Quadrupole Splitting Research Articles

Особенности магнитной структуры поликристаллов Y-=SUB=-3-=/SUB=-Fe-=SUB=-5-=/SUB=-O-=SUB=-12-=/SUB=-, синтеризованных методом радиационно-термического спекания

The Mössbauer spectroscopy (MS) method was used to study polycrystalline iron yttrium garnet (YIG) samples synthesized by radiation thermal sintering (RTS) technology and by standard ceramic technology (CT). The best option for decomposing the MS spectra of the objects of study was selected, which is a model of the experimental spectrum with five sextets. An additional fifth sextet is caused by Fe3 + ions, surrounded by oxygen vacancies which lead to distortion of the Fe tetrahedra which reflected by an increase in the quadrupole splitting of Fe3+. An increase in the density of s-electrons on Fe ions in distorted tetrahedra lead to a decrease in the isomeric chemical shift δ of Fe ions up to a values closed to the δ value for Fe4+ ions. It was shown that the optimal crystalline structure realizes for Y3Fe5O12 polycrystals upon sintering by RTS method in the temperature range of 1350–1400 ° C for a time from 40 to 60 min.

Comparative Study of the Electronic Structures of μ-Oxo, μ-Nitrido, and μ-Carbido Diiron Octapropylporphyrazine Complexes and Their Catalytic Activity in Cyclopropanation of Olefins.

The electronic structure of three single-atom bridged diiron octapropylporphyrazine complexes (FePzPr8)2X having Fe(III)-O-Fe(III), Fe(III)-N-Fe(IV) and Fe(IV)-C-Fe(IV) structural units was investigated by Mössbauer spectroscopy and density functional theory (DFT) calculations. In this series, the isomer shift values decrease, whereas the values of quadrupole splitting become progressively greater indicating the increase of covalency of Fe-X bond in the μ-oxo, μ-nitrido, μ-carbido row. The Mössbauer data point to low-spin systems for the three complexes, and calculated data with B3LYP-D3 show a singlet state for μ-oxo and μ-carbido and a doublet state for μ-nitrido complexes. An excellent agreement was obtained between B3LYP-D3 optimized geometries and X-ray structural data. Among (FePzPr8)2X complexes, μ-oxo diiron species showed a higher reactivity in the cyclopropanation of styrene by ethyl diazoacetate to afford a 95% product yield with 0.1 mol % catalyst loading. A detailed DFT study allowed to get insight into electronic structure of binuclear carbene species and to confirm their involvement into carbene transfer reactions.

Effect of etching on spin canting in hydrothermally synthesized Co-Ni ferrite particles

Spinel cobalt-nickel ferrite nanoparticles, having a particle size of around 40 nm, were produced by undergoing a succession of synthesis routes comprised of chemical co-precipitation, hydrothermal treatment (HT), and etching in hydrochloric acid (ET) of concentration 2.0 mol/L. The room temperature high-field Mossbauer spectra were analyzed in detail and significant spin canting associated with Fe3+ ions at both A- and B-sites were observed for both the HT and ET samples in the presence of external magnetic field of 5 T. The effect of etching on various Mossbauer parameters such as the hyperfine field distribution, isomer shift, quadrupole splitting and the linewidth was noted. The A site and B site canting angles of the HT sample were respectively 17° and 22°, while canting angle of approximately 13° was observed for both A and B sites in the ET sample.

57Fe conversion electron Mössbauer study on tin oxide films doped with dilute two metal ions

Tin dioxide (SnO2) films codoped with 1% mole ratio of (Co, Fe) ions, (Cu, Fe) ions, and (Ti, Fe) ions were prepared on SnO2 (1% Sb) thin film on glass by a spray pyrolysis. Fe species included in these films were characterized by 57Fe conversion electron Mossbauer (CEM) spectra measured at room temperature (RT) and low temperatures (50 K and 16 K). Three paramagnetic Fe3+ species were observed in RT CEM spectra of two metal ions codoped SnO2 films. The broad magnetic sextet was observed in 16 K CEM spectra of (Co, Fe) codoped SnO2 and (Cu, Fe) codoped SnO2 films, but not even at low temperature for (Ti, Fe) codoped SnO2 film. The relationship between isomer shift and quadrupole splitting were shown and compared with the results of ab initio calculations based on the configuration models of metal ions and oxygen defects.

