Comparison of Quinone‐Based Catholytes for Aqueous Redox Flow Batteries and Demonstration of Long‐Term Stability with Tetrasubstituted Quinones

Quinones are appealing targets as organic charge carriers for aqueous redox flow batteries (RFBs), but their utility continues to be constrained by limited stability under operating conditions. The present study evaluates the stability of a series of water‐soluble quinones, with redox potentials ranging from 605–885 mV versus NHE, under acidic aqueous conditions (1 m H₂SO₄). Four of the quinones are examined as cathodic electrolytes in an aqueous RFB, paired with anthraquinone‐2,7‐disulfonate as the anodic electrolyte. The RFB data complement other solution stability tests and show that the most stable electrolyte is a tetrasubstituted quinone containing four sulfonated thioether substituents. The results highlight the importance of substituting all C–H positions of the quinone in order to maximize the quinone stability and set the stage for design of improved organic electrolytes for aqueous RFBs.     

A Phosphonate‐Functionalized Quinone Redox Flow Battery at Near‐Neutral pH with Record Capacity Retention Rate

A highly stable phosphonate‐functionalized anthraquinone is introduced as the redox‐active material in a negative potential electrolyte (negolyte) for aqueous redox flow batteries operating at nearly neutral pH. The design and synthesis of 2,6‐DPPEAQ, (((9,10‐dioxo‐9,10‐dihydroanthracene‐2,6‐diyl)bis(oxy))bis(propane‐3,1‐diyl))bis(phosphonic acid), which has a high solubility at pH 9 and above, is described. Chemical stability studies demonstrate high stability at both pH 9 and 12. By pairing 2,6‐DPPEAQ with a potassium ferri/ferrocyanide positive electrolyte across an inexpensive, nonfluorinated permselective polymer membrane, this near‐neutral quinone flow battery exhibits an open‐circuit voltage of 1.0 V and a capacity fade rate of 0.00036% per cycle and 0.014% per day, which is the lowest ever reported for any flow battery in the absence of rebalancing processes. It is further demonstrated that the negolyte pH drifts upward upon atmospheric oxygen penetration but, when oxygen is excluded, oscillates reversibly between 9 and 12 during cycling. These results enhance the suitability of aqueous‐soluble redox‐active organics for use in large‐scale energy storage, potentially enabling massive penetration of intermittent renewable electricity.     

Charge Storage Capacity of Renewable Biopolymer/Conjugated Polymer Interpenetrating Networks Enhanced by Electroactive Dopants

Renewable materials are requested for large scale electrical storage, a coming necessity with the growth of intermittent solar and wind renewable electricity generation. Biopolymers are a source of inexpensive materials, in particular through the use of black liquor from paper production, a waste product. Interpenetrating networks of the biopolymer lignosulfonate (Lig) and conjugated polymer polypyrrole (Ppy) are synthesized by galvanostatic polymerization from pyrrole/lignosulfonate mixture in acidic aqueous electrolyte. Methoxy and phenolic functional group present in the non‐conducting lignosulfonate are converted to quinone groups. The redox chemistry of quinones is used for charge storage, along with charge storage in polypyrrole. A large variation of the electrochemical activity between lignosulfonates obtained from different sources is observed. The charge storage capacities are significantly enhanced by also including another electroactive dopant, anthraquinone sulfonate (AQS). AQS redox peaks act as an internal reference (standard) to probe the redox electrochemistry of Lig. The synthesized Ppy(Lig) and Ppy(Lig‐AQS) electrodes are characterized by cyclic voltammetry, galvanostatic charge‐discharge cycling, electrochemical quartz crystal microbalance, and atomic force microscopy.     

Humic substance-mediated reduction of iron(III) oxides and degradation of 2,4-D by an alkaliphilic bacterium,Corynebacterium humireducens MFC-5

