Electron Donation Mechanism Research Articles

Accelerating electron transfer dynamics for efficient nitrogen photoreduction on nitrogenase-mimicking metal-organic framework/Fe-dispersed MXene

Achieving efficient artificial nitrogen fixation similar to nitrogenase under mild conditions has always been a goal pursued by human beings, but it remains a huge challenge. Herein, a novel photothermal catalyst UiO-66(Zr-Fe)/Fe-dispersed MXene having nitrogenase-mimicking electron transfer structure and electron donation mechanism is fabricated and applied in a designed gas-vapor-solid bi-phase system for efficient N2-to-NH3 conversion. The complementary role playing of UiO-66(Zr-Fe), Ti3C2 MXene and well-dispersed Fe sites are capable of simulating nitrogenase triple tandem system that in charge of electron generation, electron transfer and N2 fixation, respectively. In this well-designed catalyst, a remarkably high NH3 formation rate of 133.2 μmol g−1 h−1 under solar level illumination is recorded. This superior performance of the biomimetic structure is related to the enhanced light-to-matter interaction for effective multi-electron extraction and promoted photothermal response for accelerating surface reaction dynamics.

Impact of Plant Origin on Eurasian Propolis on Phenolic Profile and Classical Antioxidant Activity.

Propolis is a bee product with known medical properties, including antioxidant activity. The scope of the study is profiling 19 different Eurasian propolis samples (mostly from Russia and Kazakhstan, Kyrgyzstan, Poland, Ukraine, and Slovakia). Profiles of propolises were investigated by ultra-high-performance liquid chromatography–diode array detector–mass spectrometry (UPLC-DAD-MS). Classical antioxidant properties, which are based on electron donation mechanism, were assessed by DPPH, ferric reducing antioxidant power (FRAP), and oxygen radical absorbance capacity (ORAC) assays. Total phenolic and flavonoid contents were also evaluated by colorimetric tests. Most of the samples exhibited significant content of polyphenols (from 30.28 to 145.24 mg GAE/g of propolis) and flavonoids (from 10.45 to 82.71 mg GAE/g of propolis). Most of the propolis samples exhibited potent antiradical (DPPH test—from 8.83 to 64.47 mg GAE/g of propolis) and reducing activity (FRAP test—from 0.08 to 1.17 mmol Fe2+/g of propolis). Based on the occurrence of marker compounds, propolis samples were classified as poplar, aspen–birch, aspen–poplar, and aspen–birch–poplar type. Main markers present in propolis of poplar (e.g., chrysin, pinocembrin, galangin, and 3-O-acetyl-pinobanksin), birch (ermanin and acacetin) and aspen (2-acetyl-1,3-di-p-coumaroylglycerol) origin were used. DPPH, FRAP, and ORAC tests results were correlated with flavonoids, total polyphenols, or the polyphenols other than flavonoids content. In term of activity, poplar propolis type was variable, while aspen–birch–poplar type usually exhibited high DPPH and FRAP activity.

Electron donation mechanism of superior Cs-supported oxides for catalytic soot combustion

Soot particulates from diesel engines cause serious environmental and health problems. Catalytic combustion removal of soot by alkali metal oxides is one of the most promising aftertreatment techniques because no precious metals are used. However, the activation mechanism of gaseous oxygen and/or soot by alkali metal oxides remains unclear. Here, Cs-supported typical oxides were reported to exhibit superior catalytic performance for soot combustion to the most studied K-supported counterparts. The active species were characterized as Cs2O, whilst ketene was still distinguished as the reaction intermediate as in the case of the K-supported catalysts. The easier electron donation from Cs2O to O2 than from K2O results in the more active oxygen species with higher charge density. On the other hand, soot activation by chemisorption on the Cs2O surface is easier than on the K2O surface via the oxygen-containing group at the edge of soot, leading to more electrons donated from Cs2O to soot. These theoretical calculation results were further confirmed by in situ IR of soot combustion. The understanding of the electron donation mechanism provides an insight into the essence of soot combustion catalyzed by alkali metal oxides.

Characterization of chalcogen bonding interactions via an in-depth conceptual quantum chemical analysis.

