Range Of Redox Potentials Research Articles

Experimental and Theoretical Predictors for Redox Potentials of Bispyridinylidene Electron Donors.

Bispyridinylidenes are neutral organic molecules capable of two-electron oxidation at a range of redox potentials that are widely tunable by choice of substituent, making them attractive as homogeneous organic reductants and active materials in redox flow batteries. In an effort to readily predict the redox potentials of this important class of compounds, we have developed correlations between the experimental redox potentials and both experimental and theoretical predictors. On the experimental side, we show that multinuclear NMR chemical shifts of related pyridinium ions correlate well with the redox potentials of bispyridinylidenes, with R2 and standard errors (S) reaching 0.9810 and 0.048 V, respectively, when the 13C (N-CH3) and 1H (ortho) chemical shifts are used together. Theoretical studies of the bispyridinylidenes and their doubly oxidized bipyridinium ions gave a range of predictively valuable equations at various levels of computational cost. This ranged from a simple model using only the EHOMO of the bispyridinylidenes (R2=0.9689; S=0.060 V), to a more computationally intensive model which include solvation effects for both redox states which gave the highest predictive value for all methods (R2=0.9958; S=0.022 V). This work will guide further studies of this important class of molecules.

Asymmetric photoredox catalytic formal de Mayo reaction enabled by sensitization-initiated electron transfer.

Visible-light-driven photoredox catalysis is known to be a powerful tool for organic synthesis. Its occurrence critically depends on the twice exothermic single-electron transfer processes of photosensitizers, which are governed by the redox properties of the species involved. Hence, the inherently narrow range of redox potentials of photosensitizers inevitably constrains their further availability. Sensitization-initiated electron transfer has recently been found to effectively overcome this substantial challenge. However, feasible and practical strategies for designing such complicated catalytic systems are rather scarce. Herein we report an elaborate dual-catalyst platform, with dicyanopyrazine as a visible light photosensitizer and a pyrenyl-incorporated chiral phosphoric acid as a co-sensitizer, and we demonstrate the applicability of this sensitization-initiated electron transfer strategy in an asymmetric formal de Mayo-type reaction. The catalysis platform enables otherwise thermodynamically unfavourable electron transfer processes to close the redox cycle and allows for precise access to valuable enantioenriched 1,5-diketones with a wide substrate range.

Gallionellaceae pangenomic analysis reveals insight into phylogeny, metabolic flexibility, and iron oxidation mechanisms.

Neutrophilic iron-oxidizing bacteria (FeOB) produce copious iron (oxyhydr)oxides that can profoundly influence biogeochemical cycles, notably the fate of carbon and many metals. To fully understand environmental microbial iron oxidation, we need a thorough accounting of iron oxidation mechanisms. In this study, we show the Gallionellaceae FeOB genomes encode both characterized iron oxidases as well as uncharacterized multiheme cytochromes (MHCs). MHCs are predicted to transfer electrons from extracellular substrates and likely confer metabolic capabilities that help Gallionellaceae occupy a range of different iron- and mineral-rich niches. Gallionellaceae appear to specialize in iron oxidation, so it would be advantageous for them to have multiple mechanisms to oxidize various forms of iron, given the many iron minerals on Earth, as well as the physiological and kinetic challenges faced by FeOB. The multiple iron/mineral oxidation mechanisms may help drive the widespread ecological success of Gallionellaceae.

Effect of the Cu2+/1+ Redox Potential of Non-Macrocyclic Cu Complexes on Electrochemical CO2 Reduction

Cu2+/1+ complexes facilitate the reduction of CO2 to valuable chemicals. The catalytic conversion likely involves the binding of CO2 and/or reduction intermediates to Cu2+/1+, which in turn could be influenced by the electron density on the Cu2+/1+ ion. Herein we investigated whether modulating the redox potential of Cu2+/1+ complexes by changing their ligand structures influenced their CO2 reduction performance significantly. We synthesised new heteroleptic Cu2/1+ complexes, and for the first time, studied a (Cu-bis(8-quinolinolato) complex, covering a Cu2+/1+ redox potential range of 1.3 V. We have found that the redox potential influenced the Faradaic efficiency of CO2 reduction to CO. However, no correlation between the redox potential and the Faradaic efficiency for methane was found. The lack of correlation could be attributed to the presence of a Cu-complex-derived catalyst deposited on the electrodes leading to a heterogeneous catalytic mechanism, which is controlled by the structure of the in situ deposited catalyst and not the redox potential of the pre-cursor Cu2+/1+ complexes.

