X-band Electron Paramagnetic Resonance Spectra Research Articles

EPR studies of ferredoxin in spinach and cyanobacterial thylakoids related to photosystem I-driven NADP+ reduction.

Photosynthetic light-dependent reactions occur in thylakoid membranes where embedded proteins capture light energy and convert it to chemical energy in the form of ATP and NADPH for use in carbon fixation. One of these integral membrane proteins is Photosystem I (PSI). PSI catalyzes light-driven transmembrane electron transfer from plastocyanin (Pc) to oxidized ferredoxin (Fd). Electrons from reduced Fd are used by the enzyme ferredoxin-NADP+ reductase (FNR) for the reduction of NADP+ to NADPH. Fd and Pc are both small soluble proteins whereas the larger FNR enzyme is associated with the membrane. To investigate electron shuttling between these diffusible and embedded proteins, thylakoid photoreduction of NADP+ was studied. As isolated, both spinach and cyanobacterial thylakoids generate NADPH upon illumination without extraneous addition of Fd. These findings indicate that isolated thylakoids either (i) retain a "pool" of Fd which diffuses between PSI and membrane bound FNR or (ii) that a fraction of PSI is associated with Fd, with the membrane environment facilitating PSI-Fd-FNR interactions which enable multiple turnovers of the complex with a single Fd. To explore the functional association of Fd with PSI in thylakoids, electron paramagnetic resonance (EPR) spectroscopic methodologies were developed to distinguish the signals for the reduced Fe-S clusters of PSI and Fd. Temperature-dependent EPR studies show that the EPR signals of the terminal [4Fe-4S] cluster of PSI can be distinguished from the [2Fe-2S] cluster of Fd at > 30K. At 50K, the cw X-band EPR spectra of cyanobacterial and spinach thylakoids reduced with dithionite exhibit EPR signals of a [2Fe-2S] cluster with g-values gx = 2.05, gy = 1.96, and gz = 1.89, confirming that Fd is present in thylakoid preparations capable of NADP+ photoreduction. Quantitation of the EPR signals of P700+ and dithionite reduced Fd reveal that Fd is present at a ratio of ~ 1 Fd per PSI monomer in both spinach and cyanobacterial thylakoids. Light-driven electron transfer from PSI to Fd in thylakoids confirms Fd is functionally associated (< 0.4 Fd/PSI) with the acceptor end of PSI in isolated cyanobacterial thylakoids. These EPR experiments provide a benchmark for future spectroscopic characterization of Fd interactions involved in multistep relay of electrons following PSI charge separation in the context of photosynthetic thylakoid microenvironments.

Comparative EPR Studies on the Influence of Genistein on Free Radicals in Non-Irradiated and UV-Irradiated MCF7, T47D and MDA-MB-231 Breast Cancer Cells.

The antioxidant activity and the association of genistein with carcinogenesis are widely documented. Few studies directly measure the number of free radicals generated in cells, either during the action of factors stimulating their formation, e.g., ultraviolet (UV), or after exposure to antioxidants. The most suitable method for analysing free radicals is electron paramagnetic resonance (EPR) spectroscopy. The EPR method detects a paramagnetic centre with a single electron. Antioxidants neutralize free radicals, therefore, EPR analysis of antioxidant efficacy is as valuable and important as studying the paramagnetic centres of radicals. The aim of the study was to determine the influence of genistein on free radicals basal level and after UV exposure in breast cancer cell lines MCF7, T47D and MDA-MB-231 cell lines. The impact of genistein on cell viability was investigated at concentrations of 0.37 μM, 3.7 μM, 37 μM and 370 μM. Genistein at a concentration of 370 μM revealed a cytotoxic effect on the cells of all three tested breast cancer lines. Genistein at a concentration of 0.37 μM showed no significant effect on the cell viability of all tested breast cancer lines. Therefore, cell proliferation and antioxidant properties were examined using genistein at a concentration of 0.37 μM and 37 μM. X-band (9.3 GHz) EPR spectra of three different types of breast cancer cells (ER-positive, PR-positive and HER-2 negative: MCF7 and T47D and triple-negative MDA-MB-231) were compared. UV irradiation was used as a factor to generate free radicals in cells. The effect of free radical interactions with the antioxidant genistein was tested for non-UV-irradiated (corresponding to the basal level of free radicals in cells) and UV-irradiated cells. The levels of free radicals in the non-irradiated cells studied increased in the following order in breast cancer cells: T47D < MDA-MB-231 < MCF7 and UV-irradiated breast cancer cells: MDA-MB-231 < MCF7 < T47D. UV-irradiation altered free radical levels in all control and genistein-cultured cells tested. UV irradiation caused a slight decrease in the amount of free radicals in MCF7 cells. A strong decrease in the amount of free radicals was observed in UV-irradiated MDA-MB-231 breast cancer cells. The amount of free radicals in T47D cancer cells increased after UV irradiation. Genistein decreased the amount of free radicals in non-irradiated and UV-irradiated MCF7 cells, and only a weak effect of genistein concentrations was reported. Genistein greatly decreased the amount of free radicals in UV-irradiated T47D cancer cells cultured with genistein at a concentration of 3.7 μM. The effect of genistein was negligible in the other samples. Genistein at a concentration of 3.7 μM decreased the amount of free radicals in non-irradiated MDA-MB-231 cancer cells, but genistein at a concentration of 37 μM did not change the amount of free radicals in these cells. An increase in the amount of free radicals in UV-irradiated MDA-MB-231 cancer cells was observed with increasing genistein concentration. The antioxidant efficacy of genistein as a potential plant-derived agent supporting the treatment of various cancers may be determined by differences in signalling pathways that are characteristic of breast cancer cell line subtypes and differences in activation of oxidative stress response pathways.

