Model Quinone Research Articles

Molecular Unravelling of the Structural Effect of Quinone Redox Mediators on Oxygen Reduction Reaction in Aprotic Lithium-Oxygen Batteries.

Redox mediators (RMs) provide tantalizing solutions to unlock the energy capabilities of aprotic lithium-oxygen (Li-O2) batteries by driving solution-mediated Li2O2 growth. However, the structural effect of RMs on the catalytic efficiency of the oxygen reduction reaction remains incompletely understood. Herein, we present the interplay between the structure of RMs and their discharge capabilities by a comparative study of model quinone (Q)-based RMs. Specifically, at low current densities, incorporating electron-withdrawing groups onto the Q ring can positively move the discharge potential and deliver larger discharge capacity by extending the lifespan of the LiQO2 intermediate and allowing for Li2O2 growth into deeper electrolyte regions. Conversely, at high current densities, the absence of electron-withdrawing groups facilitates homogeneous reaction kinetics from LiQ to regenerate Q (i.e., decreased lifespan of LiQO2), mitigating electrode potential polarization and preserving catalytic activity of Q for higher discharge capacity. The work establishes structure-property relationships that guide the rational design of RMs toward next-generation Li-O2 batteries.

Identifying Quinones in Complex Aqueous Environmental Media (Biochar Extracts) through Tagging with Cysteine and Cysteine-Contained Peptides and High Resolution Mass Spectrometry Analysis.

Quinones are among the most important components in natural organic matter (NOM) for redox reactions; however, no quinones in complex environmental media have been identified. To aid the identification of quinone-containing molecules in ultracomplex environmental samples, we developed a chemical tagging method that makes use of a Michael addition reaction between quinones and thiols (-SH) in cysteine (Cys) and cysteine-contained peptides (CCP). After the tagging, candidates of quinones in representative aqueous environmental samples (water extractions of biochar) were identified through high-resolution mass spectrometry (HRMS) analysis. The MS and UV spectra analysis showed rapid reactions between Cys/CCP and model quinones with β-carbon from the same benzene ring available for Michael addition. The tagging efficiency was not influenced by other co-occurring nonquinone representative compounds, including caffeic acid, cinnamic acid, and coumaric acid. Cys and CCP were used to tag quinones in water extractions of biochars, and possible candidates of quinones (20 and 53 based on tagging with Cys and CCP, respectively) were identified based on the HRMS features for products of reactions with Cys/CCP. This study has successfully demonstrated that such a Michael addition reaction can be used to tag quinones in complex environmental media and potentially determine their identities. The method will enable an in-depth understanding of the redox chemistry of NOM and its critical chemical compositions and structures.

Interaction between peptide and solid lignin suggest mechanisms of abiotic covalent bond formation

The mechanisms behind the formation of soil organic matter (SOM) and associated nitrogen immobilization remain partially elusive, while ongoing research continues to shed light on carbon and nitrogen sequestration in the environment. Studies show that quinone-like structures within alkali-soluble humic extracts of SOM can bond covalently with nitrogen-containing molecules derived from proteinaceous organic matter. However, direct molecular evidence for this chemical bonding with lignin in the solid form and non-solvent-extracted SOM is lacking. Using gel-state 1H-15N heteronuclear single quantum coherence (HSQC) high-resolution magic angle spinning (HRMAS) nuclear magnetic resonance (NMR) spectroscopy, we demonstrate that a 15N-labeled peptide (glycine-glycine-glycine-arginine, GGGR), can covalently bind to solid, untreated, brown rotted wood as well as 1,2-naphthoquinone, a quinone-model molecule typically found in degraded lignin. The new peaks in both reactions represent a change in the environment surrounding the N-functional groups. Interaction of the 15N-labeled peptide with the model quinone produced NMR cross peaks that are similar to those of the solid lignin. This suggests that the quinone-like structures are the most likely functional groups to form covalent bonds with peptides in the degraded lignin. Both Michael addition and Schiff base formation is proposed when the peptide GGGR interacts with white oak lignin and 1,2-naphthoquinone. These processes are of considerable importance to N incorporation into SOM and offer insights into how proteinaceous molecules could potentially be preserved and sequestered through covalent bond formation.

