaryotic Ia enzymes, appears to be a countermeasure of certain pathogens against iron deprivation imposed by their hosts. Here, we report a metal-free type of class I RNR (subclass e) from two human pathogens. The Cys• in its α subunit is generated by a stable, tyrosine-derived dihydroxyphenylalanine radical (DOPA•) in β. The three-electron oxidation producing DOPA• occurs in Escherichia coli only if the β is coexpressed with the NrdI activase encoded adjacently in the pathogen genome. The independence of this new RNR from transition metals, or the requirement for a single metal ion only transiently for activation, may afford the pathogens an even more potent countermeasure against transition metal-directed innate immunity. [ABSTRACT FROM AUTHOR]

The natural formation of hydroxyl radicals ( OH) is important for the attenuation of organic contaminants. In this study, seven model polyphenols were selected to react with four types of smectite clays with varied Fe contents in the presence of H 2 O 2 . Diethyl phthalate (DEP) was selected as a model organic contaminant due to its wide distribution in environment. The results show the appearance of Fe-bearing smectite clays can significantly promote ·OH formation with polyphenols and H 2 O 2 under anoxic conditions; clay particle size, the content and location of lattice Fe in smectite clays greatly affect OH formation. Hydrogen bond between phenolic group and smectite surfaces, and cation assisted hydrogen bond between carboxylic group and clay surfaces are important types of complexation. Electrons can be transferred from coordinated polyphenols to structural Fe(III) atoms in tetrahedral layers or at broken edges to form structural Fe(II) and/or semiquinone radicals, both of which can induce H 2 O 2 decomposition to OH. DEP can be degraded by OH attack, and the main products are proposed as phthalic acid, monomethyl phthalate, hydroxyl-diethyl phthalates. Our findings suggest that Fe(III)-bearing smectite clay can be reduced by polyphenol and produce OH in anoxic environments, which can induce organic contaminants transformation. [ABSTRACT FROM AUTHOR]

Dopamine (DA), a simplified mimic of mussel proteins, can be employed as a reductant in the preparation of Au nanoparticles (AuNPs) due to its inherent catechol building block. The widely accepted mechanism of AuNP formation using DA as the reductant assumes that the reduction of Au(III) ions involves the two-electron oxidation of DA, where the corresponding phenol and phenolates serve as the reductive species to yield quinone. We herein report a novel insight into the mechanism of formation of AuNPs using DA as the reductant. We demonstrate that the synthesis of AuNPs requires the prior oxidation of the DA to form quinone units, which then catalyze the formation of semiquinones. These semiquinone radicals (SMQs) reduce the Au(III) ions to form the initial AuNPs, and further growth is then catalyzed by the first AuNPs, with nucleation occurring where the SMQs, phenols, and phenolates can serve as reductive species. In addition, DA oxidizes and polymerizes to form a polydopamine capping layer on the AuNPs. We therefore expect that the novel mechanism proposed herein may promote us to furthermore explore the production of noble metal NPs using other polyphenols. [ABSTRACT FROM AUTHOR]

Formation of protonated 3,4,6-triisopropylsemiquinone radicals in solution of a mixture of 3,4,6- triisopropyl-о-benzoquinone and 3,4,6-triisopropylcatechol was shown using ESR spectroscopy. Heating the mixture to 110°С leads to the formation of the condensation products, additional amount of 3,4,6-triisopropylcatechol, and evolution of propylene. Mathematical simulation of the kinetics of the reaction was performed. [ABSTRACT FROM AUTHOR]