With the use of an alkaliphilic bacterium, Corynebacterium humireducens MFC-5, this study investigated the reduction of goethite (α-FeOOH) and degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) mediated by different humic substances (humics) and quinones in alkaline conditions (pH of 9.0). The results indicated that (i) using sucrose as the electron donor, the strain MFC-5 was capable of reducing anthraquinone-2,6-disulfonic acid (AQDS), anthraquinone-2-disulfonic acid (AQS), anthraquinone-2-carboxylic acid (AQC), humic acid (HA) and fulvic acid (FA), and its reducing capability ranked as AQC > AQS > AQDS > FA > HA; (ii) the anaerobic reduction of α-FeOOH and 2,4-D by the strain was insignificant, while the reductions were greatly enhanced by the addition of quinones/humics serving as redox mediators; (iii) the Fe(III) reduction rate was positively related to the content of quinone functional groups and the electron-accepting capacities (EAC) of quinones/humics based on fourier-transform infrared spectroscopy (FT-IR) and electrochemical analyses; however, such a relationship was not found in 2,4-D degradation probably because quinone reduction was not the rate-limiting step of quinone-mediated reduction of 2,4-D. Using the example of α-FeOOH and 2,4-D, this study well demonstrated the important role of humics reduction on the Fe(III)/Fe(II) biogeochemical cycle and chlorinated organic compounds degradation in alkaline reducing environments.Funding Information This study was supported by the National Natural Science Foundation of China (Nos 41101211, 31070460, 41101477), and The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.     

Enhancing the electron transfer capacity and subsequent color removal in bioreactors by applying thermophilic anaerobic treatment and redox mediators

The effect of temperature, hydraulic retention time (HRT) and the redox mediator anthraquinone-2,6-disulfonate (AQDS), on electron transfer and subsequent color removal from textile wastewater was assessed in mesophilic and thermophilic anaerobic bioreactors. The results clearly show that compared with mesophilic anaerobic treatment, thermophilic treatment at 55 degrees C is an effective approach for increasing the electron transfer capacity in bioreactors, and thus improving the decolorization rates. Furthermore, similar color removals were found at 55 degrees C between the AQDS-free and AQDS-supplemented reactors, whereas a significant difference (up to 3.6-fold) on decolorization rates occurred at 30 degrees C. For instance, at an HRT of 2.5 h and in the absence of AQDS, the color removal was 5.3-fold higher at 55 degrees C compared with 30 degrees C. The impact of a mix of mediators with different redox potentials on the decolorization rate was investigated with both industrial textile wastewater and the azo dye Reactive Red 2 (RR2). Color removal of RR2 in the presence of anthraquinone-2-sulfonate (AQS) (standard redox potential E(0)' of -225 mV) was 3.8-fold and 2.3-fold higher at 30 degrees C and 55 degrees C, respectively, than the values found in the absence of AQS. Furthermore, when the mediators 1,4-benzoquinone (BQ) (E(0)' of +280 mV), and AQS were incubated together, there was no improvement on the decolorization rates compared with the bottles solely supplemented with AQS. Results imply that the use of mixed redox mediators with positive and negative E(0)' under anaerobic conditions is not an efficient approach to improve color removal in textile wastewaters.     

Near Neutral pH Redox Flow Battery with Low Permeability and Long‐Lifetime Phosphonated Viologen Active Species

A highly stable phosphonate‐functionalized viologen is introduced as the redox‐active material in a negative potential electrolyte for aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 m and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching ?0.462 V versus SHE at pH = 9. The negative charges in both the oxidized and the reduced states of 1,1′‐bis(3‐phosphonopropyl)‐[4,4′‐bipyridine]‐1,1′‐diium dibromide ( BPP?Vi ) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP?Vi with a ferrocyanide‐based positive potential electrolyte across an inexpensive, non‐fluorinated cation exchange membrane at pH = 9 exhibits an open‐circuit voltage of 0.9 V and a capacity fade rate of 0.016% per day or 0.00069% per cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low‐permeating and low reduction potential redox active materials into near neutral ARFBs.     

The transformation and toxicity of anthraquinone dyes during thermophilic (55 degrees C) and mesophilic (30 degrees C) anaerobic treatments