Analytical fingerprint of the interactions between quinones and bioenergetic membranes in Chlamydomonas reinhardtii

The use of intact photosynthetic organisms (e.g. microalgae or cyanobacteria) for biotechnological approaches is a promising avenue to extract sustainable energy from oxygenic photosynthesis. More particularly, exogenous quinones can act as electron shuttles to reroute part of the photosynthetic electron flow of living cells to an outer collecting electrode. This encouraging approach is however hampered by reported poisoning or side-effects of exogenous quinones on the cell bioenergetics. In order to contribute to understand those effects, we investigated the modes and sites of interaction of two model quinones (2,6-DCBQ and 2,6-DMBQ) with the respiratory and photosynthetic electron transfer chains of the green alga Chlamydomonas reinhardtii. By considering different analytical tools (chlorophyll a fluorescence, transient absorption spectrometry, O2 consumption rate), the two exogenous quinones are shown to hamper the photosynthetic electron transfer from photosystem II (PSII) to the cytochrome b6f and, at longer term, PSII damage. In addition, the investigated quinones initiate the suppression of mitochondrial respiration, illustrated by the decrease of O2 consumption. This results in the diminution of the ATP exchanges between mitochondrion and chloroplast responsible for the generation of the proton motive force across the thylakoid in darkness, and in turn affects the performances of the CF1FO ATPase. For all those effects, 2,6-DCBQ was more effective than 2,6-DMBQ in agreement with its higher redox potential and partition coefficient values. This work provides a new framework for the study of biophotovoltaic devices using photosynthetic organisms and quinones as mediators and could be extended to find the best candidates combining efficient bioelectricity production and limited toxicity.

A Unified Charge Storage Mechanism to Rationalize the Electrochemical Behavior of Quinone-Based Organic Electrodes in Aqueous Rechargeable Batteries.

Due to their eco-sustainability and versatility, organic electrodes are promising candidates for large-scale energy storage in rechargeable aqueous batteries. This is notably the case of aqueous hybrid batteries that pair the low voltage of a zinc anode with the high voltage of a quinone-based (or analogue of quinone-based) organic cathode. However, the mechanisms governing their charge-discharge cycles remain poorly understood and are even a matter of debate and controversy. No consensus exists on the charge carrier in mild aqueous electrolytes, especially when working in an electrolyte containing a multivalent metal cation such as Zn2+. In this study, we comprehensively investigate the electrochemical reactivity of two model quinones, chloranil, and duroquinone, either diluted in solution or incorporated into carbon-based composite electrodes. We demonstrate that a common nine-member square scheme proton-coupled electron transfer mechanism allows us to fully describe and rationalize their electrochemical behavior in relation to the pH and chemical composition of the aqueous electrolyte. Additionally, we highlight the crucial role played by the pKas associated with the reduced states of quinones in determining the nature of the charge carrier that compensates for the negative charges reversibly injected in the active material. Finally, contrary to the widely reported findings for Zn/organic batteries, we unequivocally establish that the predominant solid-state charge carriers in Zn2+-based mild aqueous electrolytes are not multivalent Zn2+ cations but rather protons supplied by the weakly acidic hexaaqua metal ions (i.e., [Zn(H2O)6]2+]).

Magnetite-equipped algal-rich sediments for microbial fuel cells: Remediation of sediment organic matter pollution and mechanisms of remote electron transfer