Highly efficient organic-inorganic hybrid planar heterojunction perovskite solar cells with an architecture of indium tin oxide (ITO)/PEDOT:PSS-NH 2 -OH/CH 3 NH 3 PbI 3 /PCBM/Al are fabricated via one-step, solution process by employing the dopamine modified PEDOT:PSS (PEDOT:PSS-NH 2 -OH) as the hole transport layer, of which the power conversion efficiency of 15.34% is achieved with negligible hysteresis regardless of different scanning directions and scanning speeds. The average efficiency of twenty devices with PEDOT:PSS-NH 2 -OH is increased to 14.16% from 10.67% with PEDOT:PSS, and the former devices exhibit good reproducibility with small standard deviations. Cyclic voltammetry results show that the highest occupied molecular orbital energy level of PEDOT:PSS-NH 2 -OH (5.32 eV) matches well with the valence band of CH 3 NH 3 PbI 3 absorbers (5.4 eV). Photoluminescence measurements indicate that the CH 3 NH 3 PbI 3 film deposited on PEDOT:PSS-NH 2 -OH has strong charge extraction capability than that on PEDOT:PSS. Electrochemical impedance spectroscopy results suggest that the photovoltaic devices exhibit efficient hole transport and excellent electron blocking employing PEDOT:PSS-NH 2 -OH as the hole transport layer, as well as have low series, contact resistance and large recombination resistance. The research demonstrates that the stable semiquinone and amine radical modified PEDOT:PSS can acts as an outstanding hole transport material for fabricating high-efficient perovskite solar cells. [ABSTRACT FROM AUTHOR]

Dimeric cytochromes bc are central components of photosynthetic and respiratory electron transport chains. In their catalytic core, four hemes b connect four quinone (Q) binding sites. Two of these sites, Q i sites, reduce quinone to quinol (QH 2 ) in a step-wise reaction, involving a stable semiquinone intermediate (SQ i ). However, the interaction of the SQ i with the adjacent hemes remains largely unexplored. Here, by revealing the existence of two populations of SQ i differing in paramagnetic relaxation, we present a new mechanistic insight into this interaction. Benefiting from a clear separation of these SQ i species in mutants with a changed redox midpoint potential of hemes b , we identified that the fast-relaxing SQ i (SQ iF ) corresponds to the form magnetically coupled with the oxidized heme b H (the heme b adjacent to the Q i site), while the slow-relaxing SQ i (SQ iS ) reflects the form present alongside the reduced (and diamagnetic) heme b H . This so far unreported SQ iF calls for a reinvestigation of the thermodynamic properties of SQ i and the Q i site. The existence of SQ iF in the native enzyme reveals a possibility of an extended electron equilibration within the dimer, involving all four hemes b and both Q i sites. This substantiates the predicted earlier electron transfer acting to sweep the b-chain of reduced hemes b to diminish generation of reactive oxygen species by cytochrome bc 1 . In analogy to the Q i site, we anticipate that the quinone binding sites in other enzymes may contain yet undetected semiquinones which interact magnetically with oxidized hemes upon progress of catalytic reactions. [ABSTRACT FROM AUTHOR]

To evaluate the metabolic responses in tumour cells exposed to ionizing radiation, oxygen consumption rate (OCR), cellular lipid peroxidation, cellular energy status (intracellular nucleotide pool and ATP production), and mitochondrial reactive oxygen species (ROS), semiquinone (SQ), and iron-sulphur (Fe−S) cluster levels were evaluated in human cervical carcinoma HeLa cells at 12 and 24 h after X-irradiation. LC/MS/MS analysis showed that levels of 8-iso PGF2α and 5-iPF2α-VI, lipid peroxidation products of membrane arachidonic acids, were not altered significantly in X-irradiated cells, although mitochondrial ROS levels and OCR significantly increased in the cells at 24 h after irradiation. LC/UV analysis revealed that intracellular AMP, ADP, and ATP levels increased significantly after X-irradiation, but adenylate energy charge (adenylate energy charge (AEC) = [ATP + 0.5 × ADP]/[ATP + ADP + AMP]) remained unchanged after X-irradiation. In low-temperature electron spin resonance (ESR) spectra of HeLa cells, the presence of mitochondrial SQ at g = 2.004 and Fe-S cluster at g = 1.941 was observed and X-irradiation enhanced the signal intensity of SQ but not of the Fe-S cluster. Furthermore, this radiation-induced increase in SQ signal intensity disappeared on treatment with rotenone, which inhibits electron transfer from Fe-S cluster to SQ in complex I. From these results, it was suggested that an increase in OCR and imbalance in SQ and Fe-S cluster levels, which play a critical role in the mitochondrial electron transport chain (ETC), occur after X-irradiation, resulting in an increase in ATP production and ROS leakage from the activated mitochondrial ETC. [ABSTRACT FROM AUTHOR]

