Triggers Electron Transfer Research Articles

Built-in Electric Field in 1D/2D Heterostructure Boosts Zinc Air Battery Performance.

The realization of a rechargeable zinc-air battery (ZAB) is hindered by the low intrinsic oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities. In this work, an abundant built-in electric field is noticed in a 1D/2D CoO/CoS2 heterostructure, triggering electron transfer from CoO to CoS2 associated with a downshifted d band center of the Co atom mitigating the strong electrochemical adsorption of *OH species on active sites; thereby, boosted OER and ORR performance are achieved. Namely, the OER specific activity of CoO/CoS2 is enhanced by 3.8- and 2.2-fold compared to the counterpart of CoO and CoS2, respectively. Furthermore, the kinetic current density of CoO/CoS2, a fingerprint of intrinsic ORR activity, is promoted by 46 and 6.6 times relative to CoO and CoS2. The rechargeable ZAB performance attains 215.6 mW cm-2, 1.6-times better than Pt/C-IrO2. Moreover, the superior performance remained for 600 h. Besides, the battery performance of the all-solid-state ZAB reaches 83.8 mW cm-2, revealing its promising application in wearable device.

Nanotransistor-based gas sensing with record-high sensitivity enabled by electron trapping effect in nanoparticles

Highly sensitive, low-power, and chip-scale H2 gas sensors are of great interest to both academia and industry. Field-effect transistors (FETs) functionalized with Pd nanoparticles (PdNPs) have recently emerged as promising candidates for such H2 sensors. However, their sensitivity is limited by weak capacitive coupling between PdNPs and the FET channel. Herein we report a nanoscale FET gas sensor, where electrons can tunnel between the channel and PdNPs and thus equilibrate them. Gas reaction with PdNPs perturbs the equilibrium, and therefore triggers electron transfer between the channel and PdNPs via trapping or de-trapping with the PdNPs to form a new balance. This direct communication between the gas reaction and the channel enables the most efficient signal transduction. Record-high responses to 1–1000 ppm H2 at room temperature with detection limit in the low ppb regime and ultra-low power consumption of ~\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}300 nW are demonstrated. The same mechanism could potentially be used for ultrasensitive detection of other gases. Our results present a supersensitive FET gas sensor based on electron trapping effect in nanoparticles.

Photothermal flows activated CsPbBr3 ⊆ Pb-TCPP S-scheme heterojunction: In-situ converted interface for robust CO2 reduction coupling with benzylamine oxidation

By the self-sacrificial conversion approach, the perovskite CsPbBr3 nanocrystals (CPB NCs) are impregnated from a lead-porphyrin metal–organic frameworks (Pb-TCPP). Such rational design not only shapes the firm heterojunction to optimize interface electronic environment and rapid reaction kinetics. The difference on Fermi level between the parent Pb-TCPP and derived CPB dramatically could trigger electron transfer, generating a strong internal electric field. The S-scheme heterojunction not only contributes to promote the photogenerated electron transfer separation, but also endows its excellent photothermal properties, which in turn improve the reoxidation ability of the heterojunction. In-situ experimental characterization and theoretical calculations unveil electron-rich CPB acts as the CO2 activation sites and the Pb-TCPP side plays a vital role in benzylamine (BA) oxidation. The as-transferred 0.1CPB ⊆ Pb-TCPP delivers the outperformed photochemical property with a yield rate of 143.3 μmol·g−1·h−1 of CO and 18.6 μmol·g−1·h−1 of CH4, and BDA productive rate of 21.4 mmol·g−1·h−1. This work puts forward to a luxurious booster to interface phase conversion and photothermal effect in the photosynthetic fields.

