Coupled Electron Transfer Research Articles

Constraint Coupling of Redox Cascade and Electron Transfer Synchronization on Electrode-Nanosensor Interface for Repeatable Detection of Tumor Biomarkers.

Quantitative analysis of up-regulated biomarkers in pathological tissues is helpful to tumor surgery yet the loss of biomarker extraction and time-consuming operation limited the accurate and quick judgement in preoperative or intraoperative diagnosis. Herein, an immobilization-free electrochemical sensing platform is developed by constraint coupling of electron transfer cascade on electrode-nanosensor interface. Specifically, electrochemical indicator (Ri)-labeled single-stranded DNA on electroactive nanodonor (polydopamine, PDA) can be responsively detached by formation of DNA complex through the recognition and binding with targets. By applying the oxidation potential of Ri, nanosensor collisions on electrode surface trigger a cascade redox cycling of PDA and Ri through synchronous electron transfer, which boost the amplification of current signal output. The developed nanosensor exhibit excellent linear response toward up-regulated biomarkers (miRNA-21, ATP, and VEGF) with low detection limits (32 fM, 386pM, and 2.8 pM). Moreover, background influence from physiological interferent is greatly reduced by restricted electron transfer coupling on electrode. The practical applicability is illustrated in sensitive and highly repeatable profiling of miRNA-21 in lysate of tumor cells and tumor tissue, beneficial for more reliable diagnosis. This electrochemical platform by employing electron transfer cascades at heterogeneous interfaces offers a route to anti-interference detection of biomarkers in tumor tissues.

Direct Electrocatalytic Methanol Oxidation on MoO3/Ni(OH)2: Exploiting Synergetic Effect of Adjacent Mo and Ni.

Ni-based materials have been widely investigated as methanol oxidation reaction (MOR) catalysts. The formation of NiOOH and its reduction to Ni(OH)2 are generally regarded as essential steps for methanol oxidation. However, in such an indirect route, the efficiency of proton coupled electron transfer is fundamentally limited by the rate of transition from Ni(OH)2 to NiOOH back and forth. Herein we demonstrate a direct MOR pathway on MoO3/Ni(OH)2 without the formation of a NiOOH mediator. The MoO3/Ni(OH)2 exhibits a benchmark electrocatalytic MOR current density of 1000 mA cm-2 at 1.52 V vs. RHE with a nearly 100% faradic efficiency, outperforming all the state of art MOR electrocatalysts. In-situ Raman spectroscopy confirms that NiOOH is not formed during the electrocatalytic MOR process on the MoO3/Ni(OH)2. Density functional theory calculations suggest that Ni2+ in MoO3/Ni(OH)2 serves as the methanol adsorption site while the doped Mo6+ plays a key role in capturing the deprotonated H·. Benefiting from the Mo-Ni synergistic effect, the energy barrier of the CH2O* → CHO* + H* process is significantly reduced, avoiding the NiOOH formation and leading to the direct MOR. Our research unravels a direct electrochemical MOR pathway that does not rely on NiOOH formation and provides a facile strategy of regulating the intermediate process barrier for MOR.

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity.

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

Rationally designed oxygen vacant TiO2 decorated with covalent organic framework for enhanced electrocatalytic nitrogen reduction to ammonia

This study focuses on the rational design of uniformly distributed oxygen vacant TiO2−x nanoparticles supported on an anthraquinone-based covalent organic framework (TiO2−x@COF-Aq), aiming to address the challenges of Haber-Bosch process. The N-philic nature of Ti and the presence of abundant oxygen vacant sites on the TiO2−x@COF-Aq promote the adsorption of nitrogen, facilitating the electrocatalytic NRR process. The theoretical study reveals that TiO2−x@COF-Aq suppresses the H+ adsorption, facilitates oxygen vacancy formation and favors the N2 adsorption at oxygen vacant sites. The synthesized nanocatalyst exhibits many folds higher electrochemical NRR activity than bulk oxygen vacant TiO2−x with an ammonia yield of ∼30 μg mg-1 h–1 and ∼16% Faradaic efficiency. Long term stability study manifests the robustness and industrial significance of the catalyst. The overall synergism between the stabilized nanoparticle and functionalized COF has established the fundamental understanding of the proton coupled electron transfer for N2 fixation through experiments and theoretical findings.