Hydrogen bonding between semiquinone (SQ) intermediates and side-chain or backbone nitrogens in protein quinone processing sites (Q-sites) is a common motif. Previous studies on SQs from multiple protein environments have reported specific features in the 15N HYSCORE spectra not reproducible by a theory based on fixed hyperfine parameters, and the source of these lineshape distortions remained unknown. In this work, using the spectra of the SQ in the Q-sites of wild-type and mutant D75H cytochrome bo3 ubiquinol oxidase from Escherichia coli, we have explained the observed additional features as originating from a-strain of the isotropic hyperfine coupling. In two-dimensional spectra, the a-strain manifests as well-resolved lineshape distortions of the basic cross-ridges and accompanying lines of low intensity in the opposite quadrant that allow its direct analysis. We have shown that their appearance is regulated by the relative values of the strain width, Δa, and parameter, δ = |2a + T| - 4v15N. α-strain provides a direct measure of the structural dynamics and heterogeneity of the O···H···N bond in the SQ systems. [ABSTRACT FROM AUTHOR]

Abstract: Highly efficient planar heterojunction perovskite solar cells (PVSCs) with dopamine (DA) semiquinone radical modified poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (DA‐PEDOT:PSS) as a hole transporting layer (HTL) were fabricated. A combination of characterization techniques were employed to investigate the effects of DA doping on the electron donating capability of DA‐PEDOT:PSS, perovskite film quality and charge recombination kinetics in the solar cells. Our study shows that DA doping endows the DA‐PEDOT:PSS‐modified PVSCs with a higher radical content and greater perovskite to HTL charge extraction capability. In addition, the DA doping also improves work function of the HTL, increases perovskite film crystallinity, and the amino and hydroxyl groups in DA can interact with the undercoordinated Pb atoms on the perovskite crystal, reducing charge‐recombination rate and increasing charge‐extraction efficiency. Therefore, the DA‐PEDOT:PSS‐modified solar cells outperform those based on PEDOT:PSS, increasing open‐circuit voltage (Voc) and power conversion efficiency (PCE) to 1.08 V and 18.5%, respectively. Even more importantly, the efficiency of the unencapsulated DA‐PEDOT:PSS‐based PVSCs are well retained with only 20% PCE loss after exposure to air for 250 hours. These in‐depth insights into structure and performance provide clear and novel guidelines for the design of effective HTLs to facilitate the practical application of inverted planar heterojunction PVSCs. [ABSTRACT FROM AUTHOR]

The key regularities of redox isomerism of six-coordinate bis-semiquinonato cobalt complexes in the crystalline phase are considered. The factors determining the temperature of transition between the redox isomers of various cobalt complexes (mononuclear and binuclear complexes, coordination polymers) were described. The transition parameters were shown to depend not only on the electronic and spatial structure of the ligands in a particular complex, but also on the crystal structure of the complex and intermolecular interactions in the lattice. [ABSTRACT FROM AUTHOR]

The article reports on the proactivity of the semiquinone and carbon-centered radicals in eumelanin through in situ photo-induced electron paramagnetic resonance technique. Topics include the light and water-driven production of semiquinone which demonstrate the electrical and biologically relevant photoreactivity, the analysis of the spin property and stability of free radicals, and the association between microscopic spin behavior and observable microscopic electrical conductivity.