Using the bio-electrochemical methods for the restoration of high algae sediments is full of potential and challenges. How to promote extracellular electron transfer (EET) process in microbial fuel cells (MFC) is the key bottleneck. The study had explored the potential application of magnetite on accelerating electron transfer for improving the output of MFC and sediment pollution remediation. The results indicated that the organic matter degradation rate showed a remarkable increase of 27.45 %, and the voltage output was approximately 1.68 times higher compared to the MFC configured with regular sediment. Abundant electroactive bacteria (EABs), such as Geobacter and Burkholderiaceae, and fermentative bacteria were responsible for these results, accompanied by the enhanced fluorescence of humic substances (HS), increased concentration and activity of cytochrome C (25.05 % and 21.12 %), as well as elevated extracellular polymeric substance content. Moreover, the intrinsic EET mechanisms among Fe-oxides, HS, and EABs were explored. According to the electrochemical analysis and substance transformation, the EET process involved four stages: magnetite-enhanced direct electron transfer via strong conductivity, iron respiration mediating electron transfer to the electrode, the model quinone substance acting as an electron shuttle facilitating EET and iron reduction, and iron cycling mediating electron transfer. This study provides an effective strategy for pollution remediation in algal-rich sediment, which was beneficial for the harmless treatment and resource utilization of both algae and sediment, simultaneously.

Quantification of Quinones in Environmental Media by Chemical Tagging with Cysteine-Containing Peptides Coupled to Size Exclusionary Separation.

Quinones are one of the most important redox-reactive organic compounds in natural environments, such as soil, water, and sediment, playing an important role in regulating the environmental processes and biogeochemical cycles of critical elements under climate change, including the influences of extreme events such as wildfires. However, to date, no existing methods can quantify quinones in complex environmental media. To overcome this challenge, a quantification method was developed by coupling chemical tagging of quinones by cysteine-containing nonaromatic peptides (Cpep) through a Michael addition reaction with size exclusionary chromatography (SEC) separation and ultraviolet (UV) analysis─leveraging on the characteristic absorbance of aromatic rings at 254 nm and molecular size of peptide. The method was demonstrated using model quinones, including 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 1,4-anthraquinone (AQ), with a detection limit of 3.3, 0.7, and 0.2 μM, respectively. Concentrations of quinones in water extractions of biochars, soils, and wildfire-derived ashes were determined to range from 0.8 to 14 μM and were positively correlated with their redox reactivity determined by a chemical assay. This method provides a novel rapid quantification of quinones in complex environmental media as well as a quick assessment for redox reactivity and opens up new avenues for studying environmental transformation and remediation of contaminants.

Insight into the model quinone compound AQDS mediated the Mn(II) abiotic oxidation and mineral mineralization on hematite surface under oxic and neutral conditions

Oxidation of Mn(II) and the subsequent mineral mineralization can significantly influence the transport and fate of trace metals and organic pollutants in the soil and sediment environments. Abiotic oxidation of Mn(II) on the surface of Fe mineral in the open air is a vital formation pathway of Mn (oxyhydr)oxides; however, the impact of electron shuttle substances at neutral pH has seldomly been reported. Anthraquinone-2, 6-disulfonate (AQDS) is a typical electron shuttle and might affect the abiotic oxidation of Mn(II). In this study, abiotic oxidation of Mn(II) on the surface of a hematite substrate and mineralization of oxidation product in the presence of AQDS (0–1 mM) were investigated at pH 7.5 in the open air. In the Mn(II)-AQDS treatments, most Mn(II) remained in solution, suggesting that AQDS shows little catalytic activity in the abiotic oxidation of Mn(II) without hematite present. By contrast, Mn(II) was rapidly oxidized, and oxidation rates increased from 4.2 × 10−2 to 8.8 × 10−2 h−1 with increasing AQDS concentration from 0 to 1 mM in the hematite-Mn(II)-AQDS treatments. Abiotic oxidation of Mn(II) produced granular-like Mn3O4, fibrous-like β-MnOOH, and rod-like γ-MnOOH, and no other secondary Fe minerals were found. The proposed mechanism of the AQDS-mediated abiotic oxidation of Mn(II) on a hematite surface is the AQDS serving as an electronic medium with semiconductor hematite as a foreign substrate, promoting electron transfer between oxygen and Mn(II) on its surface. Our results revealed that electron mediators impart crucial control on not only the rate but also the compositions of Mn(II) oxidation product. The findings of this study provide new insights into the geochemical functions of electron shuttles on the dynamics of redox-sensitive elements under oxic and neutral conditions and expand the understanding of interfacial heterogeneous nucleation reactions at the molecular scale.

