Among the intermediate catalytic steps of the water‐oxidizing Mn4CaO5 cluster of photosystem II (PSII), the final metastable S3 state is critically important because it binds one substrate and ...precedes O2 evolution. Herein, we combine X‐ and Q‐band EPR experiments on native and methanol‐treated PSII of Spinacia oleracea and show that methanol‐treated PSII preparations of the S3 state correspond to a previously uncharacterized high‐spin (S=6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate‐spin conformation (S=3). The high‐spin intermediate is assigned to a water‐unbound form, with a MnIV3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated MnIV ion. These results resolve and define the structural heterogeneity of the S3 state, providing constraints on the S3 to S4 transition, on substrate identity and delivery pathways, and on the mechanism of O−O bond formation.
Electron paramagnetic resonance spectroscopy reveals that the final metastable catalytic state (S3) in biological water oxidation in higher plants is a mixture of a high‐spin and an intermediate‐spin form. These two forms represent distinct structural components related to water binding and activation on the path towards oxygen evolution.
We report here on the implementation of arbitrary waveform generation (AWG) capabilities at ∼200 GHz into an Electron Paramagnetic Resonance (EPR) and Dynamic Nuclear Polarization (DNP) instrument ...platform operating at 7 Tesla. This is achieved with the integration of a 1 GHz, 2 channel, digital to analogue converter (DAC) board that enables the generation of coherent arbitrary waveforms at K
u
-band frequencies with 1ns resolution into an existing architecture of a solid state amplifier multiplier chain (AMC). This allows for the generation of arbitrary phase- and amplitude-modulated waveforms at 200 GHz with > 150 mW power. We find that the non-linearity of the AMC poses significant difficulties in generating amplitude-modulated pulses at 200 GHz. We demonstrate that in the power-limited regime of ω
1
< 1MHz phase-modulated pulses were sufficient to achieve significant improvements in broadband (>10MHz) spin manipulation in incoherent (inversion), as well as coherent (echo formation) experiments. Highlights include the improvement by one order of magnitude in inversion bandwidth compared to that of conventional rectangular pulses, as well as a factor of two in improvement in the refocused echo intensity at 200GHz.
There have recently been advances in methods for detecting local secondary structures of membrane protein using electron paramagnetic resonance (EPR). A three pulsed electron spin echo envelope ...modulation (ESEEM) approach was used to determine the local helical secondary structure of the small hole forming membrane protein, S21 pinholin. This ESEEM approach uses a combination of site-directed spin labeling and 2H-labeled side chains. Pinholin S21 is responsible for the permeabilization of the inner cytosolic membrane of double stranded DNA bacteriophage host cells. In this study, we report on the overall global helical structure using circular dichroism (CD) spectroscopy for the active form and the negative-dominant inactive mutant form of S21 pinholin. The local helical secondary structure was confirmed for both transmembrane domains (TMDs) for the active and inactive S21 pinholin using the ESEEM spectroscopic technique. Comparison of the ESEEM normalized frequency domain intensity for each transmembrane domain gives an insight into the α-helical folding nature of these domains as opposed to a π or 310-helix which have been observed in other channel forming proteins.
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•A three pulse ESEEM approach is applied to determine the local helical secondary structure of hole forming membrane protein, S21 pinholin.•The local helical secondary structure was confirmed for both transmembrane domains (TMDs) of S21 pinholin using the ESEEM spectroscopic technique.•The use of this ESEEM technique is advantageous over other conventional biophysical structural techniques.
Nitrate is a pervasive aquatic contaminant of global environmental concern. In nature, the most effective nitrate reduction reaction (NRR) is catalyzed by nitrate reductase enzymes at neutral pH, ...using a highly‐conserved Mo center ligated mainly by oxo and thiolate groups. Mo‐based NRR catalysts mostly function in organic solvents with a low water stability. Recently, an oxo‐containing molybdenum sulfide nanoparticle that serves as an NRR catalyst at neutral pH was first reported. Herein, in a nanoparticle‐catalyzed NRR system a pentavalent MoV(=O)S4 species, an enzyme mimetic, served as an active intermediate for the NRR. Potentiometric titration analysis revealed that a redox synergy among MoV−S, S radicals, and MoV(=O)S4 likely play a key role in stabilizing MoV(=O)S4, showing the importance of secondary interactions in facilitating NRR. The first identification and characterization of an oxo‐ and thiolate‐ligated Mo intermediates pave the way to the molecular design of efficient enzyme mimetic NRR catalysts in aqueous solution.