Integrating AgFeO2 and Bi5O7I to establish an S-scheme heterojunction with significantly boosted norfloxacin photocatalytic degradation

Developing highly efficient heterojunction photocatalysts for high performance antibiotic removal is of immense importance but still remains a challenge. Herein, AgFeO2-Bi5O7I (AF-BI), a promising oxygen vacancies rich binary S-scheme heterojunction photocatalyst was synthseized via co-precipitation and hydrothermal treatment. The spherical AgFeO2 spheres were uniformly affixed on the nanorods of Bi5O7I via an intimate contact, as verified through electron microscopy. The photodegradation efficiency of optimized 15 %AgFeO2-Bi5O7I (15AF-BI) for norfloxacin (NFX) was 98.2 % (120 min) with apparent rate constant 0.022 min−1 which was 4 times pure AgFeO2. In municipal water, removal efficiency reached 90.4 % and a high 76.1 % and 65.2 % total organic carbon removal in 3 h in visible light and solar light respectively was achieved. The coupling of AgFeO2 and Bi5O7I triggers electron transfer, forming an internal electric field (leading to a directional charge transfer efficient separation of the photo-induced carriers as confirmed by PL, EIS, TPCR results. The validation of existence of S-scheme mechanism and oxygen vacancies was achieved by in-situ X-ray photoelectron spectroscopy (XPS) and EPR analysis. The free radical scavenging experiments and electron spin resonance (ESR) spectroscopy revealed ●OH and ●O2– radicals were the major reactive species involved in the photodegradation process. The heterojunction is magnetic and can be separated under guidance of magnetic field which makes reusability and separation easy. Based on ESR findings, band structure analysis and mass spectrometry NFX degradation route was predicted. This work provides effective potential solution for preparing new magnetic S-scheme heterojunctions for effective antibiotics degradation and further will expand to realm of clean energy storage and conversion.

Structural basis of human NOX5 activation

NADPH oxidase 5 (NOX5) catalyzes the production of superoxide free radicals and regulates physiological processes from sperm motility to cardiac rhythm. Overexpression of NOX5 leads to cancers, diabetes, and cardiovascular diseases. NOX5 is activated by intracellular calcium signaling, but the underlying molecular mechanism of which — in particular, how calcium triggers electron transfer from NADPH to FAD — is still unclear. Here we capture motions of full-length human NOX5 upon calcium binding using single-particle cryogenic electron microscopy (cryo-EM). By combining biochemistry, mutagenesis analyses, and molecular dynamics (MD) simulations, we decode the molecular basis of NOX5 activation and electron transfer. We find that calcium binding to the EF-hand domain increases NADPH dynamics, permitting electron transfer between NADPH and FAD and superoxide production. Our structural findings also uncover a zinc-binding motif that is important for NOX5 stability and enzymatic activity, revealing modulation mechanisms of reactive oxygen species (ROS) production.

Electron Transfer from Encapsulated Fe3C to the Outermost N‐Doped Carbon Layer for Superior ORR

AbstractEncapsulating Fe3C in carbon layers has emerged as an innovative strategy for protecting Fe3C while preserving its high oxygen reduction activity. However, fundamental questions persist regarding the active sites of encapsulated Fe3C due to the restricted accessibility of oxygen molecules to the metal sites. Herein, the intrinsic electron transfer mechanisms of Fe3C nanoparticles encapsulated in N‐doped carbon materials are unveiled for oxygen reduction electrocatalysis. The precision‐structured C1N1 material is used to synthesize N‐doped carbons with encapsulated Fe3C, significantly enhancing catalytic activity (EONSET = 0.98 V) and achieving near‐100% operational stability. In anion‐exchange membrane fuel cells, an excellent peak power density of 830 mW cm−2 is reached at 60 °C. The experimental and computational results revealed that the presence of Fe3C cores dynamically triggers electron transfer to the outermost carbon layer. This phenomenon amplifies the oxygen reduction reaction performance at N sites, contributing significantly to the observed catalytic enhancement.