Synthesis, Characterization, DNA/HSA Interaction, and Cytotoxic Activity of a Copper(II) Thiolate Schiff Base Complex and Its Corresponding Water-Soluble Stable Sulfinato-O Complex Containing Imidazole as a Co-ligand.

A Cu(II) thiolato complex [CuL(imz)] (1) (H2L = o-HOC6H4C(H)=NC6H4SH-o) and the corresponding water-soluble stable sulfinato-O complex [CuL'(imz)] (2) (H2L' = o-HOC6H4C(H)=NC6H4S(=O)OH) were synthesized and characterized using physicochemical techniques. Compound 2 is found to be a dimer in the solid state as characterized using single-crystal X-ray crystallography. XPS studies clearly showed the differences in the sulfur oxidation states in 1 and 2. Both compounds are found to be monomers in solution as revealed from their four-line X-band electron paramagnetic resonance spectra in CH3CN at room temperature (RT). 1-2 were tested to assess their ability to exhibit DNA binding and cleavage activity. Spectroscopic studies and viscosity experiments suggest that 1-2 bind to CT-DNA through the intercalation mode having moderate binding affinity (Kb ∼ 104 M-1). This is further supported by molecular docking studies of complex 2 with CT-DNA. Both complexes display significant oxidative cleavage of pUC19 DNA. Complex 2 also showed hydrolytic DNA cleavage. The interaction of 1-2 with HSA revealed that they have strong ability to quench the intrinsic fluorescence of HSA by a static quenching mechanism (kq ∼ 1013 M-1 s-1). This is further complemented by Förster resonance energy transfer studies that revealed binding distances of r = 2.85 and 2.75 nm for 1 and 2, respectively, indicating high potential for energy transfer from HSA to complex. 1-2 were capable of inducing conformational changes of HSA at secondary and tertiary levels as observed from synchronous and three-dimensional fluorescence spectroscopy. Molecular docking studies with 2 indicate that it forms strong hydrogen bonds with Gln221 and Arg222 located near the entrance of site-I of HSA. 1-2 showed potential toxicity in human cervical cancer HeLa cells, lung cancer A549 cells, and cisplatin-resistant breast cancer MDA-MB-231 cells and appeared to be most potent against HeLa cells (IC50 = 2.04 μM for 1 and 1.86 μM for 2). In HeLa cells, 1-2 mediated cell cycle arrest in S and G2/M phases, which progressed into apoptosis. Apoptotic features seen from Hoechst and AO/PI staining, damaged cytoskeleton actin viewed from phalloidin staining, and increased caspase-3 activity upon treatment with 1-2 collectively suggested that they induced apoptosis in HeLa cells via caspase activation. This is further supported by western blot analysis of the protein sample extracted from HeLa cells treated with 2.

Mixed Ligand Mononuclear Copper(II) Complex as a Promising Anticancer Agent: Interaction Studies with DNA/HSA, Molecular Docking, and In Vitro Cytotoxicity Studies.