We studied in batch assays the transformation and toxicity of anthraquinone dyes during incubations with anaerobic granular sludge under mesophilic (30 degrees C) and thermophilic (55 degrees C) conditions. Additionally, the electron shuttling capacity of the redox mediator anthraquinone-2-sulfonic acid (AQS) and subsequent increase on decolourisation rates was investigated on anthraquinone dyes. Compared with incubations at 30 degrees C, serum bottles at 55 degrees C presented distinctly higher decolourisation rates not only with an industrial wastewater containing anthraquinone dyes, but also with model compounds. Compared with batch assays at 30 degrees C, the first-order rate constant "k" of the Reactive Blue 5 (RB5) was enhanced 11-fold and 6-fold for bottles at 55 degrees C supplemented and free of AQS, respectively. However, the anthraquinone dye Reactive Blue 19 (RB19) demonstrated a very strong toxic effect on volatile fatty acids (VFA) degradation and methanogenesis at both 30 degrees C and 55 degrees C. The apparent inhibitory concentrations of RB19 exerting 50% reduction in methanogenic activity (IC50-value) were 55 mg l(-1) at 30 degrees C and 45 mg l(-1) at 55 degrees C. Further experiments at both temperatures revealed that RB19 was mainly toxic to methanogens, because the glucose oxidizers including acetogens, propionate-forming, butyrate-forming and ethanol-forming microorganisms were not affected by the dye toxicity.     

Enhancement of nitroaromatic compounds anaerobic biotransformation using a novel immobilized redox mediator prepared by electropolymerization

Functionalized polypyrrole (PPy) composites were prepared by incorporation of a model redox mediator, anthraquinonedisulphonate (AQDS), as doping anion during the electropolymerization of pyrrole (Py) monomer on active carbon felt (ACF) electrode. Then, the resulting composite, ACF/PPy/AQDS as a novel immobilized redox mediator for catalyzing anaerobic biotransformation of the model nitroaromatic compounds (NACs), such as nitrobenzene (NB), 2,4- and 2,6-dinitrotoluene (DNT), were investigated in detail. The results showed that ACF/PPy/AQDS exhibited good catalytic activity and stability, and its addition effectively accelerated the NACs anaerobic reduction to the corresponding amino compounds. In order to estimate the relationship between community dynamics and the function of immobilized redox mediator, a combined method based on fingerprints (ribosomal intergenic spacer analysis, RISA) and 16S rRNA gene sequencing was used. The results indicated that the existence of ACF/PPy/AQDS made the potent AQDS-reducing bacteria keeping predominant in the catalytic systems. Based on the results above, it can be concluded that this novel immobilized redox mediator is feasible and potentially useful to enhance NACs anaerobic reduction.     

Accelerated removal of Sudan dye by Shewanella oneidensis MR-1 in the presence of quinones and humic acids

Although there have been many studies on bacterial removal of soluble azo dyes, much less information is available for biological treatment of water-insoluble azo dyes. The few bacterial species capable of removing Sudan dye generally require a long time to remove low concentrations of insoluble dye particles. The present work examined the efficient removal of Sudan I by Shewanella oneidensis MR-1 in the presence of redox mediator. It was found that the microbially reduced anthraquinone-2,6-disulfonate (AQDS) could abiotically reduce Sudan I, indicating the feasibility of microbially-mediated reduction. The addition of 100 μM AQDS and other different quinone compounds led to 4.3–54.7 % increase in removal efficiencies in 22 h. However, adding 5-hydroxy-1,4-naphthoquinone into the system inhibited Sudan I removal. The presence of 10, 50 and 100 μM AQDS stimulated the removal efficiency in 10 h from 26.4 to 42.8, 54.9 and 64.0 %, respectively. The presence of 300 μM AQDS resulted in an eightfold increase in initial removal rate from 0.19 to 1.52 mg h^?1 g^?1 cell biomass. A linear relationship was observed between the initial removal rates and AQDS concentrations (0–100 μM). Comparison of Michaelis–Menten kinetic constants revealed the advantage of AQDS-mediated removal over direct reduction. Different species of humic acid could also stimulate the removal of Sudan I. Scanning electronic microscopy analysis confirmed the accelerated removal performance in the presence of AQDS. These results provide a potential method for the efficient removal of insoluble Sudan dye.     

