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We studied the reactions of sterically hindered bis-semiquinonates of copper(II) with 4,4′-bipyridine. Coordination polymer consisting of metallamacrocyclic binuclear copper(II) cage-like moieties bridged with 4,4′-bipyridine linkers was isolated in the reaction of copper(II) bis-semiquinonate metallamacrocycle with 4,4′-bipyridine. It is important that the initial and final metallamacrocycles in this reaction are different by the disposition of semiquinone units in the coordination surroundings of copper (II) ions. The transformation of one metallamacrocycle into another could only be achieved by a two-fold sequential cleavage of the metal-semiquinone bond. [ABSTRACT FROM AUTHOR]

We have investigated the kinetic behavior of the electronbifurcating crotonyl-CoA-dependent NADH: ferredoxin oxidoreductase EtfAB:bcd from Megasphaera elsdenii. The overall behavior of the complex in both the reductive and the oxidative half-reactions is consistent with that previously determined for the individual EtfAB and bcd components. This includes an uncrossing of the half-potentials of the bifurcating flavin of the EtfAB component in the course of ferredoxinreducing catalysis, ionization of the bcd flavin semiquinone and the appearance of a charge transfer complex upon binding of the high potential acceptor crotonyl-CoA. The observed rapid-reaction rates of ferredoxin reduction are independent of [NADH], [crotonyl-CoA], and [ferredoxin], with an observed rate of ~0.2 s-1, consistent with the observed steady-state kinetics. In enzyme-monitored turnover experiments, an approach to steady-state where the complex's flavins become reduced but no ferredoxin is generated is followed by a steadystate phase characterized by extensive ferredoxin reduction but little change in overall levels of flavin reduction. The approach to the steady-state phase can be eliminated by prior reduction of the complex, in which case there is no lag in the onset of ferredoxin reduction; this is consistent with the et FAD needing to be reduced to the level of the (anionic) semiquinone for bifurcation and concomitant ferredoxin reduction to occur. Single-turnover experiments support this conclusion, with the accumulation of the anionic semiquinone of the et FAD apparently required to prime the system for subsequent bifurcation and ferredoxin reduction. [ABSTRACT FROM AUTHOR]

Laccases are multi‐copper oxidases (MCOs) that oxidize a broad range of substrates while reducing molecular oxygen to water. Although much effort has been made to elucidate the catalytic mechanism of laccases, information about reactive catalytic intermediates during catalysis is not yet fully understood. Herein, hydroquinone (HQ) polymerization catalyzed by prokaryotic small laccase (SLAC) was investigated by electron paramagnetic resonance (EPR) spectroscopy to provide more information on the catalytic mechanism. Briefly, free radical intermediates during the catalysis were investigated by real‐time in situ EPR measurement to monitor the formation and transformation of free radicals. A paramagnetic species was identified which was attributed to a copolymer formed by hydroquinone, benzoquinone, and semiquinone radical. In addition, the low‐temperature EPR spectrum of the trinuclear copper center during catalysis not only revealed tentative rotational motion of the histidine imidazole rings coordinating the TNC upon electron uptake but also provided evidence of the formation of oxygen bridge at TNC during the reaction, represented by the observation of a new type 2 copper component featured larger g// values and smaller A// values. Together, EPR spectroscopic insights into the catalytic intermediates during enzymatic hydroquinone polymerization have extended the knowledge of the biophysical characteristics and catalytic mechanism of SLAC. [ABSTRACT FROM AUTHOR]

Currently, the copolymer of dopamine (DA) and pyrrole (PY) via chemical and electrochemical oxidation usually requires additional oxidants, and lacks flexibility in regulating the size and morphology, thereby limiting the broad applications of DA‐PY copolymer in biomedicine. Herein, the semiquinone radicals produced by the self‐oxidation of DA is ingeniously utilized as the oxidant to initiate the following copolymerization with PY, and a series of quinone‐rich polydopamine‐pyrrole copolymers (PDAm‐nPY) with significantly enhanced absorption in near‐infrared (NIR) region without any additional oxidant assistance is obtained. Moreover, the morphology and size of PDAm‐nPY can be regulated by changing the concentration of DA and PY, thereby optimizing nanoscale PDA0.05‐0.15PY particles (≈ 150 nm) with excellent NIR absorption and surface modification activity are successfully synthesized. Such PDA0.05‐0.15PY particles show effective photoacoustic (PA) imaging and photothermal therapy (PTT) against 4T1 tumors in vivo. Furthermore, other catechol derivatives can also copolymerize with PY under the same conditions. This work by fully utilizing the semiquinone radical active intermediates produced through the self‐oxidation of DA reduces the dependence on external oxidants in the synthesis of composite materials and predigests the preparation procedure, which provides a novel, simple, and green strategy for the synthesis of other newly catechol‐based functional copolymers. [ABSTRACT FROM AUTHOR]

The paper is devoted to the measurements of one‐electron redox potentials of transient free radicals Eo. These values can be measured in the reversible redox reactions of radicals under investigation with a reference pair, which Eo is known. Several Eo of transient radicals are presented. The convenient and important objects for Eo measurement are quinone/semiquinone radical‐anions. A general problem of measurement of thermodynamic properties of short‐lived transients is discussed. [ABSTRACT FROM AUTHOR]

The flavoprotein Cytochrome P450 reductase (CPR) is the unique electron pathway from NADPH to Cytochrome P450 (CYPs). The conformational dynamics of human CPR in solution, which involves transitions from a "locked/closed" to an "unlocked/open" state, is crucial for electron transfer. To date, however, the factors guiding these changes remain unknown. By Site‐Directed Spin Labelling coupled to Electron Paramagnetic Resonance spectroscopy, we have incorporated a non‐canonical amino acid onto the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) domains of soluble human CPR, and labelled it with a specific nitroxide spin probe. Taking advantage of the endogenous FMN cofactor, we successfully measured for the first time, the distance distribution by DEER between the semiquinone state FMNH• and the nitroxide. The DEER data revealed a salt concentration‐dependent distance distribution, evidence of an "open" CPR conformation at high salt concentrations exceeding previous reports. We also conducted molecular dynamics simulations which unveiled a diverse ensemble of conformations for the "open" semiquinone state of the CPR at high salt concentration. This study unravels the conformational landscape of the one electron reduced state of CPR, which had never been studied before. [ABSTRACT FROM AUTHOR]

Flavin is one of the most prevalent redox molecules utilized by electroactive bacteria. Electroactive bacteria form a three-dimensional architecture with multiple cell assemblages on electrodes in bioelectrochemical systems. This provokes the importance of unveiling the redox chemistry of flavins during electron transfer not only at the bacteria/electrode interface but also inside cell assemblages. However, it has been difficult to directly compare the redox species contributing to each electron transfer reaction. In this study, to simultaneously detect the flavin redox species at the electrode surface and those in cell assemblages, we conducted bipotentiometric cyclic voltammetry on a colony of Shewanella oneidensis MR-1. The bipotentiometric data showed that flavin mononucleotide proceeds the redox cycle at − 0.43 V (vs. standard hydrogen electrode) in the MR-1 colony assignable to the semiquinone/hydroquinone redox cycle, which was supported by experiments with semiquinone scavenger and gene deletion mutants. Notably, the peak at − 0.43 V was not detected at the electrode surface, indicating that the flavin redox cycles and redox potentials involved in the electron transfer inside MR-1 assemblages differ from those at the MR-1/electrode interface. The measurement system presented herein offers a platform to clarify the redox reactions in cell assemblages as well as at the bacteria/electrode interface.

