Search Results (67)

Ozone is a powerful destructive agent in the oxidative process of polymer composites. The destructive ability of ozone depends primarily on its concentration, duration of exposure, the type of polymer, and its matrix structure. In this work, nonwoven PLA/NR fibers with natural rubber contents of 5, 10, and 15 wt.% were obtained, which were then subjected to ozone oxidation for 800 min. The effect of ozone treatment was estimated using various methods of physicochemical analysis. The visual effect was manifested in the form of a change in the color of PLA/NR fibers. The method of differential scanning calorimetry revealed a change in the thermophysical characteristics. The glass transition and cold crystallization temperatures of polylactide shifted toward lower temperatures, and the degree of crystallinity increased. It was found that in PLA/NR fiber samples, the degradation process predominates over the crosslinking process, as an increase in the melt flow rate by 1.5–1.6 times and a decrease in the correlation time determined by the electron paramagnetic resonance method were observed. The IR Fourier method recorded a change in the chemical structure during ozone oxidation. The intensity of the ether bond bands changed, and new bands appeared at 1640 and 1537 cm⁻¹, which corresponded to the formation of –C=C– bonds. Full article

Acetone carboxylase (AC) from Xanthobacter autotrophicus is a 360 KDa α₂β₂γ₂ heterohexamer that catalyzes the ATP-dependent formation of phosphorylated acetone and bicarbonate intermediates that react at Mn(II) metal active sites to form acetoacetate. Structural models of X. autotrophicus AC (XaAC) with and without nucleotides reveal that the binding and phosphorylation of the two substrates occurs ~40 Å from the Mn(II) active sites where acetoacetate is formed. Based on the crystal structures, a significant conformational change was proposed to open and close a tunnel that facilitates the passage of reaction intermediates between the sites for nucleotide binding and phosphorylation of substrates and Mn(II) sites of acetoacetate formation. We have employed electron paramagnetic resonance (EPR), kinetic assays, and hydrogen/deuterium exchange mass spectrometry (HDX-MS) of poised ligand-bound states and site-specific amino acid variants to complete an in-depth analysis of Mn(II) coordination and allosteric communication throughout the catalytic cycle. In contrast with the established paradigms for carboxylation, our analyses of XaAC suggested a carboxylate shift that couples both local and long-range structural transitions. Shifts in the coordination mode of a single carboxylic acid residue (αE89) mediate both catalysis proximal to a Mn(II) center and communication with an ATP active site in a separate subunit of a 180 kDa α₂β₂γ₂ complex at a distance of 40 Å. This work demonstrates the power of combining structural models from X-ray crystallography with solution-phase spectroscopy and biophysical techniques to elucidate functional aspects of a multi-subunit enzyme. Full article

Catechol-like compounds are found throughout biology in the form of both redox-active and metal-binding functional groups. Within the marine environment, catechol groups are known to coordinate strongly with vanadate and ferric ions, and this binding is regulated through redox mechanisms. While investigating marine melanin formation in vitro, we found that DOPA, a catechol-containing amino acid, reacts with both metals differently when provided with sulfite, a weak reductant, and selenite, a weak oxidant. Both compounds interacted with the DOPA–vanadium complex, but only selenite, the more redox-labile chalcogenide, led to the creation of melanin particulates. When DOPA, vanadate, and selenite are present together, a metal-binding spectra shift and a melanin variant are rapidly observed. This variant was found to form large, elongated filaments with a low carboxylic acid content and a unique electron paramagnetic resonance signature. When compared to enzymatically produced melanin, this chemically synthesized variant was more thermally and biologically inert, exhibiting a lower redox activity. The results demonstrate that the regulation of the redox environment from metal–catechol interactions can help to control both the chemical and physical properties of melanin aggregates, suggesting a scalable and cell- and enzyme-free synthesis pathway for applications that may require inert materials of strict composition. Full article

This study explores the structural, magnetic, and magnetocaloric properties of ${\mathrm{Ce}}_2$(Fe, Co${)}_{17}$ (x = 0, 0.5, 0.6, and 0.7) compounds synthesized via arc melting under high temperatures exceeding 2300 K. The as-cast ingots are subsequently sealed and subjected to a heat treatment at 1323 K to improve homogeneity and crystallinity. Detailed analyses using X-ray diffraction and magnetometry reveal that cobalt substitution significantly impacts the structural and magnetic behavior, enabling precise tuning of the magnetic transition temperature and magnetic order. The substitution induces an anisotropic increase in cell parameters and shifts the magnetocaloric effect (MCE) from low temperatures (200 K for x = 0) to near room temperature (285 K for x = 0.7), enhancing the operating temperature range. The magnetocaloric effect is studied across different magnetic transitions: a metamagnetic and ferro-antiferromagnetic transition followed by a paramagnetic state in one sample, and a direct ferro-paramagnetic transition in another. The compounds exhibit a second-order magnetic phase transition, ensuring a reversible MCE, with a relative cooling power (RCP) that is approximately 85% of that of pure Gd. Moreover, the use of cerium, the most cost-effective rare-earth element (5 $/kg), combined with its low atomic concentration (10%) in these intermetallics, enhances the sustainability and affordability of these materials. These findings underline the potential of iron-rich Ce-based compounds for next-generation refrigeration and energy-harvesting applications. Full article