An approach to evaluate two-electron reduction of 9,10-phenanthraquinone and redox activity of the hydroquinone associated with oxidative stress

Quinones are widely used as medicines or redox agents. The chemical properties are based on the reactions against an electron donor. 9,10-Phenanthraquinone (PQ), which is a quinone contaminated in airborne particulate matters, forms redox cycling, not Michael addition, with electron donors. Redox cycling of PQ contributes to its toxicity, following generation of reactive oxygen species (ROS). Detoxification of quinones is generally thought to be two-electron reduction forming hydroquinones. However, a hydroquinone of PQ, 9,10-dihydroxyphenanthrene (PQH(2)), has been never detected itself, because it is quite unstable. In this paper, we succeeded in detecting PQH(2) as its stable derivative, 9,10-diacetoxyphenanthrene (DAP). However, higher concentrations of PQ (>4 microM) form disproportionately with PQH(2), producing the 9,10-phenanthraquinone radical (PQ(-)) which is a one-electron reducing product of PQ. In cellular experiments using DAP as a precursor of PQH(2), it was shown that PQH(2) plays a critical role in the oxidative protein damage and cellular toxicity of PQ, showing that two-electron reduction of PQ can also initiate redox cycling to cause oxidative stress-dependent cytotoxicity.     

Microbial reduction of structural iron in interstratified illite-smectite minerals by a sulfate-reducing bacterium

Clay minerals are ubiquitous in soils, sediments, and sedimentary rocks and could coexist with sulfate‐reducing bacteria (SRB) in anoxic environments, however, the interactions of clay minerals and SRB are not well understood. The objective of this study was to understand the reduction rate and capacity of structural Fe(III) in dioctahedral clay minerals by a mesophilic SRB, Desulfovibrio vulgaris and the potential role in catalyzing smectite illitization. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu‐2, mixed‐layer illite‐smectite RAr‐1 and ISCz‐1, and illite IMt‐1) were exposed to D. vulgaris in a non‐growth medium with and without anthraquinone‐2,6‐disulfonate (AQDS) and sulfate. Our results demonstrated that D. vulgaris was able to reduce structural Fe(III) in these clay minerals, and AQDS enhanced the reduction rate and extent. In the presence of AQDS, sulfate had little effect on Fe(III) bioreduction. In the absence of AQDS, sulfate increased the reduction rate and capacity, suggesting that sulfide produced during sulfate reduction reacted with the phyllosilicate Fe(III). The extent of bioreduction of structural Fe(III) in the clay minerals was positively correlated with the percentage of smectite and mineral surface area of these minerals. X‐ray diffraction, and scanning and transmission electron microscopy results confirmed formation of illite after bioreduction. These data collectively showed that D. vulgaris could promote smectite illitization through reduction of structural Fe(III) in clay minerals.     

Quinone-Mediated Microbial Goethite Reduction and Transformation of Redox Mediator,Anthraquinone-2,6-Disulfonate (AQDS)

The aim of current study is to identify the kinetic characteristics and elucidate the possible transformation pathways of the interaction between the redox mediator (anthraquinone-2,6-disulfonate, AQDS) and goethite during the process of microbial goethite reduction by Shewanella putrefaciens, a dissimilatory iron reduction bacterium (DIRB). Speciations of both AQDS and microbially reduced ferrous iron are used to characterize the interaction process among S. putrefaciens, AQDS and goethite. Due to the complexities of the natural environment, two pre-incubation reaction systems of the “DIRB–goethite” and the “DIRB–AQDS” are introduced to investigate the dynamics of goethite reduction and redox transformation of AQDS. Results show that the characteristics of the microbial goethite reduction and the kinetic transformation between two species of the redox mediator are different in two pre-incubation reaction systems. Both abiotic and enzymatic reactions and their coupling regulate the kinetic process for “redox mediatoriron” interaction in the presence of DIRB. This study will help to understand the characteristics and mechanism of microbial reduction of the Fe(III) oxide and transformation of redox mediator.     

A Stable Vanadium Redox‐Flow Battery with High Energy Density for Large‐Scale Energy Storage

The all‐vanadium redox flow battery is a promising technology for large‐scale renewable and grid energy storage, but is limited by the low energy density and poor stability of the vanadium electrolyte solutions. A new vanadium redox flow battery with a significant improvement over the current technology is reported in this paper. This battery uses sulfate‐chloride mixed electrolytes, which are capable of dissolving 2.5 M vanadium, representing about a 70% increase in energy capacity over the current sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of ?5 to 50 °C, potentially eliminating the need for electrolyte temperature control in practical applications. This development would lead to a significant reduction in the cost of energy storage, thus accelerating its market penetration.     