Facilitating Proton Coupled Electron Transfer Reaction through the Interfacial Micro Electric Field with Fe─N4 ─C in FeMOFs Glass.

The proton-coupled electron transfer(PCET) reaction plays a crucial role in the chemical transformation process andhas become one of the most concerned elementary reactions. However, the complex kinetics of PCET reaction, which requires the simultaneous transfer of protons and electrons, leads to the dilemma that thermodynamics and kinetics cannot bebalanced and restricts its further development. In this, an interface micro-electric field (IMEF) basedon Fe─N4 in FeMOFs (Fe-Based Metal-Organic Frameworks) glass is designed tosynchronize proton/electron interface behavior for the first time to realizeefficient PCET reaction and optimize reaction thermodynamics and kinetics. The IMEF facilitates the separation of photogenerated electrons and holes, and accelerates Fe(III)/Fe(II) cycle. Driven by near-surface electric field force, the protons near surfacemigrate to Fe sites and participate in Fe(IV)═O formation and reaction, lowering the reaction energy barrier. Based on the interface regulation ofIMEF, a high-efficiency PCET reaction is realized, and kinetic reactionrate constant of photocatalytic oxidation of emerging contaminants is increasedby 3.7times. This study highlights a strategy for IMEFs to modulate PEC Treactions for a wide range of potential applications, including environmental and ecological applications.

A cascade of in situ conversion of bicarbonate to CO2 and CO2 electroreduction in a flow cell with a Ni-N-S catalyst

Combination of CO2 capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO3- to high-value chemicals is a promising route of low cost and high efficiency. The electrochemical reduction of HCO3- is challenging due to the inaccessible of negatively charged molecular groups to the electrode surface. Herein, we adopt a comprehensive strategy to tackle this challenge, i.e., cascade of in situ chemical conversion of HCO3- to CO2 and CO2 electrochemical reduction in a flow cell. With a tailored Ni-N-S single atom catalyst (SACs), where sulfur (S) atoms located in the second shell of Ni center, the CO2 electroreduction (CO2ER) to CO is boosted. The experimental results and density functional theory (DFT) calculations reveal that the introduction of S increases the p electron density of N atoms near Ni atom, thereby stabilizing *H over N and boosting the first proton coupled electron transfer process of CO2ER, i.e., *+e–+*H+*CO2→*COOH. As a result, the obtained catalyst exhibits a high faradaic efficiency (FECO ∼98%) and a low overpotential of 425 mV for CO production as well as a superior turnover frequency (TOF) of 47397 h−1, outcompeting most of the reported Ni SACs. More importantly, an extremely high FECO of 90% is achieved at 50 mA cm−2 in the designed membrane electrode assembly (MEA) cascade electrolyzer fed with liquid bicarbonate. This work not only highlights the significant role of the second coordination on the first coordination shell of the central metal for CO2ER, but also provides an alternative and feasible strategy to realize the electrochemical conversion of HCO3- to high-value chemicals.

Conformational coupling of redox-driven Na+-translocation in Vibrio cholerae NADH:quinone oxidoreductase

In the respiratory chain, NADH oxidation is coupled to ion translocation across the membrane to build up an electrochemical gradient. In the human pathogen Vibrio cholerae, the sodium-pumping NADH:quinone oxidoreductase (Na+-NQR) generates a sodium gradient by a so far unknown mechanism. Here we show that ion pumping in Na+-NQR is driven by large conformational changes coupling electron transfer to ion translocation. We have determined a series of cryo-EM and X-ray structures of the Na+-NQR that represent snapshots of the catalytic cycle. The six subunits NqrA, B, C, D, E, and F of Na+-NQR harbor a unique set of cofactors that shuttle the electrons from NADH twice across the membrane to quinone. The redox state of a unique intramembranous [2Fe-2S] cluster orchestrates the movements of subunit NqrC, which acts as an electron transfer switch. We propose that this switching movement controls the release of Na+ from a binding site localized in subunit NqrB.