The reaction rate bottleneck during interconversion between insulating S8 (S) and Li2S fundamentally leads to incomplete conversion and restricted lifespan of Li−S battery, especially under high S loading and lean electrolyte conditions. Herein, we demonstrate a new catalytic chemistry: soluble semiquinone, 2‐tertbutyl‐semianthraquinone lithium (Li+TBAQ⋅−), as both e‐/Li+ donor and acceptor for simultaneous S reduction and Li2S oxidation. The efficient activation of S and Li2S by Li+TBAQ⋅− in the initial discharging/charging state maximizes the amount of soluble lithium polysulfide, thereby substantially improve the rate of solid–liquid‐solid reaction by promoting long‐range electron transfer. With in situ Raman spectra and theoretical calculations, we reveal that the activation of S/Li2S is the rate‐limiting step for effective S utilization under high S loading and low E/S ratio. Beyond that, the S activation ratio is firstly proposed as an accurate indicator to quantitatively evaluate the reaction rate. As a result, the Li−S batteries with Li+TBAQ⋅− deliver superior cycling performance and over 5 times higher S utilization ratio at high S loading of 7.0 mg cm−2 and a current rate of 1 C compared to those without Li+TBAQ⋅−. We hope this study contributes to the fundamental understanding of S redox chemical and inspires the design of efficient catalysis for advanced Li−S batteries. [ABSTRACT FROM AUTHOR]

A complete theoretical analysis using first the simple Hückel model followed by more sophisticated multi‐reference calculations on a trinuclear Ni(II) complex (Tp#Ni3HHTP), bearing the non‐innocent bridging ligand HHTP3−, is carried out. The three semiquinone moieties of HHTP3− couple antiferromagnetically and lead to a single unpaired electron localized on one of the moieties. The calculated exchange coupling integrals together with the zero‐field parameters allow, when varied within a certain range, reproducing the experimental data. These results are generalized for two similar other trinuclear complexes containing Ni(II) and Cu(II). The electronic structure of HHTP3− turns out to be independent of both the chemical nature and the geometry of the metal ions. We also establish a direct correlation between the geometrical and the electronic structures of the non‐innocent ligand that is consistent with the results of calculations. It allows experimentalists to get insight into the magnetic behavior of this type of complexes by an analysis of their X‐ray structure. [ABSTRACT FROM AUTHOR]

It has been shown using experimental (electron paramagnetic resonance) and quantum chemical (density functional theory) research methods that the previously observed changes in the electron paramagnetic resonance spectrum of the semiquinone Tiron radical in seawater are due to its interaction with Mg2+ ions. The formation of a complex between Mg2+ and the Tiron radical is due to the electrostatic attraction of ions carrying large charges of the opposite sign (+2 and –3), on the one hand, and the ability of Mg2+ ions to bind effectively to bidentate oxygen-containing ligands, on the other hand. The formation of tight contact ion pairs leads to a redistribution of the electron and spin density in the Tiron radical, which can be judged by the observed changes in the electron paramagnetic resonance spectrum of this radical. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide is an important intra- and inter-cellular messenger. It has been reported that beverages such as tea and coffee generate H2O2. The aim of this study was to examine whether H2O2 is formed in infusions of medicinal herbs and to explain the mechanism of its formation. Infusions of sixteen herbs were studied and the presence of H2O2 was found in all cases. The highest concentrations of H2O2, exceeding 50 µM, were found in the infusions of leaves of Betula and Polygonum, as well as of the inflorescence of Tilia. Considerable amounts of diverse phenolic compounds were detected in the infusions. Two successive one-electron oxidations of phenolics are the main source of H2O2 in these infusions; as expected, the generation of semiquinone and superoxide radicals was confirmed. The infusions of medicinal herbs were also able to scavenge H2O2. The herbal infusions were cytotoxic to human ovary cancer SKOV-3 and PEO1 cells; this cytotoxicity was compromised by catalase added to the growth medium, demonstrating the contribution of H2O2 to the cytotoxic action of herbal extracts. [ABSTRACT FROM AUTHOR]

We have investigated the equilibrium properties and rapidreaction kinetics of the isolated butyryl-CoA dehydrogenase (bcd) component of the electron-bifurcating crotonyl-CoAdependent NADH:ferredoxin oxidoreductase (EtfAB-bcd) from Megasphaera elsdenii. We find that a neutral FADH- semiquinone accumulates transiently during both reduction with sodium dithionite and with NADH in the presence of catalytic concentrations of EtfAB. In both cases full reduction of bcd to the hydroquinone is eventually observed, but the accumulation of FADH- indicates that a substantial portion of reduction occurs in sequential one-electron processes rather than a single two-electron event. In rapid-reaction experiments following the reaction of reduced bcd with crotonyl-CoA and oxidized bcd with butyryl-CoA, long-wavelength-absorbing intermediates are observed that are assigned to bcdred:crotonyl-CoA and bcdox:butyryl-CoA charge-transfer complexes, demonstrating their kinetic competence in the course of the reaction. In the presence of crotonyl-CoA there is an accumulation of semiquinone that is unequivocally the anionic FAD--rather than the neutral FADH- seen in the absence of substrate, indicating that binding of substrate/product results in ionization of the bcd semiquinone. In addition to fully characterizing the rapid-reaction kinetics of both the oxidative and reductive half-reactions, our results demonstrate that oneelectron processes play an important role in the reduction of bcd in EtfAB-bcd. [ABSTRACT FROM AUTHOR]

A new type of lubricating material (BTA-P4444-Lig) was synthesized by combining lignin with tetrabutylphosphorus and benzotriazole. The tribological properties, corrosion resistance, and anti-oxidation properties of BTA-P4444-Lig as a lubricant were investigated. The lubricating material exhibits excellent friction reduction and wear resistance, as well as good thermal stability and excellent oxidation resistance. Mechanistic analysis reveals that the active elements N and P in the lubricating material react with the metal substrate, and the reaction film effectively blocks direct contact between the friction pairs, affording excellent friction reduction and wear resistance. At the same time, the phenolic hydroxyl group in lignin reacts with oxygen free radicals to form a resonance-stable semi-quinone free radical, which interrupts the chain reaction and affords good anti-oxidant activity. [ABSTRACT FROM AUTHOR]

