Mediator compounds for the electrochemical study of biological redox systems: a compilation

Size-Dependent Reduction Kinetics of Iron Oxides in Single and Mixed Mineral Systems

Iron(III) (oxyhydr)oxide minerals with varying particle sizes commonly coexist in natural environments and are susceptible to both chemical and microbial reduction, affecting the fate and mobility of trace elements, nutrients, and pollutants. The size-dependent reduction behavior of iron (oxyhydr)oxides in single and mixed mineral systems remains poorly understood. In this study, we used microbial and mediated electrochemical reduction approaches to investigate the reduction kinetics and extents of goethite and hematite. We found that small particles were preferentially reduced relative to their large counterparts in single and mixed mineral systems regardless of microbial or electrochemical treatments, which is attributed to the combined effect of higher thermodynamic favorability and greater surface availability. In mixed mineral systems, small particles were reduced slightly faster, whereas large particles were reduced notably slower and less extensively than solely predicted from single mineral systems. Specifically, when reduced alone, small particles showed Fe(III) reduction rate constants that were 1.5- to 3.6-fold higher than large particles, while when reduced together, the reduction rate constants for small particles were 6- to 21-fold higher than the rate constants for large particles. These collective findings provide new insights into the pivotal role of nanoparticulate iron (oxyhydr)oxides in environmental redox reactions.

Discovery and characterization of an FAD-dependent glucose 6-dehydrogenase

Many patients with diabetes use self-measurement devices for blood glucose to understand their blood glucose levels. Most of these devices use FAD-dependent glucose dehydrogenase (FAD-GDH) to determine blood glucose levels. For this purpose, FAD-GDHs specifically oxidizing glucose among the sugars present in blood are required. Many FAD-GDHs with high substrate specificity have been reported previously; however, their substrate specificity is insufficient as they also react with xylose. Therefore, we aimed to identify FAD-GDHs without xylose reactivity. We screened and obtained a new enzyme from Colletotrichum plurivorum (CpGDH). CpGDH showed high activity to glucose in the presence of electron mediators but low activity to xylose. We prepared the glucose oxidation products using CpGDH and subjected to TLC, HPLC, mass spectrometry, and NMR analyses. The results demonstrated that CpGDH is a previously unknown FAD-dependent glucose 6-dehydrogenase (FAD-G6DH) that oxidizes glucose to glucuronic acid. The stoichiometric ratio of the substrate and electron mediator was 1:2, suggesting that CpGDH catalyzes two-step oxidation reactions, including oxidation of primary alcohols to aldehydes and of aldehydes to carboxylic acids. We concluded that CpGDH has the unique substrate-binding manner based on the result of docking simulation of CpGDH with a substrate glucose. We then constructed a phylogenetic tree of carbohydrate-related flavoproteins including FAD-G6DHs, indicating that FAD-G6DHs are different from the known FAD-dependent oxidoreductases. Overall, this study is the first to report FAD-G6DHs. These results will likely contribute to the development of more accurate blood glucose sensors and further research on the metabolisms of glucosides and their metabolites.

Redox Properties of Structural Fe in Clay Minerals: 4. Reinterpreting Redox Curves by Accounting for Electron Transfer and Structural Rearrangement Kinetics

Iron-bearing smectite clay minerals can act as electron sources and sinks in the environment. Previous studies using mediated electrochemical analyses to determine the reduction potential (EH) values of smectites observed that the relationship between the structural Fe2+(s)/FeTotal ratio in the smectite and EH varied based on the redox history of the smectite. We hypothesize that this behavior, referred to as redox hysteresis, results from the smectite particles not equilibrating with the applied EH over the course of the experiment (∼30 min). To test this hypothesis, we developed a model incorporating interfacial electron transfer kinetics and charge redistribution within the particle to simulate the mediated electrochemical experiments from previous studies. The simulated redox curves accurately matched the previously reported experimental redox curves of the smectite SWa-1, demonstrating that longer equilibration periods led to a decrease in redox hysteresis. We validated this experimentally by measuring the redox curve of SWa-1 after an equilibration period of at least 12 h. Furthermore, we extended the simulations to three other smectites (NAu-1, NAu-2, and SWy-2) and extracted their respective thermodynamic and kinetic parameters. This work offers a framework for interpreting and modeling redox reactions on clay surfaces, along with key parameters for four commonly studied smectites.