Modular Metal-Quinone Networks with Tunable Architecture and Functionality.

Nanostructured materials with tunable structures and functionality are of interest in diverse areas. Herein, metal ions are coordinated with quinones through metal-acetylacetone coordination bonds to generate a class of structurally tunable, universally adhesive, hydrophilic, and pH-degradable materials. A library of metal-quinone networks (MQNs) is produced from five model quinone ligands paired with nine metal ions, leading to the assembly of particles, tubes, capsules, and films. Importantly, MQNs show bidirectional pH-responsive disassembly in acidic and alkaline solutions, where the quinone ligands mediate the disassembly kinetics, enabling temporal and spatial control over the release of multiple components using multilayered MQNs. Leveraging this tunable release and the inherent medicinal properties of quinones, MQN prodrugs with a high drug loading (>89 wt %) are engineered using doxorubicin for anti-cancer therapy and shikonin for the inhibition of the main protease in the SARS-CoV-2 virus.

Modular Metal‐Quinone Networks with Tunable Architecture and Functionality

AbstractNanostructured materials with tunable structures and functionality are of interest in diverse areas. Herein, metal ions are coordinated with quinones through metal‐acetylacetone coordination bonds to generate a class of structurally tunable, universally adhesive, hydrophilic, and pH‐degradable materials. A library of metal‐quinone networks (MQNs) is produced from five model quinone ligands paired with nine metal ions, leading to the assembly of particles, tubes, capsules, and films. Importantly, MQNs show bidirectional pH‐responsive disassembly in acidic and alkaline solutions, where the quinone ligands mediate the disassembly kinetics, enabling temporal and spatial control over the release of multiple components using multilayered MQNs. Leveraging this tunable release and the inherent medicinal properties of quinones, MQN prodrugs with a high drug loading (>89 wt %) are engineered using doxorubicin for anti‐cancer therapy and shikonin for the inhibition of the main protease in the SARS‐CoV‐2 virus.

Mechanical Insights into Activation of Peroxides by Quinones: Formation of Oxygen-Centered Radicals or Singlet Oxygen.

In this work, the mechanism of the activation of peroxides by quinones has been investigated through quantum chemical calculations. Hydrogen peroxide (H2O2), peroxomonosulfate (PMS), peracetic acid (PAA), and CH3OOH were chosen as the model peroxides and p-benzoquinone (p-BQ) and tetrachloro-1,4-benzoquinone (TCBQ) as the model quinones. The nucleophilic attack of peroxides can occur on the carbonyl and olefinic carbons of quinones. For p-BQ, the nucleophilic attack of HO2-, CH3OO-, PMS, and PAA might prefer to occur on the carbonyl carbons, which have more positive atomic charges. Then, further transformation could not be induced from the addition of HO2- and CH3OO- to p-BQ. Comparatively, singlet oxygen (1O2) could be generated in the cases of PMS and PAA. For TCBQ, the chlorine atoms cause the olefinic carbons to carry more positive atomic charges, and then, HO2- preferred to add to the olefinic carbons, which might induce the formation of the hydroxyl radical (•OH). The activation of PMS by TCBQ was similar to that by p-BQ, with the kinetical feasibility of 1O2 formation. These findings may provide some theoretical insights into the reaction of peroxides with quinones, especially into the interconnection between the substitutes and the formation of oxygen-centered radicals (e.g., •OH) and 1O2.

Redox transformation of structural iron in nontronite induced by quinones under anoxic conditions