Copper amine oxidases are enzymes that exhibit in their active site a mononuclear copper complex and a 2,4,5‐trihydroxyphenylalanine quinone (TPQ) cofactor; in the oxidative half of the catalytic cycle, the enzymes regulate their activity by a temperature‐dependent electron transfer equilibrium between the CuII complex with the reduced, aminoquinol form of the cofactor and the reactive CuI complex with the corresponding oxidized, semiquinone form of the cofactor. Here, we report the first mononuclear copper complex with redox‐active ligands showing quantitative, reversible electromerism between a CuII eletromer with reduced, neutral ligand and a CuI electromer with an oxidized, radical monocationic ligand. The CuII form, being exclusively present at low temperature, exhibits a lower enthalpy (like the enzymes), but the CuI complex exhibits a higher entropy and is exclusively present at room temperature in CH2Cl2 solution. Further analysis, based on six different copper complexes, discloses a large solvent effect on electromerism. [ABSTRACT FROM AUTHOR]

Bifurcating electron transferring flavoproteins (Bf-ETFs) tune chemically identical flavins to two contrasting roles. To understand how, we used hybrid quantum mechanical molecular mechanical calculations to characterize noncovalent interactions applied to each flavin by the protein. Our computations replicated the differences between the reactivities of the flavins: the electron transferring flavin (ETflavin) was calculated to stabilize anionic semiquinone (ASQ) as needed to execute its single-electron transfers, whereas the Bf flavin (Bfflavin) was found to disfavor the ASQ state more than does free flavin and to be less susceptible to reduction. The stability of ETflavin ASQ was attributed in part to H-bond donation to the flavin O2 from a nearby His side chain, via comparison of models employing different tautomers of His. This H-bond between O2 and the ET site was uniquely strong in the ASQ state, whereas reduction of ETflavin to the anionic hydroquinone (AHQ) was associated with side chain reorientation, backbone displacement, and reorganization of its H-bond network including a Tyr from the other domain and subunit of the ETF. The Bf site was less responsive overall, but formation of the Bfflavin AHQ allowed a nearby Arg side chain to adopt an alternative rotamer that can H-bond to the Bfflavin O4. This would stabilize the anionic Bfflavin and rationalize effects of mutation at this position. Thus, our computations provide insights on states and conformations that have not been possible to characterize experimentally, offering explanations for observed residue conservation and raising possibilities that can now be tested. [ABSTRACT FROM AUTHOR]

In the field of molecular magnetism, the lanthanide–radical (Ln–Rad) method has become one of the most appealing tactics for introducing strong magnetic interactions and has spurred on the booming development of heterospin single-molecule magnets (SMMs). The article is a timely retrospect on the research progress of Ln–Rad heterospin systems and special attention is invested on low dimensional Ln–Rad compounds with SMM behavior, primarily concerning with nitrogen-based radicals, semiquinone and nitroxide radicals. Rational design, molecular structures, magnetic behaviors and magneto-structural correlations are highlighted. Meanwhile, particular attention is focused on the influence of exchange couplings on the dynamic magnetic properties, with the purpose of helping to guide the design of prospective radical-based Ln–SMMs. [ABSTRACT FROM AUTHOR]

Our study proposes a pharmacological strategy to target cancerous mitochondria via redox-cycling "mitocans" such as quinone/ascorbate (Q/A) redox-pairs, which makes cancer cells fragile and sensitive without adverse effects on normal cells and tissues. Eleven Q/A redox-pairs were tested on cultured cells and cancer-bearing mice. The following parameters were analyzed: cell proliferation/viability, mitochondrial superoxide, steady-state ATP, tissue redox-state, tumor-associated NADH oxidase (tNOX) expression, tumor growth, and survival. Q/A redox-pairs containing unprenylated quinones exhibited strong dose-dependent antiproliferative and cytotoxic effects on cancer cells, accompanied by overproduction of mitochondrial superoxide and accelerated ATP depletion. In normal cells, the same redox-pairs did not significantly affect the viability and energy homeostasis, but induced mild mitochondrial oxidative stress, which is well tolerated. Benzoquinone/ascorbate redox-pairs were more effective than naphthoquinone/ascorbate, with coenzyme Q0/ascorbate exhibiting the most pronounced anticancer effects in vitro and in vivo. Targeted anticancer effects of Q/A redox-pairs and their tolerance to normal cells and tissues are attributed to: (i) downregulation of quinone prenylation in cancer, leading to increased mitochondrial production of semiquinone and, consequently, superoxide; (ii) specific and accelerated redox-cycling of unprenylated quinones and ascorbate mainly in the impaired cancerous mitochondria due to their redox imbalance; and (iii) downregulation of tNOX. [ABSTRACT FROM AUTHOR]

Protons participate in many reactions. In proteins, protons need paths to move in and out of buried active sites. The vectorial movement of protons coupled to electron transfer reactions establishes the transmembrane electrochemical gradient used for many reactions, including ATP synthesis. Protons move through hydrogen bonded chains of waters and hydroxy side chains via the Grotthuss mechanism and by proton binding and release from acidic and basic residues. MCCE analysis shows that proteins exist in a large number of protonation states. Knowledge of the equilibrium ensemble can provide a rational basis for setting protonation states in simulations that fix them, such as molecular dynamics (MD). The proton path into the QB site in the bacterial reaction centers (RCs) of Rb. sphaeroides is analyzed by MD to provide an example of the benefits of using protonation states found by the MCCE program. A tangled web of side chains and waters link the cytoplasm to QB. MCCE analysis of snapshots from multiple trajectories shows that changing the input protonation state of a residue in MD biases the trajectory shifting the proton affinity of that residue. However, the proton affinity of some residues is more sensitive to the input structure. The proton transfer networks derived from different trajectories are quite robust. There are some changes in connectivity that are largely restricted to the specific residues whose protonation state is changed. Trajectories with QB•− are compared with earlier results obtained with QB [Wei et. al Photosynthesis Research volume 152, pages153–165 (2022)] showing only modest changes. While introducing new methods the study highlights the difficulty of establishing the connections between protein conformation. [ABSTRACT FROM AUTHOR]

Two new cobalt bis-o-semiquinonato complexes, (Pyz-Phen)Co(3,6-DBSQ)2 (I) and (Bpyz)Co(3,6-DBSQ)2 (II) (Pyz-Phen = pyrazino[2,3-f][1,10]phenanthroline, Bpyz = bipyrazine, 3,6-DBSQ = 3,6-di-tert-butyl-o-benzoquinone radical anion), were synthesized. According to X-ray diffraction data, both complexes have a trigonal-prismatic geometry of the inner coordination sphere. The distribution of C–O and Co–O bond lengths, which reflects the valence state of the metal and the ligands, indicates that the complexes are formed by cobalt(II) surrounded by two semiquinone radical anions. The results of magnetochemical measurements show that the pyrazino[2,3-f][1,10]phenanthroline complex is a derivative of low-spin divalent cobalt, whereas its bipyrazine structural analogue is a high-spin cobalt(II) derivative. [ABSTRACT FROM AUTHOR]