Revealing the activity of inert Zn-based ZIF-L/aerogel composites in Fenton-like catalysis: Electron-deficient N ([sbnd]C[dbnd]N[sbnd]C) triggering 1O2 evolution

The inertness of Zn with fully occupied 3d10 configurations makes original Zn-based MOFs inactive, which are rarely reported in Fenton-like reactions. Herein, the prepared ZIF-L (Zn) was immobilized on foaming gelatin aerogel to fabricate composite catalysts (ZIF-L/FGA150) for PMS-based Fenton-like reactions. Unexpectedly, ZIF-L/FGA150 exhibited high Fenton-like activity in activating PMS for antibiotic norfloxacin (NOF) removal, by which over 99 % NOF could be degraded in 30 min. 1O2 was proved to be the sole contributor to mediating the non-radical oxidization process without the participation and evolution of free radicals. Mechanism investigation further revealed that the uncoordinated N (CNC) at edge 2-methylimidazole bonding with one Zn core were electron-deficient as a result of the deviation of electron density towards Zn-N4 coordination, and the electron-deficient N were identified as the active sites responsible for triggering electron transfer from the nucleophilic peroxy bonds of PMS accompanied by PMS decomposition into SO5− to generate singlet oxygen (1O2), thus driving NOF degradation. Owing to the 1O2-mediating non-radical oxidization, ZIF-L/FGA150 possessed high Fenton-like activity, unaffected by the interference of various environmental factors and in multiple actual water matrixes. This work might provide a novel insight into the reactivity of Zn-based MOFs in Fenton-like catalysis and develop an effective approach for antibiotic remediation.

Characterization of A π–π stacking cocrystal of 4-nitrophthalonitrile directed toward application in photocatalysis

Photoexcitation of the electron-donor-acceptor complexes have been an effective approach to achieve radicals by triggering electron transfer. However, the catalytic version of electron-donor-acceptor complex photoactivation is quite underdeveloped comparing to the well-established utilization of electronically biased partners. In this work, we utilize 4-nitrophthalonitrile as an electron acceptor to facilitate the efficient π-stacking with electron-rich aromatics to form electron-donor-acceptor complex. The characterization and energy profiles on the cocrystal of 4-nitrophthalonitrile and 1,3,5-trimethoxybenzene disclose that the electron transfer is highly favorable under the light irradiation. This electron acceptor catalyst can be efficiently applied in the benzylic C−H bond photoactivation by developing the Giese reaction of alkylanisoles and the oxidation of the benzyl alcohols. A broad scope of electron-rich aromatics can be tolerated and a mechanism is also proposed. Moreover, the corresponding π-anion interaction of 4-nitrophthalonitrile with potassium formate can further facilitate the hydrocarboxylation of alkenes efficiently.

RuSe2-CoTe Heterogeneous Surfaces Coated with NC Layer for Excellent HER Performance under Alkaline Condition.

Electrocatalytic hydrogen production has been a promising high-purity hydrogen production technology, attracting a large number of researchers' research interest. Ru has a hydrogen binding capacity similar to Pt, but its price is far lower than Pt, making it a promising alternative to Pt. However, a single Se electronic structure modulation is not sufficient to enable RuSe2 to be used for practical applications on a large scale due to the lack of electrons. Therefore, choosing a suitable way to electronically modulate the Ru atoms in RuSe2 can effectively improve the activity of the catalyst. Cobalt telluride (CoTe) can significantly enhance electrocatalytic performance due to tellurium's low electronegativity and excellent metal properties. In this work, the NC layer possesses excellent electrical conductivity and CoTe acts as an electron donor to optimize the electronic structure locally and trigger electron transfer efficiently. The RuSe2-CoTe/NC electrode requires an overpotential of only 25.4 mV (10 mA cm-2), which is superior to that of RuSe2/NF (65 mV) and CoTe/NC (115 mV). Meanwhile, the Tafel slope of RuSe2-CoTe/NC (67.8 mV dec-1) was better than that of RuSe2/NF (113.6 mV dec-1) and CoTe/NC (209.5 mV dec-1), showing that the build-up of the superior heterojunction makes the RuSe2-CoTe/NC with better hydrogen evolution reaction (HER) reaction kinetics. In addition, after 30 h of long-term stability testing, no significant decrease in catalytic activity was observed, proving the good stability of the RuSe2-CoTe/NC catalyst.