Investigating the Electrochemically Driven Capture and Release of Long-Chain PFAS by Redox Metallopolymer Sorbents.

The remediation of perfluoroalkyl substances (PFAS) is an urgent challenge due to their prevalence and persistence in the environment. Electrosorption is a promising approach for wastewater treatment and water purification, especially through the use of redox polymers to control the binding and release of target contaminants without additional external chemical inputs. However, the design of efficient redox electrosorbents for PFAS faces the significant challenge of balancing a high adsorption capacity while maintaining significant electrochemical regeneration. To overcome this challenge, we investigate redox-active metallopolymers as a versatile synthetic platform to enhance both electrochemical reversibility and electrosorption uptake capacity for PFAS removal. We selected and synthesized a series of metallopolymers bearing ferrocene and cobaltocenium units spanning a range of redox potentials to evaluate their performance for the capture and release of perfluorooctanoic acid (PFOA). Our results demonstrate that PFOA uptake and regeneration efficiency increased with more negative formal potential of the redox polymers, indicating possible structural correlations with the electron density of the metallocenes. Poly(2-(methacryloyloxy)ethyl cobaltoceniumcarboxylate hexafluorophosphate) (PMAECoPF6) showed the highest affinity toward PFOA, with an uptake capacity of more than 90 mg PFOA/g adsorbent at 0.0 V vs Ag/AgCl and a regeneration efficiency of more than 85% at -0.4 V vs Ag/AgCl. Kinetics of PFOA release showed that electrochemical bias greatly enhanced the regeneration efficiency when compared to open-circuit desorption. In addition, electrosorption of PFAS from different wastewater matrices and a range of salt concentrations demonstrated the capability of PFAS remediation in complex water sources, even at ppb levels of contaminants. Our work showcases the synthetic tunability of redox metallopolymers for enhanced electrosorption capacity and regeneration of PFAS.

Intracytoplasmic membranes develop in Geobacter sulfurreducens under thermodynamically limiting conditions

Geobacter sulfurreducens is an electroactive bacterium capable of reducing metal oxides in the environment and electrodes in engineered systems1,2. Geobacter sp. are the keystone organisms in electrogenic biofilms, as their respiration consumes fermentation products produced by other organisms and reduces a terminal electron acceptor e.g. iron oxide or an electrode. To respire extracellular electron acceptors with a wide range of redox potentials, G. sulfurreducens has a complex network of respiratory proteins, many of which are membrane-bound3–5. We have identified intracytoplasmic membrane (ICM) structures in G. sulfurreducens. This ICM is an invagination of the inner membrane that has folded and organized by an unknown mechanism, often but not always located near the tip of a cell. Using confocal microscopy, we can identify that at least half of the cells contain an ICM when grown on low potential anode surfaces, whereas cells grown at higher potential anode surfaces or using fumarate as electron acceptor had significantly lower ICM frequency. 3D models developed from cryo-electron tomograms show the ICM to be a continuous extension of the inner membrane in contact with the cytoplasmic and periplasmic space. The differential abundance of ICM in cells grown under different thermodynamic conditions supports the hypothesis that it is an adaptation to limited energy availability, as an increase in membrane-bound respiratory proteins could increase electron flux. Thus, the ICM provides extra inner-membrane surface to increase the abundance of these proteins. G. sulfurreducens is the first Thermodesulfobacterium or metal-oxide reducer found to produce ICMs.

Redox leveling of the Kok cycle of photosystem II established by water ligand binding to the oxygen evolving complex.

Photosystem II (PSII) produces dioxygen at the oxygen-evolving complex (OEC) by catalyzing light-driven water oxidation. The OEC Mn4CaO5 cluster cycles through a series of redox state intermediates Si (i = 0-4) along the Kok cycle, as the tyrosine Z radical oxidizes the cluster and accumulates multiple oxidizing equivalents. All S-state transitions occur within a narrow range of redox potentials and involve alternating oxidation and deprotonation steps. Here, we address the origin of the redox leveling mechanism as determined by the structure of hydrogen bonding surrounding the OEC and binding of water ligands to the OECs. We focus on the S2 to S3 transition to investigate the relative oxidation potential of the tyrosine radical as compared to the oxidation potential of high valent Mn centers regulated by water binding. Free energy calculations at the DFT QM/MM level of theory show that water binding lowers the free energy change for the S2/S3 transition by 13.7 kcal/mol, demonstrating the regulatory role of water binding on the reaction mechanism. The dynamics of water binding to the Mn is further analyzed by running molecular dynamics simulations where the interaction potential of water ligands to the Mn centers is parametrized at the DFT level. Our analysis demonstrates the capabilities of our simulations for studies of substrate water ligand exchange to the OEC that enables direct comparisons to time-resolved mass spectroscopic measurements.