Chemoorganotrophic electrofermentation by Cupriavidus necator using redox mediators

The non-pathogenic β-proteobacterium Cupriavidus necator has the ability to switch between chemoorganotrophic, chemolithoautotrophic and electrotrophic growth modes, making this microorganism a widely used host for cellular bioprocesses. Oxygen usually acts as the terminal electron acceptor in all growth modes. However, several challenges are associated with aeration, such as foam formation, oxygen supply costs, and the formation of an explosive gas mixture in chemolithoautotrophic cultivation with H2, CO2 and O2. Bioelectrochemical systems in which O2 is replaced by an electrode as a terminal electron acceptor offer a promising solution to these problems. The aim of this study was to establish a mediated electron transfer between the anode and the metabolism of living cells, i.e. anodic respiration, using fructose as electron and carbon source. Since C. necator is not able to transfer electrons directly to an electrode, redox mediators are required for this process. Based on previous observations on the extracellular electron transfer enabled by a polymeric mediator, we tested 11 common biological and non-biological redox mediators for their functionality and inhibitory effect for anodic electron transfer in a C. necator-based bioelectrochemical system. The use of ferricyanide at a concentration of 15 mM resulted in the highest current density of 260.75µAcm-2 and a coulombic efficiency of 64.1 %.

Photo-Electro-Biochemical H2 Production Using the Carbon Material-Based Cathode Combined with Genetically Engineered Escherichia coli Whole-Cell Biocatalysis

Abio/bio hybrids, which incorporate biocatalysts that promote efficient and selective material conversions under mild conditions into existing catalytic reactions, have attracted considerable attention for developing new catalytic systems. This study constructed a H2 -forming biocathode based on a carbon material combined with whole-cell biocatalysis of genetically-engineered-hydrogenase-overproducing Escherichia coli for the photoelectrochemical water splitting for clean H2 production. Low-cost and abundant carbon materials are generally not suitable for H2 -forming cathode due to their high overpotential for proton reduction; however, the combination of the reduction of an organic electron mediator on the carbon electrode and the H2 formation with the reduced mediator by the redox enzyme hydrogenase provides a H2 -forming cathodic reaction comparable to that of the noble metal electrode. The present study demonstrates that the recombinant E. coli whole cell can be employed as a part of the H2 -forming biocathode system, and the biocathode system wired with TiO2 photoanode can be a photoelectrochemical water-splitting system without external voltage assistance under natural pH. The findings of this study expand the feasibility of applications of whole-cell biocatalysis and contribute to obtaining solar-to-chemical conversions by abio/bio hybrid systems, especially for low-cost, noble-metal-free, and clean H2 production.

Triple-Barrel Ultramicroelectrodes for Multipurpose, Submilliliter Electroanalysis

Here, we have developed and applied a triple-barrel microelectrode. This device incorporates a platinum disk working electrode, a platinum disk counter electrode, and a low-leakage Ag/AgCl reference electrode into a small probe. We demonstrate that the incorporated low-leakage reference electrode shows similar voltammetry, potentiometry, and drift when compared to a commercial reference electrode in bulk solution. We also demonstrate the versatility of such a small three-channel system via voltammetry in nanoliter droplets and through electroanalysis of captured aerosols. Finally, we demonstrate the probe's potential utility in single-cell electroanalysis by making measurements within salmon eggs.

Electro-fermentation: Sustainable bioproductions steered by electricity

The market of biobased products obtainable via fermentation processes is steadily increasing over the past few years, driven by the need to create a decarbonized economy. To date, industrial fermentation (IF) employs either pure or mixed microbial cultures (MMC) whereby the type of the microbial catalysts and the used feedstock affect metabolic pathways and, in turn, the type of product(s) generated. In many cases, especially when dealing with MMC, the economic viability of IF is hindered by factors such as the low attained product titer and selectivity, which ultimately challenge the downstream recovery and purification steps. In this context, electro-fermentation (EF) represents an innovative approach, based on the use of a polarized electrode interface to trigger changes in the rate, yield, titer or product distribution deriving from traditional fermentation processes. In principle, the electrode in EF can act as an electron acceptor (i.e., anodic electro-fermentation, AEF) or donor (i.e., cathodic electro-fermentation, CEF), or simply as a mean to control the oxidation-reduction potential of the fermentation broth. However, the molecular and biochemical basis underlying the EF process are still largely unknown. This review paper provides a comprehensive overview of recent literature studies including both AEF and CEF examples with either pure or mixed microbial cultures. A critical analysis of biochemical, microbiological, and engineering aspects which presently hamper the transition of the EF technology from the laboratory to the market is also presented.

Thermodynamic controls on rates of iron oxide reduction by extracellular electron shuttles

Anaerobic microbial respiration in suboxic and anoxic environments often involves particulate ferric iron (oxyhydr-)oxides as terminal electron acceptors. To ensure efficient respiration, a widespread strategy among iron-reducing microorganisms is the use of extracellular electron shuttles (EES) that transfer two electrons from the microbial cell to the iron oxide surface. Yet, a fundamental understanding of how EES-oxide redox thermodynamics affect rates of iron oxide reduction remains elusive. Attempts to rationalize these rates for different EES, solution pH, and iron oxides on the basis of the underlying reaction free energy of the two-electron transfer were unsuccessful. Here, we demonstrate that broadly varying reduction rates determined in this work for different iron oxides and EES at varying solution chemistry as well as previously published data can be reconciled when these rates are instead related to the free energy of the less exergonic (or even endergonic) first of the two electron transfers from the fully, two-electron reduced EES to ferric iron oxide. We show how free energy relationships aid in identifying controls on microbial iron oxide reduction by EES, thereby advancing a more fundamental understanding of anaerobic respiration using iron oxides.