In this study, the analysis of XRD patterns reveals that both natural melanin and copper oxide-doped melanin exhibit amorphous structures. The absence of new peaks for Cu₂O confirms that the copper oxide nanoparticles are effectively integrated into the melanin matrix. Utilizing Scherrer’s equation, we calculated the crystallite size, which increased from 2.02 nm for natural melanin to 3.01 nm after doping, suggesting a significant influence of copper ions on melanin structure. Additionally, the VSM measurements demonstrate a shift from ferromagnetic behavior in the natural melanin to paramagnetism in the doped samples, highlighting the impact of copper oxide on the magnetic properties of melanin. These findings provide valuable insights into the biological and functional transformations of natural melanin, furthering our understanding of its role in various medical applications. Full article

Folic acid (FA) is an essential vitamin involved in crucial metabolic processes, while copper(II) ions play significant roles in various biological functions. This study aims to investigate the interaction between FA and Cu²⁺ using ¹H and ¹³C NMR spectroscopy under different pH levels and concentrations. The research employed detailed NMR analysis to explore how Cu²⁺ binds to FA, focusing on changes in chemical shifts, diffusion coefficients, and copper-induced paramagnetic effects. The key findings reveal that Cu²⁺ predominantly coordinates with the pteridine ring (PTE) of FA, with minimal involvement from the glutamic acid (Glu) moiety. The interaction is strongly concentration-dependent: at lower FA concentrations, Cu²⁺ binds effectively to the PTE ring, while at higher concentrations, intermolecular interactions among FA molecules hinder copper binding. The study also observed pronounced paramagnetic effects on the PTE and p-aminobenzoic acid protons, with negligible effects on Glu signals. These results provide new insights into the structural characteristics of FA-Cu²⁺ complexes, contributing to a better understanding of their biochemical interactions and implications for folate metabolism. Full article

The metallocenes ferrocene (Cp₂Fe, 1), nickelocene (Cp₂Ni, 2), and cobaltocene (Cp₂Co, 3) crystallize in the same space group (P2₁/a) and they have the same shape and similar size. Therefore, they form solid solutions with random distribution of the different molecules when crystallized from solution. Alternatively, the solid metallocenes can be ground together manually, and the solid solutions form at any molar ratio within minutes. The metallocenes 2 and 3 are paramagnetic. Solid solutions of 1/3 and 2/3 have been studied by paramagnetic solution and solid-state NMR spectroscopy. The effect of the paramagnetic species on the other components in the solid solutions has been investigated. The impact on the chemical shifts is limited. However, the halfwidths and the signal shapes, as defined by the rotational sideband intensities, change with increasing amounts of paramagnetic components. The ¹H T₁ relaxation times are shortened for diamagnetic protons in the presence of paramagnetic metallocenes in the solid solutions. It has been demonstrated that all metallocenes mix at the molecular level within the polycrystalline samples. The EPR spectra of the solid solutions are dominated by the most intensive signal of any paramagnetic metallocene in the solid samples. Full article

A series of ZnCr_2−xHo_xSe₄ microcrystalline spinels (where x = 0.05, 0.075, and 0.10) containing holmium ions in octahedral coordination were obtained by sintering of adequate reactants at high temperatures. The obtained doped materials were characterized by X-ray diffraction, Scanning Electron Microscopy, UV–Vis–NIR, molecular field approximation, and XPS spectroscopies. Their thermal properties were also investigated. The doping of the ZnCr₂S₄ matrix with paramagnetic Ho³⁺ ions with a content of not more than 0.1 and a screened 4f shell revealed a significant effect of orbital and Landau diamagnetism, a strong reduction in short-range ferromagnetic interactions, and a broadening and shift of the peak of the first critical field by simultaneous stabilization of the sharp peak in the second critical field. These results correlate well with FPLO calculations, which show that Cr sites have magnetic moments of 3.19 µ_B and Ho sites have significantly larger ones with a value of 3.95 µ_B. Zn has a negligible magnetic polarization of 0.02 µ_B, and Se induces a polarization of approximately −0.12 µ_B. Full article