Effect of the type of N-substituent in the benzo-18-azacrown-6 compound on copper(II) chelation: complexation, radiolabeling, stability in vitro, and biodistribution in vivo.

In this work, we synthesized two new benzo-18-azacrown-6 ethers bearing picolinate and pyridine pendant arms and studied the copper complexes of these ligands, as well as those of an acetate analog. All considered ligands were capable of forming mono- and dinuclear complexes due to their large size and large number of donor sites. Among all forms of complexes, the coordination of cations inside the macrocycle has only been shown for the mononuclear form of the acetate complex, while out-cage coordination has been observed for other forms. Electrochemical studies have shown the instability of the mononuclear form of the complex with the pyridine ligand to the reduction in the range of redox potentials of bioreductants. The stabilities of labeled acetate complexes with "in-cage" coordination of the cation and picolinate with "out-cage" coordination were compared in an excess of serum and superoxide dismutase; while the former turned out to be unstable to transchelation, the latter was stable throughout the experiment. Additional studies in biologically relevant media were performed for the picolinate complex and demonstrated its stability in vitro. The biodistribution of this complex in mice after 6 hours post-injection demonstrates a slow excretion from the body; however, the accumulation is noticeably lower than that of free copper cations.

Effect of H2O2 on Na,K-ATPase.

Na,K-ATPase is a member of the P-type ATPase family, which transforms the energy of ATP to the transmembrane Na/K gradient that is used to create membrane potential, support the excitability of neurons and myocytes, control pH, and transport substances. The regulation of the Na,K-ATPase function by physiological regulators also comprises a central role in the adaptation of organisms to different conditions.H2O2is one of the main signaling molecules in redox metabolism and plays important function in cellular physiology. H2O2 also regulates signaling pathways via the specific oxidation of proteins harboring redox-sensitive moieties, like metal centers or cysteine residues, which control their activity. The Na,K-ATPase is redox-sensitive with an "optimal redox potential range," where the reactive oxygen species (ROS), levels beyond this "optimal range" are responsible for enzyme inhibition. Thus reactive oxygen species manifest a hermetic effect, which is expressed by biphasic action; stimulation by low doses and inhibition by high doses. This study was aimed to reveal redox-sensitivity of brain synaptic membrane fractions Na,K-ATPase via H2O2 effects. Different concentrations of H2O2 change the kinetic parameters of the enzyme system for MgATP complex, Na+, and K+ differently. Moreover, H2O2 changes p-chloromercuribenzoic acids (PCMB) affinity. H2O2 targets thiols of the Na,K-ATPase - low and high concentrations of H2O2 change the oxidative state of thiolate (S-) from Cys differently, resulting in the corresponding activation or inhibition of the enzyme. Targeting thiols of the Na,K-ATPase tunes the activity of the Na,K-ATPase to the cellular demands and sustains the enzyme activity at the "optimal" level.

Uncovering Molecular Structure – Redox Potential Relationships for Organic Electrode Materials: A Hybrid DFT – Machine Learning Approach