The persulfate-based advanced oxidation processes employing heterogeneous photocatalysts to generate sulfate radicals (SO4•−) from peroxydisulfate ion (PDS, S2O82−) have been extensively investigated to remove organic pollutants. In this work, BiOX (X = Cl, Br, and I) photocatalysts were investigated to activate PDS and enhance the transformation rate of various organic substances under UV (398 nm) and Vis (400–700 nm) radiation. For BiOCl and BiOBr, in addition to excitability, the light-induced oxygen vacancies are decisive in the activity. Although without organic substances, the BiOI efficiency highly exceeds that of BiOBr and BiOCl for PDS activation (for BiOI, 15–20%, while for BiOBr and BiOCl, only 3–4% of the PDS transformed); each BiOX catalyst showed enhanced activity for 1,4-hydroquinone (HQ) transformation due to the semiquinone radical-initiated PDS activation. For sulfamethoxypyridazine (SMP), the transformation is driven by direct charge transfer, and the effect of PDS was less manifested. BiOI proved efficient for transforming various organic substances even under Vis radiation. The efficiency was enhanced by PDS addition (HQ is wholly transformed within 20 min, and SMP conversion increased from 40% to 90%) without damaging the catalyst; its activity did change over three consecutive cycles. Results related to the well-adsorbed trimethoprim (TRIM) and application of biologically treated domestic wastewater as a matrix highlighted the limiting factors of the method and visible light active photocatalyst, BiOI. [ABSTRACT FROM AUTHOR]

The exchange coupling, represented by the J parameter, is of tremendous importance in understanding the reactivity and magnetic behavior of open-shell molecular systems. In the past, it was the subject of theoretical investigations, but these studies are mostly limited to the interaction between metallic centers. The exchange coupling between paramagnetic metal ions and radical ligands has hitherto received scant attention in theoretical studies, and thus the understanding of the factors governing this interaction is lacking. In this paper, we use DFT, CASSCF, CASSCF/NEVPT2, and DDCI3 methods to provide insight into exchange interaction in semiquinonato copper(II) complexes. Our primary objective is to identify structural features that affect this magnetic interaction. We demonstrate that the magnetic character of Cu(II)-semiquinone complexes are mainly determined by the relative position of the semiquinone ligand to the Cu(II) ion. The results can support the experimental interpretation of magnetic data for similar systems and can be used for the in-silico design of magnetic complexes with radical ligands. [ABSTRACT FROM AUTHOR]

Dopamine has been widely used for surface modification of cardiovascular medical devices as it forms films on most substrates that provide functional groups for surface chemical modification. However, under oxidative stress, the phenolic hydroxyl group on dopamine can undergo reversible transformation into phenol-semiquinone-quinone, which can cause cytotoxicity and immunotoxicity. In this study, we measured the effects of semiquinone on the behavior of vascular wall cells and inflammatory cells under oxidative stress via ultraviolet irradiation with a hydrogen peroxide diluent. Na2S2O3 was used as a stabilizer to obtain a semiquinone-rich poly-dopamine film, then phenol-semiquinone-quinone ratio on its surface was evaluated at three irradiation-oxidation time points. We found that the poly-dopamine film with ultraviolet irradiation in hydrogen peroxide solution for 15 min had the highest semiquinone occupancy of 19.18%. In the experimental group irradiated for 15 min, endothelial cells were cultured statically for 3 days and the number of surface adherent endothelial cells in the group with added semiquinone stabilizer was reduced to 73% of that in the group without stabilizer, indicating that semiquinone rich surface inhibits adhesion and proliferation of endothelial cells; Smooth muscle cells were cultured statically for 3 days, and the number of adherent smooth muscle on surfaces without stabilizer was reduced to 75% of that on surfaces with stabilizer added, indicating that semiquinone rich surfaces promote smooth muscle proliferation. These results demonstrate that semiquinone can adversely affect the repair effect after implantation of cardiovascular materials. Therefore, our study provides a reference for the application and optimization of dopamine in cardiovascular implant materials. Graphical Abstract [ABSTRACT FROM AUTHOR]

Complex I is a key proton‐pumping enzyme in bacterial and mitochondrial respiratory electron transport chains. Using quantum chemistry and electrostatic calculations, we have examined the pKa of the reduced quinone QH−/QH2 in the catalytic cavity of complex I. We find that pKa (QH−/QH2) is very high, above 20. This means that the energy of a single protonation reaction of the doubly reduced quinone (i.e. the reduced semiquinone QH−) is sufficient to drive four protons across the membrane with a potential of 180 mV. Based on these calculations, we propose a possible scheme of redox‐linked proton pumping by complex I. The model explains how the energy of the protonation reaction can be divided equally among four pumping units of the pump, and how a single proton can drive translocation of four additional protons in multiple pumping blocks. [ABSTRACT FROM AUTHOR]

A tetradentate ligand, 1,1,2,2-tetrakis(4-(pyridin-4-yl)phenyl)ethene (TPPE), was adopted to construct a two-dimensional coordination polymer that incorporated valence tautomerism and luminescence, and the synergistic effect arising from energy transfer from TPPE to the semiquinone moieties was experimentally and theoretically uncovered. [ABSTRACT FROM AUTHOR]

Produced from root exudation activities and the decomposition of organic matter, gallic acid (GA) has been shown to be widely involved in the electron transfer reactions in soils as well as the transformation of redox-sensitive pollutants, while which was subject to the mediation of solid surfaces with the unknown mechanisms. This study aimed to investigate the catalytic effect of goethite on the Cr(VI) reduction by GA and explore the implication for the transformation of Cr(VI) in an oxisol. The results have shown that the adsorption of GA by goethite followed pseudo-second-order model. The GA was very stable at pH 4.0 in solution, whereas its oxidation by O2wasaccelerated in the presence of goethite as evidenced by the rapid appearance of IR peak of quinones. Similarly, the reduction of Cr(VI) by GA proceeded slowly with semi-quinonic species as the intermediate oxidation products of GA, while which would be significantly enhanced with the addition of goethite or oxisol. The mechanism was ascribed to that surface adsorption promoted the dissociation of –OH groups of GA and the subsequent formation of semiquinone radicals. Most importantly, these free radicals could be stabilised on the goethite surfaces, which was believed to be favourable for the reduction of Cr(VI). The catalysis of aqueous Fe(III) was excluded in this study due to its low concentration in the reaction systems. Furthermore, the majority of produced Cr(III) was adsorbed on the surfaces as the form of octahedron. The findings above indicated the environmental risk of Cr(VI)-polluted acidicsoilswould be alleviated when GA was present. [ABSTRACT FROM AUTHOR]