The isolated copper(II) complex [CuL(o-phen)]·H2O (1) [H2L = o-HO-C6H4C(H)=N-C6H4-SH-o, o-phen = 1,10-phenanthroline] was structurally characterized using single-crystal X-ray crystallography. 1 in CH3CN at liquid nitrogen temperature displayed a characteristic monomeric X-band electron paramagnetic resonance spectrum having a tetragonal character with g∥ = 2.1479 and g⊥ = 2.0691 and A∥ ≈ 18.0 mT and A⊥ ≤ 3.9 mT, respectively. 1 showed a strong binding affinity toward calf thymus DNA as reflected from its intrinsic binding constant (Kb = 7.88 × 105 M–1), and its competitive displacement of ethidium bromide suggested an intercalative DNA-binding mode (Kapp = 1.32 × 106 M–1). This was confirmed from the viscosity study that showed an increase in the viscosity of DNA with an increasing concentration of 1. Complex 1 is highly efficient in promoting oxidative and hydrolytic DNA cleavage (kobs = 1.987 h–1). 1 showed a strong binding affinity with the carrier protein human serum albumin (HSA) (Ka = 5.22 × 105 M–1). A high bimolecular quenching constant kq = 2.29 × 1013 M–1s–1 indicated a static quenching mechanism involved in the fluorescence quenching of HSA by 1. Fluorescence resonance energy transfer theory suggested that the distance (r = 3.52 nm) between 1 and HSA is very close. Molecular docking studies suggested that 1 primarily binds to HSA in subdomain IIA. A protein–ligand interaction profiler was used to visualize hydrophobic, hydrogen bonds, and π–cation interactions between HSA and 1. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay using HeLa and MDA-MB-231 cells showed a significant in vitro anticancer activity of 1 (IC50 2.63 and 2.68 μM, respectively). Nuclear staining assays suggested apoptotic cell death in HeLa cells treated with 1. The effect of 1 on the cytoskeletal actin filaments visualized using phalloidin staining showed extensive destruction of actin filaments. Flow cytometric analysis indicated that 1 inhibits the growth of HeLa cells through cell cycle arrest in the S phase. Western blot analysis showed upregulation in the expression of apoptotic marker proteins caspase 3, p53, and Bax. These results collectively indicate that 1 induces apoptosis by promoting DNA damage and has a high potential to act as an anticancer agent.

Decoding the Ambiguous Electron Paramagnetic Resonance Signals in the Lytic Polysaccharide Monooxygenase from Photorhabdus luminescens.

Understanding the structure and function of lytic polysaccharide monooxygenases (LPMOs), copper enzymes that degrade recalcitrant polysaccharides, requires the reliable atomistic interpretation of electron paramagnetic resonance (EPR) data on the Cu(II) active site. Among various LPMO families, the chitin-active PlAA10 shows an intriguing phenomenology with distinct EPR signals, a major rhombic and a minor axial signal. Here, we combine experimental and computational investigations to uncover the structural identity of these signals. X-band EPR spectra recorded at different pH values demonstrate pH-dependent population inversion: the major rhombic signal at pH 6.5 becomes minor at pH 8.5, where the axial signal dominates. This suggests that a protonation change is involved in the interconversion. Precise structural interpretations are pursued with quantum chemical calculations. Given that accurate calculations of Cu g-tensors remain challenging for quantum chemistry, we first address this problem via a thorough calibration study. This enables us to define a density functional that achieves accurate and reliable prediction of g-tensors, giving confidence in our evaluation of PlAA10 LPMO models. Large models were considered that include all parts of the protein matrix surrounding the Cu site, along with the characteristic second-sphere features of PlAA10. The results uniquely identify the rhombic signal with a five-coordinate Cu ion bearing two water molecules in addition to three N-donor ligands. The axial signal is attributed to a four-coordinate Cu ion where only one of the waters remains bound, as hydroxy. Alternatives that involve decoordination of the histidine brace amino group are unlikely based on energetics and spectroscopy. These results provide a reliable spectroscopy-consistent view on the plasticity of the resting state in PlAA10 LPMO as a foundation for further elucidating structure–property relationships and the formation of catalytically competent species. Our strategy is generally applicable to the study of EPR parameters of mononuclear copper-containing metalloenzymes.