Density functional theory and machine learning are used to investigate the structure-electrochemical performance relationships of organic moieties for use in Li-ion batteries. Namely, DFT calculations are performed to predict the redox potential of several novel organic molecules with an accuracy within ~0.1 V of experimental measurements. However, despite its ability to provide valuable insight regarding the electrochemical properties of novel organic molecules, our high efficacy DFT modeling protocol demands significant computational time and is therefore impractical for the vast screening of novel material candidate. As a result, we explore machine learning as a strategy for the accelerated discovery of novel organic materials. More critically, we use machine learning as a method for assessing the various structure-electrochemical relationships which can provide a more general guideline for the design of organic electrode materials. We are employing different learning models, including artificial neural networks, gradient-boosting regression, and kernel methods (such as kernel ridge regression and Gaussian process regression), via three different pipelines with varying sophistication with the aim of generating an advanced ML scheme for the accurate prediction and analysis of electrochemical activity. In addition to incorporating structural fingerprints, we are exploring an active learning framework, namely using the efficient global optimization scheme, to explore the materials space strategically using publicly available datasets. Through this approach, we can discover new electrode materials that could 1) have a higher probability for achieving enhanced electrochemical properties and 2) increase our learning model’s performance by increasing our dataset’s representation of the material space. Additionally, we are implementing a high-throughput virtual screening (HTVS) pipeline which consists of several surrogate learning models with increasing levels of fidelity. The material candidates are pruned at each stage to discard samples that are unlikely to possess a desirable redox potential for cathodic application, and the remaining samples move on to the next model. In this way, only a subset of the original dataset which has a higher probability for lying within the desired redox potential range is pursued using the most computationally expensive model.

Exploration of bifunctional Vanadium-based Metal-Organic framework with double active centers for Potassium-ion batteries

Exploration of suitable electrode hosts with large open channels that can reversibly accommodate K+ with large radius have been extensively investigated. Nevertheless, the reported inorganic counterparts were inevitably restricted by the difficulty of large K+ diffusion capability in crystal structure and the huge volume change. Herein, we report a bifunctional vanadium-based metal–organic framework (MIL-47) with double active centers and larger lamellar spacing that could serve as both cathode and anode material, respectively by controlling the redox potential range in potassium-ion batteries. The results suggest that the stable K-storage mechanism is the reversible rearrangement of the conjugated carboxyl groups of organic terephthalic acid into enolate and the reversible redox activity of V ions, with the specific capacity of 272 mAh g−1 (0.01–1.5 V) and 50 mAh g−1 (1.5–3.8 V) at the current density of 10 mA g−1 for MIL-47 anode and MIL-47 cathode, respectively. The unsaturated functional group of MIL-47 and the intermediate bridged V atom not only provide multi-dimensional channels for electron and ion transport but also stabilize its crystal structure. Additionally, a symmetric full-cell in potassium-ion batteries based on MIL-47 was constructed successfully by avoiding the utilization of K metal with safety concerns. Our results provide new insight into structure design for next-generation large-scale energy storage applications.

Release and mobilization of Ni, Co, and Cr under dynamic redox changes in a geogenic contaminated soil: Assessing the potential risk in serpentine paddy environments

The release dynamics and mobilization of geogenic Ni, Co, and Cr in serpentine paddy soils under fluctuating redox conditions have not yet been well studied. Here we investigated the release dynamics of Cr, Co, and Ni and controlling factors (e.g., Fe, Mn, Mg, Cl−, PO43−, SO42−, and dissolved organic carbon (DOC)) in a geogenic-contaminated serpentine soil under wide range of redox potential (EH) changes. The effects of re-oxidation process have been also investigated. The soil was incubated for 28 days and EH was controlled from oxidation (+200 mV) to reduction (−200 mV) and re-oxidation (+240 mV) using a microcosm setup in duplicates. The slurry pH increased, along with decreasing EH. The average concentration of dissolved Co (17.1–23.6 μg L−1) decreased under low EH/high pH and vice versa. The average concentration of dissolved Cr decreased sharply from 624 μg L−1 to 54.4 μg L−1 with decreasing EH from +200 mV to 0 mV and the associated increase of pH from 7.8 to 8.5; then, it was constant around 24.5 μg L−1. Concentration of dissolved Ni was lower (73.5–84.6 μg L−1) under high EH at the first week of incubation; then, increased to 108.5 μg L−1 under low EH (−200 mV); thereafter, increased more at the end up to 124.5 μg L−1 at high EH (+240 mV), because of the pH decrease. A factor analysis identified that Cr and Co formed one group with Mn and Mg, while Ni was clustered together with Cl−, DOC, and SO42−. This indicates that the redox-induced release dynamic of Cr and Co was mainly governed by MnMg compounds, while the release of Ni was mainly affected by the aliphatic compounds of DOC and the redox chemistry of chlorides and sulfur in this soil. The re-oxidation increased the mobilization of Ni and Co and did not affect the release of Cr. These findings suggest that the redox-induced mobilization of geogenic Co, Ni, and Cr from soil to water in serpentine rice soils should be considered due to the high solubility and thus the associated bioavailability and potential environmental and human health risks, when such metal-enriched soils will be used for agricultural flood-dry cycle systems.