Carbonyl and defect of metal-free char trigger electron transfer and O2•− in persulfate activation for Aniline aerofloat degradation

Residual flotation reagents in mineral processing wastewater can trigger severe ecological threats to the local groundwater if they are discharged without treatment. Metal-free biochar-induced persulfate-advanced oxidation processes (KCBC/PS) were used in this study to elucidate the degradation of aniline aerofloat (AAF) – a typical flotation reagent. In KCBC/PS system, AAF can be removed at low doses of catalyst (KCBC, 0.05 g/L) and oxidant (PS, 0.3 mM) additions with high efficiency. The analysis revealed the dominance of O2•− among the identified reactive oxygen species (ROS), which achieved deeper mineralization for the AAF degradation in the KCBC/PS system. The role of the electron transfer mechanism was equally important; the importance was corroborated by the chemical quenching experiments, electron spin resonance (ESR) detection, probe experiments, and electrochemical analysis. It benefited from the electron transfer mechanism in the KCBC/PS system and exhibited a wide pH adaptation (3.5–11) and high resistance to inorganic anions for real mining wastewater treatment. Combined with theoretical calculations and other analyses, the carbonyl group was deemed to be the active site of the non-radical pathway of biochar, while the site of the conversion of SO4•− to superoxide radicals by biochar activation represented a defect. These findings revealed a synergistic effect of multiple active sites on PS activation in biochar-based materials. Moreover, the intermediate degradation products of AAF from mass spectrometry indicated a possible pathway through the density functional theory (DFT) method, which was effective in reducing the environmental toxicity of pollutants for the first time according to the T.E.S.T software and seed germination experiments. Overall, our study proposed a novel modification strategy for cost-effective and environmentally friendly biochar-based catalysts, while also deepening our understanding of the mechanism of activation of persulfate by metal-free carbon-based materials.

Self-Driven Electron Transfer Biomimetic Enzymatic Catalysis of Bismuth-Doped PCN-222 MOF for Rapid Therapy of Bacteria-Infected Wounds.

In this work, a biomimetic nanozyme catalyst with rapid and efficient self-bacteria-killing and wound-healing performances was synthesized. Through an in situ reduction reaction, a PCN-222 metal organic framework (MOF) was doped with bismuth nanoparticles (Bi NPs) to form Bi-PCN-222, an interfacial Schottky heterojunction biomimetic nanozyme catalyst, which can kill 99.9% of Staphylococcus aureus (S. aureus). The underlying mechanism was that Bi NP doping can endow Bi-PCN-222 MOF with self-driven charge transfer through the Schottky interface and the capability of oxidase-like and peroxidase-like activity, because a large number of free electrons can be captured by surrounding oxygen species to produce radical oxygen species (ROS). Furthermore, once bacteria contact Bi-PCN-222 in a physiological environment, its appropriate redox potential can trigger electron transfer through the electron transport pathway in bacterial membranes and then the interior of the bacteria, which disturbs the bacterial respiration process and subsequent metabolism. Additionally, Bi-PCN-222 can also accelerate tissue regeneration by upregulating fibroblast proliferation and angiogenesis genes (bFGF, VEGF, and HIF-1α), thereby promoting wound healing. This biomimetic enzyme-catalyzed strategy will bring enlightenment to the design of self-bacterial agents for efficient disinfection and tissue reconstruction simultaneously.

Triphenylphosphine oxide promoting visible-light-driven C-C coupling via desulfurization.

Triphenylphosphine oxide (TPPO) and triphenylphosphine (TPP) can form a complex in solution, promoting visible light absorption to trigger electron transfer within the complex and generate radicals. Subsequent radical reactions with thiols enable desulfurization to produce carbon radicals that react with aryl alkenes to yield new C-C bonds. Since ambient oxygen can easily oxidize TPP to TPPO, the reported method requires no explicit addition of a photocatalyst. This work highlights the promise of using TPPO as a catalytic photo-redox mediator in organic synthesis.