TUNING OF CRYSTALLITE SIZE, ENERGY GAP, AND MAGNETIC PROPERTY OF Mn-DOPED CoFe2O4 NANOPARTICLES

This paper focuses on the synthesis and structural, optical, and magnetic characterization of Mn-doped CoFe2O4 nanoparticles synthesized by a simple chemical co-precipitation method. Synthesized magnetic nanoparticles were characterized by XRD, FTIR, UV-Vis, PL, TEM, VSM, and EPR spectroscopy. XRD analysis confirmed the significant reduction in the crystallite size from [Formula: see text][Formula: see text]nm to 10[Formula: see text]nm as the Mn content is increased from 0 to 1. UV-Vis spectra confirmed that the Co–Mn ferrite is a direct bandgap magnetic material that possesses an energy gap from 3.92[Formula: see text]eV to 4.33[Formula: see text]eV. FTIR vibrational frequency observed between 468 and 548 [Formula: see text]confirmed the existence of metal–oxygen bond at tetrahedral and octahedral sites. Photoluminescence spectra confirmed the red emission of the samples from the peak at 680[Formula: see text]nm. TEM analysis suggests that the single domain of CoFe2O4 nanoparticles may vary between 38 and 72[Formula: see text]nm. Composition analysis confirmed the homogeneous mixing of Co, Mn Fe, and O atoms in the synthesized samples. The VSM study confirmed that Mn substitution favors transition from ferromagnetic to superparamagnetic. VSM analysis also confirmed the lessening in saturation magnetization and coercivity on Mn doping. The X-band electron paramagnetic resonance spectrum recorded at room temperature conveys that the superexchange interaction is increased with the increase in Mn concentration.

Multi-Frequency and Single-Crystal EPR on V4+ in W-Doped \u03b2-Vanadyl(V) Phosphate: Hyperfine Coupling- and g-Tensor Values and Orientation

Different structure types of vanadyl(V) orthophosphate [i.e. (VV ≡O)3+ orthophosphate] have been subjects of research due to their catalytic activity in the oxidation of n-butane to maleic anhydride. Electron paramagnetic resonance (EPR) spectroscopy can be exploited to elucidate the electronic structure of such compounds. When tuning the oxidation state of vanadium in (V1-xWx)OPO4, X-band EPR spectra have confirmed the presence of paramagnetic V4+ ions. However, some of the features in these spectra could not be explained. Here, powder samples of β-(VIV0.01VV.98WVI.01)OPO4 are investigated at S-, X- and Q- band, along with X-band EPR measurements on single crystals. Thereby, the discrepancies between the spectra and their simulations could be resolved. In particular, it could be shown that the g and A tensors are not coaxial. The resulting consistent EPR picture and the refined paramagnetic parameters are reported. The work underlines the indispensability of a multi-frequency approach in EPR for unequivocal conclusions.

New copper(II) complexes with (Z)-N′-{(2-hydroxynaphthalen-1-yl}methylene)acetohydrazide]: X-ray structure, Hirshfeld analysis, X-band electron paramagnetic resonance spectra, TD-DFT calculations and superoxide dismutase mimetic activity

In this paper, two ternary copper(II) complexes having the general formula [Cu(HL)(X)] have been synthesized by reacting equimolar solutions of Cu(ClO4)2·6H2O, HL and X (HL = (Z)-N′-{(2-hydroxynaphthalen-1-yl}methylene)acetohydrazide], X = 2,9-dimethyl-1,10-phenanthroline (DMPHEN) and BZI = benzimidazole) in methanol. The complexes have been characterized by microanalysis (C, H and N), magnetic susceptibility and spectral (IR, UV–Vis and X-band epr) measurements. The molecular structures of both complexes have been determined by single-crystal X-ray diffraction analysis. Most interestingly, complex 1, which contains of both hydrogen bonding and π∙∙∙π (chelate-chelate) interactions, forms a supramolecular architecture. Hirshfeld analysis and fingerprint plots verified weak CH∙∙∙π and π∙∙∙π non-covalent interactions that lead both complexes to build supramolecular architectures. To relate to the experimental environment, TD-DFT calculations have been carried out. The TD-DFT findings suggest that transitions in the lowest-energy region are mixed absorption bands with d-d and ligand to metal transitions (LMCT). Room temperature magnetic measurements have been performed to disclose the paramagnetic nature of the complexes. An X-band epr spectral study has been carried out to verify the paramagnetic and bonding behavior of both complexes. The stability of the metal center was explored using electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques. In addition, the superoxide dismutase activities of both complexes have been evaluated using nitro blue tetrazolium assays at pH 7.4. The SOD activity data rank among the best values reported for low molecular weight mononuclear copper(II) complexes reported in the literature.