The chalcogen bond has been acknowledged as an influential noncovalent interaction (NCI) between an electron-deficient chalcogen (donor) and a Lewis base (acceptor). This work explores the main features of chalcogen bonding through a large-scale computational study on a series of donors and acceptors spanning a wide range in strength and character of this type of bond: (benzo)chalcogenadiazoles (with Ch = Te/Se/S) versus halides and neutral Lewis bases with O, N, and C as donor atoms. We start from Pearson's hard and soft acids and bases (HSAB) principle, where the hard nature of the chalcogen bond is quantified through the molecular electrostatic potential and the soft nature through the Fukui function. The σ-holes are more pronounced when going down in the periodic table and their directionality matches the structural orientation of donors and acceptors in the complexes. The Fukui functions point toward an n→σ*-type interaction. The initial conjectures are further scrutinized using quantum mechanical methods, mostly relating to the systems' electron density. A Ziegler-Rauk energy decomposition analysis shows that electrostatics plays a distinctly larger role for the soft halides than for the hard, uncharged acceptors, associated with the softness matching within the HSAB principle. The natural orbital for chemical valence analysis confirms the n→σ* electron donation mechanism. Finally, the electron density and local density energy at the bond critical point in the quantum theory of atoms in molecules study and the position of the spikes in the reduced density gradient versus density plot in the NCI theory situate the chalcogen bond in the same range as strong hydrogen bonds. © 2017 Wiley Periodicals, Inc.

Novel Carrier Doping Mechanism for Transparent Conductor: Electron Donation from Embedded Ag Nanoparticles to the Oxide Matrix.

A trade-off between the carrier concentration and carrier mobility is an inherent problem of traditional transparent conducting oxide (TCO) films. In this study, we demonstrate that the electron concentration of TCO films can be increased without deteriorating the carrier mobility by embedding Ag nanoparticles (NPs) into Al-doped ZnO (AZO) films. An increment of Ag NP content up to 0.7 vol % in the AZO causes the electron concentration rising to 4 × 1020 cm-3. A dependence of the conductivity on temperature suggests that the energy barrier for the electron donation from Ag NPs at room temperature is similar to the Schottky barrier height at the Ag-AZO interface. In spite of an increase in the electron concentration, embedded Ag NPs do not compromise the carrier mobility at room temperature. This is evidence showing that this electron donation mechanism by Ag NPs is different from impurity doping, which produces both electrons and ionized scattering centers. Instead, an increase in the Fermi energy level of the AZO matrix partially neutralizes Al impurities, and the carrier mobility of Ag NP embedded AZO film is slightly increased. The optical transmittance of mixture films with resistivity less than 1 × 10-3 Ω·cm still maintains above 85% in visible wavelengths. This opens a new paradigm to the design of alternative TCO composite materials which circumvent an inherent problem of the impurity doping.

3,4-Dihydro-1,3-2H-benzoxazines: Novel reducing agents through one electron donation mechanism and their application as the formation of nano-metallic silver coating

3,4-dihydro-1,3-2H-benzoxazines as novel one-electron donators for silver(I) ion into nano-metallic silver was firstly found and reported. The silver formation from nano-spherical particles to coral-like and dendrite-like structures was presented. With respect to the characterization results, the feasible reaction mechanism of the silver formation was proposed as an electron donated from benzoxazine to silver(I) ion, resulting in a radical cationic species of benzoxazine and silver(0). Based on this reduction process, a new approach for nano-silver coating on various surfaces such as fumed silica (SiO2), titanium dioxide (TiO2), carbon black (CB), chitosan (CS) including plastic sheet (polycarbonate, PC) and pellet (polyvinyl alcohol, PVA), was also revealed. Besides the nano-silver coated products were applied as antimicrobials fillers for Staphylococcus aureus ATCC 25923, Bacillus subtilis ATCC 6633, Micrococcus luteus ATCC 9341, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 2785 and Candida albicans ATCC 10231.

