Mechanism Of Electron Transfer Reaction Research Articles

Regulation Mechanism of Electron Transfer Reaction from Cytochrome C to Cytochrome C Oxidase

The respiratory chain in mitochondria is quite essential for the energy generation in cells, effectively producing “the energy currency in living organisms”, ATP. The driving force for this system is a series of electron transfers form Complex I (NADH:quinone oxidoreductase) to Complex IV (Cytochrome c Oxidase; CcO) in inner mitochondria membrane, and these electron transfer reactions promote the proton pumping from the matrix to the intermembrane space in mitochondria, and, by use of the resulting proton concentration gradient across the membrane, Complex V (ATP synthase) produces ATP. These series of electron transfer reactions terminate at CcO to reduce molecular oxygen to water molecules. The electrons required for this four-electron reduction of molecular oxygen in CcO are donated from a small soluble electron transfer hemoprotein, Cytochrome c (Cyt c), and this electron transfer is also associated with the proton pumping for the ATP generation in the respiratory chain. To examine the molecular regulation mechanism of the electron transfer reaction to terminate the electron transport chain for four-electron reduction of molecular oxygen and proton pumping for the ATP generation, we kinetically followed the electron transfer reaction and analyzed the electron transfer reaction by the Michaelis-Menten equation. By utilizing the Cyt c mutants, we identified the amino acid residues to regulate the electron transfer reactions (Biochem. J., 2020, 477,1565). The kinetic analysis of the electron transfer reaction also revealed that the apparent rate for the electron transfer reaction determined by the steady-state kinetics is much slower than the intramolecular electron transfer rate in the Cyt c – CcO complex. On the other hand, we successfully determined the interaction site between Cyt c and CcO by using NMR (Proc. Natl. Acad. Sci., 2011, 108, 12271) and protein-protein docking simulation (J. Biol. Chem., 2016, 291, 15320). The Cyt c – CcO complex structure estimated from the NMR and protein-protein docking simulation clearly showed that the two redox centers, heme iron in Cyt c and CuA in subunit II of CcO, were located more than 20 Å apart, and the complexes exhibiting shorter redox center distances were energetically unstable. Combined with the results from kinetic and structural studies, we proposed a “conformationally gated electron transfer mechanism”, where the thermal fluctuation of the protein structure controls the electron transfer rate. However, direct experimental evidence that protein conformational fluctuations regulate electron transfer reactions has not been obtained. In this study, we focused on the viscosity of the protein solution as a factor to regulate the conformational fluctuation of the proteins, based on the assumption that high viscosity of the solution suppresses the conformational fluctuation. The electron transfer reaction from Cyt c to CcO was examined in the high viscosity solution and NMR relaxation measurements under high viscosity were also performed to identify the local structural fluctuation responsible for the regulation of the electron transfer rate. Together with the transient complex structures between Cyt c and CcO estimated by the MD simulation, the regulation mechanism of the electron transfer from Cyt c to CcO based on the “conformational gating” will be discussed.

Electrochemical characterization and performance of phenothiazine derivatives as redox mediators

Two phenothiazine derivatives were synthesized as new redox mediators. Their electrochemical properties were compared with conventional redox mediators, such as thionine, ferricyanide(III), and hexaammineruthenium(III). Cyclic voltammetry experiments were performed under various conditions on glassy carbon and ultramicroelectrodes to investigate the proton-coupled electron transfer reaction mechanism, electrochemical reversibility, and chemical stability of the redox mediators. The number of intervening electrons and protons in the redox processes of redox mediators at each temperature was estimated accordingly. Furthermore, thermodynamic parameters (standard enthalpy change, standard entropy change, and standard Gibbs free energy) of redox mediators could be evaluated by analyzing their electrochemical behavior according to the reaction temperatures. The mediator performance was also analyzed for the oxidation of reduced nicotinamide adenine dinucleotide. These experimental and analytical results confirm that the performance of redox mediators is similar or superior to those of other mediators.

Cyclic voltammetric studies of electron transfer reactions associated with bioactive iron–KA complexation at different biological pH

This study explores electrochemical interactions of a pharmaceutical drug, kojic acid (5-hydroxy-2-hydroxymethyl-4 H-pyran-4-one, KA), with bioactive iron Fe(III) at different pH systems by using cyclic voltammetric technique. For this study, buffers relevant to physiological pH of blood and digestive system have been selected. Bioactive iron was found to form an electroactive complex [Fe(III)L3] in the presence of KA at all studied pH, whereas electrochemical behavior of KA was also changed by complex formation. This electrochemical investigation reveals that under the experimental conditions, [Fe(III)L3] complex undergoes a diffusion controlled, one electron redox process that is quasi-reversible in nature. It was also found that [Fe(III)L3] complex undergoes coupled chemical reaction according to EC (an electron transfer reaction followed by a chemical reaction) mechanism, and this interaction is pH dependent. Modified Nicholson and Shain, Gileadi, and Kochi methods were used to determine standard heterogeneous electron transfer rate constant ( k0) of [Fe(III)L3] complex. It was found to be in the range of 10−3 to 10−5 cm/s.