Structural, Optical and EPR Study of Zn1−xFexO Nanocrystals

Iron-doped ZnO nanocrystals, ZnO (Zn1−xFexO), with doping atomic ratio x = 0.0–0.05, have been synthesized using the sol–gel method to obtain nanoparticles with an average crystal size of around 38.0 nm. Materials were characterized by X-ray fluorescence, Raman and Fourier transform infrared (FTIR) spectroscopies, diffuse reflectance spectroscopy and continuous-wave electron paramagnetic resonance (EPR). FTIR spectroscopies suggest minor change in the Zn–O band occurring in the ZnO environment when the ZnO is Fe doped. X-ray diffraction measurements indicate that lattice parameters of Fe-doped ZnO do not change for all doping levels x. Raman studies show that no peaks disappear when Fe concentration increases, indicating that the incorporation of Fe does not cause an important local structural disarray in the crystalline ZnO lattice. Two broad bands appear at ≈ 570 cm−1 and ≈ 650 cm−1, which are related to the Zn substitution by iron. The band gap does not seem to be influenced by the average crystal size or the cell parameter. It could be because the crystal size is large enough to prevent quantum confinement. X-band EPR spectra recorded at 10 K are similar for all doping levels, x = 0.01–0.05, and are consistent with a spin Hamiltonian composed by isotropic Zeeman and second-order axial symmetry zero-field splitting (ZFS) interactions. Due to the small size of the particles and the difference between the ionic radii of Fe3+ and Zn2+, the EPR spectra are strongly influenced by a static distribution of ZFS parameters, leading to a severe lineshape broadening. An efficient strategy for the simulation process is explained in detail.

New copper(II) complexes with (Z)-N´-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide]: X-ray structure, Hirshfeld analysis, X-Band electron paramagnetic resonance spectra, TD-DFT calculations and superoxide dismutase mimetic activity

In this paper, two ternary copper(II) complexes having the general formula [Cu(HL)(X)] have been synthesized by reacting equimolar solution of Cu(ClO 4 ) 2 .6H 2 O, HL and X (HL = (Z)-N´-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide], X = (DMPHEN = 2, 9-dimethyl-1,10-phenanthroline and BZI = benzimidazole) in methanol. The complexes have been characterized by microanalysis (C, H, and N), magnetic susceptibility and spectral (IR, UV-Vis and X-band epr) measurements. The molecular structures of both complexes have been determined by single-crystal X-ray diffraction analysis. Most interestingly, complex 1 , which consist of both hydrogen bonding and π∙∙∙π (chelate-chelate) interactions form supramolecular architecture. The Hirshfeld analysis and the fingerprint plots verified weak CH∙∙∙π and π∙∙∙π non-covalent interactions that lead both complexes to build supramolecular architectures. To relate to the experimental environment, TD-DFT calculations have been carried out. TD-DFT finding suggests that the transitions on the lowest-energy region are mixed absorption bands with d-d transition and ligand to metal transition (LMCT). The room temperature magnetic measurements have been performed to disclose the paramagnetic nature of complexes. X-band epr spectral study has been done to verify the paramagnetic and bonding behavior of both complexes. The stability of the metal center was explored using electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques. In addition, superoxide dismutase activities of both complexes have been evaluated using nitro blue tetrazolium assays at pH 7.4. The SOD activity data rank among the best values reported for low molecular weight mononuclear copper(II) complexes reported in the literature.In this paper, two ternary copper(II) complexes having the general formula [Cu(HL)(X)] have been synthesized by reacting equimolar solution of Cu(ClO 4 ) 2 .6H 2 O, HL and X (HL = (Z)-N´-{(2-hydroxynapthalen-1-yl}methylene)acetohydrazide], X = (DMPHEN = 2, 9-dimethyl-1,10-phenanthroline and BZI = benzimidazole) in methanol. The complexes have been characterized by microanalysis (C, H, and N), magnetic susceptibility and spectral (IR, UV-Vis and X-band epr) measurements. The molecular structures of both complexes have been determined by single-crystal X-ray diffraction analysis. Most interestingly, complex 1 , which consist of both hydrogen bonding and π∙∙∙π (chelate-chelate) interactions form supramolecular architecture. The Hirshfeld analysis and the fingerprint plots verified weak CH∙∙∙π and π∙∙∙π non-covalent interactions that lead both complexes to build supramolecular architectures. To relate to the experimental environment, TD-DFT calculations have been carried out. TD-DFT finding suggests that the transitions on the lowest-energy region are mixed absorption bands with d-d transition and ligand to metal transition (LMCT). The room temperature magnetic measurements have been performed to disclose the paramagnetic nature of complexes. X-band epr spectral study has been done to verify the paramagnetic and bonding behavior of both complexes. The stability of the metal center was explored using electrochemical (cyclic voltammetry and differential pulse voltammetry) techniques. In addition, superoxide dismutase activities of both complexes have been evaluated using nitro blue tetrazolium assays at pH 7.4. The SOD activity data rank among the best values reported for low molecular

