EPR Studies Research Articles

Reductive nitrosylation of Iron(III) complexes of tetradentate ligands

Two iron (III) complexes, 1 and 2 with L1 and L2, [ L1 = N,N′-bis(2- hydroxybenzyl) - 1, 2- diaminobenzene; L2 = N, N′- bis(2- hydroxybenzyl) ethylenediamine], respectively, were synthesized and characterized. In methanol solution of the complexes, the Fe(III) centre was found to reduce in presence of NO gas. The formation of [FeIII-NO] intermediate prior to the reduction of Fe(III) center was evidenced by UV-visible, solution FT-IR, X-band EPR studies. The presence of excess NO gas leads to the reductive nitrosylation of the complexes leading to the formation of corresponding Fe(II)-nitrosyl complexes, 3 and 4, respectively. The Fe(II)-nitrosyls were isolated as solid and characterized spectroscopically. Density Functional Theory (DFT) calculations were performed to optimize the structures of all the complexes. All the complexes are fully optimized using PBE functional and DNP basis set in the presence of methanol solvent. The Conductor-like Screening Model (COSMO) as incorporated into the DMol (Maron et al., 2013) [3] program with dielectric constant of 37.5 is adopted to study the solvent effect.

Acceptorless or Transfer Dehydrogenation of Glycerol Catalyzed by Base Metal Salt Cobaltous Chloride - Facile Access to Lactic Acid and Hydrogen or Isopropanol.

The dehydrogenation of glycerol to lactic acid (LA) under both acceptorless and transfer dehydrogenation conditions using readily available, inexpensive, environmentally benign and earth-abundant base metal salt CoCl2 is reported here. The CoCl2 (0.5 mol %) catalyzed acceptorless dehydrogenation of glycerol at 160 °C in the presence of 0.75 equiv. of KOH, gave up to 33 % yield of LA in 44 % selectivity apart from hydrogen. Alternatively, with acetone as a sacrificial hydrogen acceptor, the CoCl2 (0.5 mol %) catalyzed dehydrogenation of glycerol at 160 °C in the presence of 1.1 equiv. of NaOt Bu resulted in up to 93 % LA with 96 % selectivity along with another value-added product isopropanol. Labelling studies revealed a modest secondary KIE of 1.68 which points to the involvement of C-H bond activation as a part of the catalytic cycle but not as a part of the rate-determining step. Catalyst poisoning experiments with PPh3 and CS2 are indicative of the homogeneous nature of the reaction mixture involving molecular species that are likely to be in-situ formed octahedral Co(II) as inferred from EPR, HRMS and Evans magnetic moment studies. The net transfer dehydrogenation activity is attributed to exclusive contribution from the alcoholysis step.

Synthesis and EPR study of highly stable trityl-nitroxide biradicals

Synthesis and EPR study of highly stable trityl-nitroxide biradicals

Development of highly sensitive and selective trace acetone sensor using perovskite yttrium ferrite: Mechanism, kinetics and phase dependence study

Detection and monitoring of exhale breath acetone, bio-marker of diabetes is the need of the day to save millions human life. Trace acetone sensor having capability to detect breath acetone has been developed using perovskite YFeO3 powder (YFO) synthesized through a facile solution combustion technique. It is observed that YFO started to form hexagonal phase at 700 °C (YF-7) and ultimately converted to orthorhombic phase at 1000 °C (YF-10). Amongst others, YF-10 delivered best trace acetone sensing performance including a eight-fold response (S= Rg/Ra∼10.25 folds) to 1ppm acetone with ultrafast response-recovery time (∼0.6s-2s). Further, YF-10 sensor shows excellent selectivity, long-term stability more than 180 days, almost insensitive towards moisture and lower detection limit as low as 100 ppb. X-ray photoelectron spectroscopy (XPS) reveals that along with Fe3+ there exist Fe2+ ions which become maximum at 1000 °C. Formation of Fe2+ gives rise to oxygen vacancies which is maximum for YF-10 which is confirmed by PL and EPR studies. Enhanced acetone sensing performance of orthorhombic YFO is explained using quantitative phase analysis, honey-comb morphology, regulation of oxygen vacancies and bivalency of iron in different phases of yttrium ferrite. Eley−Rideal model has been invoked to explain the affinity towards acetone in comparison with other interfering gases. DFT calculation demonstrate that adsorption energy, a major parameter for gas sensing performance of any materials is lower for orthorhombic phase. In totality, it is observed that YFeO3 powder based sensor have huge potential for exhale breath analysis to monitor diabetes.