Unexpected role of electron-transfer hub in direct degradation of pollutants by exoelectrogenic bacteria.

Exoelectrogenic bacteria (EEB) are capable of anaerobic respiration with diverse extracellular electron acceptors including insoluble minerals, electrodes and flavins, but the detailed electron transfer pathways and reaction mechanisms remain elusive. Here, we discover that CymA, which is usually considered to solely serve as an inner-membrane electron transfer hub in Shewanella oneidensis MR-1 (a model EEB), might also function as a reductase for direct reducing diverse nitroaromatic compounds (e.g. 2,4-dichloronitrobenzene) and azo dyes. Such a process can be accelerated by dosing anthraquinone-2,6-disulfonate. The CymA-based reduction pathways in S. oneidensis MR-1 for different contaminants could be functionally reconstructed and strengthened in Escherichia coli. The direct reduction of lowly polar contaminants by quinol oxidases like CymA homologues might be universal in diverse microbes. This work offers new insights into the pollutant reduction mechanisms of EEB and unveils a new function of CymA to act as a terminal reductase.

Elucidating the Mechanism of Bimolecular Photoinduced Electron Transfer Reactions.

The current developments in photoredox chemistry are stimulating a renewed interest for bimolecular photoinduced electron transfer reactions. Their investigation, initiated in the 1960s using conventional photochemical tools, resulted in a relatively simple reaction scheme. More recent studies, using not only spectroscopic techniques with better time resolution and extended spectral/temporal windows but also molecular dynamics simulations, reveal a more complex picture. This Perspective focuses on the results of these latest studies with neutral organic reactants, highlighting the time dependence of the quenching rate, the effect of mutual orientation of the reactants on the electronic coupling, and their consequence on the nature of the reaction product. Remaining questions, such as the occurrence of distant electron transfer in nonviscous liquids are also addressed, and possible directions toward their answer are proposed.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)33+-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG0ET and ΔG0PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG0ET and ΔG0PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPTlim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPTlim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

Green One-Pot Synthesis of Coumarin-Hydroxybenzohydrazide Hybrids and Their Antioxidant Potency.

Compounds from the plant world that possess antioxidant abilities are of special importance for the food and pharmaceutical industry. Coumarins are a large, widely distributed group of natural compounds, usually found in plants, often with good antioxidant capacity. The coumarin-hydroxybenzohydrazide derivatives were synthesized using a green, one-pot protocol. This procedure includes the use of an environmentally benign mixture (vinegar and ethanol) as a catalyst and solvent, as well as very easy isolation of the desired products. The obtained compounds were structurally characterized by IR and NMR spectroscopy. The purity of all compounds was determined by HPLC and by elemental microanalysis. In addition, these compounds were evaluated for their in vitro antioxidant activity. Mechanisms of antioxidative activity were theoretically investigated by the density functional theory approach and the calculated values of various thermodynamic parameters, such as bond dissociation enthalpy, proton affinity, frontier molecular orbitals, and ionization potential. In silico calculations indicated that hydrogen atom transfer and sequential proton loss–electron transfer reaction mechanisms are probable, in non-polar and polar solvents respectively. Additionally, it was found that the single-electron transfer followed by proton transfer was not an operative mechanism in either solvent. The conducted tests indicate the excellent antioxidant activity, as well as the low potential toxicity, of the investigated compounds, which makes them good candidates for potential use in food chemistry.

Enhanced photocatalytic performance of Cu2O/MoS2/ZnO composites on Cu mesh substrate for nitrogen reduction

Cu2O nanoparticles and MoS2 nanoflowers decorated with ZnO nanospheres were successfully co-deposited on Cu mesh via a mild electrodeposition method to build a dual direct Z-scheme heterostructure. The prepared materials can effectively synthesize ammonia with N2 and H2O in the liquid membrane reactor under simulated visible light. The results indicate that 3D nanomaterials exhibit better performance compared to a pure semiconductor due to the synergistic effect of enhanced visible light absorption, longer photogenerated carrier lifetime and the specific charge transfer path of dual direct Z-scheme structure. Meanwhile, the hydrophilicity of Cu2O/MoS2/ZnO rapidly makes the surface of the catalyst wet when it participates in the photo reaction, which promotes the contact between the reactant and exciton. This work proposes the electron transfer and possible reaction mechanism corresponding to the designed catalyst, which can provide a reference for other photocatalytic applications using a semiconductor heterojunction as a catalyst.