Influence of Mn dopant on the crystallite size, optical and magnetic behaviour of CoFe2O4 magnetic nanoparticles

Cubic spinel structured Co1-xMnxFe2O4 (x = 0, 0.25, 0.5, 0.75 & 1.0) nanoparticles were successfully synthesized by the chemical precipitation method. Increase in Mn content shows a significant reduction on the crystallite size from 16.7 to 10.19 nm. FTIR vibrational bonds between 464 and 585 cm−1 confirm the interaction of metal-oxygen at tetrahedral and octahedral sites, a de-notified vibrations stretch. UV–visible diffuse reflectance spectrum indicates that Co–Mn ferrite is a direct bandgap material with bandgap ranges from 3.84 to 4.02 eV. The red-emission of the samples was confirmed form the peak at 680 nm. Formation of single domain cobalt ferrite nanoparticles of size ~38–72 nm was confirmed by TEM which exhibited morphological transition from nanobar to nanoparticles. Inductively coupled plasma emission - optical emission spectrometer result confirmed the homogeneous mixing of Co, Mn, Fe, and O atoms. The vibrating sample magnetometer study confirmed that Mn concentration favours the magnetic transition from ferromagnetism to superparamagnetism. Saturation magnetization gets reduced from 34.20 to 0.5 emu/g and the and coercivity gets reduced from 0.1112 to 0.0328 T. The X-band electron paramagnetic resonance spectrum recorded at room temperature imparts the increase of superexchange interaction with the increase in manganese concentration.

Purely Spectroscopic Determination of the Spin Hamiltonian Parameters in High-Spin Six-Coordinated Cobalt(II) Complexes with Large Zero-Field Splitting.

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac)2(bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (gx = 2.388, gy = 2.417, gz = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm-1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

Post-polymerization reactivity of free radicals trapped in resin-based dental restorative materials by ESR spectroscopy

The propagating and allylic free radicals generated when polymerizing dimethacrylate resin-based materials used in dentistry are detected by electron spin resonance (ESR) spectroscopy as a nine-line spectrum. This study aimed to probe their post-polymerization reactivity with a novel methodology to quantify these radicals separately. X-band ESR spectra were recorded from a photo-cure dental resin composite (~ 20 days) and a dual-cure resin cement (~ 15 days). Spectra simulations were applied to separate and determinate the contribution from each radical to the ESR signal according to the method presented. The decay curves of the free radicals were then obtained to probe their post-polymerization reactivity as function of composition and initiation mode (photo-cure and self-cure). The allylic radical’s post-polymerization half-life time was approximately twice than that of the propagating radical, independently the composition and the activation mode, indicating that their relative reactivity does not depend of these parameters. The methodology presented for individual quantification of free radicals showed to be adequate to probe the kinetics of the free radicals generated and trapped in resin-based dental.

Understanding the Roles of Triethylaluminum in Phosphinimide-Supported Titanium Catalyst Systems for Ethylene Polymerization

Treatment of [(Ind)(tBu3P═N)TiCl2] (1; Ind = indenyl) with AlEt3 in a range of solvents affords the emerald green heterobimetallic complex [(Ind)(tBu3P═N)Ti(μ2-Cl)2AlEt2] (2). Single-crystal X-ray diffraction studies, as well as nuclear magnetic and X-band electron paramagnetic resonance spectra, support formulation of 2 as a paramagnetic, formally [TiIIIAlIII] complex. Both 1 and 2 are active precatalysts for ethylene polymerization in the presence of AlEt3 and a solid superacid; the product polyethylene materials are found to have similar molecular weight distributions in each case (3.29 with 1 and 3.05 with 2), implicating involvement of similar active catalysts in both. Electrochemical studies reveal that 1 can undergo one-electron reduction to TiIII near −2.0 V versus ferrocenium/ferrocene in tetrahydrofuran electrolyte, whereas 2 undergoes one-electron oxidation at a significantly more positive potential (ca. −1.30 V). Spectroelectrochemical studies show that reduction of 1 in the presence of AlEt3 ...