Nanoporous Polytetrafluoroethylene/Silica Composite Separator as a High‐Performance All‐Vanadium Redox Flow Battery Membrane

A novel low‐cost nanoporous polytetrafluoroethylene (PTFE)/silica composite separator has been prepared and evaluated for its use in an all‐vanadium redox flow battery (VRB). The separator consists of silica particles enmeshed in a PTFE fibril matrix. It possesses unique nanoporous structures with an average pore size of 38 nm and a porosity of 48%. These pores function as the ion transport channels during redox flow battery operation. This separator provides excellent electrochemical performance in the mixed‐acid VRB system. The VRB using this separator delivers impressive energy efficiency, rate capability, and temperature tolerance. In additon, the flow cell using the novel separator also demonstrates an exceptional capacity retention capability over extended cycling, thus offering excellent stability for long‐term operation. The characteristics of low cost, excellent electrochemical performance and proven chemical stability afford the PTFE/silica nanoporous separator great potential as a substitute for the Nafion membrane used in VRB applications.     

Single‐Step Spray Printing of Symmetric All‐Organic Solid‐State Batteries Based on Porous Textile Dye Electrodes

A symmetric solid‐state battery based on organic porous electrodes is fabricated using scalable spray‐printing. The active electrode material is based on a textile dye (disperse blue 134 anthraquinone) and is capable of forming divalent cations and anions in oxidation and reduction processes. The resulting molecule can be used in both negative and positive electrode reactions. After spray printing an inter‐connected pore honeycomb electrode, a solid‐state electrolyte (σ_Li: × 10^?4 S cm^?1) based on a polymeric ionic liquid is spray‐printed as a second layer and infiltrated through the porous electrodes. A symmetric all‐organic battery is then formed with the addition of another identical set of electrode and electrolyte layers. Both density functional theory calculations and charge‐discharge profiles show that the potentials for the negative and positive electrode reactions are amongst the lowest (≈2.0 V vs Li) and the highest (≈3.5 V vs Li), respectively, for quinone‐type molecules. Over the C‐rate range 0.2 to 5 C, the battery has a discharge cell voltage of more than 1 V even up to 250 charge‐discharge cycles and capacities are in the range 50–80 mA h g^?1 at 0.5 C.     

A Metal‐Free and Biotically Degradable Battery for Portable Single‐Use Applications

This article presents a new approach for environmentally benign, low‐cost batteries intended for single‐use applications. The proposed battery is designed and fabricated using exclusively organic materials such as cellulose, carbon, and wax and features an integrated quinone‐based redox chemistry to generate electricity within a compact form factor. This primary capillary flow battery is activated by the addition of a liquid sample and has shown continuous operation up to 100 min with an output voltage that can be conveniently scaled to match the voltage needs of portable electronic devices (1.5–3.0 V). Once depleted, the battery can be disposed of without the need for any recycling facility, as its components are nontoxic and shown to be biotically degradable in a standardized test. The practical utility of the battery is demonstrated by direct substitution of a lithium ion coin cell in a diagnostic application.     

中国希瓦氏菌D14^T的厌氧腐殖质呼吸

实验证明,希瓦氏菌新种(ShewanellacinicaD14T)在厌氧条件下可以利用多种有机酸盐和甲苯等环境有毒污染物作为电子供体,以腐殖质作为唯一末端电子受体进行厌氧呼吸(即醌呼吸)。电子在细胞膜呼吸链的传递过程中,偶联能量的产生来支持菌体的生长,1mmol/LAQDS可支持细胞增殖约60倍。电子供体的氧化和唯一电子受体腐殖质还原之间存在着动态的偶联过程,随着电子供体量的增加腐殖质还原的量也随之增加。典型呼吸链抑制剂诸如:抑制Fe-S中心的Cu2 ,甲基萘醌类似物标桩菌素,抑制甲基萘醌氧化型向还原型转化的双香豆素和细胞色素P450的专一抑制物甲吡酮等对腐殖质的还原有着极为显著的抑制作用,为进一步证明希瓦氏菌(Shewanellacinica)D14T可利用腐殖质进行厌氧呼吸提供了有力的佐证。而D14T在进行腐殖质呼吸的同时,对于甲苯,苯胺等环境有毒物质的有效降解则具有着重要的环境学意义。     