Preparation and characterization of spin crossover thin solid films

Iron(II) spin crossover complexes display a reversible transition from low-spin (LS) state to high-spin (HS) state by e.g. variation of temperature, pressure or by irradiation with light. Therefore, these systems are promising candidates for information storage materials. In view of practical device applications thin films of these materials are needed. The SCO-compound [Fe(Htrz)2(trz)] (BF4) (1) switches between the LS and the HS state with a 50 K wide thermal hysteresis loop above room temperature. We have prepared thin films of 1 on a SiO2 substrate by spin coating. The spin states of the films have been characterized by Mossbauer spectroscopy in reflection mode using a MIMOS II spectrometer. A low quadrupole splitting (LS state) at 300 K and a high quadrupole splitting (HS state) at 400 K were found for the film, as well as for bulk powder of 1. This confirms that a spin crossover occurs above room temperature. Furthermore, synchrotron based nuclear resonance scattering measurements from 80 K to 400 K indicate that the hyperfine parameters are similar to those of the bulk powder of 1. DFT calculations reproduce the experimentally determined Fe-vibrational density of states of the bulk and of the thin film sample of 1. These results indicate that a higher fraction of HS Fe atoms is present in the film of 1. Therefore, we conclude different SCO properties of the thin film and the bulk material of 1.

A 119Sn Mössbauer-spectroscopic characterization of the diamagnetic birefringence material Sn2B5O9Cl

Abstract The crystal structure of diamagnetic borate chloride Sn2B5O9Cl exhibits two crystallographically independent tin sites which both show pronounced lone-pair activity of the tin atoms. This is reflected in substantial quadrupole splitting in the 119Sn Mössbauer spectrum. The isomer shifts of 3.887(8) and 4.137(7) mm s−1 clearly indicate divalent tin. In agreement with bond valence calculations, the Sn1 atoms have a lower charge and the higher isomer shift, compatible with a higher electron density at the tin nuclei.

Magnetically recoverable catalyst based on porous nanocrystalline HoFeO3 for processes of n-hexane conversion

Nowadays, a wide class of rare earth orthoferrites is of great interest among acid-base catalysts due to their high activity and the possibility of magnetic recovery. However, the synthesis of chemically and phase-pure rare-earth orthoferrites in the form of nanocrystals with developed surface and microstructure is still a difficult task. In this work, we report for the first time a successful synthesis of phase-pure holmium orthoferrite (HoFeO3) nanocrystals with a porous microstructure and a branched surface via simple glycine-nitrate combustion approach. Holmium orthoferrite nanocrystals were characterized by using energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), 57Fe Mössbauer spectroscopy and low-temperature sorption-desorption of nitrogen. Acid-base properties of the surface and catalytic activity of porous HoFeO3 in process of n-hexane conversion were analyzed as well. Finally, a magnetic recovery procedure was performed for the spent catalyst to analyze its efficiency for synthesized HoFeO3. The elemental composition of the sample corresponds to holmium orthoferrite (48.8 and 51.2 at. %, for Ho and Fe, respectively) with the absence of any impurities as it was shown by EDX. The PXRD results show that the synthesized HoFeO3 nanocrystals have an orthorhombic structure (space group Pbnm) and an average crystallite size of 62 ± 5 nm. The Rietveld method was used to refine the unit cell parameters as follows: a = 5.254(1) Å, b = 5.575(3) Å, c = 7.598(4) Å; Rwp = 2.45%. The Mössbauer spectrum of HoFeO3 evidenced the magnetically ordered state of nanocrystals and it is represented by a sextet with quadrupole splitting (QS), of 0 mm/s, an isomeric shift (IS) of 0.36 mm/s, and an effective magnetic field (Heff) of 497 kOe. Sorption-desorption of nitrogen shows that the holmium orthoferrite has a predominantly macro- and mesoporous structure, also confirmed by SEM, with specific surface area and total porosity values of 31 m2/g and 0.071 cm3/g, respectively. The analysis of the acid-base properties of the surface of the porous HoFeO3 evidences a high concentration of the aprotic (Lewis) acidic (pKa = 14.2) and basic (pKa = -4.4) sites, as well as of the Brønsted weak acid (pKa = 6.4) sites. The catalytic activity of holmium nanocrystalline in process of n-hexane conversion was analyzed and compared with commercial dehydrogenation (CD-1) and cracking (ZSM-5) catalysts. The n-hexane conversion and cracking products selectivity displayed by HoFeO3 are comparable to those of the reference catalysts, and the selectivity to dehydrogenation and especially isomerization products significantly exceeds the values of CD-1 and ZSM-5; low n-hexane oligomerization product selectivity of HoFeO3 catalyst was observed as well. A high recovery rate (~97%) achieved as a result of the magnetic separation of the spent o-HoFeO3-based catalyst from the reaction mixture, followed by heat treatment at 600 °C for 1 h. These results allow us to conclude that synthesized porous HoFeO3 can be considered as a promising basis for new catalytic materials for cracking, dehydrogenation and isomerization processes with a possibility of highly efficient magnetic recovery.