Binding of Oxovanadium(IV) Complexes to Blood Serum Albumins

In this work the binding of VIVO2+ and VIVO-complexes to serum albumins {human serum albumin (HSA), bovine serum albumin (BSA) and porcine serum albumin (PSA)} are studied using circular dichroism (CD), electron paramagnetic resonance (EPR) and visible absorption spectroscopy. The results confirm previous findings that VIVO2+ occupies at least two types of binding sites on albumin: ‘the strong vanadium binding site’ (designated by VBS1) and ‘the weak vanadium binding sites’ (designated by VBS2). VBS1 binds 1 mol equivalent of VIVO2+. On the other hand VBS2 correspond to binding of several mol equivalents of VIVO, and studies done with PSA in the presence of excess ZnII ions indicate that VSB2 corresponds to two distinct types of sites. The hyperfine coupling constant Az for VIVO2+ binding at VBS2 on HSA and BSA are all very similar (~168 × 10-4 cm-1) but differ slightly on PSA (~166 × 10-4 cm-1) due to differences in the binding sets. When (VIVO)-HSA systems are titrated with maltol ternary species of (maltol)m(VIVO)mHSA and (maltol)2m(VIVO)mHSA stoichiometry form which are clearly distinguishable from the binary (VIVO)-HSA system by the type and intensity of the CD spectra recorded. Changes are also observable in the intensity of the X-band EPR spectra, but not much in the hyperfine coupling constants Az, which are all in the range 166-167 × 10-4 cm-1. The results further demonstrate that the presence of maltol may enhance the binding of VIVO to albumin.

Structural influence of mixed transition metal ions on lithium bismuth borate glasses

Lithium bismuth borate glasses containing mixed transition metals having composition 7CoO·23Li2O·20Bi2O3·50B2O3 (CLBB), 7V2O5·23Li2O·20Bi2O3·50B2O3 (VLBB) and x(2CoO·V2O5)·(30 - x)Li2O·20Bi2O3·50B2O3 (x = 0.0 (LBB) and x = 2.0, 5.0, 7.0, 10.0 mol% (CVLBB1-4)) are synthesized via melt quench route. The synthesized compositions are investigated for their physical properties using density (D) and molar volume (Vm), thermal properties by analyzing DSC/TG thermo-graphs, structural properties using IR absorption spectra in the mid-IR range and optical properties using UV–Vis–NIR spectroscopy. The Electron Paramagnetic Resonance (EPR) spectra of vanadyl and cobalt ion have been analyzed to study compositional effects on spin-Hamiltonian parameters. The non linear variations in physical properties depict a strong structural influence of Co/V- oxides on the glassy matrix. The compositional variations in characteristic temperatures (glass transition temperature Tg, glass crystallization temperature Tp and glass melting temperature Tm) reveals that Tg for glass samples CLBB is relatively less than that of pure lithium bismuth borate (LBB) glass sample wherein Tg for sample VLBB is higher than that of LBB. The increase in Tg (as compared with LBB) with an enhanced substitution of mixed transition metal oxides (2CoO·V2O5) shows a progressive structure modification of bismuth borate matrix. These predictions are very well corroborated by corresponding compositional trends of Tp and Tm. FTIR studies reveal that Co2+& VO2+ ions lead to structural rearrangements through the conversion of three-coordinated boron into four coordinated boron and thereby reducing number of non-bridging oxygen atoms. Bismuth is found to exist in [BiO6] octahedral units only, whereas boroxol rings are not present in the glass network. The theoretical values of optical basicity (Λth) and corresponding oxide ion polarizability (αo2−) have also been calculated to investigate oxygen covalency of glass matrix. Trends in both these parameters suggested an increase in ionic bonding on substitution of divalent transition metal cations causing a more bonding compaction in glass structure. The UV–Vis–NIR spectra suggest that cobalt ions exist as Co2+ states in octahedral coordination in glass network. Inter-electronic repulsion parameter and crystal field splitting energy were evaluated to understand the site symmetry around Co2+-ion in glass. X-band EPR spectra suggest that vanadium ions (V4+) exists as VO2+-ions in octahedral coordination with tetragonal compression. Spin Hamiltonian parameters g-values and A-values of VO2+ ions in glass were calculated. For sample CLBB two resonance lines in EPR spectrum attribute to octahedral symmetry around Co2+-ions were observed.