Integration of manganese ores with activated carbon granules into CW-MFC to trigger anoxic electron transfer and removal of ammonia nitrogen

Anoxic ammonium removal is limited by the lack of an electron acceptor. Six different microcosms were tested in parallel for a long-term operation. A constructed wetland-microbial fuel cell (CW-MFC) filled with manganese ore (MO) and activated carbon granules (AC) boosts the biochemistry of N-transformations under anoxic condition. The highest nitrogen removal was about 7.5 gN/(m2 d). With a COD/N = 1.7 in the influent, total nitrogen (TN) and chemical oxygen demand (COD) removal efficiencies of the coupling system increased by 53.5% and 29.6%, as compared with the control, respectively. Notably, the introduction of MO caused 891 ± 23 mV voltage output. The occurrence of different removal paths for ammonium in the presence of MO include adsorption, chemical oxidation and microbial nitrification. Altered extracellular polymeric substances (EPS) was involved in extracellular electron transfer (EET) processes and the cycle of Mn(Ⅳ)/Mn(Ⅱ) for accelerated nitrogen removal. The presence of AC expands the interaction between exoelectrogens and MO. Further study showed that Geobacter and Geothrix associated with electricity production and manganese reduction have been found generally in Mn-rich microcosms. This study provides valuable guidance for a removal of NH4+ under anoxic conditions and highlights the potential effect of triggering electron transfer for the development of energy production and pollutant remediation.

Indirect (hydrogen-driven) electrodeposition of porous silver onto a palladium membrane

Hydrogen permeation through a pure palladium film (25 μm thickness, optically dense) is employed to trigger electron transfer (hydrogen-driven) reactions at the external palladium | aqueous electrolyte interface of a two-compartment electrochemical cell. Two systems are investigated to demonstrate feasibility for (i) indirect hydrogen-mediated silver electrodeposition with externally applied potential and (ii) indirect hydrogen-mediated silver electrodeposition driven by external formic acid decomposition. In both cases, porous metal deposits form as observed by optical and electron microscopies. Processes are self-limited as metal deposition blocks the palladium surface and thereby slows down further hydrogen permeation. The proposed methods could be employed for a wider range of metals, and they could provide an alternative (non-electrochemical or indirect) procedure for metal removal or metal recovery processes or for indirect metal sensing.Graphical abstract

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Gold-supported ceria nanoparticles (CeOx/Au), constituting an inverse system with respect to the more commonly studied ceria-supported gold nanoparticles, were previously identified as an excellent catalyst for water–gas shift reaction, CO oxidation, and steam reforming of methanol. However, the electronic structure and reactivity of such inverse catalysts have not been well understood. To probe the inherent nanoparticle–support interactions and their mechanistic role for the catalytic CO oxidation over this composite catalyst, ab initio molecular dynamics simulations and static density functional theory computations have been carried out for Au(111)-supported ceria clusters (Ce10O20/19), as a realistic model system of an inverse CeOx/Au catalyst. We have identified the perimeter of the supported ceria nanoparticle as the most favorable O vacancy formation site; however, the vacancy further migrates to an inner interface site during the thermalization process, simultaneously triggering electron transfer f...