Electrochemical Performance of NASICON-structured Na3-x V2-xTix(PO4)3 (0.0 < x < 1.0) as aqueous Na-ion battery positive electrodes

Phosphate frameworks with NASICON structure are among the most studied and applied Li- and Na-ion battery electrode and electrolyte materials. In this work, the NASICON-structured Na3-xV2-xTix(PO4)3 with x = 0.0, 0.25, 0.5, 0.75 and 1.0 are successfully prepared by conventional solid-state synthesis and characterized in detail as potential aqueous Na-ion battery positive electrodes with improved charge capacity and cycling stability. Structural analysis using powder X-ray diffractometry indicates that titanium substitutes vanadium at arbitrary concentration without significant distortion of the NASICON structure. The results show that titanium content in this system directly correlates with its aqueous stability when cycled in simple 1 M Na2SO4 aqueous electrolyte within the vanadium redox potential range. Electrochemical kinetics and charge capacity measurements show Na2VTi(PO4)3 as well as Na2.25V1.25Ti0.75(PO4)3 to be stable positive electrodes in simple aqueous electrolyte solutions. Hybrid density functional theory analysis of V-O chemical bonding suggests that it is stabilized by the presence of titanium in the NASICON structure. In this work, we show that the observed capacity loss in full symmetric cells is caused by the capacity imbalance between positive and negative electrodes which progresses during cycling but not the aqueous materials stability per se. This imbalance is caused by several parasitic reactions, the most important being the oxygen reduction reaction catalyzed by Ti(III) species. Careful mitigation and management of this reaction could, in principle, allow for the preparation of truly capacity balanced cells (i.e. without a need of any electrode overcapacity), and superior cycling stability.

A universal small-molecule organic cathode for high-performance Li/Na/K-ion batteries

Organic solids are mainly composed by organic molecules interacted by Van der Waals forces, which indicates that every organic molecule can independently store electrons and counter metal ions during redox reaction in rechargeable metal-ion batteries. However, the serious dissolution problem as well as the solvent structure in electrolytes limits this universal capability for organic electrodes. In this article, a novel small-molecule organic compound namely 1,1′-(1,4-phenylene)-bis(perylene-3,4,9,10-tetracarboxylic dianhydride) (2PTCDA, 125 mAh g−1) is intentionally designed, in order to increase the insolubility against liquid electrolytes and partially extend its π-π aromatic skeleton without sacrificing the redox activity and specific capacity. Based on the insolubility and the properly-selected electrolytes, 2PTCDA can exhibit almost the same cathode performance in all Li/Na/K-ion half and full cells. Specifically, 2PTCDA can deliver the redox potential range of 1.4–3.6 V (vs. Li/Na/K) and discharge capacities of 120–137 mAh g−1 in Li/Na/K-ion half cells.

Rice husk and charred husk amendments increase porewater and plant Si but water management determines grain As and Cd concentration

PurposeRice is a staple crop worldwide and a silicon (Si) hyperaccumulator with Si levels reaching 5–10% of its mass; this can result in desilication and Si-deficiency if plant residues are not managed correctly. Rice is also uniquely subject to arsenic (As) and cadmium (Cd) contamination depending on soil conditions. Our goal is to quantify the effects of rice husk (a Si-rich milling byproduct) amendments and different water management strategies on rice uptake of Si, As, and Cd.MethodsWe employed 4 husk amendment treatments: Control (no husk), Husk (untreated husk), Biochar (husk pyrolyzed at 450 °C), and CharSil (husk combusted at > 1000 °C). Each of these amendments was studied under nonflooded, alternate wetting and drying (AWD), and flooded water management in a pot study. Porewater chemistry and mature plant elemental composition were measured.ResultsHusk and Biochar treatments, along with flooding, increased porewater and plant Si. Vegetative tissue As decreased with increasing porewater Si, but grain As and plant Cd were primarily controlled by water management. Grain As and Cd were inversely correlated and are simultaneously minimized in a redox potential (Eh) range of 225–275 mV in the studied soil. Ferrihydrite in root iron plaque decreased As translocation from porewater to grain, but amendments were not able to increase plaque ferrihydrite content.ConclusionWe conclude moderate husk amendment rates (i.e., 4 years’ worth) with minimal pretreatment strongly increases rice Si content but may not be sufficient to decrease grain As in low Si and As soil.