Bacteria-driven copper redox reaction coupled electron transfer from Cr(VI) to Cr(III): A new and alternate mechanism of Cr(VI) bioreduction

Cr(VI) released into the environment inevitably co-exists with other contaminants, such as heavy metal ions, thus altering the performance of bacteria for Cr(VI) reduction; however, the mechanism underlying Cr(VI)-reducing bacterial response to heavy metal ions remains elusive. Herein, we investigate the toxic effects of Cu(II) and Cr(VI) on Cr(VI)-reducing bacterium Pannonibacter phragmitetus D-6 (hereafter D-6), which changes the primary metabolic pattern of Cr(VI). At Cu(II) concentrations of 10–100 mg/L, the efficiency of Cr(VI) reduction increases significantly. The co-exposure of Cr(VI) and Cu(II) induces D-6 to preferentially respond to Cu(II) through electrostatic forces, which is then reduced to Cu(I) outside and inside the bacterial cells. The original pathways for Cr(VI) reduction are weakened via downregulating genes related to Cr(VI) transport and reduction. A new mechanism involving Cu(II)-mediated electron transfer from D-6 to Cr(VI) is elucidated. Specially, Cu(II) accumulates around the cells as an electron shuttle and promotes Cr(VI) reduction. Genes encoding cytochromes involved in electron transfer are significantly up-regulated, thus promoting Cu(II) reduction. The Cu(II)/Cu(I) redox cycle ensures the continuous bioremoval of Cr(VI) in a cycle test. This study reveals an overlooked mechanism for Cr(VI) reduction, which provides theoretical guidance for designing practical microbial process to remediate Cr(VI) contamination.

Development of a Small Molecule Hydrogen Evolution Catalyst

Based on prior work done by Diane K. Smith et al on redox responsive 3 hydrogen bond array dimers, a fundamental investigation on the nature of proton coupled electron transfer (PCET) was done by surveying a series of new electroactive “host” H-bond dimers. In these dimers, electron transfer should induce proton transfer across a H-bond, increasing the overall strength of hydrogen bond binding the “guest” and “host”. In particular, the goal was to investigate how electrochemical behavior and binding strength changes as the relative acidity of the electro-inactive “guest” changes in a 2-hydrogen bond array. For this task, amido pyridinium derivatives will be used as the redox active "hosts" and a series of different carboxylic acid, 2-hydroxypyridine, and amide derivatives as the electro-inactive "guests". Unexpectedly, in the presence of carboxylic acids, the amido pyridinium acts as a hydrogen evolution catalyst. During guest host cyclic voltammogram (CV) titrations, there was a large anodic shift of 500 mV and a catalytic increase in current, which is characteristic of an EC mechanism (figure 2). A series of investigations into this mechanism were done following the initial discovery, including scan rate and concentration dependence CV’s, spectro-electrochemical studies, bulk electrolysis, DEMS, and DFT calculations. These studies allowed us to propose the mechanism below (figure 1) for hydrogen evolution. This presentation will detail our efforts to characterize this hydrogen evolution catalyst, the relevance of small molecule hydrogen evolution catalyst, and how we intend to modify it for synthetic purposes.Figure 1 (left image): Reduction of pyridinium amide with and without guests and proposed mechanism of hydrogen evolutionFigure 2 (right image): CV of pyridinium amide in the presence of dodecanoic acid Figure 1

Impact of Catalyst Morphology on the Adsorption and Electrocatalytic Hydrogenation of Benzaldehyde