Structural Basis of Vitamin K Antagonism

The vitamin K cycle supports blood coagulation, bone mineralization, and vascular calcium homeostasis. A key enzyme in this cycle, vitamin K epoxide reductase (VKOR), is the target of vitamin K antagonists (VKAs). Despite their extensive clinical use, the dose of VKAs (e.g., warfarin) is hard to regulate and overdose can lead to fatal bleeding. Improving the dose regulation requires understanding how VKAs inhibit VKOR, which is a membrane-embedded enzyme difficult to characterize with structural and biochemical studies. Here we achieve a long-standing goal of obtaining crystal structures of human VKOR with warfarin, which represents coumarin-based VKAs; with phenindione, which represents indandione-based VKAs; with superwarfarins, the most commonly used rodenticides; and with vitamin K epoxide in a reaction intermediate state. We have also solved structures of a VKOR-like homolog with warfarin, with vitamin K substrates, and without ligand. These structures show that human VKOR adopts an overall fold with four transmembrane helices (TM) and a large ER-luminal region. VKAs are bound at the active site of HsVKOR, which is formed by the surrounding four-TM bundle and a cap domain on top. The cap domain is stabilized by a linked anchor domain that interacts with the membrane surface. VKOR binds specifically to VKAs through hydrogen bonding to their diketone groups. Mutating VKOR residues recognizing the diketones render strong warfarin resistance. Except the hydrogen bonds, the binding pocket is largely hydrophobic. This pocket is incompatible with warfarin metabolite, explaining the inactivation of warfarin through CYP2C9 metabolism; CYP2C9 and VKOR genotypes can explain 30-50% of the patient variability in warfarin dose. In addition, the high potency of superwarfarins is due to the interaction of their side group with a tunnel where the isoprenyl chain of vitamin K is bound. For VKOR catalysis, the same residues affording the VKA-binding specificity also facilitate substrate reduction Initiation of the catalysis requires a reactive cysteine to form a substrate adduct. Interactions from this stably bound adduct induces a closed conformation, thereby triggering electron transfer to reduce the substrate. Importantly, the open to closed conformational change during catalysis similar to that induced by the binding of VKAs. Taken together, VKAs achieve inhibition through mimicking key interactions and conformational changes required for VKOR catalytic cycle. Understanding of these mechanisms will enable improved strategy to regulate warfarin dose and have a broad impact on thromboembolic diseases and bone disorders. Disclosures No relevant conflicts of interest to declare.

Theoretical evidence for the sensitivity of charge-rearrangement-enhanced x-ray ionization to molecular size

It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangement-enhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.

Highly reversible Li-O2 battery induced by modulating local electronic structure via synergistic interfacial interaction between ruthenium nanoparticles and hierarchically porous carbon

The interfacial interaction between catalysts and substrates has been considered as a pivotal factor determining the activities in different applications. However, interfacial interaction-dependent catalytic activities are currently neglected in understanding oxygen involved reaction in Li-O2 battery, seriously restricting the development of high performance Li-O2 battery. Herein, Ru@hierarchically porous carbon (Ru@HPC) with well-defined interfaces are fabricated and employed as cathode in Li-O2 battery. Notably, Ru@HPC based Li-O2 battery demonstrate better performance than that based on cathode formed by simply mixing Ru nanoparticles with hierarchically porous carbon (Ru/HPC). The Ru@HPC improved performance with a large specific capacity of ~7100 mA h g−1, high round-trip efficiency of 71%, high rate capability, and excellent cyclic stability of over 125 cycles (over 40 days). Density functional theory calculations indicated that the interfacial interaction between Ru nanoparticles and carbon substrate in Ru@HPC, which triggers electron transfer from ruthenium to carbon and thus optimizes the surface adsorption energy of the reactants and intermediate, facilitates the performance improvement of Li-O2 battery. Our study for the first time reveals that the interfacial interaction between catalyst and substrate is a critical factor determining the Li-O2 battery performance and offers a promising design strategy of catalyst for Li-O2 batteries.

Energy Transduction in Nitrogenase.