High-pressure X-ray diffraction and Mössbauer spectroscopy study of Fe1.087Te

The compression mechanism of Fe1.087Te was studied by high-pressure X-ray diffraction at ambient temperature. Fe1.087Te retains tetragonal symmetry up to the highest measured pressure of 25.67 GPa and the structural parameters obtained by high-pressure X-ray diffraction were used as input parameters for electronic structure calculation at the DFT level. The electronic structure calculations show that the chemical bonding between Te atoms across the van der Waals gap is enhanced at elevated pressures and the Fermi surface undergoes at topological change at ≈4 GPa reflecting a change from 2- to 3-dimensional character of the electronic structure. Mössbauer spectra of Fe1.087Te recorded at elevated pressures and ambient temperature showed no indication of a pressure-induced change of the spin-state and the obtained isomer shifts and quadrupole splittings are consistent with Fe2+ ion with S = 1.

Radiofrequency excitation-related 23 Na MRI signal loss in skeletal muscle, cartilage, and skin.

To assess the sodium MRI signal loss resulting from typically used RF excitation pulses in human skeletal muscle, patellar cartilage, and skin. A double flip-angle experiment was performed 3 times on the knees of 5 healthy volunteers with prescribed ω1 = γB1 of 1.67 kHz, 0.333 kHz, and 0.167 kHz. This was done to search for ω1 -dependent increased rates of sodium-23 central resonance flipping known to result from residual quadrupole splitting (ωQ ), as this flip-angle effect is associated with signal loss. This study facilitated in vivo regression of Gaussian-distributed residual quadrupole splitting SD (ωQ(SD) ) as well as T2fast and T2slow . Signal loss predicted from simulation was then compared with images acquired using 90° RF pulse lengths of 0.5 ms, 0.25 ms, and 0.15 ms. Sodium-23 central resonance flipping was significantly greater than prescribed (44% cartilage, 23% skin, 9% muscle) using ω1 = 0.167 kHz, but only 4% cartilage, 5% skin, and 2% muscle using ω1 = 1.67 kHz. Regression yielded ωQ(SD) = 420 ± 50 Hz for cartilage but no significant ωQ(SD) for skin or muscle. This points to rapid biexponential relaxation as the cause of the flip-angle effect for skin/muscle. The T2fast(60%) /T2slow(40%) values were 1.6 ± 0.8 ms/16.1 ± 2.5 ms for muscle, 2.7 ± 0.9 ms/18.4 ± 2.5 ms for cartilage, and 0.4 ± 0.1 ms/9.3 ± 1.7 ms for skin. Simulation predicted signal loss of 6% ± 3%, 3% ± 1%, and 2% ± 1% for muscle, 16% ± 3%, 6% ± 1%, and 3% ± 1% for cartilage, and 26% ± 7%, 15% ± 4%, and 10% ± 3% for skin when using 90° RF pulse lengths of 0.5 ms, 0.25 ms, and 0.15 ms, matching experiment. High-power (short) RF pulses are necessary to reduce excitation-related signal loss, particularly for sodium-23 imaging of cartilage and skin.

Electric Hyperfine Interactions of 57Fe Impurity Atoms in ACrO3 Perovskite-Type Chromites (A = Sc, In, Tl, Bi)

Perovskite-type chromites ACr0.9557Fe0.05O3 (A = Sc, In, Tl, Bi) have been studied by probe Mossbauer spectroscopy on 57Fe nuclei. The spectra of the ACr0.9557Fe0.05O3 samples (A = Sc, In, Bi) measured above the Neel temperature TN indicate the presence of several nonequivalent positions occupied by impurity iron atoms in all chromites. To explain significant differences in quadrupole splittings, semi-empirical calculations of the electric field gradient have been performed for all studied compounds within the ionic model taking into account monopole and dipole contributions. The performed calculations are in satisfactory agreement with experimental data.