Catalytic Control of Spiroketal Formation in Rubromycin Polyketide Biosynthesis

The medically important bacterial aromatic polyketide natural products typically feature a planar, polycyclic core structure. An exception is found for the rubromycins, whose backbones are disrupted by a bisbenzannulated [5,6]‐spiroketal pharmacophore that was recently shown to be assembled by flavin‐dependent enzymes. In particular, a flavoprotein monooxygenase proved critical for the drastic oxidative rearrangement of a pentangular precursor and the installment of an intermediate [6,6]‐spiroketal moiety. Here we provide structural and mechanistic insights into the control of catalysis by this spiroketal synthase, which fulfills several important functions as reductase, monooxygenase, and presumably oxidase. The enzyme hereby tightly controls the redox state of the substrate to counteract shunt product formation, while also steering the cleavage of three carbon‐carbon bonds. Our work illustrates an exceptional strategy for the biosynthesis of stable chroman spiroketals.

A Kinetic Investigation of the Early Steps in Cytochrome c Nitrite Reductase (ccNiR)-Catalyzed Reduction of Nitrite

The decaheme enzyme cytochrome c nitrite reductase (ccNiR) catalyzes reduction of nitrite to ammonium in a six-electron, eight-proton process. With a strong reductant as the electron source, ammonium is the sole product. However, intermediates accumulate when weaker reductants are employed, facilitating study of the ccNiR mechanism. Herein, the early stages of Shewanella oneidensis ccNiR-catalyzed nitrite reduction were investigated by using the weak reductants N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and ferrocyanide. In stopped-flow experiments, reduction of nitrite-loaded ccNiR by TMPD generated a transient intermediate, identified as Fe_H1^II(NO₂^-), where Fe_H1 represents the ccNiR active site. Fe_H1^II(NO₂^-) accumulated rapidly and was then more slowly converted to the two-electron-reduced moiety {Fe_H1NO}⁷; ccNiR was not reduced beyond the {Fe_H1NO}⁷ state. The midpoint potentials for sequential reduction of Fe_H1^III(NO₂^-) to Fe_H1^II(NO₂^-) and then to {Fe_H1NO}⁷ were estimated to be 130 and 370 mV versus the standard hydrogen electrode, respectively. Fe_H1^II(NO₂^-) does not accumulate at equilibrium because its reduction to {Fe_H1NO}⁷ is so much easier than the reduction of Fe_H1^III(NO₂^-) to Fe_H1^II(NO₂^-). With weak reductants, free NO• was released from nitrite-loaded ccNiR. The release of NO• from {Fe_H1NO}⁷ is exceedingly slow (k ∼ 0.001 s^-1), but it is somewhat faster (k ∼ 0.050 s^-1) while Fe_H1^III(NO₂^-) is being reduced to {Fe_H1NO}⁷; then, the release of NO• from the undetectable transient {Fe_H1NO}⁶ can compete with reduction of {Fe_H1NO}⁶ to {Fe_H1NO}⁷. CcNiR appears to be optimized to capture nitrite and minimize the release of free NO•. Nitrite capture is achieved by reducing bound nitrite with even weak electron donors, while NO• release is minimized by stabilizing the substitutionally inert {Fe_H1NO}⁷ over the more labile {Fe_H1NO}⁶.

Translational research of hemoglobin vesicles as a transfusion alternative

Clinical situations arise in which blood for transfusion becomes scarce or unavailable. Considerable demand for a transfusion alternative persists because of various difficulties posed by blood donation and transfusion systems. Hemoglobin-vesicles (HbV) are artificial oxygen carriers being developed for use as a transfusion alternative. Just as biomembranes of red blood cells (RBCs) do, phospholipid vesicles (liposomes) for Hb encapsulation can protect the human body from toxic effects of molecular Hb. The main HbV component, Hb, is obtained from discarded human donated blood. Therefore, HbV can be categorized as a biologic agent targeting oxygen for peripheral tissues. The purification procedure strictly eliminates the possibility of viral contamination. It also removes all concomitant unstable enzymes present in RBC for utmost safety from infection. The deoxygenated HbVs, which are storable for over years at ambient temperature, can function as an alternative to blood transfusion for resuscitation from hemorrhagic shock and O₂ therapeutics. Moreover, a recent study clarified beneficial effects for anti-oxidation and anti-inflammation by carbon monoxide (CO)-bound HbVs. Autoxidation of HbV (HbO₂ → metHb + O₂-.) is unavoidable after intravenous administration. Co-injection of methylene blue can extract the intraerythrocytic glycolytic electron energy effectively and reduce metHb. Other phenothiazine dyes can also function as electron mediators to improve the functional life span of HbV. This review paper summarizes recent progress of the research and development of HbV, aimed at clinical applications.