The tpyPhOH ligand supports multiple redox processes, the first is hypothesized to be metal/ligand based PCET, and the second involves a resonance stabilized semiquinone-like species. These redox couples overlap in the absence of proton acceptors— the addition of water to acetonitrile solutions of Ru-tpyPhOH complexes teases out the PCET process. [Display omitted] Ligands play a central role in dictating the electronic properties of metal complexes to which they are coordinated. A fundamental understanding of changes in ligand properties can be used as design principles for more efficient catalysts. Designing ligands that have multiple protonation states that will change the properties of the coordination complex would be useful as potential ways of controlling catalysis, for example, as an on/off switch where one redox state exists below thermodynamic potential and another exists above. Thus, phenol moieties built into strongly coordinating ligands, like that of tpyPhOH (4′-(4-hydroxyphenyl)-2,2′:6′,2′'-terpyridine) may provide such a handle. Herein, we report the electrochemical and spectral characterization, and the crystallographic and computational analysis of two ruthenium analogs: [Ru(tpy)(tpyPhOH)](PF 6) 2 and [Ru(tpyPhOH) 2 ](PF 6) 2 (tpy = 2,2′:6′,2′'-terpyridine). Cyclic voltammetry and differential pulse voltammetry indicate that two redox events occur, one of which is pH independent and we hypothesize that these follow an electrochemical-chemical-electrochemical (ECE) mechanism. XRD results of the ruthenium complexes' protonated forms are generally consistent with expected bond lengths and angles and are in agreement with computational modeling. The properties are compared to a previously reported analog that contains the –OH group directly connected to terpyridine, [Ru(tpyOH) 2 ](PF 6) 2 , where tpyOH is 4′-hydroxy-2,2′:6′,2′'-terpyridine, with some intriguing differences. Overall, these data indicate that the phenyl-substituent decouples the phenol such that it behaves both as an electron withdrawing substituent and a location for a ligand centered oxidation event to occur. [ABSTRACT FROM AUTHOR]

• Riboflavin (RF) enhances current production via outer-membrane cytochromes (OMCs). • Whole-cell circular dichroism spectroscopy reveals heme oxidation state in OMCs. • Fumarate or dimethyl sulfoxide oxidizes OMCs even in the presence of lactate. • Bound RF as semiquinone state in reduced OMCs highly increases current production. • Upon the oxidation of OMCs, I c sharply decreases, and RF dissociates from OMCs. The rate enhancement of extracellular electron transfer is critical for microbial fuel cells (MFCs). Flavins have been a promising additive to enhance current production even with a few µM through the formation of bound semiquinone cofactor in the outer membrane cytochromes (OMCs). However, the effect of flavins vastly decreases upon the dissociation from OMCs to be soluble electron shuttle. Therefore, identifying a critical factor to stabilize the bound flavin cofactor is essential. Herein, we demonstrated that the reduced heme centers in OMCs promote riboflavin (RF) binding to OMCs by modulating the intracellular electron pathways in Shewanella oneidensis MR-1. UV–Vis and circular dichroism spectroscopy in situ showed that fumarate or dimethyl sulfoxide (DMSO) oxidizes heme centers in OMCs even at low concentrations with lactate as an electron donor, resulted in the defect of current enhancement by RF. Differential pulse voltammetry detected more soluble flavins and less bound semiquinone in the presence of fumarate or DMSO in the wild type. However, mutant strains lacking a reductase for fumarate or DMSO recovered the effect of RF. These results strongly suggest that reduced heme centers promote RF to bind OMCs, and alternative electron acceptors suppress power generation in MFCs even more than the stoichiometric ratio. [ABSTRACT FROM AUTHOR]