A Mössbauer spectroscopy study of Fe based cemented carbides

Mössbauer spectroscopy has been used as a novel characterization technique to investigate Fe charge states, Fe complexes and hyperfine interaction parameters of different phases in WC-10Fe and WC-10(FeNi) materials sintered at three different temperatures (1350, 1430 and 1510 °C). The materials were characterized using standard cemented carbide quality control, and spectroscopic techniques to evaluate the structural changes and magnetic effects of the binder. The WC-10Fe grade had the highest Vickers hardness ranging from 1282 to 1320 HV30, for the different sintering temperatures. X-ray diffraction data showed the presence of WC and the metal binder phase, α-Fe and γ-FeNi. Transmission Mӧssbauer spectroscopy spectra obtained for the milled powders revealed only the α-Fe phase with a hyperfine magnetic field, Bhf ~33 T. Conversion electron Mössbauer spectroscopy on the sintered compacts revealed the presence of multiple fields, suggesting the possibility of minor phases present in the binder which were not detected using X-ray diffraction. The corresponding spectra for the sintered WC-Fe grades exhibited two magnetic fields with hyperfine parameters of ~33 T and ~17 T, respectively. These fields were assigned to α-Fe with some W atoms in solution and a W-rich Fe phase, respectively. The Mӧssbauer spectrum for the FeNi binder sample at the lowest sintering temperature of 1340 °C showed a paramagnetic doublet with an isomer shift, δ = −0.08 mm/s and electric quadrupole splitting, ∆EQ = 0.00 mm/s and a weak hyperfine magnetic field, Bhf = 15.7 T. The doublet has been assigned to γ-FeNi and the magnetic component to a W-rich γ-FeNi phase. For the higher sintering temperature, a distribution of magnetic fields (~33 T, 25 T and 9 T) was evident in the Mössbauer spectrum. These magnetic fields were tentatively assigned to multiple W-rich γ-FeNi phases.

Quadrupole coupling in alkali metal amides MNH2 ([formula omitted]A1): An experimental and computational study

Rotational spectra of LiNH2 and NaNH2 have been recorded using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the range 22 – 59 GHz. The species were created from the reaction of metal vapor and ammonia, diluted in argon, using a Discharge-Assisted Laser Ablation Source (DALAS). The JKa,Kc = 101 → 000 transition was measured for both molecules, as well as the JKa,Kc = 202 → 101 transition for NaNH2, all of which exhibited quadrupole coupling splittings. The two data sets were each analyzed with an S-reduced asymmetric top Hamiltonian, establishing the lithium and sodium electric quadrupole coupling parameter, χaa for the first time, and refining previous rotational constants. Quadrupole and nuclear-spin rotation interactions were also computationally investigated at the MP2/6-311G++(3df,2pd) level for LiNH2, NaNH2 and KNH2. These calculations suggest that the major contributor to the quadrupole interactions is the alkali metal nucleus, not that of nitrogen, as confirmed experimentally. Comparison of quadrupole coupling constants suggest that LiNH2 and NaNH2 are principally ionic molecules with a charge distribution similar to LiF and NaF.

Density Functional Theory (DFT)-Based Bonding Analysis Correlates Ligand Field Strength with 99Ru Mössbauer Parameters of Ruthenium-Nitrosyl Complexes.

We applied density functional theory calculations to ruthenium-nitrosyl complexes, which are known to exist in high-level radioactive waste generating during reprocessing of spent nuclear fuel, to give a theoretical correlation between 99Ru Mössbauer spectroscopic parameters and ligand field strength for the first time. The structures of the series of complexes, [Ru(NO)L5] (L = Br-, Cl-, NH3, CN-), were modeled based on the corresponding single-crystal X-ray coordinates. The comparisons of the geometries and total energies between the different spin states suggested that the singlet spin state of [Ru(II)(NO+)L5] complexes were the most stable. This result was supported by the benchmark calculations of the 99Ru Mössbauer isomer shift (δ) and quadrupole splitting (ΔEQ) values. The calculated results of both the δ and ΔEQ values reproduced the experimental results by reported previously and increased in the order of L = Br-, Cl-, NH3, CN-. Finally, we estimated the ligand field strength (Δo) based on molecular orbitals, assuming C4v symmetry and showed the increase of Δo values in that order, being consistent with well-known spectrochemical series of ligands. The increase attributes to the strengthening of the abilities of σ-donor and π-acceptor of the L-ligands to the Ru atom, resulting in the increase of the δ values. Furthermore, the increase of the σ-type donation into Ru dx2-y2 orbital and the π-type back-donation from Ru dxz, dyz orbitals in that order caused the decrease of the electron density along the Ru-NO axis, resulting in the increase of the ΔEQ values. This study is expected to contribute to the ligand design for the ruthenium-nitrosyl complexes, leading to the drug design for NO carrier and the decontamination of radioactive ruthenium from the ecological system, as well as the recovery of platinum-group metals from high-level radioactive waste.