Nitrogenase is a complicated two-component enzyme system that uses ATP binding and hydrolysis energy to achieve one of the most difficult chemical reactions in nature, the reduction of N2 to NH3. One component of the Mo-based nitrogenase system, Fe protein, delivers electrons one at a time to the second component, the catalytic MoFe protein. This process occurs through a series of synchronized events collectively called the "Fe protein cycle". Elucidating details of the events associated with this cycle has constituted an important challenge in understanding the nitrogenase mechanism. Electron delivery is a multistep process involving three metal clusters with intra- and interprotein events. It is proposed that the first electron transfer event is a gated intraprotein transfer of one electron from the MoFe protein P-cluster to the FeMo cofactor. Measurement of the effect of osmotic pressure on the rate of this electron transfer process revealed that it is gated by protein conformational changes. This first electron transfer is activated by binding of the Fe protein containing two bound ATP molecules. The mechanism of how this protein-protein association triggers electron transfer remains unknown. The second electron transfer event is proposed to be a rapid interprotein "backfill" with transfer of one electron from the reduced Fe protein 4Fe-4S cluster to the oxidized P-cluster. In this way, electron delivery can be viewed as a case of "deficit spending". Such a deficit-spending electron transfer process can be envisioned as a way to achieve one-direction electron flow, limiting the potential for back electron flow. Hydrolysis of two ATP molecules associated with the Fe protein occurs after the electron transfer and therefore is not used to directly drive the electron transfer. Rather, ATP hydrolysis is proposed to contribute to relaxation of the "activated" conformational state associated with the ATP form of the complex, with the free energy from ATP hydrolysis being used to pay back energy associated with component protein association and electron transfer. Release of inorganic phosphate (Pi) and protein-protein dissociation follow electron transfer and ATP hydrolysis. The rate-limiting step for the Fe protein cycle is not dissociation of the two proteins, as previously believed, but rather is release of Pi after ATP hydrolysis, which is then followed by rapid protein-protein complex dissociation. Nitrogenase is composed of two catalytic halves that do not function independently but rather exhibit anticooperative nuclear motion in which electron transfer in one-half of the complex partially inhibits electron transfer and ATP hydrolysis in the other half. Calculations indicated the existence of anticooperative interactions across the entire nitrogenase complex, suggesting a mechanism for the control of events on opposite ends of this large complex. The mechanistic necessity for this anticooperative process remains unknown. This Account presents a working model for how all of these processes work together in the nitrogenase "machine" to transduce the energy from ATP binding and hydrolysis to drive N2 reduction.

Intriguing I2 Reduction in the Iodide for Chloride Ligand Substitution at a Ru(II) Complex: Role of Mixed Trihalides in the Redox Mechanism.

The compound [Ru(CN(t)Bu)4(Cl)2], 1, reacts with I2, yielding the halogen-bonded (XB) 1D species {[Ru(CN(t)Bu)4(I)2]·I2}n, (2·I2)n, whose building block contains I(-) ligands in place of Cl(-) ligands, even though no suitable redox agent is present in solution. Some isolated solid-state intermediates, such as {[Ru(CN(t)Bu)4(Cl)2]·2I2}n, (1·2I2)n, and {[Ru(CN(t)Bu)4(Cl)(I)]·3I2}n, (3·3I2)n, indicate the stepwise substitution of the two trans-halide ligands in 1, showing that end-on-coordinated trihalides play a key role in the process. In particular, the formation of ClI2(-) triggers electron transfer, possibly followed by an inverted coordination of the triatomic species through the external iodine atom. This allows I-Cl separation, as corroborated by Raman spectra. The process through XB intermediates corresponds to reduction of one iodine atom combined with the oxidation of one coordinated chloride ligand to give the corresponding zerovalent atom of I-Cl. This redox process, explored by density functional theory calculations (B97D/6-31+G(d,p)/SDD (for I and Ru atoms)), is apparently counterintuitive with respect to the known behavior of the corresponding free halogen systems, which favor iodide oxidation by Cl2. On the other hand, similar energy barriers are found for the metal-assisted process and require a supply of energy to be passed. In this respect, the control of the temperature is fundamental in combination with the favorable crystallizations of the various solid-state products. As an important conclusion, trihalogens, as XB adducts, are not static in nature but are able to undergo dynamic inner electron transfers consistently with implicit redox chemistry.