Revisiting Understanding of Electrochemical CO2 Reduction on Cu(111): Competing Proton-Coupled Electron Transfer Reaction Mechanisms Revealed by Embedded Correlated Wavefunction Theory.

Copper (Cu) electrodes, as the most efficacious of CO2 reduction reaction (CO2RR) electrocatalysts, serve as prototypes for determining and validating reaction mechanisms associated with electrochemical CO2 reduction to hydrocarbons. As in situ electrochemical mechanism determination by experiments is still out of reach, such mechanistic analysis typically is conducted using density functional theory (DFT). The semilocal exchange-correlation (XC) approximations most often used to model such catalysis unfortunately engender a basic error: predicting the wrong adsorption site for CO (a key CO2RR intermediate) on the most ubiquitous facet of Cu, namely, Cu(111). This longstanding inconsistency casts lingering doubt on previous DFT predictions of the attendant CO2RR kinetics. Here, we apply embedded correlated wavefunction (ECW) theory, which corrects XC functional error, to study the CO2RR on Cu(111) via both surface hydride (*H) transfer and proton-coupled electron transfer (PCET). We predict that adsorbed CO (*CO) reduces almost equally to two intermediates, namely, hydroxymethylidyne (*COH) and formyl (*CHO) at -0.9 V vs the RHE. In contrast, semilocal DFT approximations predict a strong preference for *COH. With increasing applied potential, the dominance of *COH (formed via potential-independent surface *H transfer) diminishes, switching to the competitive formation of both *CHO and *COH (both formed via potential-dependent PCET). Our results also demonstrate the importance of including explicitly modeled solvent molecules in predicting electron-transfer barriers and reveal the pitfalls of overreliance on simple surface *H transfer models of reduction reactions.

Harnessing Interfacial Electron Transfer in Redox Flow Batteries

Summary Redox flow batteries (RFBs) have garnered increasing attention for their potential to enable the widespread adoption of renewable electricity. However, a critical need associated with the continued development of this technology involves designing electrode-electrolyte interfaces that exhibit rapid, stable electron transfer kinetics. This targeted review outlines key challenges associated with measuring and enhancing the electron transfer kinetics of established and emerging flow battery active materials. We discuss several promising opportunities for advancing flow battery science and technology using the tools of applied electroanalysis and catalysis science. These challenges and opportunities are broadly relevant for future research directed at advancing the commercial adoption of RFBs for grid-scale energy storage.

Tetrathiafulvalene: A Gate to the Mechanochemical Mechanisms of Electron Transfer Reactions

This report describes aspects of our previous studies of the mechanochemical synthesis of charge transfer complexes of the electron donor tetrathiafulvalene, which are relevant to the use of laboratory X-ray powder diffraction for ex situ monitoring of mechanochemical reactions toward investigating their mechanisms. In particular, the reaction of tetrathiafulvalene and chloranil was studied under neat mechanochemical conditions and liquid-assisted grinding with diethyl ether (1 μL/mg). The product in both cases is the green tetrathiafulvalene chloranil polymorph and the mechanism of the redox reaction is presumably the same. However, while the kinetic profile of the neat mechanochemical synthesis was fitted with a second-order rate law, that of the overall faster liquid-assisted grinding reaction was fitted with the Ginstling-Brounshtein 3D diffusion-controlled model. Hence, the diffusional processes and mass transfer bringing the reactants together and separating them from products must be different. Diffraction measurements sensitive to crystalline phases and amorphous material, combined with in situ monitoring by spectroscopic techniques, will ultimately afford a better understanding of mechanochemical reaction mechanisms, a hot topic in mechanochemistry.

Electron Transfer in Nitrogenase.

Nitrogenase is the only enzyme capable of reducing N2 to NH3. This challenging reaction requires the coordinated transfer of multiple electrons from the reductase, Fe-protein, to the catalytic component, MoFe-protein, in an ATP-dependent fashion. In the last two decades, there have been significant advances in our understanding of how nitrogenase orchestrates electron transfer (ET) from the Fe-protein to the catalytic site of MoFe-protein and how energy from ATP hydrolysis transduces the ET processes. In this review, we summarize these advances, with focus on the structural and thermodynamic redox properties of nitrogenase component proteins and their complexes, as well as on new insights regarding the mechanism of ET reactions during catalysis and how they are coupled to ATP hydrolysis. We also discuss recently developed chemical, photochemical, and electrochemical methods for uncoupling substrate reduction from ATP hydrolysis, which may provide new avenues for studying the catalytic mechanism of nitrogenase.