Reversible H2 Oxidation and Evolution by Hydrogenase Embedded in a Redox Polymer Film

Efficient electrocatalytic energy conversion requires the devices to function reversibly, i.e. deliver a significant current at minimal overpotential. Redox-active films can effectively embed and stabilise molecular electrocatalysts, but mediated electron transfer through the film typically makes the catalytic response irreversible. Here, we describe a redox-active film for bidirectional (oxidation or reduction) and reversible hydrogen conversion, consisting of [FeFe] hydrogenase embedded in a low-potential, 2,2’-viologen modified hydrogel. When this catalytic film served as the anode material in a H₂/O₂ biofuel cell, an open circuit voltage of 1.16 V was obtained - a benchmark value near the thermodynamic limit. The same film also acted as a highly energy efficient cathode material for H₂ evolution. We explained the catalytic properties using a kinetic model, which shows that reversibility can be achieved despite intermolecular electron transfer being slower than catalysis. This understanding of reversibility simplifies the design principles of highly efficient and stable bioelectrocatalytic films, advancing their implementation in energy conversion.

Catalytic bias in oxidation-reduction catalysis

Cataytic bias refers to the propensity of a reaction catalyst to effect a different rate acceleration in one direction versus the other in a chemical reaction under non-equilibrium conditions. In biocatalysis, the inherent bias of an enzyme is often advantagous to augment the innate thermodynamics of a reaction to promote efficiency and fidelity in the coordination of catabolic and anabolic pathways. In industrial chemical catalysis a directional cataltyic bias is a sought after property in facilitating the engineering of systems that couple catalysis with harvest and storage of for example fine chemicals or energy compounds. Interestingly, there is little information about catalytic bias in biocatalysis likely in large part due to difficulties in developing tractible assays sensitive enough to study detailed kinetics. For oxidation-reduction reactions, colorimetric redox indicators exist in a range of reduction potentials to provide a mechanism to study both directions of reactions in a fairly facile manner. The current short review attempts to define catalytic bias conceptually and to develop model systems for defining the parameters that control catalytic bias in enzyme catalyzed oxidation-reduction catalysis.

Photo-biohydrogen Production by Photosensitization with Biologically Precipitated Cadmium Sulfide in Hydrogen-Forming Recombinant Escherichia coli

An inorganic-biological hybrid system that integrates features of both stable and efficient semiconductors and selective and efficient enzymes is attractive for facilitating the conversion of solar energy to hydrogen. In this study, we aimed to develop a new photocatalytic hydrogen-production system based on Escherichia coli whole-cell genetically engineered as a biocatalysis for highly active hydrogen formation. The photocatalysis part was obtained by bacterial precipitation of cadmium sulfide (CdS), which is a visible-light-responsive semiconductor. The recombinant E. coli cells were sequentially subjected to CdS precipitation and heterologous [FeFe]-hydrogenase synthesis to yield a CdS@E. coli hybrid capable of light energy conversion and hydrogen formation in a single cell. The CdS@E. coli hybrid achieved photocatalytic hydrogen production with a sacrificial electron donor, thus demonstrating the feasibility of our system and expanding the current knowledge of photosensitization using a whole-cell biocatalyst with a bacterially precipitated semiconductor.

Controlled-Potential-Based Electrochemical Sulfide Sensors: A Review

Due to their potential applications in industry and potent toxicity to the environment, sulfides and their detection have attracted the attention of researchers. To date, a large number of controlled-potential techniques for electrochemical sulfide sensors have been developed, thanks to their simplicity, reasonable limit of detection (LOD), and good selectivity. Different researchers have applied different strategies for developing selective and sensitive sulfide sensors. However, there has been no systematic review on controlled-potential techniques for sulfide sensing. In light of this absence, the main aim of this review article is to summarize various strategies for detecting sulfide in different media. The efficiencies of the developed sulfide sensors for detecting sulfide in its various forms are determined, and the essential parameters, including sensing strategies, working electrodes, detection media, pH, LOD, sensitivity, and linear detection range, are emphasized in particular. Future research in this area is also recommended. It is expected that this review will act as a basis for further research on the fabrication of sulfide sensors for practical applications.