Binary Co(II) complexes having anthraquinoe-based ligands show ct -DNA and human serum albumin binding afiinity and highly significant in vitro photo-enhanced cytotoxicty in HeLa and A549 cancer cells with low dark toxicity via generation of ROS. [Display omitted] • Anthraquinone-based binary Co(II) complexes display excellent light-activated cytotoxicity in cancer cells. • They generate cytotoxic reactive oxygen species on excitation with visible light. • They show significant binding affinity to DNA and human serum albumin. Co(II) is a bioessential ion and its complexes are less prone to reduction by cellular thiols thereby reducing the probability of dark toxicity. Based on the above analogy, we have designed and synthesized five anthraquinone-based binary cobalt(II) complexes viz. [Co(HL1) 2 ] (1), [Co(HL2) 2 ] (2), [Co(HL3) 2 ] (3), [Co(HL4) 2 ] (4) and [Co(HL5) 2 ] (5), where H 2 L1 = 2-(2-hydroxyphenyl)-1 H -anthra[1,2- d ]imidazole-6,11-dione; H 2 L2 = 2-(4-(diethylamino)-2-hydroxyphenyl)-1 H -anthra[1,2- d ]imidazole-6,11-dione; H 2 L3 = (2-(2-hydroxy-3-methoxyphenyl)-1 H -anthra[1,2- d ]imidazole-6,11-dione; H 2 L4 = 2-(3-hydroxy-6-(hydroxymethyl)-2-methylpyridin-4-yl)-1 H anthra[1,2- d ]imidazole-6,11 dione and H 2 L5 = 2-(3-hydroxynaphthalen-2-yl)-1 H -anthra[1,2- d ]imidazole-6,11-dione. The complexes were thoroughly characterized by spectroscopic and analytical methods and their potential application in photoactivated chemotherapy have been evaluated. The complexes 1 – 5 showed an irreversible voltammetric response at 0.43 V to 0.88 V corresponding to Co(II)/Co(III) couple. Three other responses were observed at ∼ -0.92 V, ∼ -1.16 V and ∼ -1.48 V for the ligands present in the binary Co(II) complexes due to quinone (Q)/ semiquinone (SQ -) and semiquinone (SQ -)/ catecholate (Cat2-) redox couples. The complexes display significant binding affinity to human serum albumin (HSA) displaying binding constants in the range 6.70 (±0.10) × 104 to 3.13 (±0.05) × 105 M−1. They also bind to calf-thymus (ct) DNA showing K b values in the order ∼ 105-106 M−1. In vitro light-activated cytotoxicity studies showed significant photocytotoxicity against human cervical carcinoma, HeLa (IC 50 ∼ 1.6–4.2 µM) and human lung carcinoma, A549 (IC 50 ∼ 2.1–6.1 µM) cells with minimal dark toxicity (IC 50 > 70 µM). Flow cytometric experiments reveal that complex 5 is able to produce reactive oxygen species (ROS) on exposure to visible light which responsible for photoactivated cytotoxicity. [ABSTRACT FROM AUTHOR]

The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO2+) generation has been reported as one of the primary oxidants. FeO2+ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO2+ in the Fenton reaction depends on factors such as pH and Fe: H 2 O 2 ratio. Reaction mechanisms have been proposed to generate FeO2+, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO2+ (using xylidine as a selective substrate). The findings revealed that FeO2+ production is increased compared to the classical Fenton reaction and that FeO2+ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO2+ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO2+ through this pathway. [Display omitted] • Fe(IV) was produced at pH = 2.0 and 4.0 through two pathways by Fenton reaction. • In Fenton–like systems driven by catechol, Fe(IV) production is higher than in Fenton systems. • In Fenton-like systems, Fe(IV) is only produced by HO·-dependent pathways. [ABSTRACT FROM AUTHOR]

[Display omitted] • The addition of Fe 2 (SO 4) 3 promoted the composting humification process. • Fe 2 (SO 4) 3 effectively regulated the evolution of free radicals from HS. • Abundance of iron-reducing bacteria greatly increased during thermophilic phase. • Extensive generation of free radicals was produced via humification process. The aim of this paper was to investigate the influence of Fe (III) on humification and free radicals evolution. The experimental data showed that the experimental group (CT) with Fe 2 (SO 4) 3 had a better degree of humification than the control group (CK). The humic substances (HS) content was 10% higher in CT (23.94 mg·g−1) than in CK (21.54 mg·g−1) in the final. Fe (III) contributed significantly to the formation of free radicals in HS. The amount of H 2 O 2 in CT increased to 74.8 mmol·kg−1, while CK was only 46.5 mmol·kg−1. The content of semiquinone free radical was 10.32 × 1011 spins/mm3 in CT, 5.11 × 1011 spins/mm3 in CK in the end. Several iron-reducing bacteria were detected in composting, among which Paenibacillus was dominant. The above findings suggested that the application of Fe 2 (SO 4) 3 enhanced the iron reduction synergistic quinone redox cycling and promoted the generation of free radicals during the humification of composting. [ABSTRACT FROM AUTHOR]