The mechanisms by which enzymes promote catalytic reactions efficiently through their structural changes remain to be fully elucidated. Recent progress in serial femtosecond X‐ray crystallography (SFX) using X‐ray free‐electron lasers (XFELs) has made it possible to address these issues. In particular, mix‐and‐inject serial crystallography (MISC) is promising for the direct observation of structural changes associated with ongoing enzymic reactions. In this study, SFX measurements using a liquid‐jet system were performed on microcrystals of bacterial copper amine oxidase anaerobically premixed with a substrate amine solution. The structure determined at 1.94 Å resolution indicated that the peptidyl quinone cofactor is in equilibrium between the aminoresorcinol and semiquinone radical intermediates, which accumulate only under anaerobic single‐turnover conditions. These results show that anaerobic conditions were well maintained throughout the liquid‐jet SFX measurements, preventing the catalytic intermediates from reacting with dioxygen. These results also provide a necessary framework for performing time‐resolved MISC to study enzymic reaction mechanisms under anaerobic conditions. [ABSTRACT FROM AUTHOR]

Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum(III) ion serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e--reduction of lanthanum-activated ortho-quinone to a semiquinone-lanthanum complex La(SQ-.)2, which undergoes a coupled O-H/C-H (PCHT: proton coupled hydride transfer) dehydrogenation for aerobic oxidation of alcohols with up to 330 h−1 TOF. A decade ago the first rare-earth-metal dependent enzyme was discovered, in which a non-redox lanthanide ion is central in the active site of a methanol dehydrogenase. Inspired by this discovery, here the authors show that an open-shell semi-quinone anionic radical species, complexed with lanthanum, could serve as a very efficient aerobic oxidation catalyst under ambient conditions. [ABSTRACT FROM AUTHOR]

PURPOSE: The extremophile group of microorganisms that has the ability to live in extreme conditions. These microbes have made beneficial adaptations in their genetic and metabolic makeup to survive the hostile environment. Extremophiles possess unique genes and know how to breed in extreme conditions. These characteristics make extremophiles unique and are now widely used to produce important biomolecules that are stable at high or low temperature, extreme pH values, and even in the presence of lethal pollutants. This chapter seeks to evaluate and provide sufficient evidence for the antioxidant and protective capabilities of the novel bacterial metabolites Semiquinone glucoside derivative and N-acetyl tryptophan glucoside as a microbial species isolated from Bacillus sp. INM-1 fermented broth and their possible therapeutic action. [ABSTRACT FROM AUTHOR]

We explored the capability of low-frequency Electron Paramagnetic Resonance (EPR) to noninvasively detect melanin (a stable semiquinone free radical) in the human skin. As previous in vitro studies on biopsies suggested that the EPR signal from melanin was different when measured in skin melanomas or benign nevi, we conducted a prospective first-in-man clinical EPR study in patients with skin lesions suspicious of melanoma. EPR spectra were obtained using a spectrometer operating at 1 GHz, with a surface coil placed over the area of interest. Two clinical studies were carried out: 1) healthy volunteers (n = 45) presenting different skin phototypes; 2) patients (n = 88) with skin lesions suspicious of melanoma (n = 100) requiring surgical resection. EPR data obtained before surgery were compared with histopathology results. The method was not sensitive enough to measure differences in melanin content due to changes in skin pigmentation. In patients, 92% of the spectra were analyzable. The EPR signal of melanin was significantly higher (p < 0.0001) in melanoma lesions (n = 26) than that in benign atypical nevi (n = 62). A trend toward a higher signal intensity (though not significant) was observed in high Breslow depth melanomas (a marker of skin invasion) than in low Breslow lesions. To date, no naturally occurring free radicals have been detected by low-frequency EPR systems adapted for clinical studies. Here, we demonstrated for the first time the ability of this technology to detect an endogenous free radical, opening new avenues for evaluating clinical EPR as a potential aid in the diagnosis of pigmented skin lesions. [Display omitted] • Skin lesion characterization before surgery could accelerate patient management. • Melanin is an endogenous stable semiquinone free radical. • First-in-man clinical EPR study to detect noninvasively melanin in skin lesions. • Study on 100 lesions suspicious of melanoma requiring surgical resection. • EPR signal of melanin was higher in melanoma compared to atypical nevi. [ABSTRACT FROM AUTHOR]

The EtfAB components of two bifurcating flavoprotein systems, the crotonyl-CoA-dependent NADH:ferredoxin oxidoreductase from the bacterium Megasphaera elsdenii and the menaquinone-dependent NADH:ferredoxin oxidoreductase from the archaeon Pyrobaculum aerophilum, have been investigated. With both proteins, we find that removal of the electron-transferring flavin adenine dinucleotide (FAD) moiety from both proteins results in an uncrossing of the reduction potentials of the remaining bifurcating FAD; this significantly stabilizes the otherwise very unstable semiquinone state, which accumulates over the course of reductive titrations with sodium dithionite. Furthermore, reduction of both EtfABs depleted of their electron-transferring FAD by NADH was monophasic with a hyperbolic dependence of reaction rate on the concentration of NADH. On the other hand, NADH reduction of the replete proteins containing the electrontransferring FAD was multiphasic, consisting of a fast phase comparable to that seen with the depleted proteins followed by an intermediate phase that involves significant accumulation of FAD⋅−, again reflecting uncrossing of the half-potentials of the bifurcating FAD. This is then followed by a slow phase that represents the slow reduction of the electron-transferring FAD to FADH−, with reduction of the now fully reoxidized bifurcating FAD by a second equivalent of NADH. We suggest that the crossing and uncrossing of the reduction half-potentials of the bifurcating FAD is due to specific conformational changes that have been structurally characterized. [ABSTRACT FROM AUTHOR]

We present the structure of a photoactivated animal (6-4) photolyase in its radical pair state, captured by serial crystallography. We observe how a conserved asparigine moves towards the semiquinone FAD chromophore and stabilizes it by hydrogen bonding. Several amino acids around the final tryptophan radical rearrange, opening it up to the solvent. The structure explains how the protein environment stabilizes the radical pair state, which is crucial for function of (6-4) photolyases and cryptochromes. [ABSTRACT FROM AUTHOR]

Bifurcating electron transfer flavoproteins (Bf ETFs) are important redox enzymes that contain two flavin adenine dinucleotide (FAD) cofactors, with contrasting reactivities and complementary roles in electron bifurcation. However, for both the "electron transfer" (ET) and the "bifurcating" (Bf) FADs, the only charged amino acid within 5 Å of the flavin is a conserved arginine (Arg) residue. To understand how the two sites produce different reactivities utilizing the same residue, we investigated the consequences of replacing each of the Arg residues with lysine, glutamine, histidine, or alanine. We show that absence of a positive charge in the ET site diminishes accumulation of the anionic semiquinone (ASQ) that enables the ET flavin to act as a single electron carrier, due to depression of the oxidized versus. ASQ reduction midpoint potential, E°OX/ASQ. Perturbation of the ET site also affected the remote Bf site, whereas abrogation of Bf FAD binding accelerated chemical modification of the ET flavin. In the Bf site, removal of the positive charge impaired binding of FAD or AMP, resulting in unstable protein. Based on pH dependence, we propose that the Bf site Arg interacts with the phosphate(s) of Bf FAD or AMP, bridging the domain interface via a conserved peptide loop ("zipper") and favoring nucleotide binding. We further propose a model that rationalizes conservation of the Bf site Arg even in non-Bf ETFs, as well as AMP's stabilizing role in the latter, and provides a mechanism for coupling Bf flavin redox changes to domain-scale motion. [ABSTRACT FROM AUTHOR]