Multisensor Systems and Arrays for Medical Applications Employing Naturally-Occurring Compounds and Materials

The significant improvement of quality of life achieved over the last decades has stimulated the development of new approaches in medicine to take into account the personal needs of each patient. Precision medicine, providing healthcare customization, opens new horizons in the diagnosis, treatment and prevention of numerous diseases. As a consequence, there is a growing demand for novel analytical devices and methods capable of addressing the challenges of precision medicine. For example, various types of sensors or their arrays are highly suitable for simultaneous monitoring of multiple analytes in complex biological media in order to obtain more information about the health status of a patient or to follow the treatment process. Besides, the development of sustainable sensors based on natural chemicals allows reducing their environmental impact. This review is concerned with the application of such analytical platforms in various areas of medicine: analysis of body fluids, wearable sensors, drug manufacturing and screening. The importance and role of naturally-occurring compounds in the development of electrochemical multisensor systems and arrays are discussed.

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset-from SCV and SWV on all NOMs and model compounds-revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.

Fourier Transform Infrared Spectrovoltammetry and Quantitative Modeling of Analytes in Kinetically Constrained Redox Mixtures

Redox-active analytes that do not support direct electron transfer on the electrode, such as proteins with buried redox centers, pose challenges to characterization of their structural and thermodynamic properties. Investigations of indirect transitions in analytes supported by complex redox mixtures require a careful balance between kinetic limitations and spectral interference from the mediators. Using methylene green and thionine acetate as redox mediators and myoglobin as the analyte, we demonstrate that normal pulse spectrovoltammetry (NPSV) with Fourier transform infrared (FT-IR) detection and subsequent global spectral regression analysis can resolve structural and thermodynamic properties simultaneously with little a priori information. Both the E_1/2 and unbiased redox difference FT-IR spectra of the Fe(II)/Fe(III) redox couple of myoglobin in reduction and oxidation NPSV modes were in good agreement with those reported earlier by independent techniques. The thermodynamic and kinetic limitations of mediators/analyte interactions were investigated using comprehensive semiempirical kinetic simulation models. This modeling effort yielded a flexible computational tool capable of quantitatively predicting the redox response in mediated electrochemical studies and defining its limitations, thus greatly expanding the range and precision of the formal mediator/analyte concentration ratio rule.

Metal Transformation by a Novel Pelosinus Isolate From a Subsurface Environment

The capability of microorganisms to alter metal speciation offers potential for the development of new strategies for immobilization of toxic metals in the environment. A metal-reducing microbe, "Pelosinus lilae" strain UFO1, was isolated under strictly anaerobic conditions from an Fe(III)-reducing enrichment established with uncontaminated soil from the Department of Energy Oak Ridge Field Research Center, Tennessee. "P. lilae" UFO1 is a rod-shaped, spore-forming, and Gram-variable anaerobe with a fermentative metabolism. It is capable of reducing the humic acid analog anthraquinone-2,6-disulfonate (AQDS) using a variety of fermentable substrates and H2. Reduction of Fe(III)-nitrilotriacetic acid occurred in the presence of lactate as carbon and electron donor. Ferrihydrite was not reduced in the absence of AQDS. Nearly complete reduction of 1, 3, and 5 ppm Cr(VI) occurred within 24 h in suspensions containing 108 cells mL-1 when provided with 10 mM lactate; when 1 mM AQDS was added, 3 and 5 ppm Cr(VI) were reduced to 0.1 ppm within 2 h. Strain UFO1 is a novel species within the bacterial genus Pelosinus, having 98.16% 16S rRNA gene sequence similarity with the most closely related described species, Pelosinus fermentans R7T. The G+C content of the genomic DNA was 38 mol%, and DNA-DNA hybridization of "P. lilae" UFO1 against P. fermentans R7T indicated an average 16.8% DNA-DNA similarity. The unique phylogenetic, physiologic, and metal-transforming characteristics of "P. lilae" UFO1 reveal it is a novel isolate of the described genus Pelosinus.

Anodic reactions of NADH model compound by utilizing both light irradiation and riboflavin as a redox mediator

Both light and a redox mediator riboflavin (RF) were utilized to promote the electro-oxidation of an NADH model compound (1-benzyl-1,4-dihydronicotinamide, BNAH), which is a key process for enzymatic biofuel cells to obtain a high performance. At the cathode, H⁺ ions were simultaneously reduced to produce H₂ gas. To elucidate the cell reactions of this photogalvanic cell, which is significant information about the fabrication of enzymatic biofuel cells with a high performance, the effect of the BNAH and RF concentrations on the cell current, the light wavelength dependence on the current, and reduction of the RF concentration were evaluated. The obtained results strongly suggest that the anodic reactions were composed of the following reactions: 1) the photo-excitation of RF, 2) the attack of the excited RF on the BNAH and the generation of the radical species of BNAH and RF, and 3) the chain reactions between the radical species.