One of the less understood parts of the catalytic cycle of cytochrome bc 1 / b 6 f complexes is the mechanism of electronic bifurcation occurring within the hydroquinone oxidation site (Q o site). Several models describing this mechanism invoke a phenomenon of formation of an unstable semiquinone. Recent studies with isolated cytochrome bc 1 or b 6 f revealed that a relatively stable semiquinone spin-coupled to the reduced Rieske cluster (SQ-FeS) is generated at the Q o site during the oxidation of ubi- or plastohydroquinone analogs under conditions of continuous turnover. Here, we identified the EPR transition of SQ-FeS formed upon oxidation of ubihydroquinone in native photosynthetic membranes from purple bacterium Rhodobacter capsulatus . We observed a significant amount of SQ-FeS generated when the antimycin-inhibited enzyme experiences conditions of non-equilibrium caused by the continuous light activation of the reaction center. We also noted that SQ-FeS cannot be detected under equilibrium redox titrations in dark. The non-equilibrium redox titrations of SQ-FeS indicate that this center has a higher apparent redox midpoint potential when compared to the redox midpoint potential of the quinone pool. This suggests that SQ-FeS is stabilized, which corroborates a recently proposed mechanism in which the SQ-FeS state is metastable and functions to safely hold electrons at the local energy minimum during the oxidation of ubihydroquinone and limits superoxide formation. Our results open new possibilities to study the formation and properties of this state in cytochromes bc under close to physiological conditions in which non-equilibrium is attained by the light activation of bacterial reaction centers or photosystems. [ABSTRACT FROM AUTHOR]

Abstract: Semiquinones (SQ) are generated in photosynthetic organisms upon photoinduced electron transfer to quinones (Q). They are stabilized by hydrogen bonding (HB) with the neighboring residues, which alters the properties of the reaction center. We designed, synthesized, and investigated resorcin[4]arene cavitands inspired by this function of SQ in natural photosynthesis. Cavitands were equipped with alternating quinone and quinoxaline walls bearing hydrogen bond donor groups (HBD). Different HBD were analyzed that mimic natural amino acids, such as imidazole and indole, along with their analogues, pyrrole and pyrazole. Pyrroles were identified as the most promising candidates that enabled the cavitands to remain open in the Q state until strengthening of HB upon reduction to the paramagnetic SQ radical anion provided stabilization of the closed form. The SQ state was generated electrochemically and photochemically, whereas properties were studied by UV/Vis spectroelectrochemistry, transient absorption, and EPR spectroscopy. This study demonstrates a photoredox‐controlled conformational switch towards a new generation of molecular grippers. [ABSTRACT FROM AUTHOR]

The driving force for the electron transfer (ET) step in the catalytic cycle of cytochrome P450BM3 is investigated using three sets of 1μs molecular dynamic simulations for the resting state of P450 in complex with the flavin (FMN) in the reductase domain. These sets involve the following: (i) substrate-free (SF), (ii) substrate (N-palmitoyl glycine, i.e., NPG)-bound (SB), and (iii) SB with the semiquinone radical anion (SQ-) of FMN. Starting from the X-ray structure of the SF heme domain, we observe that the α1-helix (of the reductase) and the C-helix (of the heme) undergo reorientation to a parallel orientation, which is the thermochemically stable form. The reorientation of the helices pushes away the FMN to a distance of 18.4 Å from the heme's center. When the substrate binds it causes the I-helix of the heme domain to kink and push the C-helix toward the α1-helix, thereby locking the latter two into a stabilized perpendicular conformation, wherein the FMN -- heme distance is 12 Å. The distance drops further in the SQ- form, and upon QM/MM geometry optimization the two moieties approach 8.8 Å, which enhances the ET rate (by 104-106 fold) to the heme's Fe[sup 3+] ion. These motions are driven by hydrogen bond strengthening between the C- and the α1-helices. Finally, substrate binding leads to formation of an organized water chain connecting the FMN and heme moieties. The water channel assists the ET and couples it to the proton transfer steps that should activate O2 and create the oxo -- iron active species. [ABSTRACT FROM AUTHOR]