A study of defect structures in Fe-alloyed ZnO: Morphology, magnetism, and hyperfine interactions

In order to study the effect of Fe cation substitution on the local structure, defect formation, and hyperfine interactions in ZnO, Mössbauer spectroscopy measurements of the microwave processed Zn1−xFexO (x=0.05, 0.10, 0.15, and 0.20) nanoparticles, together with ab initio calculations, were performed. Complementary information on the distribution of particle size and morphology, as well as magnetic properties, were obtained by X-ray diffraction, transmission electron microscopy, and squid-magnetometry. The selected model for analyzing the Mössbauer spectra of our samples is a distribution of quadrupole splittings. The fitting model with two Lorentz doublets was rejected due to its failure to include larger doublets. The Fe3+ ions do not yield magnetic ordering in the samples at room temperature. The results from first-principles calculations confirm that the major component of the Mössbauer spectra corresponds to the Fe-alloyed ZnO with Zn vacancy in the next nearest neighbor environment. The magnetic measurements are consistent with the description of the distribution of iron ions over the randomly formed clusters in the ZnO host lattice. While at room temperature all the samples are paramagnetic, magnetic interactions cause a transition into a cluster spin-glass state at low temperatures.

Influence of Lipid Saturation, Hydrophobic Length and Cholesterol on Double-Arginine-Containing Helical Peptides in Bilayer Membranes.

Membrane proteins are essential for many cell processes yet are more difficult to investigate than soluble proteins. Charged residues often contribute significantly to membrane protein function. Model peptides such as GWALP23 (acetyl-GGALW5 LAL8 LALALAL16 ALW19 LAGA-amide) can be used to characterize the influence of specific residues on transmembrane protein domains. We have substituted R8 and R16 in GWALP23 in place of L8 and L16, equidistant from the peptide center, and incorporated specific 2 H-labeled alanine residues within the central sequence for detection by solid-state 2 H NMR spectroscopy. The resulting pattern of [2 H]Ala quadrupolar splitting (Δνq ) magnitudes indicates the core helix for R8,16 GWALP23 is significantly tilted to give a similar transmembrane orientation in thinner bilayers with either saturated C12:0 or C14:0 acyl chains (1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)) or unsaturated C16:1 Δ9 cis acyl chains. In bilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC; C18:1 Δ9 cis) multiple orientations are indicated, whereas in longer, unsaturated 1,2-dieicosenoyl-sn-glycero-3-phosphocholine (DEiPC; C20:1 Δ11 cis) bilayers, the R8,16 GWALP23 helix adopts primarily a surface orientation. The inclusion of 10-20 mol % cholesterol in DOPC bilayers drives more of the R8,16 GWALP23 helix population to the membrane surface, thereby allowing both charged arginines access to the interfacial lipid head groups. The results suggest that hydrophobic thickness and cholesterol content are more important than lipid saturation for the arginine peptide dynamics and helix orientation in lipid membranes.

Berichtigung: Bioinspired, Size‐Tunable Self‐Assembly of Polymer—Lipid Bilayer Nanodiscs