Mediator Preference of Two Different FAD-Dependent Glucose Dehydrogenases Employed in Disposable Enzyme Glucose Sensors

Most commercially available electrochemical enzyme sensor strips for the measurement of blood glucose use an artificial electron mediator to transfer electrons from the active side of the enzyme to the electrode. One mediator recently gaining attention for commercial sensor strips is hexaammineruthenium(III) chloride. In this study, we investigate and compare the preference of enzyme electrodes with two different FAD-dependent glucose dehydrogenases (FADGDHs) for the mediators hexaammineruthenium(III) chloride, potassium ferricyanide (the most common mediator in commercial sensor strips), and methoxy phenazine methosulfate (mPMS). One FADGDH is a monomeric fungal enzyme, and the other a hetero-trimeric bacterial enzyme. With the latter, which contains a heme-subunit facilitating the electron transfer, similar response currents are obtained with hexaammineruthenium(III), ferricyanide, and mPMS (6.8 µA, 7.5 µA, and 6.4 µA, respectively, for 10 mM glucose). With the fungal FADGDH, similar response currents are obtained with the negatively charged ferricyanide and the uncharged mPMS (5.9 µA and 6.7 µA, respectively, for 10 mM glucose), however, no response current is obtained with hexaammineruthenium(III), which has a strong positive charge. These results show that access of even very small mediators with strong charges to a buried active center can be almost completely blocked by the protein.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

The 1,6,7,12-Tetraazaperylene Bridging Ligand as an Electron Reservoir and Its Disulfonato Derivative as Redox Mediator in an Enzyme-Electrode Process

The homodinuclear ruthenium(II) complex [{Ru(l-N₄ Me₂ )}₂ (μ-tape)](PF₆ )₄ {[1](PF₆ )₄ } (l-N₄ Me₂ =N,N'-dimethyl-2,11-diaza[3.3](2,6)-pyridinophane, tape=1,6,7,12-tetraazaperylene) can store one or two electrons in the energetically low-lying π* orbital of the bridging ligand tape. The corresponding singly and doubly reduced complexes [{Ru(l-N₄ Me₂ )}₂ (μ-tape^.- )](PF₆ )₃ {[2](PF₆ )₃ } and [{Ru(l-N₄ Me₂ )}₂ (μ-tape^2- )](PF₆ )₂ {[3](PF₆ )₂ }, respectively, were electrochemically generated, successfully isolated and fully characterized by single-crystal X-ray crystallography, spectroscopic methods and magnetic susceptibility measurements. The singly reduced complex [2](PF₆ )₃ contains the π-radical tape^.- and the doubly reduced [3](PF₆ )₂ the diamagnetic dianion tape^2- as bridging ligand, respectively. Nucleophilic aromatic substitution at the bridging tape in [1]⁴⁺ by two sulfite units gave the complex [{Ru(l-N₄ Me₂ )}₂ {μ-tape-(SO₃ )₂ }]²⁺ ([4]²⁺ ). Complex dication [4]²⁺ was exploited as a redox mediator between an anaerobic homogenous reaction solution of an enzyme system (sulfite/sulfite oxidase) and the electrode via participation of the low-energy π*-orbital of the disulfonato-substituted bridging ligand tape-(SO₃ )₂^2- (E_red1 =-0.1 V versus Ag/AgCl/1 m KCl in water).

Modeling Microbial Electrosynthesis

Mathematical modeling is an overarching approach for assessing the complexity of microbial electrosynthesis (MES) and for complementing the relevant experimental research. By describing and linking compartments, components, and processes with appropriate mathematical equations, MES and the corresponding bioelectrodes and complete bioelectrochemical systems can be analyzed and predicted across several temporal and local scales. Thereby, insights into fundamental phenomena and mechanisms, in addition to process engineering and design can be obtained. However, a substantial lack of knowledge about extracellular electron transfer mechanisms and electrotrophic microorganisms presumably prevented the development of adequate models of MES, especially of biocathodes, so far. To propel efforts regarding this demanding task, this chapter provides a comprehensive overview of the relevant compartments, components and processes, appropriate model strategies, and a discussion on potential modeling pitfalls. By adapting an established approach to assessing the energetics of microorganism, an instruction for calculating stoichiometry, thermodynamics, and kinetics, with the example of electro-autotrophic growth at cathodes, is presented. Models of bioanodes and fundamental electrochemical equations are described to provided strategies for calculating cathodic electron-uptake reactions and connecting them to the microbial metabolism. Finally, differential equations are detailed for coupling the distinct compartments of a bioelectrochemical system. Although MES comprises anodic and cathodic reactions, the present chapter focuses on biocathodes representing a functional connection between cathode and electron-accepting microorganisms. Graphical Abstract.