Bacterial reaction centers (RC) from Rhodobacter sphaeroides have been widely used to functionalize electrodes and to generate photocurrent. However, in most studies, direct electron transfer from the semiquinone to the electrode was not observed because the H subunit of the RC shields the semiquinone. It is demonstrated in the current work that removal of the H subunit effectively exposes the semiquinone sites in the LM dimer. This is demonstrated by measuring the second-order rate constant for the reaction between ferricyanide and the anionic semiquinone Q formed by an actinic flash. The rate constant increases 1000-fold for Q oxidation by ferricyanide in the LM dimer compared to the intact RC. The second-order rate constant approaches the diffusion limit of 6 × 10 M·s at low pH, but it decreases steadily when the pH is above 6.5. This pH dependence suggests that the protonation state of the LM dimer plays an important role in controlling the electron transfer kinetics. It is also shown that the addition of exogenous ubiquinone to replenish the Q site, which is mostly empty in the LM dimer, leads to oxidation of Q by O following an actinic flash. It is concluded that removal of the H subunit results in exposure of the semiquinone sites of the LM dimer to externally added oxidants and may provide a strategy for enhancing direct electron transfer from the RC to an electrode. [ABSTRACT FROM AUTHOR]

The strong effect of magnetic field on the electrochemical (EC) reduction of a diamagnetic species was monitored in situ in a 600 MHz (14 T) NMR spectrometer. Throughout EC-NMR experiments, the diamagnetic species is influenced by the Lorentz force (cross product of current density and magnetic field), which in turn acts on analyte transport and, as a result, enhances reaction rates. This phenomenon, known as magnetoelectrolysis, has not been considered in several in situ EC-NMR studies in solution, electron paramagnetic resonance (EC-EPR) spectroscopy, and magnetic resonance imaging (EC-MRI) involving the oxidation and reduction of organic compounds and lithium ion batteries. Recently, we have demonstrated the presence of this effect in the electroplating of a paramagnetic ion species by monitoring it in situ in a low-field (0.23 T) NMR spectrometer. In this report, a ca. five-fold enhancement in the electroreduction rate of benzoquinone was observed when the analyses were performed in situ in the NMR spectrometer. Therefore, this work has the objective of informing the scientific community that before every electrochemical reaction carried out in situ in NMR, EPR and MRI apparatuses, the influence of the magnetic field on the reactions must be evaluated, since it can alter the mechanism and kinetics of the reaction which, if not taken into account may lead to wrong interpretations of the data. [ABSTRACT FROM AUTHOR]

Flavin-based electron transfer bifurcation is emerging as a fundamental and powerful mechanism for conservation and deployment of electrochemical energy in enzymatic systems. In this process, a pair of electrons is acquired at intermediate reduction potential (i.e. intermediate reducing power), and each electron is passed to a different acceptor, one with lower and the other with higher reducing power, leading to "bifurcation." It is believed that a strongly reducing semiquinone species is essential for this process and it is expected that this species should be kinetically short-lived. We now demonstrate that the presence of a short-lived anionic flavin semiquinone (ASQ) is not sufficient to infer the existence of bifurcating activity, although such a species may be necessary for the process. We have used transient absorption spectroscopy to compare the rates and mechanisms of decay ofASQgenerated photochemically in bifurcating NADH-dependent ferredoxin-NADP+ oxidoreductase and the non-bifurcating flavoproteins nitroreductase, NADH oxidase and flavodoxin. We found that different mechanisms dominate ASQ decay in the different protein environments, producing lifetimes ranging over 2 orders of magnitude. Capacity for electron transfer among redox cofactors versus charge recombination with nearby donors can explain the range of ASQ lifetimes that we observe. Our results support a model wherein efficient electron propagation can explain the short lifetime of the ASQ of bifurcating NADH-dependent ferredoxin-NADP+ oxidoreductase I and can be an indication of capacity for electron bifurcation. [ABSTRACT FROM AUTHOR]