In neutral: The nitrate reduction reaction (NRR) is essential for water remediation. In an artificial NRR catalyst, an enzyme mimetic intermediate has been identified for the first time in the activation of the NRR at neutral pH. The newly identified intermediate is stable in neutral water, and the secondary electronic interactions were found to play a key role.
Three different perfluoroalkylated borafluorenes (FBf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the ...borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo‐aryl moieties. They differ with regard to the para substituents on their exo‐aryl moieties, being a proton (FXylFBf, FXyl: 2,6‐bis(trifluoromethyl)phenyl), a trifluoromethyl group (FMesFBf, FMes: 2,4,6‐tris(trifluoromethyl)phenyl) or a dimethylamino group (p‐NMe2‐FXylFBf, p‐NMe2‐FXyl: 4‐(dimethylamino)‐2,6‐bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron‐deficient derivative FMesFBf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor‐substituted derivative p‐NMe2‐FXylFBf exhibits thermally activated delayed fluorescence (TADF) from a charge‐transfer (CT) state, whereas the FMesFBf and FXylFBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition‐dipole moments.
Having your cake and eating it too: Functionalization of 9‐borafluorenes with trifluoromethyl groups makes them exceptionally easy to reduce while maintaining excellent stability towards hydrolysis. The systems also exhibit long‐lived fluorescence or thermally activated delayed fluorescence (TADF) depending on the exo‐aryl para‐substituent.
•EPR spectroscopy is a reliable tool for studying EM-heating effect on heavy oil EOR.•EM heating increases saturated hydrocarbons in heavy oil after 50 min exposure.•EM heating decreases resins and ...asphaltenes in heavy oil after 50 min exposure.•EM heating improves normal in heavy oil by alkyl substituents elimination.
Heavy and extra-heavy oil are generating considerable interest in terms of replacing light oil role in the next few years. Nowadays, various enhanced oil recovery (EOR) methods are under development for an effective exploration and oil refining of unconventional oil sources. Electrical methods are among the most commonly discussed types of EOR techniques. Despite this interest, no one as far as we know has studied the effectiveness and mechanisms of electromagnetic (EM) heating influence on heavy oil conversion. It is common knowledge that heavy oils often contain stable paramagnetic centers, which potentially can serve as at least signaling species to follow the EOR upgrading degree. For the first time, to the best of our knowledge, pulsed and high-frequency (high-field, with 3.4 T detection magnetic field) electron paramagnetic resonance (EPR) techniques were exploited to investigate the EM heating influence on Bazhenov formation heavy oil by subjecting it to 50 Hz (0.75 T) electromagnetic field in laboratory conditions. The obtained samples were analyzed by a set of physico-chemical methods including SARA analysis (S: Saturates, A: Aromatics, R: Resins, A: Asphaltenes), gas chromatography–mass spectrometry (GCMS) in addition to pulsed and conventional (cw) EPR spectroscopy. SARA analysis has shown an increase in the content of asphaltenes from 1.2 ± 0.3 wt% in the initial oil to 1.4 ± 0.4, 1.4 ± 0.3, 1.3 ± 0.2, 1.3 ± 0.5 wt% in the samples obtained at 10, 20, 30 and 40 min respectively. Interestingly, this content has drastically dropped to 0.2 ± 0.05 and 0.3 ± 0.02 wt% in the samples obtained at 50 and 60 min of EM heating. Analogically for resins content change which has firstly increased from 1.5 ± 0.5 wt% in the initial oil to 1.9 ± 0.5, 2.6 ± 0.6, 3.0 ± 0.2, 3.0 ± 0.6 wt% in the samples obtained at 10, 20, 30 and 40 min respectively. Then this content has decreased to 0.5 ± 0.05 and 1.0 ± 0.04 wt% in the samples obtained at 50 and 60 min of EM heating. The content of saturated and aromatic compounds has been found to reach 99.3 ± 0.6 wt% in the sample obtained at 50 min comparing to 97.3 ± 1.1 wt% in the initial oil. Moreover, the GCMS data have showed a significant increase in the content of normal alkanes, which resulted from the processes of oil high molecular compounds fractions conversion as a consequence of alkyl substituents elimination. Our data points towards the idea that the investigated oil sample EPR signal intensity and their spectroscopic characteristics do not change with the EM heating either in the X- or W-band frequencies. g|| and g⊥ were found equal to 2.0024 and 2.0015 respectively at 3357 and 3358 mT for the initial sample and the obtained sample after 60 min of EM heating. Similar results (g||=2.0004 and g⊥=1.9991) were obtained for these samples at 3360 and 3362 mT. The findings of this study can help for better understanding of the mechanisms ruling the impact of electromagnetic heating on enhancing oil recovery.