The University of Michigan found through its internal investigation1 that Sudheer Kumar Ramadugu, one of the co-authors on the paper, had falsified data in the spectra in Figures 4E and 4F in this Communication. The experiments have been repeated and the correct 14N NMR spectra (Fig. 4E and Fig. 1F) demonstrate the magnetic alignment of the macro-nanodiscs as originally reported. The 14N quadrupole couplings in Figure 4E and 4F have been corrected from 8 kHz and 16 kHz to 9 kHz and 18 kHz, respectively. 31P NMR spectra (Fig. 4B and 4C) for the new samples were also recorded. The chemical shift values have been corrected from −17.9 ppm to −13 ppm in Figure 4B and from 20 ppm to 22 ppm in Figure 4C. The corrected Figure 4 and associated caption are as follows. Magnetic-alignment of SMA-EA polymer based macro-nanodiscs: Static 31P (A–C) and 14N (D–F) NMR spectra demonstrate the presence of isotropic nanodiscs (ca. 10 nm diameter) (A,D), the magnetic-alignment of macro-nanodiscs (ca. 50 nm) with the lipid bilayer perpendicular to the external magnetic field (B,E), and flipped macro-nanodiscs (95:5 DMPC:DMPE-DTPA) in the presence of Yb3+ ions with the lipid bilayer parallel to the external magnetic field (C,F). G) 2D 15N–1H TROSY sQC spectrum of isotropic SMA-EA nanodiscs containing 15N-cytochrome b5 reconstituted in DMPC lipid bilayers. All spectra were recorded at 35 °C, where the DMPC lipids are in a fluid lamellar phase bilayer. H) 2D 1H–15N PISEMA spectrum of SMA-EA macro-nanodiscs containing 15N-cytochrome b5 reconstituted in DMPC lipid bilayers with the bilayer normal perpendicular to the external magnetic field. The new data have also resulted in a few minor changes to the text: On page 11626 the sentence should read: The observation of a peak at −13 ppm in the 31P NMR spectrum (Figure 4B) and approximately 9 kHz 14N quadrupole splitting (Figure 4E) suggest that the lipid bilayer normal is oriented perpendicular to the external magnetic field direction. On page 11627 the sentence should read: the 31P peak appeared at around 22 ppm (Figure 4C) and the 14N quadrupole coupling splitting increased by a factor of 2 to become about 18 kHz (Figure 4F). On page 11627 Figure 4I is no longer relevant and thus the sentence “The ability to control the extent of flip of macro-nanodiscs is also demonstrated by recording 31P spectra for various concentrations of Yb3+ (Figure 4I).” is no longer applicable. An updated Supporting Information for this paper is made available online along with this Corrigendum. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Mössbauer studies of mixed-valence manganite Pb3(Mn0.965Fe0.035)7O15

Pb3Mn7O15 manganite with mixed valence (Mn3+/Mn4+) was studied by Mossbauer effect on 57Fe impurity atoms in the temperature range of 80–673 K. The experimental spectra consisted of two main quadrupole doublets corresponding to trivalent Fe atoms replacing Mn atoms in different positions in the crystal lattice. Identification of the doublets was made on the basis of calculations of quadrupole splitting within the point charges model and taking possible polarization of oxygen ions into account. In two temperature ranges: T ≈ 370–510 K, corresponding to the structural phase transition, and at T ≈ 140–230 K, where strong anomalies of the magnetic and electrical properties were observed, the sharp changes in the hyperfine quadrupole splitting were observed. It is assumed that these changes are caused by changes in the valence of manganese ions and structural deformations.

Breaking the Backbone: Central Arginine Residues Induce Membrane Exit and Helix Distortions within a Dynamic Membrane Peptide.

Transmembrane domains of membrane proteins sometimes contain conserved charged or ionizable residues which may be essential for protein function and regulation. This work examines the molecular interactions of single Arg residues within a highly dynamic transmembrane peptide helix. To this end, we have modified the GW4,20ALP23 (acetyl-GGAW4(AL)7AW20AGA-amide) model peptide framework to incorporate Arg residues near the center of the peptide. Peptide helix formation, orientation and dynamics were analyzed by means of solid-state NMR spectroscopy to monitor specific 2H- or 15N-labeled residues. GW4,20ALP23 itself adopts a tilted orientation within lipid bilayer membranes. Nevertheless, the GW4,20ALP23 helix exhibits moderate to high dynamic averaging of NMR observables, such as 2H quadrupolar splittings or 15N-1H dipolar couplings, due to competition between the interfacial Trp residues on opposing helix faces. Here we examine how the helix dynamics are impacted by the introduction of a single Arg residue at position 12 or 14. Residue R14 restricts the helix to low dynamic averaging and a well-defined tilt that varies inversely with the lipid bilayer thickness. To compensate for the dominance of R14, the competing Trp residues cause partial unwinding of the helix at the C-terminal. By contrast, R12GW4,20ALP23 exits the DOPC bilayer to an interfacial surface-bound location. Interestingly, multiple orientations are exhibited by a single residue, Ala-9. Quadrupolar splittings generated by 2H-labeled residues A3, A5, A7, and A9 do not fit to the α-helical quadrupolar wave plot defined by residues A11, A13, A15, A17, A19, and A21. The discontinuity at residue A9 implicates a helical swivel distortion and an apparent 310-helix involving the N-terminal residues preceding A11. These molecular features suggest that, while arginine residues are prominent factors controlling transmembrane helix dynamics, the influence of interfacial tryptophan residues cannot be ignored.