DFT study of the interaction between ethanethiol and Fe-containing ionic liquids for desulfuration of natural gasoline

The interaction between ethanethiol molecule and either anhydrous FeIII chloride anions or 1-butyl-3-methylimidazolium ([BMIM]+) cations of ionic liquids (ILs) was investigated using Density Functional Theory approach and results were correlated with our previous experimental results on extractive desulfurization (EDS), where ILs containing anionic FeIII species show excellent performance to remove sulfur compounds from natural gasoline, especially when there exists an excess of FeCl3 in the [BMIM]+[FeCl4]− IL since this mixture contains binuclear anions [Fe2Cl7]−, whose Fe―Cl―Fe bonds are larger and less strong than Fe―Cl bonds of mononuclear anions [FeCl4]−, being then the former bonds activated for ethanethiol chemisorption. Molecular frontier orbitals and Mulliken atomic charges reveal that the high desulfurization performance could be due to a Dewar–Chatt–Duncanson-like mechanism of electron donation–backdonation among ethanethiol sulfur and transition metal centers of [Fe2Cl7]− anions, and this mechanism is promoted because of the symmetry affinity among the ethanethiol HOMO and the atomic orbital t2g on Fe sites in [Fe2Cl7]− LUMO. On the other hand, when there is no excess of FeCl3, EDS increases with the size of N-alkyl substitutes in [BMIM]+ due to ethanethiol physisorption by these cations.

Inhibition of reactive nitrogen species invitro and exvivo by thioredoxin h2 from sweet potato ‘Tainong 57’ storage roots

In this study, the ability of thioredoxin h2 (TRX h2) expressed in Escherichia coli to scavenge ON and ONOO− were investigated. The data obtained show that TRX h2 generated a dose-dependent inhibition on production of nitrite and superoxide radicals. TRX h2 also caused a dose-dependent inhibition of the oxidation of dihydrorhodamine 123 (DHR) by peroxynitrite. Spectrophotometric analyses revealed that TRX h2 suppressed the formation of ONOO−-mediated tyrosine nitration through an electron donation mechanism. In further studies, TRX h2 also showed a significant ability of inhibiting the nitration of bovine serum albumin (BSA) in a dose-dependent manner. In vivo TRX h2 inhibited LPS-induced nitrite production in macrophage in a concentration-dependent manner. The present study suggested that TRX h2 had an efficient reactive nitrogen species scavenging ability. TRX h2 might be a potential effective NO and ONOO− scavenger useful for the prevention of the NO and ONOO− involved diseases.

Peroxynitrite scavenging activity of lithospermate B from Salvia miltiorrhiza.

Peroxynitrite (ONOO-) is produced by the reaction of superoxide (O2-) with nitric oxide. ONOO- damages proteins through nitration or oxidation. For protection from ONOO- induced protein modifications, ONOO- scavengers should be supplemented. Evidence was obtained that lithospermate B extracted from Salvia miltiorrhiza showed the strongest scavenging activity among its constituents. Its ONOO- scavenging activity is via an electron donation mechanism. A dihydroxyl group and a double bond seem to be essential structure requirements. The data from the experiments further confirmed a protective effect of lithospermate B on bovine serum albumin and low-density lipoprotein against ONOO-. This study demonstrated that lithospermate B with hydroxyl groups and double bonds exerts an anti-nitration effect by scavenging ONOO-.

Inhibition of Reactive Nitrogen Species in Vitro and ex Vivo by Trypsin Inhibitor from Sweet Potato ‘Tainong 57' Storage Roots

Peroxynitrite (ONOO-), formed from a reaction of superoxide and nitric oxide, is one of the most potent cytotoxic species known to oxidize cellular constituents including essential proteins, lipids, and DNA. ONOO- induces cellular and tissue injury, resulting in several human diseases such as Alzheimer's disease, atherosclerosis, and stroke. Due to the lack of endogenous enzymes responsible for ONOO- scavenging activity, finding a specific ONOO- scavenger is of considerable importance. In this study, the ability of trypsin inhibitor (TI), isolated from sweet potato storage roots (SPTI), to scavenge *ON and ONOO- was investigated. The data obtained show that TI generated a dose-dependent inhibition on production of nitrite and superoxide radicals. The IC50 value of TI on superoxide radical was 143.2 +/- 4.29 microg/mL. SOD activity staining was used to confirm SOD activity of SPTI. SPTI also caused a dose-dependent inhibition of the oxidation of dihydrorhodamine 123 (DHR) by peroxynitrite. A calculated IC50 value of 809.1 +/- 32.36 microg/mL was obtained on the inhibition of peroxynitrite radical. Spectrophotometric analyses revealed that TI suppressed the formation of ONOO--mediated tyrosine nitration through an electron donation mechanism. In further studies, TI also showed a significant ability to inhibit nitration of bovine serum albumin (BSA) in a dose-dependent manner. In vivo TI inhibited lipopolysaccharide-induced nitrite production in macrophages in a concentration-dependent manner with an IC50 value of 932.8 +/- 29.85 microg/mL. The present study suggested that TI had an efficient reactive nitrogen species scavenging ability. TI might be a potential effective NO and ONOO- scavenger useful for the prevention of NO- and ONOO--involved diseases.