EPR and Structural Characterization of Water-Soluble Mn2+-Doped Si Nanoparticles.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM–1 s–1 and r2 relaxivity of 32.7 ± 4.7 mM–1 s–1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM–1 s–1 and r2 relaxivity of 1078.5 ± 1.9 mM–1 s–1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP’s. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP’s.

Synthesis, Characterization, Optical Properties, and Electron Paramagnetic Resonance for Nano Zn 0 . 5 Co 0 . 5 Fe 2 − x Pr x O 4

Nano ferrite samples of type Zn0.5Co0.5Fe2−x PrxO4, 0 ≤ x ≤ 0.2, were prepared by co-precipitation method for different concentration of Pr ions at a calcination temperature of 823 K. The samples were characterized using x-ray diffraction and transmission electron microscope (TEM), UV spectroscopy, FTIR, and electron paramagnetic resonance (EPR). The increase in concentration of Pr ions shows a significant change in the formed nano ferrite particles size, degree of aggregation, energy band gap, and lattice parameters. X-band (9.24 GHz) EPR spectra at room temperature showed a decrease in the g factor as the Pr content was increased. Furthermore, the variation of the Pr3+ concentration in the samples led to different aggregation states, ranging from isolated nanoparticles with average crystal size of 28.5 to about 15 nm, as confirmed by x-ray diffraction and high-resolution transmission electron microscopy.

Dynamic nuclear polarization-magnetic resonance imaging at low ESR irradiation frequency for ascorbyl free radicals.

Highly water-soluble ubiquinone-0 (CoQ0) reacts with ascorbate monoanion (Asc) to mediate the production of ascorbyl free radicals (AFR). Using aqueous reaction mixture of CoQ0 and Asc, we obtained positively enhanced dynamic nuclear polarization (DNP)-magnetic resonance (MR) images of the AFR at low frequency (ranging from 515 to 530 MHz) of electron spin resonance (ESR) irradiation. The shape of the determined DNP spectrum was similar to ESR absorption spectra with doublet spectral peaks. The relative locational relationship of spectral peaks in the DNP spectra between the AFR (520 and 525 MHz), 14N-labeled carbamoyl-PROXYL (14N-CmP) (526.5 MHz), and Oxo63 (522 MHz) was different from that in the X-band ESR spectra, but were similar to that in the 300-MHz ESR spectra. The ratio of DNP enhancement to radical concentration for the AFR was higher than those for 14N-CmP, Oxo63, and flavin semiquinone radicals. The spectroscopic DNP properties observed for the AFR were essentially the same as those for AFR mediated by pyrroloquinoline quinone. Moreover, we made a success of in vivo DNP-MR imaging of the CoQ0-mediated AFR which was administered by the subcutaneous and oral injections as an imaging probe.

Revealing the Cu2+ ions localization at low symmetry Bi sites in photorefractive Bi12GeO20 crystals doped with Cu and V by high frequency EPR

The sites of incorporation of Cu2+ impurity ions in Bi12GeO20 single crystals co-doped with copper and vanadium have been investigated by electron paramagnetic resonance (EPR). While the X-band EPR spectra consist of a simple broad (ΔB ∼50mT) line with anisotropic lineshape, the W-band EPR spectra exhibit well resolved, strongly anisotropic lines, due to transitions within the 3d9–2D ground manifold of the Cu2+ ions. The most intense group of lines, attributed to the dominant Cu2+(I) center, displays a characteristic four components hyperfine structure for magnetic field orientations close to a 〈110〉 direction. The g and A tensor main axes are very close to one of the 12 possible sets of orthogonal 〈1−10〉, 〈00−1〉 and 〈110〉 crystal directions. Several less intense lines, with unresolved hyperfine structure and similar symmetry properties, mostly overlapped by the Cu2+(I) spectrum, were attributed to Cu2+(II) centers. The two paramagnetic centers are identified as substitutional Cu2+ ions at Bi3+ sites with low C1 symmetry, very likely resulting from different configurations of neighboring charge compensating defects.