Photosystem II (PSII), the water/plastoquinone photo-oxidoreductase, plays a key energy input role in the biosphere. QA•-, the reduced semiquinone form of the nonexchangeable quinone, is often considered capable of a side reaction with O2, forming superoxide, but this reaction has not yet been demonstrated experimentally. Here, using chlorophyll fluorescence in plant PSII membranes, we show that O2 does oxidize QA•- at physiological O2 concentrations with a t1/2 of 10 s. Superoxide is formed stoichiometrically, and the reaction kinetics are controlled by the accessibility of O2 to a binding site near QA•-, with an apparent dissociation constant of 70 ± 20 µM. Unexpectedly, QA•- could only reduce O2 when bicarbonate was absent from its binding site on the nonheme iron (Fe2+) and the addition of bicarbonate or formate blocked the O2-dependant decay of QA•-. These results, together with molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations, indicate that electron transfer from QA•- to O2 occurs when the O2 is bound to the empty bicarbonate site on Fe2+. A protective role for bicarbonate in PSII was recently reported, involving long-lived QA•- triggering bicarbonate dissociation from Fe2+ [Brinkert et al., Proc. Natl. Acad. Sci. U.S.A. 113, 12144-12149 (2016)]. The present findings extend this mechanism by showing that bicarbonate release allows O2 to bind to Fe2+ and to oxidize QA•-. This could be beneficial by oxidizing QA•- and by producing superoxide, a chemical signal for the overreduced state of the electron transfer chain. [ABSTRACT FROM AUTHOR]

Because cancer cells grow at an extremely fast rate,their internal oxygen demand far exceeds the blood supply, resulting in hypoxia in the tissues. In solid tumors, hypoxia leads to over-expression of some enzymes, among which the concentrations of nitroreductase, azo red uctase and diaphorase change the most. In this study, we took diaphorase as the detection target and synthesized a novel fluorescent probe Milla 1,with the benzoquinone as a recognition group.With nicotinamide adenine dinuclectide,it underwent a reduction reaction to generate hydroquinone (or semiquinone) and released the fluorophore through a bond-breaking reaction,achieving the optical detection effects towards diaphorase.In the specific recognition reaction,the diaphorase content fluctuation was directly detected by the changes of fluorescence values at 635 nm,and then the hypoxia degree in the relevant hypoxic area was deduced.Because probe Milla 1 has the advantages of high selectivity, good photostability, and low toxicity, it was successfully used in detection and optical imaging experiments of endogenous and exogenous diaphorase in HeLa cells and Caenorhabditis elegans. [ABSTRACT FROM AUTHOR]

Cyclic voltammetry was used to provide the electrochemical evidence of catalytic catechol oxidation by ruthenium organometallics of chloro-substituted 2'-hydroxychalcones and to get partial insight into the mechanistic process. The catalytic properties of organometallics were measured in methanol, ethanol, and acetonitrile and the solvent effect was confirmed. The assignation of peaks in cyclic voltammograms was made systematically by characterizing all individual components of the catalytic mixture. The cathodic peak arising from electrochemical reduction of catalytically formed quinone near − 0.4 V vs. Ag/AgCl evolves over time and can be used for kinetic measurements. The formation of new peak at + 0.1 V vs. Ag/AgCl indicates that catechol oxidation to quinone proceeds via semiquinone formation in the presence of ruthenium organometallics. The kinetic profile of the catechol oxidation in the presence of ruthenium organometallics differs from each other and the rates of oxidation are well correlated with spectrophotometric measurements. Several advantages of electrochemistry over spectrophotometry were addressed. The obtained results demonstrate that electrochemistry can be used as a comparative method to spectrophotometry to measure the catalytic catechol oxidation and provide useful qualitative and quantitative data on kinetics and mechanism of catechol oxidation. [ABSTRACT FROM AUTHOR]

Quinones are ubiquitous in nature and form one of the largest class of antitumor agents approved for clinical use. They are known to be efficient in inhibiting cancer cells growth. Under physiological conditions they can undergo non‐enzymatic one‐electron reduction to give the moderately toxic species of semiquinone radical‐anion. Thus, electrochemical study of quinones might provide a basic knowledge on semi‐quinone radicals formation in both in vivo and in vitro under different media. Several processes are outlined briefly and discussed in the present article. Previously we investigated the electrochemical and spectral properties of ω‐N‐quinonyl amino acids. Such quinone‐bearing peptides are known to be cytotoxic and of potential clinical significance. We were able to prove that the ω‐amino quinonyl compounds are very effective in producing stable semiquinone radicals. Moreover, a direct relation was found between the first reduction potentials of the quinonyl moiety and their reactivity towards the ω‐amino acids. In order to increase our knowledge of such amino quinonyl compounds and enlarge the arsenal of such cytotoxic compounds, a series of N,N‐diquinonyl amines (1–6) bearing an internal proton (stems from the NH moiety) were synthesized. Their electron‐transfer capabilities were probed by cyclic voltammetry measurements, in dichloromethane. It was found that the acidic NH group linking the two quinonyl moieties undergoes an initial electrochemical reduction step and generates a nitride anion. This step is followed by further reductions to yield quasi‐stable semiquinone radicals and polyanions, Since these acidic diquinones (1–6) serve also as a source of internal proton donors even in non‐polar medium, they might cause protonation of basic radical‐anions and polyanion intermediates during the various electrochemical stages. The processes are demonstrated and discussed by analyzing different mechanistic schemes. The successful generation of relatively stable semiquinone radicals is a prerequisite for the manifestation of site directed antitumor activity by these bis‐quinonyl amino derivatives. Based on the values of their redox potentials some of them could be promising candidates for clinical development. [ABSTRACT FROM AUTHOR]