Aqueous Electrochemical Energy Storage with a Mediator‐Ion Solid Electrolyte

This study presents a battery concept with a “mediator‐ion” solid electrolyte for the development of next‐generation electrochemical energy storage technologies. The active anode and cathode materials in a single cell can be in the solid, liquid, or gaseous form, which are separated by a sodium‐ion solid‐electrolyte separator. The uniqueness of this mediator‐ion strategy is that the redox reactions at the anode and the cathode are sustained by a shuttling of a mediator sodium ion between the anolyte and the catholyte through the solid‐state electrolyte. Use of the solid‐electrolyte separator circumvents the chemical‐crossover problem between the anode and the cathode, overcomes the dendrite‐problem when employing metal‐anodes, and offers the possibility of using different liquid electrolytes at the anode and the cathode in a single cell. The battery concept is demonstrated with two low‐cost metal anodes (zinc and iron), two liquid cathodes (bromine and potassium ferricyanide), and one gaseous cathode (air/O₂) with a sodium‐ion solid electrolyte. This novel battery strategy with a mediator‐ion solid electrolyte is applicable to a wide range of electrochemical energy storage systems with a variety of cathodes, anodes, and mediator‐ion solid electrolytes.     

Kinetics during the redox biotransformation of pollutants mediated by immobilized and soluble humic acids

The aim of this study was to elucidate the kinetic constraints during the redox biotransformation of the azo dye, Reactive Red 2 (RR2), and carbon tetrachloride (CT) mediated by soluble humic acids (HA_s) and immobilized humic acids (HA_i), as well as by the quinoid model compounds, anthraquinone-2,6-disulfonate (AQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). The microbial reduction of both HA_s and HA_i by anaerobic granular sludge (AGS) was the rate-limiting step during decolorization of RR2 since the reduction of RR2 by reduced HA_i proceeded at more than three orders of magnitute faster than the electron-transferring rate observed during the microbial reduction of HA_i by AGS. Similarly, the reduction of RR2 by reduced AQDS proceeded 1.6- and 1.9-fold faster than the microbial reduction of AQDS by AGS when this redox mediator (RM) was supplied in soluble and immobilized form, respectively. In contrast, the reduction of NQS by AGS occurred 1.6- and 19.2-fold faster than the chemical reduction of RR2 by reduced NQS when this RM was supplied in soluble and immobilized form, respectively. The microbial reduction of HA_s and HA_i by a humus-reducing consortium proceeded 1,400- and 790-fold faster than the transfer of electrons from reduced HA_s and HA_i, respectively, to achieve the reductive dechlorination of CT to chloroform. Overall, the present study provides elucidation on the rate-limiting steps involved in the redox biotransformation of priority pollutants mediated by both HA_s and HA_i and offers technical suggestions to overcome the kinetic restrictions identified in the redox reactions evaluated.     

Molecular interactions between Geobacter sulfurreducens triheme cytochromes and the redox active analogue for humic substances

The bacterium Geobacter sulfurreducens can transfer electrons to quinone moieties of humic substances or to anthraquinone-2,6-disulfonate (AQDS), a model for the humic acids. The reduced form of AQDS (AH₂QDS) can also be used as energy source by G. sulfurreducens. Such bidirectional utilization of humic substances confers competitive advantages to these bacteria in Fe(III) enriched environments. Previous studies have shown that the triheme cytochrome PpcA from G. sulfurreducens has a bifunctional behavior toward the humic substance analogue. It can reduce AQDS but the protein can also be reduced by AH₂QDS. Using stopped-flow kinetic measurements we were able to demonstrate that other periplasmic members of the PpcA-family in G. sulfurreducens (PpcB, PpcD and PpcE) also showed the same behavior. The extent of the electron transfer is thermodynamically controlled favoring the reduction of the cytochromes. NMR spectra recorded for ¹³C,¹⁵N-enriched samples in the presence increasing amounts of AQDS showed perturbations in the chemical shift signals of the cytochromes. The chemical shift perturbations on cytochromes backbone NH and ¹H heme methyl signals were used to map their interaction regions with AQDS, showing that each protein forms a low-affinity binding complex through well-defined positive surface regions in the vicinity of heme IV (PpcB, PpcD and PpcE) and I (PpcE). Docking calculations performed using NMR chemical shift perturbations allowed modeling the interactions between AQDS and each cytochrome at a molecular level. Overall, the results obtained provided important structural-functional relationships to rationalize the microbial respiration of humic substances in G. sulfurreducens.