Levels and particle-size distributions of environmentally persistent free radicals (EPFRs) in haze-associated atmospheric particulate matter (PM) have not been highlighted, even though they may enter the human body along with PM and adversely affect human health. This study quantified the levels of EPFRs in airborne PM with different aerodynamic diameters (d ae) using electron paramagnetic resonance (EPR) spectroscopy. EPR spectra showed a single, unstructured signal from persistent semiquinone radicals. The average concentration of EPFRs in the airborne PM during haze events was 2.18 × 1220 spins/g (range: 3.06 × 1019-6.23 × 1020 spins/g), approximately 2 orders of magnitude higher than that reported previously in the US atmosphere. Particle-size distributions of EPFRs in four different PM fractions (d ae > 10 μm, 10 μm < d ae < 2.5 μm, 2.5 μm

The natural antioxidants of olive oil have phenolic structure and their activities are related to the formation of stable derivatives. In this study, the single components of the phenolic fraction of olive oil (1,4-hydroquinone, Semiquinone and 1,4-benzoquinone) have been studied as theoretical by using DFT (Density functional Theory). The behaviors of phenolic compounds of olive against to the alkyl peroxy radicals were investigated. Our data show that 1,4-benzoquinone is the best electron transfer agent in primary metabolic processes to human life. The frontier orbital gap, namely HOMO (highest occupied molecular orbital) eLUMO (lowest unoccupied molecular orbital) gap is the smallest for 1,4-benzoquinone. Hence, it is more stable than the others in blood. The natural phenolic compound's mechanism of many plants can be explained by using DFT method without consuming time and money. In this study, we have indicated the behaviors of natural antioxidants of olive oil's single components phenolic structure in blood phase. [ABSTRACT FROM AUTHOR]

This study was conducted to reconcile the controversial scientific evidence indicating the dual role of phenolic compounds in oxidative changes in fruit products. The hypothesis has been substantiated, according to which the functioning of flavonoids as redox-systems, is strictly connected with the ability of fruit phenolic compounds to inhibit the free-radical processes of the oxidation of substrates; this is due to the semiquinone presence in the equilibrium system. Terminology and appropriate methods of studying, in respect of the antioxidant effect of natural, as well as added compounds in fruit products, were analysed. The effect of redoxproperties of the antioxidant agent, on studying the inhibition of the enzymatic oxidation of phenolic compounds with this agent, has been analysed. It was shown that this field of study is vital for improving the concepts of browning inhibitors in foodstuffs, as well as for improving the theory of interactions between components in canned foodstuffs. [ABSTRACT FROM AUTHOR]

The Q-cycle mechanism of the bc1 complex is now well enough understood to allow application of advanced computational approaches to the study of atomistic processes. In addition to the main features of the mechanism, these include control and gating of the bifurcated reaction at the Qo-site, through which generation of damaging reactive oxygen species is minimized. We report a new molecular dynamics model of the Rhodobacter sphaeroides bc1 complex implemented in a native membrane, and constructed so as to eliminate blemishes apparent in earlier Rhodobacter models. Unconstrained MD simulations after equilibration with ubiquinol and ubiquinone respectively at Qo- and Qi-sites show that substrate binding configurations at both sites are different in important details from earlier models. We also demonstrate a new Qo-site intermediate, formed in the sub-ms time range, in which semiquinone remains complexed with the reduced iron sulfur protein. We discuss this, and a spring-loaded mechanism for modulating interactions of the iron sulfur protein with occupants of the Qo-site, in the context of control and gating roles. Such atomistic features of the mechanism can usefully be explored through simulation, but we stress the importance of constraints from physical chemistry and biology, both in setting up a simulation and in interpreting results. [ABSTRACT FROM AUTHOR]