The M17-F aptamer sequences can significantly improve the affinity of cubic Ag 2 O nanoparticles to the substrate 2,4-dichlorophenol and then enhance their laccase-mimic activity, which facilitate to establish a novel and stable smartphone-integrated colorimetric biosensor for the monitoring of malathion residue in food. [Display omitted] • Enhancement effect of aptamer on laccase mimetics was first studied. • Aptamer accelerates the generation of semiquinone radicals in laccase-like system. • Smartphone-integrated biosensor enables the result to be recognized by naked eye. • The colorimetric biosensor is rapid, stable, cost-effective and selective. Malathion causes a serious threat to human health due to its widespread use in the environment. Herein, a novel and stable smartphone-integrated colorimetric biosensor for malathion detection is firstly established based on aptamer-enhanced laccase-mimicking activity. The results indicate that the M17-F aptamer can increase the affinity of Ag 2 O nanoparticles to the substrate 2,4-dichlorophenol and enhance their laccase-mimicking activity. Thus, abundant semiquinone radicals are produced in the catalytic system, which are combined with chromogenic agent to generate dark red products. The corresponding RGB values for the colour change of the solution can be easily obtained using smartphones, which is used for the rapid detection of malathion. The established biosensor for malathion has a limit of detection as low as 5.85 nmol·L−1, and displays good selectivity for other competitive pesticides. Moreover, further studies have verified the applicability of the biosensor in actual samples, indicating that it may have the potential for application in malathion detection in food. [ABSTRACT FROM AUTHOR]

NewPHL is a recently discovered subgroup of ancestral DNA photolyases. Its domain architecture displays pronounced differences from that of canonical photolyases, in particular at the level of the characteristic electron transfer chain, which is limited to merely two tryptophans, instead of the "classical" three or four. Using transient absorption spectroscopy, we show that the dynamics of photoreduction of the oxidized FAD cofactor in the NewPHL begins similarly as that in canonical photolyases, i.e., with a sub-ps primary reduction of the excited FAD cofactor by an adjacent tryptophan, followed by migration of the electron hole towards the second tryptophan in the tens of ps regime. However, the resulting tryptophanyl radical then undergoes an unprecedentedly fast deprotonation in less than 100 ps in the NewPHL. In spite of the stabilization effect of this deprotonation, almost complete charge recombination follows in two phases of ~ 950 ps and ~ 50 ns. Such a rapid recombination of the radical pair implies that the first FAD photoreduction step, i.e., conversion of the fully oxidized to the semi-quinone state, should be rather difficult in vivo. We hence suggest that the flavin chromophore likely switches only between its semi-reduced and fully reduced form in NewPHL under physiological conditions. [ABSTRACT FROM AUTHOR]

Cytochrome bc1 (mitochondrial complex III) catalyzes electron transfer from quinols to cytochrome c and couples this reaction with proton translocation across lipid membrane; thus, it contributes to the generation of protonmotive force used for the synthesis of ATP. The energetic efficiency of the enzyme relies on a bifurcation reaction taking place at the Qo site which upon oxidation of ubiquinol directs one electron to the Rieske 2Fe2S cluster and the other to heme bL. The molecular mechanism of this reaction remains unclear. A semiquinone spin-coupled to the reduced 2Fe2S cluster (SQo-2Fe2S) was identified as a state associated with the operation of the Qo site. To get insights into the mechanism of the formation of this state, we first constructed a mutant in which one of the histidine ligands of the iron ion of heme bLRhodobacter capsulatus cytochrome bc1 was replaced by asparagine (H198N). This converted the low-spin, low-potential heme into the high-spin, high-potential species which is unable to support enzymatic turnover. We performed a comparative analysis of redox titrations of antimycin-supplemented bacterial photosynthetic membranes containing native enzyme and the mutant. The titrations revealed that H198N failed to generate detectable amounts of SQo-2Fe2S under neither equilibrium (in dark) nor nonequilibrium (in light), whereas the native enzyme generated clearly detectable SQo-2Fe2S in light. This provided further support for the mechanism in which the back electron transfer from heme bL to a ubiquinone bound at the Qo site is mainly responsible for the formation of semiquinone trapped in the SQo-2Fe2S state in R. capusulatus cytochrome bc1.

NADH: ubiquinone oxidoreductase (complex I) is the first enzyme complex of the respiratory chain. Complex I is a redox-driven proton pump that contributes to the proton motive force that drives ATP synthase. The structure of complex I has been analyzed by x-ray crystallography and electron cryo-microscopy and is now well-described. The ubiquinone (Q) reduction site of complex I is buried in the peripheral arm and a tunnel-like structure is thought to provide access for the hydrophobic substrate from the membrane. Several intermediate binding positions for Q in the tunnel were identified in molecular simulations. Structural data showed the binding of native Q molecules and short chain analogs and inhibitors in the access pathway and in the Q reduction site, respectively. We here review the current knowledge on the interaction of complex I with Q and discuss recent hypothetical models for the coupling mechanism.

With the aim of assessing whether polydiphenylamine (PDPA) can be used as an inductor layer for stabilizing the radical cation in the polymer matrix of poly(3-hexylthiophene) (P3HT), as used in photovoltaic devices, interphases between P3HT and PDPA films were prepared electrochemically on platinum (Pt), tin-doped indium oxide (ITO), and gold (Au) electrodes in lithium perchlorate-acetonitrile (LiClO4-ACN) solution, being denoted as Pt/PDPA:P3HT, ITO/PDPA:P3HT, and Au/PDPA:P3HT, respectively. To characterize the interphases deposited on the different electrodes, Raman spectroscopy was used to monitor the behavior of the segments in the blend matrix compared with homopolymer films deposited on the different conducting electrodes, to study the stability of the quinone and semiquinone segments of PDPA and P3HT. Bode phase diagrams obtained by electrochemical impedance spectroscopy (EIS) were used to verify the prepared systems as a function of time since preparation. It was found that the polaron structure (oxidized thiophene ring radical cation segments) was stable for 97 h when the blend was synthesized under controlled temperature (22°C) conditions at constant potential of 1.70 V applied for 60 s to the surface of the Pt electrode. For the blends synthesized on ITO and Au, these conditions resulted in greater stabilization of the bipolaron structure (dication segments). These findings reveal the effect of PDPA induction over the P3HT layer in stabilizing the semiquinone and quinone forms, compared with interfaces formed solely by homopolymers in contact with the different electrodes. [ABSTRACT FROM AUTHOR]

Light-induction of an anionic semiquinone (SQ) flavin radical in Drosophila cryptochrome (dCRY) alters the dCRY conformation to promote binding and degradation of the circadian clock protein Timeless (TIM). Specific peptide ligation with sortase A attaches a nitroxide spin-probe to the dCRY C-terminal tail (CTT) while avoiding deleterious side reactions. Pulse dipolar electron-spin resonance spectroscopy from the CTT nitroxide to the SQ shows that flavin photoreduction shifts the CTT ~1 nm and increases its motion, without causing full displacement from the protein. dCRY engineered to form the neutral SQ serves as a dark-state proxy to reveal that the CTT remains docked when the flavin ring is reduced but uncharged. Substitutions of flavin-proximal His378 promote CTT undocking in the dark or diminish undocking in the light, consistent with molecular dynamics simulations and TIM degradation activity. The His378 variants inform on recognition motifs for dCRY cellular turnover and strategies for developing optogenetic tools. Chandrasekaran et al. engineered the Drosophila circadian clock protein Cryptochrome (dCRY) to form the neutral semiquinone, which serves as a dark-state proxy. They find that the C-terminal tail of dCRY remains docked when the flavin ring is reduced but uncharged. dCRY His378 variants provide insights into the recognition motifs for dCRY turnover and strategies for optogenetic tools. [ABSTRACT FROM AUTHOR]