A new mixed-linker Fe-MOF as a multifunctional bio-photocatalyst for tandem photo-oxidation condensation reaction

A novel mixed-linker of iron-based metal–organic frameworks named Fe(INA)(H2N-BDC) has been directly synthesized by mixing 2-amino-1,4-benzenedicarboxylic acid (NH2-BDC) and isonicotinic acid (INA) using hydrothermal method. Its characterization was fully performed by a number of methods such as FT-IR, PXRD, XPS, FESEM, TEM, TGA, ICP and BET and used as a multifunctional bio-photocatalyst for the preparation of 2,4,6-triarylpyridines (TAPs) via tandem photo-oxidation condensation reactions starting from alcohols under sunlight irradiation and air atmosphere in ethanol. Mixed-linker Fe-MOF showed enhanced catalytic activity compared with its analogues with one single ligand, which indicates the synergistic effect of two linkers and Fe-O clusters in Fe(INA)(H2N-BDC) as a photocatalyst with lower band gap. This observation confirmed by an important reduction in the band gap energy and a rise in the characteristic emission of mixed-linker Fe-MOF compared with Fe-MIL-101-NH2 according to the DRS analysis and fluorescence spectrophotometry, respectively. EPR studies clearly indicated that Fe(INA)(H2N-BDC) produces 1O2 and O2•- as active oxygen units under sunlight illumination. Moreover, amine groups in NH2-BDC provide basic functional groups, which in cooperation with acidic sites (Fe in the metal nodes) promote the condensation of the in-situ formed aldehydes with acetophenone and ammonium acetate. Using sunlight irradiation as an energy-saving source, air as a safe oxidant, good yields, reusability of the photocatalyst and any requirement of additives make the present method as an environmentally-friendly method for the preparation of these important scaffolds. This is the first report of the synthesis of TAPs starting from alcohols catalyzed by MOFs.

EPR study of irradiated starch: induced radicals and dose assessment

Irradiation of starch is of much importance to get rid of microbes and for disinfection. EPR study of the ionizing radiation-induced radicals can lead to the assessment of radiation doses delivered during irradiation. The EPR spectra properties, microwave power saturation behavior, short- and long-term temporal dependencies, and other significant factors are studied. Response of EPR spectra of irradiated starch over the range (0.5–10 kGy) was investigated. The time dependence study of the radiation-induced signal radicals of starch showed that variation of the EPR signal can be noticed within ±7.7% relative to the average value over 38 days following irradiation. Results obtained indicate that gamma irradiated starch can be identified and radiation doses can be determined accurately using EPR spectroscopic techniques.

Site-directed spin label EPR studies of the structure and membrane interactions of the bacterial phospholipase ExoU.

Site-directed spin labeling (SDSL) has been invaluable in the analysis of protein structure and dynamics, and has been particularly useful in the study of membrane proteins. ExoU, an important virulence factor in Pseudomonas aeruginosa infections, is a bacterial phospholipase A2 that functions at the membrane - aqueous interface. Using SDSL methodology developed in the Hubbell lab, we find that the region surrounding the catalytic site of ExoU is buried within the tertiary structure of the protein in the soluble, apoenzyme state, but shows a significant increase in dynamics upon membrane binding and activation by ubiquitin. Continuous wave (CW) power saturation EPR studies show that the conserved serine hydrolase motif of ExoU localizes to the membrane surface in the active, holoenzyme state. SDSL studies on the C-terminal four-helix bundle (4HB) domain of ExoU similarly show a co-operative effect of ubiquitin binding and membrane association. CW power saturation studies of the 4HB domain indicate that two interhelical loops intercalate into the lipid bilayer upon formation of the holoenzyme state, anchoring ExoU at the membrane surface. Together these studies establish the orientation and localization of ExoU and the membrane surface, and illustrate the power of SDSL as applied to peripheral membrane proteins.