Microbial Survival in an Extreme Martian Analog Ecosystem: Poás Volcano, Costa Rica

Past acid-sulfate hydrothermal systems on Mars have promise in their ability to have hosted life for billions of years. One method for analyzing these systems is to study analog environments on Earth. To assess the astrobiological potential of Martian acid-sulfate hydrothermal systems, the crater lake of the active Poás Volcano, Laguna Caliente, was sampled in 2013 and 2017. Laguna Caliente presents an extremely dynamic terrestrial environment with near-ambient to boiling temperatures, pH fluctuations from −0.87 to 1.5, a wide range of chemistries and redox potential, and frequent phreatic-to-phreatomagmatic eruptions. Samples of lake fluid, sulfur clumps, and lake bottom sediment underwent 16S rRNA gene sequencing and metagenomic “shotgun” sequencing, which revealed this lake hosts an extremely low biodiversity of microorganisms dominated by Acidiphilium spp. Shotgun metagenomics of the samples suggests this community has numerous genetic adaptations that confer survival, including functional pathways to reduce the effects of toxic metals and numerous metabolic pathways utilizing a variety of simple and complex sugar molecules. The identification of these various metabolic pathways suggests adaptations related to carbon limited environments, fulfillment of high energy requirements, and survival in a hostile volcanic setting. The perseverance of life in Laguna Caliente indicates life on Mars could have thrived in analogous environments, stressing the need for the search for life in relict Martian acid-sulfate hydrothermal systems.

Leaching Chalcocite in Chloride Media—A Review

Chalcocite is the most abundant secondary copper sulfide globally, with the highest copper content, and is easily treated by conventional hydrometallurgical processes, making it a very profitable mineral for extraction. Among the various leaching processes to treat chalcocite, chloride media show better results and have a greater industrial boom. Chalcocite dissolution is a two-stage process, the second being much slower than the first. During the second stage, in the first instance, it is possible to oxidize the covellite in a wide range of chloride concentrations or redox potentials (up to 75% extraction of Cu). Subsequently, CuS2 is formed, which is to be oxidized. It is necessary to work at high concentrations of chloride (>2.5 mol/L) and/or increase the temperature to reach a redox potential of over 650 mV, which in turn decreases the thickness of the elemental sulfur layer on the mineral surface, facilitating chloride ions to generate a better porosity of this. Finally, it is concluded that the most optimal way to extract copper from chalcocite is, during the first stage, to work with high concentrations of chloride (50–100 g/L) and low concentrations of sulfuric acid (0.5 mol/L) at a temperature environment, as other variables become irrelevant during this stage if the concentration of chloride ions in the system is high. While in the second stage, it is necessary to increase the temperature of the system (moderate temperatures) or incorporate a high concentration of some oxidizing agent to avoid the passivation of the mineral.

Structural and functional insights of GSU0105, a unique multiheme cytochrome from G. sulfurreducens

Geobacter sulfurreducens possesses over 100 cytochromes that assure an effective electron transfer to the cell exterior. The most abundant group of cytochromes in this microorganism is the PpcA family, composed of five periplasmic triheme cytochromes with high structural homology and identical heme coordination (His-His). GSU0105 is a periplasmic triheme cytochrome synthetized by G. sulfurreducens in Fe(III)-reducing conditions but is not present in cultures grown on fumarate. This cytochrome has a low sequence identity with the PpcA family cytochromes and a different heme coordination, based on the analysis of its amino acid sequence. In this work, amino acid sequence analysis, site-directed mutagenesis, and complementary biophysical techniques, including ultraviolet-visible, circular dichroism, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopies, were used to characterize GSU0105. The cytochrome has a low percentage of secondary structural elements, with features of α-helices and β-sheets. Nuclear magnetic resonance shows that the protein contains three low-spin hemes (Fe(II), S = 0) in the reduced state. Electron paramagnetic resonance shows that, in the oxidized state, one of the hemes becomes high-spin (Fe(III), S = 5/2), whereas the two others remain low-spin (Fe(III), S = 1/2). The data obtained also indicate that the heme groups have distinct axial coordination. The apparent midpoint reduction potential of GSU0105 (−154 mV) is pH independent in the physiological range. However, the pH modulates the reduction potential of the heme that undergoes the low- to high-spin interconversion. The reduction potential values of cytochrome GSU0105 are more distinct compared to those of the PpcA family members, providing the protein with a larger functional working redox potential range. Overall, the results obtained, together with an amino acid sequence analysis of different multiheme cytochrome families, indicate that GSU0105 is a member of a new group of triheme cytochromes.

Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides.

Since the seminal work of Zhang in 2016, donor-acceptor cyanoarene-based fluorophores, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), have been widely applied in photoredox catalysis and used as excellent metal-free alternatives to noble metal Ir- and Ru-based photocatalysts. However, all the reported photoredox reactions involving this chromophore family are based on harnessing the energy from a single visible light photon, with a limited range of redox potentials from -1.92 to +1.79 V vs SCE. Here, we document the unprecedented discovery that this family of fluorophores can undergo consecutive photoinduced electron transfer (ConPET) to achieve very high reduction potentials. One of the newly synthesized catalysts, 2,4,5-tri(9H-carbazol-9-yl)-6-(ethyl(phenyl)amino)isophthalonitrile (3CzEPAIPN), possesses a long-lived (12.95 ns) excited radical anion form, 3CzEPAIPN•-*, which can be used to activate reductively recalcitrant aryl chlorides (Ered ≈ -1.9 to -2.9 V vs SCE) under mild conditions. The resultant aryl radicals can be engaged in synthetically valuable aromatic C-B, C-P, and C-C bond formation to furnish arylboronates, arylphosphonium salts, arylphosphonates, and spirocyclic cyclohexadienes.

Electronic Band-Engineered Nanomaterials for Biosafety and Biomedical Application

ConspectusInteraction between nanomaterials (NMs) and biological systems can be beneficial for biological functions but also can present hazards to humans. Nanotoxicology and nanomedicine, as two subdisciplines of nanotechnology, share the same goal of making safer NMs for biomedical application. NMs with unique electronic properties have been widely used for biomedical applications, such as bacterial inactivation, wound healing, tumor therapy, and Alzheimer’s disease therapy. Meanwhile, the biosafety of NMs has become a hot topic, and development of effective “safe-by-design” strategies will be beneficial for the wide applications of NMs in the biomedical field. However, it is currently hard to establish a property–activity relationship between NMs and their biosafety and biomedical applications, especially for electronic band structure including conduction band energy (Ec), valence band energy (Ev), Fermi energy (Ef), and bandgap energy (Eg). Eg determines the suitable lights used to excite NMs, and Ec and Ev determine the redox abilities of photoinduced electrons and holes, while Ef dominates the charge transfer process within NMs. Therefore, through modulating the electronic band structure of NMs, not only can the biosafety of NMs be elevated, but also the photoelectronic performance can be improved, providing a profound understanding to the design of functional NMs for the biomedical application with excellent biocompatibility.In this account, we focus on our recent progress in electronic band structure-modulated NMs for biosafety and biomedical application. First, we investigate the toxicities of NMs with different Ec levels and establish safe-by-design strategies to make safer NMs through modulating their electronic properties, such as tuning Ec values of NMs out of the biological redox potential range and tuning the Ef edge far away from the Ev edge. Second, we propose that deep level defect, resonance energy transfer, and narrow band gap intensely correlate with the photothermal performance of NMs and rationally designed heterostructures can significantly improve the photothermal conversion efficacies of these NMs. Third, we introduce a series of NMs with unique heterostructure to promote photoinduced electron–hole spatial separation and improve photodynamic performance for antibacterial and anticancer applications. Among these heterostructures, the thermally retractable heterostructure can create a favorable microenvironment for photodynamic therapy; Z-scheme heterostructure can simultaneously produce oxygen and reactive oxygen species for photodynamic therapy against hypoxic tumor; plasmon–pyroelectric heterostructure can thermally generate reactive oxygen species in an oxygen-independent manner for hypoxic tumor therapy. Furthermore, we describe the photooxidation and antioxidant abilities of NMs for treating Alzheimer’s disease through inhibiting amyloid-β self-assembly and scarifying reactive oxygen species. Finally, we propose the challenges and perspectives of electronic band structure-modulated biomedical application of NMs. We expect that this field will attract increasing research interest and create new opportunities for biosafety and biomedical studies of NMs.