Biomass represents an untapped resource for achieving a circular carbon economy. Current processes for converting biomass are limited to centralized processing facilities. Electrochemical approaches are inherently modular and enable decentralized upgrading of pyrolysis oil to value-added chemicals and fuels (products). A primary component of the aqueous phase of such feedstocks are carbonyl functionalities, where benzaldehyde (BZ) represents a model compound. Selective reduction of BZ to benzyl alcohol (BnOH) can be used to understand the catalytic upgrading of pyrolysis oil to value-added products. Electrocatalytic hydrogenation (ECH) of BZ can be carried out at mild temperatures and pressure with the hydrogen required for BZ reduction coming from the protons in the solution. However, these protons can also combine in the hydrogen evolution reaction (HER) in a competitive process. Among the noble and base metals studied in the literature, palladium (Pd) is very active and has demonstrated a high faradaic efficiency (FE) for ECH of BZ relative to HER. Palladium also forms a thermodynamically stable hydride under these conditions. However, prior work has shown that BZ inhibits the formation of palladium hydride under reaction conditions, whereas the presence of weaker adsorbing organics does not. Together, these observations raise interesting questions whether ECH occurs via a proton coupled electron transfer (PCET) pathway or via addition of H from the catalyst surface directly (Langmuir-Hinshelwood, LH pathway). The specific aims of this work investigate the effect of catalyst morphology on the surface coverage of intermediates like H and adsorbed BZ, the relative rates of PCET and LH pathways, and their competition with HER and hydride formation. For this, we investigate high surface area Pd nanoparticle electrocatalysts and a gel network (Pd Gel) of them rich with grain boundaries and strain via cyclic voltammetry. Adsorption isotherms on these Pd catalysts show changes in hydride formation and the relative affinity of H and BZ on the Pd surface. We further study the reaction rate orders and FE as a function of applied potential to better understand the contributions of PCET and LH pathways. These findings point to catalyst morphology as a further handle to manipulate activity and FE for ECH of BZ. Furthermore, these findings will enhance the mechanistic understanding of BZ ECH kinetics and will contribute to improving the overall efficacy of biomass conversion to biofuels.

Simultaneous Boosting Nitrogen Reduction Activity and Suppressing Hydrogen Evolution Reaction Activity of Transition Metal Oxide Based Electrocatalyst

Ammonia (NH3) is widely served as fertilizer, safer chemical hydrogen storage, and energy conservation materials with an annual production around 200 million tons per year. Currently, NH3 is mainly synthesized via the Haber-Bosch process, which is an extremely energy- and capital-intensive (high temperature 400–500 °C and high pressure 100–350 atm) process, consuming over 1% of the global annual energy output, emitting massive amount of greenhouse gases (~420 Mt CO2 annually), and causing serious concerns with climate change. Electrochemical nitrogen reduction reaction (eNRR) allows NH3 production from nitrogen (N2) and water (H2O) under ambient conditions. Especially if driven by renewable energies, NRR represents a potentially clean and sustainable strategy for replacing the traditional Haber-Bosch process and alleviating climate change effect. One of the most critical fundamental challenges in production of ammonia via electrochemical nitrogen reduction reaction (eNNR) is developing catalysts which are capable to boost eNRR activity and concurrently suppress the overwhelming competitive hydrogen evolution reaction (HER). This work reports a new strategy to develop such catalysts to alleviate this challenge. This new strategy for the first time takes advantage of the outstanding capability of some redox active transition metal oxides (TMOs) in their super capability in electrochemically controlled proton interstitial doping and the associated remarkable structural changes to boost their NRR activity. Further, inspired by their unique hydrogen intercalation determined HER activity, this new strategy introduced cationic substitutional doping of the TMOs with nonmetal elements, such as phosphorus (P), to modulate their proton intercalation behavior, achieving HER suppression. In this work, tungsten oxide nanosheets (WO3) have been exploited as a typical example of such TMOs to demonstrate the feasibility of this innovative strategy. It is found that cationic P doping also accompanies introduction of large quantity of oxygen vacancies (OVs) in the matrix of the WO3 and structural changes of the WO3 lattice. The synergy of the substitutional doping and interstitial doping in tuning the electronic and the local geometric structures leads to significantly enhanced eNNR performance of WO3 nanosheets. An unprecedented high Faradaic efficiency (FE) of 64.1% with NH3 yield of 24.5 µg h-1 has been reached, representing the highest NRR performance for WO3 based catalysts. It is worth mentioning that the P-doped WO3 catalyst (200 nm square with a thickness of 20-40 nm) is much larger than most of the reported catalyst. The catalytic yield is even higher than that achieved with atomic thin two-dimensional (2D) WO3 nanosheets (OV-rich) catalysts (17 µg h-1 with a FE of 7.0%), which is known for their high catalytic efficiency due to the excellent accessibility of all the catalytic active sites. It is also worthwhile to mention that the FE of 64.1% is comparable to that of the natural nitrogenase can be reached (65%), which is unprecedented high compared to most of catalysts reported so far (<25%). In addition, inclusion of Li ions in the electrolyte further increased the NH3 yield to 60.1 µg h-1 with the high FE largely remained, reached the highest eNNR performance for all the TMO based catalysts. Density functional theory calculations further revealed the synergistic role of interstitial doping of H* and substitutional cationic doping of P, and the associated generation of OVs in increasing the electronic states crossing the Fermi level of the WO3 and thus reducing reaction energy barrier in N2 activation and hydrogenation. An interesting non-PCET (proton coupled electron transfer) mechanism is revealed in the first step and the rate determining step during eNNR, providing a new design principle for TMO-based catalysts. Figure 1