Metalloporphyrins of Oil from the Severnoe Buzachi Oil Field: UV, IR, and EPR Studies

Metalloporphyrins of Oil from the Severnoe Buzachi Oil Field: UV, IR, and EPR Studies

Spin-Injection in Graphene: An EPR and Raman Study.

In this article, the enrichment of graphene and graphene oxide with free radicals through their functionalization with tyrosine is studied. In contrast with what is commonly observed in the functionalization of graphene with organic species the addition of tyrosine radicals on to the graphene substrate led to a remarkable increase of the aromatic character as indicated by the spectroscopic data. Similar behaviour was observed for the functionalization of graphene oxide. In addition, a brief analysis of the tyrosine functionalized graphene with EPR spectroscopy showed a remarkable enhancement of the spin density that could be useful in spintronics.

Optical and EPR studies of Fe-doped CaAl12O19 near-infrared emitting phosphor

In this article we report our results on optical and electron paramagnetic resonance studies of Fe-doped CaAl12O19. At room temperature, optical excitation in the ultraviolet region leads to highly intensive luminescence signal at around 810 nm that is ascribed to the emission of Fe3+. Measurements at low temperature revealed slight shift of the emission maximum and appearance of additional wide luminescence bands in the blue and near-infrared parts of spectrum. The dependence of the luminescence intensity and decay time on temperature and concentration of Fe3+ in the material has been studied. Electron paramagnetic resonance studies showed presence of at least two different paramagnetic centres related to Fe3+. The nature of the observed luminescence bands is discussed.

Effect of para‐Substituents on meso‐Functionalized Oxidovanadium(IV) Porphyrins as Catalysts for Oxygen Atom Transfer Mediated Oxidation of Benzoin to Benzil under Mild Conditions

AbstractSystematic analysis of the effect of para‐substituents (H, Cl, Br and OMe) on the meso‐phenyl group in vanadyl meso‐tetraphenylporphyrins ([VIVO(TPP)] (R=H, 1), [VIVO(TCPP)] (R=Cl, 2), [VIVO(TBPP)] (R=Br, 3) and [VIVO(TMPP)] (R=OMe, 4)) on their properties and catalytic oxygen atom transfer (OAT) for oxidation of benzoin to benzil using DMSO as well as 30 % aqueous H2O2 as the sacrificial oxygen source have been studied. Electrochemical and theoretical (density functional theory) studies are in good agreement with the influence of these substituents on the catalytic property of these complexes. Complex [VIVO(TCPP)] (2) displayed the best catalytic activity for the conversion (92 %) of benzoin to benzil in 30 h with >99 % product selectivity when DMSO was used as an oxygen source, whereas excellent conversion (~100 %) of benzoin to benzil was noticed in 18 h with 95 % product selectivity when 30 % aqueous H2O2 was used as a source of oxygen. Furthermore, among these complexes, the electron‐withdrawing nature of the chloro substituent at the p‐position of meso‐phenyl group significantly influences the oxygen atom transfer. Experimental and simulated EPR studies confirmed the +4 oxidation of vanadium in these complexes. The structure of 2, 3 and 4, confirmed by single crystal X‐ray diffraction method, are domed in shape, and the displacement of V(IV) ion from the mean porphyrin plane follows the order: 2 (0.458 Å) <3 (0.459 Å) <4 (0.479 Å). We observed that the electron‐withdrawing nature of chloro substituent at the p‐position of meso‐phenyl group influence the oxygen atom transfer from vanadyl porphyrin to dimethyl sulfide much.