In natural anoxic subsurface environments, the geochemical cycles of iron are largely associated with the migration and transformation of organic matter. Intensive attention has been paid to the redox interaction of organic matter with aqueous Fe and iron (hydr)oxides. Whereas, the abiotic redox cycling of structural Fe in clay minerals induced by quinones has not been well understood. In this study, we selected nontronite (NAu-2) as a model Fe-bearing phyllosilicate clay mineral and 1,4-hydroquinone (H2Q)/1,4-benquinone (BQ) as a model quinone couple. Our results show that the structural Fe(III) in NAu-2, with tetrahedral Fe(III) priority, can oxidize H2Q into BQ, and octahedral Fe(II) in NAu-2 can reduce BQ to H2Q, with semiquinone radicals (SQ−) as intermediate. The extent of the redox reactions depends on the reduction potential difference between NAu-2 and H2Q/BQ. However, a fraction of Fe(II)-Fe(III)-OH and Fe(II)-Fe(II)-OH groups in the octahedral sheet are difficult to be oxidized by BQ, because the reduction potential gradient decreases to a low level as the reaction proceeds. And the structure of NAu-2 can only partially restored upon re-oxidation with tetrahedral Fe(III) irreversibility. Output of this study replenishes the understanding regarding redox cycling of structural Fe in clay minerals induced by quinones.

Hydrogen Atom Transfer from HOO. to ortho-Quinones Explains the Antioxidant Activity of Polydopamine.

Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO. (k=1.5×107 and 1.1×105 M−1 s−1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Hydrogen Atom Transfer from HOO. to ortho‐Quinones Explains the Antioxidant Activity of Polydopamine

AbstractMelanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO. (k=1.5×107 and 1.1×105 M−1 s−1 in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Covalent binding with model quinone compounds unveils the environmental fate of the insensitive munitions reduced product 2,4-diaminoanisole (DAAN) under anoxic conditions

2,4-Dinitroanisole (DNAN) is an insensitive munitions compound expected to replace 2,4,6-trinitrotoluene (TNT). The product of DNAN’s reduction in the environment is 2,4-diaminoanisole (DAAN), a toxic and carcinogenic aromatic amine. DAAN is known to become irreversibly incorporated into soil natural organic matter (NOM) after DNAN’s reduction. Herein, we investigate the reactions between DAAN and NOM under anoxic conditions, using 1,4-benzoquinone (BQ) and methoxybenzoquinone (MBQ) as model humic moieties of NOM. A new method stopped the fast reactions between DAAN and quinones, capturing the fleeting intermediates. We observed that DAAN incorporation into NOM (represented by BQ and MBQ models) is quinone-dependent and occurs via Michael addition, imine (Schiff-base) formation, and azo bond formation. After dimers are formed, incorporation reactions continue, resulting in trimers and tetramers. After 20 days, 56.4% of dissolved organic carbon from a mixture of DAAN (1 mM) and MBQ (3 mM) had precipitated, indicating an extensive polymerization, with DAAN becoming incorporated into high-molecular-weight humic-like compounds. The present work suggests a new approach for DNAN environmental remediation, in which DNAN anaerobic transformation can be coupled to the formation of non-extractable bound DAAN residues in soil organic matter. This process does not require aerobic conditions nor a specific catalyst.

Covalent bonding of aromatic amine daughter products of 2,4-dinitroanisole (DNAN) with model quinone compounds representing humus via nucleophilic addition.

2,4-Dinitroanisole (DNAN) is a component of insensitive munitions (IM), which are replacing traditional explosives due to their improved safety. Incomplete IM combustion releases DNAN onto the soil, where it can leach into the subsurface with rainwater, encounter anoxic conditions, and undergo (a)biotic reduction to aromatic amines 2-methoxy-5-nitroaniline (MENA), 4-methoxy-3-nitroaniline (iMENA, isomer of MENA), and 2,4-diaminoanisole (DAAN). We report here studies of nucleophilic addition mechanisms that may account for the sequestration of aromatic amine daughter products of DNAN into soil organic matter (humus), effectively removing these toxic compounds from the aqueous environment. Because quinones are important moieties in humus, we incubated MENA, iMENA, DAAN, and related analogs with model compounds 1,4-benzoquinone and 2,3-dimethyl-1,4-benzoquinone under anoxic conditions. Mass spectrometry and ultra-high performance liquid chromatography revealed that the aromatic amines had covalently bonded to either carbonyl carbons or ring carbons β to carbonyl carbons of the quinones, producing a mixture of imines and Michael adducts, respectively. These products formed rapidly and accumulated in the one-to four-day incubations. Nucleophilic addition reactions, which do not require catalysis or oxic conditions, are proposed as a mechanism resulting in the binding of DNAN to soil observed in previous studies. To remediate sites contaminated with DNAN or other nitroaromatics, reducing conditions and humus amendments may promote their immobilization into the soil matrix.