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► An energy efficient synthesis of Fe
3O
4 nanoparticles. ► Annealing effect on morphology and magnetism of Fe
3O
4 nanoparticles. ► EPR study confirms the super paramagnetic nature ...of Fe
3O
4 nanoparticles. ► Activation energy and order of weight losses of Fe
3O
4 nanoparticles determined by TGA. ► Fe
3O
4 nanoparticles exhibit superior super paramagnetic properties.
The present article reports an energy efficient method for the synthesis of superparamagnetic ferrite (Fe
3O
4) nanoparticles (10–40
nm) and their annealing effect on the morphology, size, curie temperature and magnetic behavior at 50, 300, 400 and 500
°C. The synthesized nanoparticles were characterized by various spectroscopic techniques like FT-IR and UV–visible. The crystalline structure and particle size were estimated through solid phase as well as the liquid phase using XRD, TEM and DLS techniques. Superparamagnetic behavior of nanoparticles was confirmed by VSM. The EPR study reveals that the main feature of X-Band solid state EPR spectrum has strong transition at
g
eff
∼
3.23 (2100G) and a relatively weak transition at
g
eff
∼
2.05 (3300G). The later transition further confirms the super paramagnetic nature of these nano ferrites. The activation energy and order of weight losses of nano ferrites were found to be: 39.6
KJ
mol
−1 and 0.21 orders (600–800
°C), respectively, analyze with the help of TGA while the specific surface area (23.1
m
2
g
−1) and pore size (9
Å) were determined by Quanta chrome BET instrument.
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•Binuclear, polymeric binuclear, and 16-nuclear copper(II) complexes with Chinese lantern building blocks were studied by EPR.•EPR spectroscopy studies were performed in the X-, Q- ...and W-ranges.•Electronic exchange between two copper(II) atoms forming dimers were found.•No interactions between adjacent binuclear fragments were revealed.
In this work, binuclear, polymeric binuclear, and molecular 16-nuclear copper(II) complexes Py2Cu2(Piv)4 (1), {(EtOH)2Cu2(Chda)2·EtOH}n (2) and K2(H2O)6(Py)8Cu16(OH)2(Chda)16·EtOH·MeCN·3H2O (3) (piv is pivalate, Chda is 1,1-cyclohexanediacetate) have been studied by EPR spectroscopy in the X-, Q- and W-bands. Polynuclear copper(II) complexes are formed by binuclear fragments bonded by bridging carboxylate anions. In compounds 1–3, only pairwise interactions between copper ions are observed, despite the fact that in 2 and 3 there may be additional interactions between copper ions belonging to different dimers.
We report the first use of N‐trifluoromethylthiosaccharin as the source of SCF3 radical under photoredox catalysis. This allowed an efficient and general visible‐light‐mediated ...carbotrifluoromethylthiolation of alkenes. Under the optimized conditions using fac‐Ir(ppy)3 as the photocatalyst, various N‐aryl acrylamides as well as a wide range of substituted styrenes can readily be difunctionalized in an intra‐ or intermolecular fashion, affording the corresponding SCF3‐containing products in good to excellent yield. Importantly, the formation of this SCF3 radical along with other key radical intermediates was unambiguously demonstrated thanks to spin trapping/electron paramagnetic resonance (ST/EPR) experiments, which enabled a clear understanding of the reaction mechanism.
Electrophilic N‐trifluoromethylthiosaccharin was used for the first time as the source of the SCF3 radical. The formation of this radical, as well as other key intermediates, was highlighted by using spin trapping/EPR spectroscopy. Visible‐light‐promoted intra‐ and intermolecular carbotrifluoromethylthiolation of alkenes has been successfully developed.
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