Study of Electron Transfer Reactions Associated with Benzyl Viologen-β-Cyclodextrin Complexation in Buffer Solution of pH 7: Equilibrium and Kinetic Aspects

In this study, complex formation of benzyl viologen dication (BzV2+) with β-cyclodextrin (β-CD), in its different oxidation states, had been studied in buffer solution of pH 7, through cyclic voltammetry. In buffer solution of pH 7, extensive deposition of benzyl viologen neutral (BzVº) has been found. Further to that first redox process of BzV2+ was found reversible in buffer solution of pH 7. Benzyl viologen mono-cation (BzV+•) was found to interact with β-CD reversibly according to EC mechanism. Equilibrium constant of BzV+•-nβ-CD complex in buffer solution of pH 7, has been calculated as 5.49 M-n. The value of n was found to be 0.2. The bimolecular rate constant for the complex formation of BzV+• with β-CD was found to be 6.44 M-ns− in buffer solution of pH 7. This study is based upon an electrochemical approach to obtain the bonding equilibrium constant, the rate constants of formation and the association numbers of the complex formed between reduced forms of viologen and β-CD. This study explore the electron transfer reaction mechanism of benzyl viologen with β-CD to extend their applications at neutral pH.

Recent Advances in Photocatalysis over Metal–Organic Frameworks‐Based Materials

To meet the requirements for sustainable development, photocatalytic applications including water splitting, CO2 reduction, pollutant degradation, and organic conversion reactions have become hot topics for research, due to the predictable advantages of energy storage and transportation. Metal–organic frameworks (MOFs) featured with porosity, crystallinity, and tunable constitution offer enormous potential in photocatalytic applications. Herein, a critical overview of MOFs‐based photocatalysts is presented. Following a concise introduction of the structural characteristics of MOFs, the synthesis methods of MOFs‐based photocatalysts are summarized. Moreover, subtle modification strategies for MOFs inspired by the development of other inorganic photocatalysts are also discussed on the basis of the photocatalytic mechanisms of light absorption, electron transfer, and surface catalytic reactions. Furthermore, the applications of certain MOFs‐based photocatalysts are introduced in detail. In addition, other organic frameworks‐based photocatalytic materials, such as covalent organic frameworks and covalent triazine frameworks, are also described to provide a guided insight into the rational design and fabrication of efficient frameworks‐based photocatalysts. Finally, a brief conclusion and perspective on the opportunities and challenges for the future development of MOFs‐based photocatalysts are proposed.

Kinetics and mechanism of electron transfer reactions: Oxidative degradation of fluoroquinolone drugs in aqueous acidic/alkaline medium

Kinetics and mechanism of electron transfer reactions: Oxidative degradation of fluoroquinolone drugs in aqueous acidic/alkaline medium

Catalytic depolymerization of lignin over cesium exchanged and transition-metal substituted heterogeneous polyoxometalates

It is of great importance to develop new pathways for low-cost, high-selectivity conversion of lignin into aromatics. In this work, a series of cesium exchanged and transition-metal substituted heterogeneous polyoxometalates were prepared and applied as efficient catalysts for lignin depolymerization. The catalytic oxidation of lignin has been monitored by GPC, 2D NMR HSQC, and GC/MS technologies, and the reaction pathway was also confirmed by the examination with a dimeric β-O-4 lignin model compound. Under optimal conditions, these catalysts showed high activity toward oxidative cleavage of β-O-4 linkages, as well as β-5 and β-β CC linkages. In particular, the Co substituted polyoxometalates gave high yield of ca. 9.58% monomeric products at 150°C within 3h under an oxygen atmosphere. Results demonstrated the catalyst was easily separated from products and could be repeatedly used at least five cycles without significant loss of activity. Further, the possible reaction mechanism was proposed by a two-step oxygen-based electron transfer and oxygen transfer reaction mechanism. The design and application of the multifunctional POMs based heterogeneous catalytic system make this strategy of great interest in the production of aromatic products from lignin and inspire a new insight to utilize the entire lignocellulosic biomass.

Why Are DNA and Protein Electron Transfer So Different?