Zinc Switch in Pig Heart Lipoamide Dehydrogenase: Steady-State and Transient Kinetic Studies of the Diaphorase Reaction.

Elevation of intracellular Zn2+ following ischemia contributes to cell death by affecting mitochondrial function. Zn2+ is a differential regulator of the mitochondrial enzyme lipoamide dehydrogenase (LADH) at physiological concentrations (Ka = 0.1 µM free zinc), inhibiting lipoamide and accelerating NADH dehydrogenase activities. These differential effects have been attributed to coordination of Zn2+ by LADH active-site cysteines. A detailed kinetic mechanism has now been developed for the diaphorase (NADH-dehydrogenase) reaction catalyzed by pig heart LADH using 2,6-dichlorophenol-indophenol (DCPIP) as a model quinone electron acceptor. Anaerobic stopped-flow experiments show that two-electron reduced LADH is 15-25-fold less active towards DCPIP reduction than four-electron reduced enzyme, or Zn2+-modified reduced LADH (the corresponding values of the rate constants are (6.5 ± 1.5) × 103M-1·s-1, (9 ± 2) × 104M-1·s-1, and (1.6 ± 0.5) × 105M-1·s-1, respectively). Steady-state kinetic studies with different diaphorase substrates show that Zn2+ accelerates reaction rates exclusively for two-electron acceptors (duroquinone, DCPIP), but not for one-electron acceptors (benzoquinone, ubiquinone, ferricyanide). This implies that the two-electron reduced form of LADH, prevalent at low NADH levels, is a poor two-electron donor compared to the four-electron reduced or Zn2+-modified reduced LADH forms. These data suggest that zinc binding to the active-site thiols switches the enzyme from one- to two-electron donor mode. This zinc-activated switch has the potential to alter the ratio of superoxide and H2O2 generated by the LADH oxidase activity.

Correction to: Towards lignin-protein crosslinking: amino acid adducts of a lignin model quinone methide

The manuscript initially failed to cite the work of Dr. Cong et al., which was the basis for this effort. We apologize for this error.

Redistribution of Electron Equivalents between Magnetite and Aqueous Fe2+ Induced by a Model Quinone Compound AQDS.

The complex interactions between magnetite and aqueous Fe2+ (Fe2+(aq)) pertain to many biogeochemical redox processes in anoxic subsurface environments. The effect of natural organic matter, abundant in these same environments, on Fe2+(aq)-magnetite interactions is an additional complex that remains poorly understood. We investigated the influence of a model quinone molecule anthraquinone-2,6-disulfonate (AQDS) on Fe2+(aq)-magnetite interactions by systematically studying equilibrium Fe2+(aq) concentrations, rates and extents of AQDS reduction, and structural versus surface-localized Fe(II)/Fe(III) ratios in magnetite under different controlled experimental conditions. The equilibrium concentration of Fe2+(aq) in Fe2+-amended magnetite suspensions with AQDS proportionally changes with solution pH or initial AQDS concentration, but independent of magnetite loadings through the solid concentrations that were studied here. The rates and extents of AQDS reduction by Fe2+-amended magnetite proportionally increased with solution pH, magnetite loading, and initial Fe2+(aq) concentration, which correlates with the corresponding change of reduction potentials for the Fe2+-magnetite system. AQDS reduction by surface-associated Fe(II) in the Fe2+-magnetite suspensions induces solid-state migration of electron equivalents from particle interiors to the near-surface region and the production of nonmagnetic Fe(II)-containing species, which inhibits Fe2+(aq) incorporation or electron injection into the magnetite structure. This study demonstrates the significant influence of quinones on reductive activity of the Fe2+-magnetite system.

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO2-to-HONO Conversion on Soil Surfaces.

Humic acid (HA) is thought to promote NO2 conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO2 reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO2 to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO2 reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO2-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO2 reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO2-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.