Peroxynitrite scavenging activity of sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid) isolated from Brassica juncea.

Peroxynitrite (ONOO(-)), formed from a reaction of superoxide and nitric oxide, is one of the most potent cytotoxic species that are known to oxidize cellular constituents including essential proteins, lipids, and DNA. In this study, the ability of sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid), isolated from Brassica juncea, to scavenge ONOO(-) was investigated. The data obtained show that sinapic acid can efficiently scavenge native ONOO(-) as well as ONOO(-) derived from the peroxynitrite donor 3-morpholinosydnonimine hydrochloride (SIN-1). Spectrophotometric analyses revealed that sinapic acid suppressed the formation of ONOO(-)-mediated tyrosine nitration through an electron donation mechanism. In further studies, sinapic acid also showed a significant ability of inhibiting nitration of bovine serum albumin and low-density lipoprotein (LDL) in a dose-dependent manner. Sinapic acid decreased the LDL peroxidation induced by SIN-1-derived ONOO(-). The present study suggests that sinapic acid has an efficient ONOO(-) scavenging ability, which may well be a potent ONOO(-) oxidant scavenger for the protection of the cellular defense activity against the ONOO(-)-involved diseases.

Cycloaddition reaction of furan with Si(100)-2×1

The adsorption configuration of furan on Si(100)-2×1 at 125 K has been investigated using x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), and semiempirical molecular orbital (MO) calculation. A chemisorbed furan species is identified, which does not desorb until 300 K. Our results clearly demonstrate the covalent attachment of furan onto Si(100), possibly through a [4+2] cycloaddition reaction. Based on the frontier molecular orbital (FMO) theory and work function measurements, an electron donation mechanism from furan to Si(100)-2×1 is proposed to be involved in the cycloaddition reaction.

Studies on structure and function of photosystem II in oxygenic photosynthesis: Stoichiometry of cytochromeb-559 inSynechocystis sp. PCC 6803

Cytochromeb-559 is an integral protein of the photosystem II (PSII) reaction center from both plants and cyanobacteria. Cytochromeb-559 has the unique structure of a heme crosslinked α- and β-subunit heterodimer. The stoichiometry of cytochromeb-559 (one or two copies) per the PSII reaction center has been the subject of controversy and the molar ratio of the heme group to the special chlorophyll P-680 has a number of significant implications on our understanding of the functional role of cytochromeb-559, the mechanism of electron donation in PSII, and the stoichiometry of the other redox-active reaction center components. In order to determine the number of the cytochromeb-559 heme in the PSII reaction center ofSynechocystis sp. PCC 6803, the α- and β-subunits are covalently linked by use of the molecular genetic techniques. The resultingpsbEF fusion mutant was able to grow photoautotrophically, implying that the PSII complexes are assembled and functional in thylakoids. This result supports the fact that there are two set of cytochromeb-559 in the PSII reaction center ofSynechocystis sp. PCC 6803.