Mechanical alloying of Si and Fe: Quantum-mechanical calculations applied to Mössbauer and X-ray diffraction studies

Hyperfine parameters of iron nuclei such as isomer shift, quadrupole splitting, and asymmetry parameter are calculated for β-FeSi2 with and without vacancies, using density functional theory. They are applied, in combination with parameters of α-FeSi2 obtained earlier, for analyzing Mössbauer and X-ray diffraction experiments on the mechanical alloying of silicon with iron in the range of 1–33 at. % of Fe. Such an approach allows more detailed and precise information to be obtained on the structure of Fe–Si samples. In particular, the fraction of vacancies in α- and β-FeSi2 is estimated, and the mechanism of formation of these phases and their transformation into each other is discussed. A new model is developed for analyzing experiments on hyperfine interaction, such as Mössbauer spectroscopy, time-differential perturbed-angular correlation, and the like, for Fe–Si systems with a high Si content.

Can Density Functional Theory Predict Mössbauer Spectra in Pyrolyzed Fe-N-C Catalysts?

57Fe Mössbauer spectroscopy, based on the recoil-free emission and resonant absorption of γ-rays, allows an easier separation of the signals arising from the different Fe environments. This is in the basis of the growing interest of using 57Mössbauer spectroscopy for identification of Fe-based species present in pyrolyzed Fe-N-C catalysts. Because of their high temperature synthesis, these catalysts often comprise different coordinations of atomically-dispersed Fe ions covalently bound to the nitrogen-doped carbon matrix (FeNxCy moieties), but also metallic iron and iron carbides with long-range order.1-3 In the 57Fe Mössbauer spectra (recorded in the absence of a strong external magnetic field) metallic iron and iron carbides result in sextets (except for the paramagnetic γ-Fe structure, leading to singlet spectral component) while the fingerprint of FeNxCy moieties is a doublet spectral component. Precise interpretation of the doublets (often labeled as D1 and D2) allowing to distinguish between iron coordination geometries, spin- and oxidation state remains however challenging and has so far lacked strong theoretical or experimental reference basis for their unambiguous assignments. D1 doublet has an isomer value ranging from 0.33 to 0.4 mm/s and quadrupole splitting (QS) ranging from 0.7 to 1.2 mm/s, which is usually attributed to Fe-N4 defects with Fe2+ in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous. It is attributed to Fe-N4 defects similar to iron phthalocyanine with Fe2+ in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, these assignments have been based on experimental records of carbon-supported FeN4 macrocycles, which differ greatly from the case in which the Fe-Nx moieties are integrated into the graphene matrix. It appears, therefore, obvious that theoretical Mössbauer spectra should be highly beneficial to rationally assign the D1 and D2 spectra of Fe-N-C materials. Theoretical studies (mostly obtained with density-functional-theory, DFT, based approaches) of 57Fe Mössbauer spectra, have evidenced that the choice of the exchange-correlation functional and atomic basis sets has a strong impact on the accuracy of QS values, in particular for intermediate and high spin iron complexes. It is therefore necessary to examine for every new class of Fe-containing compounds the variation of QS with the DFT functional and other computational details. Theoretical 57Fe Mössbauer spectra in pyrolyzed Fe-N-C catalysts have not yet been made available, which prompted us toward the present DFT study of the quadrupole splitting energies in Fe-N4 defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Extensive computations of QS in Fe-N4 models with Fe in various coordinations and with different oxidation/spin states have been carried out. The use of several DFT exchange-correlation functionals and basis sets in cluster and periodic approaches are compared and will be reported. The analysis of the DFT calculated values in comparison with experimental data are used to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. Acknowledgement: This work is supported by the LabEx CheMISyst ANR-10-LABX-05-01.