Cyclic Voltammetry as an Activation Method of TiO2 Nanotube Arrays for Improvement of Photoelectrochemical Water Splitting Performance

AbstractA facile and eco‐friendly method for activating anodic TiO2 nanotubes (TNTs) by cyclic voltammetry (CV) is proposed, and photoelectrochemical properties of CV‐activated TNTs are compared with those of non‐activated TNTs and of TNTs activated by hydrogen‐thermal reduction. EPR and luminescence studies show that the pristine samples demonstrate rather large content of paramagnetic and luminescing defects, while hydrogenation and CV‐activation lead to the different type of rearrangement of defects. TNTs activated by CV‐Na2SO4 demonstrate significantly improved photocurrent density (2.25 mA cm−2) in comparison with that of the hydrogen treated and pristine ones (0.93 mA cm−2 and 0.31 mA cm−2) under NUV‐irradiation at 0.2 V (RHE). Enhanced photoactivity of Na2SO4‐activated TNTs correlates with higher luminescence quantum yield, lowest paramagnetic defects content and larger decay time of the luminescence. Thus, a decrease in the content of defects is an important factor that reduces the non‐radiative recombination of charge carriers. The activation‐induced redistribution of surface and bulk defects in nanotubes explains the increased photoelectrochemical activity of TiO2‐based anodes. Cyclic voltammetry has been proved to be a reliable method to increase the efficiency of TNTs in PEC water splitting.

Solar-Driven Reforming of Methane and Nitrogen to Methanol and Ammonium on Iron-Modified Zeolite under Ambient Conditions in Water.

Artificial photosynthesis from selective methane oxidation or nitrogen reduction to value-added chemicals provides a promising pathway for the sustainable chemical industry, while still remaining a great challenge due to the extreme difficulty in C-H and N≡N bond cleavage under ambient conditions. Catalysts that can cocatalyze these two reactions simultaneously are rarely reported. Here, Fe-ZSM-5 with highly dispersed extra-framework Fe-oxo species enables efficient and selective photocatalytic conversion of methane and nitrogen to coproduce methanol and ammonia using H2O as the redox reagent under ambient conditions. The optimized Fe-ZSM-5 photocatalyst achieves up to 0.88 mol/molFe·h of methanol products with 97% selectivity. Meanwhile, the productivity of ammonia is 0.61 mol/molFe·h. In situ EPR and DRIFT studies disclose that water serves as a redox reagent to provide hydroxyl radicals for methane oxidation and protons for nitrogen hydrogenation. Quantum chemical calculations revealed that Fe-oxo species play a significant role in the coactivation of methane and nitrogen molecules, which lowers the energy barriers of rate-determining steps for methanol and ammonia generation.

Natural deep eutectic solvents of γ-valerolactone and formic acid for oxidative desulfurization with superior efficiency

Deep eutectic solvents (DESs) are widely used in oxidative desulfurization (ODS) to produce clean fuel. However, some DESs with good ODS efficiencies are generally consisted of harmful components, and the ODS efficiencies of most DESs with green components need to be further improved. Herein, a DES with two natural compounds of γ-valerolactone (γ-Val) and formic acid (FA) was prepared and used in ODS. The hydrogen bond interaction of DES was characterized by FT-IR, 1H NMR, DSC, and calculated by density functional theory (DFT) computation. The radical quenching experiments, EPR, and kinetic study were carried out to investigate the active radical and apparent activation energy of ODS process. The desulfurization efficiencies of γ-Val and FA (molar ratio 1: 4) DES was capable of reaching 100% of dibenzothiophene (DBT) removal within 100 min at an energy saving way of room temperature (25 °C) with a low O/S molar ratio of 2. When the temperature was raised to 70 °C and the O/S molar ratio was raised to 5, 100% sulfur removal of dibenzothiophene, 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzothiophene (BT) could be achieved in 4 min, 6 min, and 10 min respectively. DFT calculations revealed that the introduction of γ-Val could reduce the transition state energy barrier of ODS compared with a single FA ODS process. This work provides a novel high ODS efficiency DES with natural compounds, indicating the potential application of green DESs in ODS catalysts.