The corpus of electron transfer (ET) theory provides considerable power to describe the kinetics and dynamics of electron flow at the nanoscale. How is it, then, that nucleic acid (NA) ET continues to surprise, while protein-mediated ET is relatively free of mechanistic bombshells? I suggest that this difference originates in the distinct electronic energy landscapes for the two classes of reactions. In proteins, the donor/acceptor-to-bridge energy gap is typically several-fold larger than in NAs. NA ET can access tunneling, hopping, and resonant transport among the bases, and fluctuations can enable switching among mechanisms; protein ET is restricted to tunneling among redox active cofactors and, under strongly oxidizing conditions, a few privileged amino acid side chains. This review aims to provide conceptual unity to DNA and protein ET reaction mechanisms. The establishment of a unified mechanistic framework enabled the successful design of NA experiments that switch electronic coherence effects on and off for ET processes on a length scale of multiple nanometers and promises to provide inroads to directing and detecting charge flow in soft-wet matter.

Comparative analysis of in vitro antioxidant capacities of mycosporine-like amino acids (MAAs)

The most common Mycosporine-like Amino Acids (MAAs) in red algae (asterina-330, shinorine, palythine, palythinol, and porphyra-334) were evaluated regarding their in vitro antioxidant capacities, comparing them with synthetic (BHT and Trolox) and natural (ascorbic acid, gallic acid, p-cumaric acid, quercetin, and rutin) antioxidants. Folin-Ciocalteu, ABTS+, and FRAP assays have the same reaction mechanism of electron transfer and usually show a positive correlation. However, the antioxidant capacities for the MAAs were discrepant between these assays. While porphyra-334, shinorine, and palythine were as active as, or more active than synthetic phenolic antioxidants (Trolox and BHT) in Folin-Ciocalteu assay, the antioxidant potentials of MAAs were lower than those of the standard compounds in ABTS+ and FRAP assays. The pH-dependent antioxidant power was hypothesized as an explanation for this distinct behavior and was evaluated through ABTS+ assay. Alkaline pH allows antioxidant capacities similar to Trolox in ABTS+ assay, supporting our hypothesis. Comparing to standard compounds, MAAs showed low activity using ORAC assay and were pro-oxidant in β-carotene/linoleic acid assay. Therefore, imino-MAAs present relevant in vitro antioxidant capacity under specific pH conditions in assays based on electron transfer.

Comparison of dye (oxazine and thiazine) materials as a photosensitizer for use in photogalvanic cells based on molecular interaction with sodium dodecyl sulphate by spectral study

The photochemistry of dye is playing a significant role for understanding the mechanism of electron transfer reactions in photoelectrochemical devices such as photogalvanic cells, DSSC, semiconductor photo-catalysis, photoconductors, etc. Oxazines (Brilliant Cresyl Blue and Nile Blue O) and thiazines (Azur A, Azur B, Azur C, Methylene Blue and Toluidine Blue O) dyes have been used widely as a photosensitizer with and without surfactants in the photogalvanic cells for solar power conversion and storage. Since, the stability and solubility of photosensitizers (dyes) are increased in the presence of surfactant and these properties lead to enhance the electrical output of the photogalvanic cells. Therefore, here we have studied the extent of interaction of different dyes with sodium dodecyl sulphate (SDS), find out the order of stability of dye–SDS on the basis of magnitudes of shifting in λmax of dye monomer and try to correlate order of dye–SDS interaction with already reported electrical output data of photogalvanic cells. Brilliant Cresyl Blue, Nile Blue O, Azur A and TB O have shown red shifting while Azur B, Azur C and MB have shown blue shifting in their λmax value with SDS, which indicates formation of dye–surfactant complex. But, the extent of formation of complex for different dyes with SDS was different due to change in their alkyl groups. Dyes with red shifting have greater stability in excited state as well as higher electrical output data of the cell than dye with blue shifting. On the basis of both red and blue shifting, order of stability of dyes–SDS complex was found as: Brilliant Cresyl Blue > Toluidine Blue O > Azur A > Nile Blue > Azur B > Methylene Blue > Azur C. The order of electrical output values of these dyes in photogalvanic cells have also been supported by literature data in the presence of SDS. Hence, the dye–surfactant complex which would have greater stability in excited state might be more useful for improvement of conversion efficiency and storage capacity of photogalvanic cells in the future.

Kinetics and Mechanism of Electron Transfer Reactions: Oxidation of Crotyl Alcohol by Peroxomonosulfate in Aqueous Acidic Medium

ABSTRACTThe kinetics and mechanism of oxidation of crotyl alcohol by peroxomonosulfate has been studied, and the species of the peroxomonosulfate are discussed to find out the role of activated species. A plausible reaction mechanism is suggested, and a derived rate law corresponds to all experimental observations. The activation parameters such as energy and entropy of activation have been calculated as 37.21 ± 0.5 kJ mol−1 and −148.91 ± 2.7 J K−1 mol−1, respectively, by employing the Eyring plot.