The Mechanism of Electron Donation to Molecular Oxygen by Phagocytic Cytochrome b558

Phagocytic cytochrome b558 is a unique heme-containing enzyme, which catalyzes one electron reduction of molecular oxygen to produce a superoxide anion with a six-coordinated heme iron. To clarify the mechanism of the superoxide production, we have analyzed oxidation-reduction kinetics of cytochrome b558 purified from porcine neutrophils by stopped-flow and rapid-scanning spectroscopy. Reduced cytochrome b558 was rapidly reoxidized by O2 showing spectral changes with clear isosbestic points, which were also observed during the reduction of ferric cytochrome b558 with Na2S2O4 under anaerobic conditions. The single turnover rate for the reaction with O2 linearly depended on the O2 concentration but was not affected by addition of CO. The rate of the reaction decreased with an increase of pH giving a pKa of 9.7. Under complete anaerobic conditions, ferrous cytochrome b558 was oxidized by ferricyanide at a rate faster than by O2. The thermodynamic analysis shows that the enthalpic energy barriers for the reactions of cytochrome b558 are significantly lower when compared to the autoxidation of native and modified myoglobins through the formation of the iron-O2 complex. These findings are most consistent with the electron transfer from the heme to O2 by an outer-sphere mechanism.

Laser flash kinetic analysis of Synechocystis PCC 6803 cytochrome c6 and plastocyanin oxidation by Photosystem I

Laser flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from reduced cytochrome c 6 and plastocyanin to photooxidized P700 in Photosystem I (PS I) particles from the cyanobacterium Synechocystis PCC 6803. Data analysis yields second-order rate constants of 1.3 · 10 7 M −1 s −1 and 1.0 · 10 7 M −1 s −1 for the heme- and copper-proteins, respectively. With the two donor proteins, the observed rate constants ( k obs) present a linear protein-concentration dependence, thus suggesting an apparent one-step bimolecular collisional mechanism. At neutral pH, the k obs values monotonically increase with increasing NaCl or MgCl 2 concentration, which is ascribed to the involvement of repulsive electrostatic interactions between the donor proteins and PS I. The difference in the effective concentration at which MgCl 2 has its maximum effect as compared with that of NaCl is attributed to the specific role played by Mg 2+ ions, which could act as electrostatic bridges between negatively charged groups. At physiological mild acid pH, cytochrome c 6 is a more efficient electron donor than plastocyanin. The inversion of the NaCl and MgCl 2 effect at pH below 5 — that is, decreasing of k obs with increasing ionic strength — is interpreted as arising from the involvement of attractive ionic interactions at pH lower than the isoelectric point of the donor proteins. Some evolutive aspects on the mechanism of electron donation to PS I are discussed.

Reevaluation of the stoichiometry of cytochrome b559 in photosystem II and thylakoid membranes.

The stoichiometry of cytochrome b559 (one or two copies) per reaction center of photosystem II (PSII) has been the subject of considerable debate. The molar ratio of cytochrome b559 has a number of significant implications on our understanding of the functional role of cytochrome b559, the mechanism of electron donation in PSII, and the stoichiometry of the other redox-active, reaction center components. We have reinvestigated the stoichiometry of cytochrome b559 in PSII-enriched and thylakoid membranes, using differential absorbance and electron paramagnetic resonance spectroscopies. The data from both quantitation procedures strongly indicate only one copy of cytochrome b559 per reaction center in PSII-enriched membranes and also suggest one copy of cytochrome b559 per reaction center in thylakoid membranes.

Flash-induced redox changes of P700 and plastocyanin in chloroplasts suspended in fluid media at sub-zero temperatures

Following a short flash of light, the reaction centre chlorophyll of photosystem I, WOO, becomes oxidised. Measurement of the dark reduction of this oxidised species generated by a single turnover give information about the mechanism of electron donation. Studies of this type have shown that the kinetics of P700’ reduction do not follow a single exponential [l-5]. As a result of this both plastocyanin and cytochrome fhave been implicated as direct donors to P700’ [ 1,2,5,6] although a claim has also been made for another more primary donor [4]. Part of the reason for the lack of clear understanding of electron donation on the oxidising side of photosystem 1 is due to the speed of the decay of the P700’ signal and complications associated with detecting it spectrophotometrically. One possible way to simplify the analysis of the signal is to measure its kinetics at low temperatures where the electron-transfer rates are likely to be slowed down. It has convincingly been shown that all properties of photosynthetic electron transport and associated proton translocation are preserved when chloroplasts are suspended in suitable aqueous/organic media at subzero temperatures [7-121. Here, we have made use of this low temperature approach to investigate the flash-induced redox changes of P700 and plastocyanin. It has been found that the electron-transfer reaction involving these components are qualitatively similar to those observed