A Comparative Multi-Frequency EPR Study of Dipolar Interaction in Tetra-Heme Cytochromes.

Distances between Fe ions in multiheme cytochromes are sufficiently short to make the intramolecular dipole-dipole interaction between hemes probable. In the analysis of EPR data from cytochromes, this interaction has thus far been ignored under the assumption that spectra are the simple sum of non-interacting components. Here, we use a recently developed low-frequency broadband EPR spectrometer to establish the extent of dipolar interaction in the example cytochromes, characterize its spectral signatures, and identify present limitations in the analysis. Broadband EPR spectra of Shewanella oneidensis MR-1 small tetraheme cytochrome (STC) have been collected over the frequency range of 0.45 to 13.11 GHz, and they have been compared to similar data from Desulfovibrio vulgaris Hildenborough cytochrome c3. The two cases are representative examples of two very different heme topologies and corresponding electron-transfer properties in tetraheme proteins. While in cytochrome c3, the six Fe-Fe distances can be sorted into two well-separated groups, those in STC are diffuse. Since the onset of dipolar interaction between Fe-Fe pairs is already observed in the X-band, the g values are determined in the simulation of the 13.11 GHz spectrum. Low-frequency spectra are analyzed with the inclusion of dipolar interaction based on available structural data on mutual distances and orientations between all hemes. In this procedure, all 24 possible assignments of individual heme spectra to heme topologies are sampled. The 24 configurations can be reduced to a few, but inspection falls short of a unique assignment, due to a remaining lack of understanding of the fine details of these complex spectra. In general, the EPR analysis suggests the four-heme system in c3 to be more rigid than that in STC, which is proposed to be related to different physiological roles in electron transfer.

Negative bias in early and late cognitive processing of coronary heart disease patients with depressive symptoms: an EPR study

BackgroundThe purpose of this research was to explore the underlying mechanisms of cognitive impairments in patients with coronary heart disease (CHD) who exhibit depressive symptoms. This was accomplished by recording Event-related potentials (ERPs) during the emotional Stroop task, with a specific focus on the temporal dynamics of attentional bias towards various emotional words.MethodsWe selected 17 CHD patients with depressive symptoms and 23 CHD patients without depression using a convenience sampling method from the Affiliated Hospital of Weifang Medical University. Each participant completed an emotional Stroop color-word task, and ERPs were recorded during the task to examine cognitive processing.ResultsCHD patients with depressive symptoms exhibited generally smaller amplitudes of N1, N2, P3 and longer latency of P3 compared to CHD patients without depression. Specifically, the N1 amplitude of negative words was smaller and the P3 amplitude of negative words was larger in the CHD with depressive group compared to the CHD group. Furthermore, within the group of CHD patients with depressive symptoms, negative words elicited a smaller N1 amplitude and larger P3 amplitude compared to positive and neutral words.ConclusionsCHD patients with depressive symptoms demonstrate decreased attentional resources, leading to cognitive impairments. Notably, significant attentional bias occurs during both early and later stages of cognitive processing. This bias is primarily characterized by an enhanced automatic processing of negative information at the early stage and difficulty disengaging from such information at the later stage. These findings contribute to the existing literature on the cognitive neural mechanisms underlying depression in CHD patients.

Synergetic Radical-Scavenging effect in the complex of copper(II) with the thiosemicarbazone of Salicylaldehyde

The radical scavenging activities of salicylaldehyde thiosemicarbazone and its copper(II) and cadmium(II) complexes were evaluated by in vitro experiments in the reaction with 2,2'-diphenyl-1-picrylhydrazyl. Salicylic aldehyde thiosemicarbazone complex with Cu(II) shows significantly higher radical-scavenging activity than Cu2+ ions, salicylaldehyde thiosemicarbazone, and Cd(II) complex. Electrochemical, EPR and kinetic studies revealed that the high rate of reaction and efficiency is due to the mechanism of electron transfer, which is enhanced by the synergetic effect of the ligand structure and the ability of the copper ion to exhibit variable valencies.