Analytical Methods for Assessing the Effects of Site-Directed Mutagenesis on Protein-Cofactor and Protein-Protein Functional Relationships

To completely understand the role of an amino acid residue that is targeted for site-directed mutagenesis a thorough analysis of the impact that the mutation has on the function of the protein is required. General methods for performing site-directed mutagenesis and expressing the recombinant protein variant are described. Protein-cofactor interactions are important because cofactors are often directly involved in facilitating catalysis by enzymes and in electron transfer by redox proteins. Many cofactors also have characteristic spectroscopic properties. As such, general methods are described to analyze the spectroscopic, redox and catalytic properties of protein-bound cofactors. Methods for assessing the effects of a mutation on protein-protein interactions are also described. Lastly, methods for assessing the overall structural integrity of the protein are described, as this is important to ensure that the mutation has not caused a global disruption of protein structure, rather than a specific effect on function.

A Comparative Analysis of the Effector Role of Redox Partner Binding in Bacterial P450s

The camphor monooxygenase, cytochrome P450cam, exhibits a strict requirement for its own redox partner, putidaredoxin (Pdx), a two-iron-sulfur ferredoxin. The closest homologue to P450cam, CYP101D1, is structurally very similar, uses a similar redox partner, and exhibits nearly identical enzymatic properties in the monooxygenation of camphor to give the same single 5-exo-hydroxy camphor product. However, CYP101D1 does not strictly require its own ferredoxin (Arx) for activity because Pdx can support CYP101D1 catalysis but Arx cannot support P450cam catalysis. We have further examined the differences between these two P450s by determining the effect of spin equilibrium, redox properties, and stability of oxygen complexes. We find that Arx shifts the spin state equilibrium toward high-spin, which is the opposite of the effect of Pdx on P450cam. In both P450s, redox partner binding destabilizes the oxy-P450 complex but this effect is much weaker with CYP101D1. In addition, resonance Raman data show that structural perturbations observed in P450cam upon addition of Pdx are absent in CYP101D1. These data indicate that Arx does not play the same effector role in catalysis as Pdx does with P450cam. The most relevant structural difference between these two P450s centers on a catalytically important Asp residue required for proton-coupled electron transfer. We postulate that with P450cam larger Pdx-assisted motions are required to free this Asp for catalysis while the smaller number of restrictions in CYP101D1 precludes the need for redox partner-assisted structural changes.

Electrochemical biosensors and nanobiosensors

Electrochemical techniques have great promise for low-cost miniaturised easy-to-use portable devices for a wide range of applications-in particular, medical diagnosis and environmental monitoring. Different techniques can be used for biosensing, with amperometric devices taking the central role due to their widespread application in glucose monitoring. In fact, glucose biosensing takes an approximately 70% share of the biosensor market due to the need for diabetic patients to monitor their sugar levels several times a day, making it an appealing commercial market.In this review, we present the basic principles of electrochemical biosensor devices. A description of the different generations of glucose sensors is used to describe in some detail the operation of amperometric sensors and how the introduction of mediators can enhance the performance of the sensors. Electrochemical impedance spectroscopy is a technique being increasingly used in devices due to its ability to detect variations in resistance and capacitance upon binding events. Novel advances in electrochemical sensors, due to the use of nanomaterials such as carbon nanotubes and graphene, are presented as well as future directions that the field is taking.

An electrochemical platform for localized pH control on demand

Solution pH is a powerful tool for regulating many kinds of chemical activity, but is generally treated as a static property defined by a pre-selected buffer. Introducing dynamic control of pH in space, time, and magnitude can enable richer and more efficient chemistries, but is not feasible with traditional methods of titration or buffer exchange. Recent reports have featured electrochemical strategies for modifying bulk pH in constrained volumes, but only demonstrate switching between two preset values and omit spatial control entirely. Here, we use a combination of solution-borne quinones and galvanostatic excitation to enable quantitative control of pH environments that are highly localized to an electrode surface. We demonstrate highly reproducible acidification and alkalinization with up to 0.1 pH s(-1) (±0.002 pH s(-1)) rate of change across the dynamic range of our pH sensor (pH 4.5 to 7.5) in buffered solutions. Using dynamic current control, we generate and sustain 3 distinct pH microenvironments simultaneously to within ±0.04 pH for 13 minutes in a single solution, and we leverage these microenvironments to demonstrate spatially-resolved, pH-driven control of enzymatic activity. In addition to straightforward applications of spatio-temporal pH control (e.g. efficiently studying pH-dependencies of chemical interactions), the technique opens completely new avenues for implementing complex systems through dynamic control of enzyme activation, protein binding affinity, chemical reactivity, chemical release, molecular self-assembly, and many more pH-controlled processes.

Bioelectrochemical reduction of volatile fatty acids in anaerobic digestion effluent for the production of biofuels

This study proves for the first time the feasibility of biofuel production from anaerobic digestion effluent via bioelectrochemical cell operation at various applied cell voltages (1.0, 1.5 and 2.0 V). An increase in cell voltage from 1 to 2 V resulted in more reduction current generation (-0.48 to -0.78 mA) at a lowered cathode potential (-0.45 to -0.84 mV vs Ag/AgCl). Various alcohols were produced depending on applied cell voltages, and the main products were butanol, ethanol, and propanol. Hydrogen and methane production were also observed in the headspace of the cell. A large amount of lactic acid was unexpectedly formed at all conditions, which might be the primary cause of the limited biofuel production. The addition of neutral red (NR) to the system could increase the cathodic reduction current, and thus more biofuels were produced with an enhanced alcohol formation compared to without a mediator.