Unraveling the Pivotal Roles of Various Metal Ion Centers in the Catalysis of Quercetin 2,4-Dioxygenases.

Quercetin 2,4-dioxygenase (QueD) with various transition metal ion co-factors shows great differences, but the internal reasons have not been illustrated in detail. In order to explore the effects of metal ion centers on the catalytic reactivity of QueD, we calculated and compared the minimum energy crossing point (MECP) of dioxygen from the relatively stable triplet state to the active singlet state under different conditions by using the DFT method. It was found that the metal ions play a more important role in the activation of dioxygen compared with the substrate and the protein environment. Simultaneously, the catalytic reactions of the bacterial QueDs containing six different transition metal ions were studied by the QM/MM approach, and we finally obtained the reactivity sequence of metal ions, Ni2+ > Co2+ > Zn2+ > Mn2+ > Fe2+ > Cu2+, which is basically consistent with the previous experimental results. Our calculation results indicate that metal ions act as Lewis acids in the reaction to stabilize the substrate anion and the subsequent superoxo and peroxo species in the reaction, and promote the proton coupled electron transfer (PCET) process. Furthermore, the coordination tendencies of transition metal ion centers also have important effects on the catalytic cycle. These findings have general implications on metalloenzymes, which can expand our understanding on how various metal ions play their key role in modulating catalytic reactivity.

Proton coupled electron transfer in tri-molecular system involving 7-Methylbenzo [a] pyrene and different phenol–base pairs: A case study for determining hydrogen bonding equilibrium constant

Photo induced proton-coupled electron transfer (PCET) reactions comprehend the concerted transfer of an electron from donor to acceptor and a proton along the hydrogen bonded axis in same time scale. Photo-oxidations of hydrogen-bonded phenols by excited-state polyarene are utilized to extract concerted proton−electron transfer reactions rate and hydrogen bonding equilibrium constant. Experiments have examined fluorescence quenching of 7-Methyl benzo[a]pyrene (7-MeB[a]P) in all possible ways in solution phase by phenol-bases pairs (hydrogen bonded adducts) for two phenols, 4-Methoxy phenol (4-MeO PhOH) and 2,6- Dimethoxy phenol (2,6-DiMeo PhOH), and different pyridine derivatives, such as simple Pyridine (Py), 2,4-Dimethylpyridine (DMPy), 2,4,6-Trimethylpyridine (TMPy), 4-(N,N-Dimethylaminopyridine) (DMAPy), along with 1-Methylimidazole (IM). Detection of 7-MeB[a]P anion in femtosecond transient absorption studies and positive kinetic isotope effect confirm the fluorescence quenching of 7-MeB[a]P is due to PCET reaction where photo-oxidations of phenols proceed by intermolecular proton movement of phenolic proton to the base along the hydrogen bond axis concerted with electron transfer to the photo excited 7-MeB[a]P. The systematic fluorescence quenching experiments of 7-MeB[a]P by hydrogen bonded adducts are performed in two different ways: Case-1, variation of base concentration for fixed and dilute solution of 7-MeB[a]P and phenol mixture and Case-2, changing phenol concentration for fixed and dilute solution of 7-MeB[a]P and base. These two data sets are thus well described by non-linear Stern-Volmer type relation, providing a strong validation for the use of these approaches for determining hydrogen bonding equilibrium constants (KHB) along with rate constant (kQB) for PCET reaction. The estimated KHB values are fairly consistence to that observed in typical spectrophotometric method following Mataga-Tsuno relation. Thus, this is a very clear demonstration that the PCET reactions can be used to determine KHB in favourable cases where ground state hydrogen bonding is the essential and necessary condition for PCET reaction to occur.