The flavin‐based electron bifurcation (FBEB) system from Acidaminococcus fermentans is composed of the electron transfer flavoprotein (EtfAB) and butyryl‐CoA dehydrogenase (Bcd). α‐FAD binds to domain II of the A‐subunit of EtfAB, β‐FAD to the B‐subunit of EtfAB and δ‐FAD to Bcd. NADH reduces β‐FAD to β‐FADH−, which bifurcates one electron to the high potential α‐FAD•− semiquinone followed by the other to the low potential ferredoxin (Fd). As deduced from crystal structures, upon interaction of EtfAB with Bcd, the formed α‐FADH− approaches δ‐FAD by rotation of domain II, yielding δ‐FAD•−. Repetition of this process leads to a second reduced ferredoxin (Fd−) and δ‐FADH−, which reduces crotonyl‐CoA to butyryl‐CoA. In this study, we measured the redox properties of the components EtfAB, EtfaB (Etf without α‐FAD), Bcd, and Fd, as well as of the complexes EtfaB:Bcd, EtfAB:Bcd, EtfaB:Fd, and EftAB:Fd. In agreement with the structural studies, we have shown for the first time that the interaction of EtfAB with Bcd drastically decreases the midpoint reduction potential of α‐FAD to be within the same range of that of β‐FAD and to destabilize the semiquinone of α‐FAD. This finding clearly explains that these interactions facilitate the passing of electrons from β‐FADH− via α‐FAD•− to the final electron acceptor δ‐FAD•− on Bcd. The interactions modulate the semiquinone stability of δ‐FAD in an opposite way by having a greater semiquinone stability than in free Bcd. [ABSTRACT FROM AUTHOR]

We show the synthesis of a redox‐active quinone, 2‐methoxy‐1,4‐hydroquinone (MHQ), from a bio‐based feedstock and its suitability as electrolyte in aqueous redox flow batteries. We identified semiquinone intermediates at insufficiently low pH and quinoid radicals as responsible for decomposition of MHQ under electrochemical conditions. Both can be avoided and/or stabilized, respectively, using H3PO4 electrolyte, allowing for reversible cycling in a redox flow battery for hundreds of cycles. [ABSTRACT FROM AUTHOR]

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2′′,3′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices. [ABSTRACT FROM AUTHOR]

Flavin-mediated photocatalytic oxidations are established in synthetic chemistry. In contrast, their use in reductive chemistry is rare. Deazaflavins with a much lower reduction potential are even better suited for reductive chemistry rendering also deazaflavin semiquinones as strong reductants. However, no direct evidence exists for the involvement of these radical species in reductive processes. Here, we synthesise deazaflavins with different substituents at C5 and demonstrate their photocatalytic activity in the dehalogenation of p-halogenanisoles with best performance under basic conditions. Mechanistic investigations reveal a consecutive photo-induced electron transfer via the semiquinone form of the deazaflavin as part of a triplet-correlated radical pair after electron transfer from a sacrificial electron donor to the triplet state. A second electron transfer from the excited semiquinone to p-halogenanisoles triggers the final product formation. This study provides first evidence that the reductive power of excited deazaflavin semiquinones can be used in photocatalytic reductive chemistry. Flavins and deazaflavins are well suited for photoredox processes but their application in photoreductions is challenging. Here, the authors provide direct evidence of the high reductive power of excited deazaflavin semiquinones and their application in catalytic photodehalogenations. [ABSTRACT FROM AUTHOR]

The fluorescence properties of FAD of Thermotoga maritima thioredoxin reductase (TmTR), taken together with the amino acid sequences and structures of similar TRs, are consistent with the interdomain rotation in the catalysis of TmTR. The standard redox potential of FAD of TmTR, -0.230 V, determined by the reactions with 3-acetylpyridine adenine dinucleotide (APAD+/APADH) redox couple, is close to that of E. coli TR. During the reduction of duroquinone with TmTR, the transient formation of neutral FAD semiquinone, and, possibly, FADH2-NAD+ complex was observed. This shows that in spite of obligatory twoelectron (hydride)-transfer between NADH and physiological disulfide oxidants, the FAD cofactor of TmTR may exist under a stable semiquinone form. [ABSTRACT FROM AUTHOR]

In the present study, the EPR spectroscopy was used to evidence differences in fruits of organically and conventionally grown bananas belonging to musaceae family. If in the investigated samples would be detected specific changes related to paramagnetic resonant centers, these could be regarded as a spectroscopic fingerprint in the differentiation of the organic and conventional fruits and vegetables. The EPR spectra were recorded from freeze-dried shell and pulp samples. The main paramagnetic species (iron, manganese and native semiquinone free radical) delivered for the investigated samples slight different EPR signals. In this stage, the results obtained by EPR testing put in evidence sensible differences between the two classes of samples, and draw the attention on differences in EPR signals recorded from banana pulp and shell. [ABSTRACT FROM AUTHOR]

Formate oxidation to carbon dioxide is a key reaction in one-carbon compound metabolism, and its reverse reaction represents the first step in carbon assimilation in the acetogenic and methanogenic branches of many anaerobic organisms. The molybdenum-containing dehydrogenase FdsABG is a soluble NAD+-dependent formate dehydrogenase and a member of the NADH dehydrogenase superfamily. Here, we present the first structure of the FdsBG subcomplex of the cytosolic FdsABG formate dehydrogenase from the hydrogen-oxidizing bacterium Cupriavidus necator H16 both with and without bound NADH. The structures revealed that the two iron-sulfur clusters, Fe4S4 in FdsB and Fe2S2 in FdsG, are closer to the FMN than they are in other NADH dehydrogenases. Rapid kinetic studies and EPR measurements of rapid freeze-quenched samples of the NADH reduction of FdsBG identified a neutral flavin semiquinone, FMNH, not previously observed to participate in NADH-mediated reduction of the FdsABG holoenzyme. We found that this semiquinone forms through the transfer of one electron from the fully reduced FMNH-, initially formed via NADH-mediated reduction, to the Fe2S2 cluster. This Fe2S2 cluster is not part of the on-path chain of iron-sulfur clusters connecting the FMN of FdsB with the active-site molybdenum center of FdsA. According to the NADH-bound structure, the nicotinamide ring stacks onto the re-face of the FMN. However, NADH binding significantly reduced the electron density for the isoalloxazine ring of FMN and induced a conformational change in residues of the FMN-binding pocket that display peptide-bond flipping upon NAD+ binding in proper NADH dehydrogenases. [ABSTRACT FROM AUTHOR]

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