Biosensors based on electrochemical lactate detection: A comprehensive review

Lactate detection plays a significant role in healthcare, food industries and is specially necessitated in conditions like hemorrhage, respiratory failure, hepatic disease, sepsis and tissue hypoxia. Conventional methods for lactate determination are not accurate and fast so this accelerated the need of sensitive biosensors for high-throughput screening of lactate in different samples. This review focuses on applications and developments of various electrochemical biosensors based on lactate detection as lactate being essential metabolite in anaerobic metabolic pathway. A comparative study to summarize the L-lactate biosensors on the basis of different analytical properties in terms of fabrication, sensitivity, detection limit, linearity, response time and storage stability has been done. It also addresses the merits and demerits of current enzyme based lactate biosensors. Lactate biosensors are of two main types – lactate oxidase (LOD) and lactate dehydrogenase (LDH) based. Different supports tried for manufacturing lactate biosensors include membranes, polymeric matrices-conducting or non-conducting, transparent gel matrix, hydrogel supports, screen printed electrodes and nanoparticles. All the examples in these support categories have been aptly discussed. Finally this review encompasses the conclusion and future emerging prospects of lactate sensors.

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Different enzymes used in lactate bio sensing have been studied.

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Support used for fabrication biosensors have been discussed.

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The linearity range, response time, detection limit, etc. have been studied.

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Merits and demerits of different supports are also discussed.

Electrochemical analyses of redox-active iron minerals: a review of nonmediated and mediated approaches

Redox-active minerals are ubiquitous in the environment and are involved in numerous electron transfer reactions that significantly affect biogeochemical processes and cycles as well as pollutant dynamics. As a consequence, research in different scientific disciplines is devoted to elucidating the redox properties and reactivities of minerals. This review focuses on the characterization of mineral redox properties using electrochemical approaches from an applied (bio)geochemical and environmental analytical chemistry perspective. Establishing redox equilibria between the minerals and working electrodes is a major challenge in electrochemical measurements, which we discuss in an overview of traditional electrochemical techniques. These issues can be overcome with mediated electrochemical analyses in which dissolved redox mediators are used to increase the rate of electron transfer and to facilitate redox equilibration between working electrodes and minerals in both amperometric and potentiometric measurements. Using experimental data on an iron-bearing clay mineral, we illustrate how mediated electrochemical analyses can be employed to derive important thermodynamic and kinetic data on electron transfer to and from structural iron. We summarize anticipated methodological advancements that will further contribute to advance an improved understanding of electron transfer to and from minerals in environmentally relevant redox processes.

Potential electron mediators to extract electron energies of RBC glycolysis for prolonged in vivo functional lifetime of hemoglobin vesicles

Developing a functional blood substitute as an alternative to donated blood for clinical use is believed to relieve present and future blood shortages, and to reduce the risks of infection and blood type mismatching. Hemoglobin vesicle (HbV) encapsulates a purified and concentrated human-derived Hb solution in a phospholipid vesicle (liposome). The in vivo safety and efficacy of HbV as a transfusion alternative have been clarified. Auto-oxidation of ferrous Hb in HbV gradually increases the level of ferric methemoglobin (metHb) and impairs the oxygen transport capabilities. The extension of the functional half-life of HbV has recently been proposed using an electron mediator, methylene blue (MB), which acts as a shuttle between red blood cells (RBC) and HbV. MB transfers electron energies of NAD(P)H, produced by RBC glycolysis, to metHb in HbV. Work presented here focuses on screening of 15 potential electron mediators, with appropriate redox potential and water solubility, for electron transfer from RBC to HbV. The results are assessed with regard to the chemical properties of the candidates. The compounds examined in this study were dimethyl methylene blue (DMB), methylene green, azure A, azure B, azure C, toluidine blue (TDB), thionin acetate, phenazine methosulfate, brilliant cresyl blue, cresyl violet, gallocyanine, toluylene blue, indigo carmine, indigotetrasulfonate, and MB. Six candidates were found to be unsuitable because of their insufficient diffusion across membranes, or overly high or nonexistent reactivity with relevant biomolecules. However, 9 displayed favorable metHb reduction. Among the suitable candidates, phenothiazines DMB and TDB exhibited effectiveness like MB did. In comparison to MB, they showed faster reduction by electron-donating NAD(P)H, coupled with showing a lower rate of reoxidation in the presence of molecular oxygen. Ascertaining the best electron mediator can provide a pathway for extending the lifetime and efficiency of potential blood substitutes.