Biomethane is produced by acetate cleavage, not direct interspecies electron transfer: genome-centric view and carbon isotope

Understanding the source of methane (CH4) is of great significance for improving the anaerobic fermentation efficiency in bioengineering, and for mitigating the emission potential of natural ecosystems. Microbes involved in the process named direct interspecies electron transfer coupling with CO2 reduction, i.e., electrons released from electroactive bacteria to reduce CO2 into CH4, have attracted considerable attention for wastewater treatment in the past decade. However, how the synergistic effect of microbiota contributes to this anaerobic carbon metabolism accompanied by CH4 production still remains poorly understood, especial for wastewater with antibiotic exposure. Results show that enhancing lower-abundant acetoclastic methanogens and acetogenic bacteria, rather than electroactive bacteria, contributed to CH4 production, based on a metagenome-assembled genomes network analysis. Natural and artificial isotope tracing of CH4 further confirmed that CH4 mainly originated from acetoclastic methanogenesis. These findings reveal the contribution of direct acetate cleavage (acetoclastic methanogenesis) and provide insightsfor further regulation of methanogenic strategies.

A Dynamic Triradical: Synthesis, Crystal Structure, and Spin Frustration.

Polyradicals, i.e., multispin organic molecules, are playing important roles in radical-based material applications for their spin-spin interaction. A dynamic covalently bonded multispin molecule may endow materials with added function such as memory and switching. However, such a species has yet to be reported. We here report the synthesis, characterization, and crystal structure of a dynamic triradical species. It is generated by the self-assembly of two molecules through a Lewis acid coupled electron transfer. The crystalline species is spin-frustrated without Jahn-Teller distortion at low temperature, while it dissociates back to diamagnetic starting material in solution at high temperature. The reversible process is tracked by variable-temperature NMR, EPR, and UV-vis-NIR spectroscopy. Isolation, property study, and dynamic bonding investigation on such a species lay the foundation for the design of functional polyradicals with potential application as memory or switching devices.

Charge Regulation in a Rieske Proton Pump Pinpoints Zero, One, and Two Proton-Coupled Electron Transfer.

The degree to which redox-driven proton pumps regulate net charge during electron transfer (ΔZET) remains undetermined due to difficulties in measuring the net charge of solvated proteins. Values of ΔZET can reflect reorganization energies or redox potentials associated with ET and can be used to distinguish ET from proton(s)-coupled electron transfer (PCET). Here, we synthesized protein "charge ladders" of a Rieske [2Fe-2S] subunit from Thermus thermophilus (truncTtRp) and made 120 electrostatic measurements of ΔZET across pH. Across pH 5-10, truncTtRp is suspected of transitioning from ET to PCET, and then to two proton-coupled ET (2PCET). Upon reduction, we found that truncTtRp became more negative at pH 6.0 by one unit (ΔZET = -1.01 ± 0.14), consistent with single ET; was isoelectric at pH 8.8 (ΔZET = -0.01 ± 0.45), consistent with PCET; and became more positive at pH 10.6 (ΔZET = +1.37 ± 0.60), consistent with 2PCET. These ΔZET values are attributed to protonation of H154 and H134. Across pH, redox potentials of TtRp (measured previously) correlated with protonation energies of H154 and H134 and ΔZET for truncTtRp, supporting a discrete proton pumping mechanism for Rieske proteins at the Fe-coordinating histidines.