Electron paramagnetic resonance spectroscopic characterization of the human KCNE3 protein in lipodisq nanoparticles for structural dynamics of membrane proteins

One of the major challenges in solubilization of membrane proteins is to find the optimal physiological environment for their biophysical studies. EPR spectroscopy is a powerful biophysical technique for studying the structural and dynamic properties of macromolecules. However, the challenges in the membrane protein sample preparation and flexible motion of the spin label limit the utilization of EPR spectroscopy to a majority of membrane protein systems in a physiological membrane-bound state. Recently, lipodisq nanoparticles or styrene-maleic acid copolymer-lipid nanoparticles (SMALPs) have emerged as a membrane mimetic system for investigating the structural studies of membrane proteins. However, its detail characterization for membrane protein studies is still poorly understood. Recently, we characterized the potassium channel membrane protein KCNQ1 voltage sensing domain (KCNQ1-VSD) and KCNE1 reconstituted into lipodisq nanoparticles using EPR spectroscopy. In this study, the potassium channel accessory protein KCNE3 containing flexible N- and C-termini was encapsulated into proteoliposomes and lipodisq nanoparticles and characterized for studying its structural and dynamic properties using nitroxide based site-directed spin labeling EPR spectroscopy. CW-EPR lineshape analysis data indicated an increase in spectral line broadenings with the addition of the styrene-maleic acid (SMA) polymer which approaches close to the rigid limit providing a homogeneous stabilization of the protein-lipid complex. Similarly, EPR DEER measurements indicated an enhanced quality of distance measurements with an increase in the phase memory time (Tm) values upon incorporation of the sample into lipodisq nanoparticles, when compared to proteoliposomes. These results agree with the solution NMR structural structure of the KCNE3 and EPR studies of other membrane proteins in lipodisq nanoparticles. This study along with our earlier studies will provide the reference characterization data that will provide benefit to the membrane protein researchers for studying structural dynamics of challenging membrane proteins.

EPR, optical and physical studies on Mn2+-doped MgO–BaO–B2O3 glasses

EPR, optical and physical studies on Mn2+-doped MgO–BaO–B2O3 glasses

Insight into Radical Initiation, Solvent Effects, and Biphenyl Production in Iron-Bisphosphine Cross-Couplings.

Iron-bisphosphines have attracted broad interest as highly effective and versatile catalytic systems for two- and three-component cross-coupling strategies. While recent mechanistic studies have defined the role of organoiron(II)-bisphosphine species as key intermediates for selective cross-coupled product formation in these systems, mechanistic features that are essential for catalytic performance remain undefined. Specifically, key questions include the following: what is the generality of iron(II) intermediates for radical initiation in cross-couplings? What factors control reactivity toward homocoupled biaryl side-products in these systems? Finally, what are the solvent effects in these reactions that enable high catalytic performance? Herein, we address these key questions by examining the mechanism of enantioselective coupling between α-chloro- and α-bromoalkanoates and aryl Grignard reagents catalyzed by chiral bisphosphine-iron complexes. By employing freeze-trapped 57Fe Mössbauer and EPR studies combined with inorganic synthesis, X-ray crystallography, reactivity studies, and quantum mechanical calculations, we define the key in situ iron speciation as well as their catalytic roles. In contrast to iron-SciOPP aryl-alkyl couplings, where monophenylated species were found to be the predominant reactive intermediate or prior proposals of reduced iron species to initiate catalysis, the enantioselective system utilizes an iron(II)-(R,R)-BenzP* bisphenylated intermediate to initiate the catalytic cycle. A profound consequence of this radical initiation process is that halogen abstraction and subsequent reductive elimination result in considerable amounts of biphenyl side products, limiting the efficiency of this method. Overall, this study offers key insights into the broader role of iron(II)-bisphosphine species for radical initiation, factors contributing to biphenyl side product generation, and protocol effects (solvent, Grignard reagent addition rate) that are critical to minimizing biphenyl generation to obtain more selective cross-coupling methods.