Synergistic coupling of NiFe-layered double hydroxide nanosheets with Co-doped porous interconnecting carbon frameworks for efficient bifunctional electrocatalysis

The design of low-cost and durable electrocatalysts with high catalytic performance of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the development of sustainable energy technologies. NiFe-layered double hydroxide (NiFe-LDH) is a promising electrocatalyst for OER, but the poor ORR activity limits its large-scale application as a bifunctional electrocatalyst in energy storage and conversion devices. The rational design of hierarchical nanohybrids is an effective strategy to construct bifunctional OER/ORR electrocatalysts. Herein, a high-performance OER/ORR bifunctional non-noble metal electrocatalyst (NiFe-LDH/CoNC-PIN) was projected. The Co-doped carbon frameworks with porous interconnecting networks (CoNC-PIN) is prepared by pyrolysis of ZIF-8/67 via a salt template strategy, followed by the uniform in situ growth of ultrathin NiFe-LDH nanosheets on CoNC-PIN to construct the hierarchical NiFe-LDH/CoNC-PIN hybrid. The molten NaCl template in pyrolysis activates the surface of ZIF-8/67 and connects them into porous carbon networks to improve the surface area, porosity and electronic conductivity of catalysts. Due to the efficient electron transfer and strong coupling between CoNC-PIN and NiFe-LDH, the NiFe-LDH/CoNC-PIN exhibits a small OER overpotential of 249 mV at 10 mA/cm2, a low Tafel slope of 27.7 mV/dec, an ORR half-wave potential of 0.80 V, and excellent durability and structural stability. This strategy provides a novel insight to fabricate advanced OER/ORR bifunctional non-noble metal electrocatalysts.

Machine-learned dynamic disorder of electron transfer coupling.

Electron transfer (ET) is a fundamental process in chemistry and biochemistry, and electronic coupling is an important determinant of the rate of ET. However, the electronic coupling is sensitive to many nuclear degrees of freedom, particularly those involved in intermolecular movements, making its characterization challenging. As a result, dynamic disorder in electron transfer coupling has rarely been investigated, hindering our understanding of charge transport dynamics in complex chemical and biological systems. In this work, we employed molecular dynamic simulations and machine-learning models to study dynamic disorder in the coupling of hole transfer between neighboring ethylene and naphthalene dimer. Our results reveal that low-frequency modes dominate these dynamics, resulting primarily from intermolecular movements such as rotation and translation. Interestingly, we observed an increasing contribution of translational motion as temperature increased. Moreover, we found that coupling is sub-Ohmic in its spectral density character, with cut-off frequencies in the range of 102 cm-1. Machine-learning models allow direct study of dynamics of electronic coupling in charge transport with sufficient ensemble trajectories, providing further new insights into charge transporting dynamics.

Respiratory complex I in mitochondrial membrane catalyzes oversized ubiquinones

NADH-ubiquinone (UQ) oxidoreductase (complex I) couples electron transfer from NADH to UQ with proton translocation in its membrane part. The UQ reduction step is key to triggering proton translocation. Structural studies have identified a long, narrow, tunnel-like cavity within complex I, through which UQ may access a deep reaction site. To elucidate the physiological relevance of this UQ-accessing tunnel, we previously investigated whether a series of oversized UQs (OS-UQs), whose tail moiety is too large to enter and transit the narrow tunnel, can be catalytically reduced by complex I using the native enzyme in bovine heart submitochondrial particles (SMPs) and the isolated enzyme reconstituted into liposomes. Nevertheless, the physiological relevance remained unclear because some amphiphilic OS-UQs were reduced in SMPs but not in proteoliposomes, and investigation of extremely hydrophobic OS-UQs was not possible in SMPs. To uniformly assess the electron transfer activities of all OS-UQs with the native complex I, here we present a new assay system using SMPs, which were fused with liposomes incorporating OS-UQ and supplemented with a parasitic quinol oxidase to recycle reduced OS-UQ. In this system, all OS-UQs tested were reduced by the native enzyme, and the reduction was coupled with proton translocation. This finding does not support the canonical tunnel model. We propose that the UQ reaction cavity is flexibly open in the native enzyme to allow OS-UQs to access the reaction site, but their access is obstructed in the isolated enzyme as the cavity is altered by detergent-solubilizing from the mitochondrial membrane.

Hydrocarbon Oxidation by a Porphyrin-π-Cation Radical Complex.

Heme and chlorin π-cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π-cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a NiII-porphyrin-π-cation complex ([NiII(P•+)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([NiII(P•+)]) working in concert with atmospheric O2 to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin-π-cation radical species oxidised substrates via a concerted PCET mechanism, where the nickel porphyrin-π-cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π-cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non-innocence could be a readily manipulated resource for oxidation catalyst development.