The present study demonstrates the effects of pH-shifting treatments and magnetic field-assisted pH-shifting treatments on the properties of myofibrillar protein (MP) in frozen meat. The solubility results indicate that the pH-shifting treatments increased the solubility of MP from 16.8% to a maximum of 21.0% (pH 9). The values of surface hydrophobicity and protein particle size distribution indicate that the pH-shifting treatment effectively inhibited protein aggregation through electrostatic interactions. However, under higher pH conditions (pH 10, 11), the treatments assisted by the magnetic field increased the degree of aggregation. The total thiol content and SDS-PAGE results further suggest that the magnetic field-assisted pH-shifting treatment accelerated the formation of covalent bonds among MPs under the alkaline environment. The results of the Differential Scanning Calorimetry (DSC) and protein secondary structure analysis indicate that the magnetic field promoted the unfolding of protein structures in an alkaline environment, markedly reducing the effective pH levels of pH-shifting. Electron paramagnetic resonance (EPR) data indicate that the phenomenon might be associated with the increased concentration of free radicals caused by the magnetic field treatment. In summary, the application of magnetic field-assisted pH-shifting treatments could emerge as a potent and promising strategy to improve the protein properties in frozen meat. Full article

Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap₃PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO₃)₃L₄ (Ln = Eu to Lu) and the structures of [Ln(NO₃)₃L₂]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO₃)₃L₂] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap₃PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by ³¹P and ¹³C NMR spectroscopy in CD₃CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the ³¹P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La–Eu) and heavier metals (Tb–Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl₃, NMR measurements show dissociation into [Ln(NO₃)₃L₂] and 2L occurs. Full article

2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO^•), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide, as a redox mediator for batteries, for the activity estimation of antioxidants, and so on. However, there is no report on the reactivity of PTIO^• in the presence of redox-inactive metal ions. In this study, it is demonstrated that the addition of scandium triflate, Sc(OTf)₃ (OTf = OSO₂CF₃), to an acetonitrile (MeCN) solution of PTIO^• resulted in an electron-transfer disproportionation to generate the corresponding cation (PTIO⁺) and anion (PTIO⁻), the latter of which is suggested to be stabilized by Sc³⁺ to form [(PTIO)Sc]²⁺. The decay of the absorption band at 361 nm due to PTIO^•, monitored using a stopped-flow technique, obeyed second-order kinetics. The second-order rate constant for the disproportionation, thus determined, increased with increasing the Sc(OTf)₃ concentration to reach a constant value. A drastic change in the cyclic voltammogram recorded for PTIO^• in deaerated MeCN containing 0.10 M Bu₄NClO₄ was also observed upon addition of Sc(OTf)₃, suggesting that the large positive shift of the one-electron reduction potential of PTIO^• (equivalent to the one-electron oxidation potential of PTIO⁻) in the presence of Sc(OTf)₃ may result in the disproportionation. When H₂O was added to the PTIO^•–Sc(OTf)₃ system in deaerated MeCN, PTIO^• was completely regenerated. It is suggested that the complex formation of Sc³⁺ with H₂O may weaken the interaction between PTIO⁻ and Sc³⁺, leading to electron-transfer comproportionation to regenerate PTIO^•. The reversible disproportionation of PTIO^• was also confirmed by electron paramagnetic resonance (EPR) spectroscopy. Full article

Gelatin nanofibers are known as wound-healing biomaterials due to their high biocompatible, biodegradable, and non-antigenic properties compared to synthetic-polymer-fabricated nanofibers. The influence of gamma radiation doses on the structure of gelatin nanofiber dressings compared to gelatin of their origin is little known, although it is very important for the production of stable bioactive products. Different-origin gelatins were extracted from bovine and donkey hides, rabbit skins, and fish scales and used for fabrication of nanofibers through electrospinning of gelatin solutions in acetic acid. Nanofibers with sizes ranging from 73.50 nm to 230.46 nm were successfully prepared, thus showing the potential of different-origin gelatin by-products valorization as a lower-cost alternative to native collagen. The gelatin nanofibers together with their origin gelatins were treated with 10, 20, and 25 kGy gamma radiation doses and investigated for their structural stability through chemiluminescence and FTIR spectroscopy. Chemiluminescence analysis showed a stable behavior of gelatin nanofibers and gelatins up to 200 °C and increased chemiluminescent emission intensities for nanofibers treated with gamma radiation, at temperatures above 200 °C, compared to irradiated gelatins and non-irradiated nanofibers and gelatins. The electron paramagnetic (EPR) signals of DMPO adduct allowed for the identification of long-life HO^● radicals only for bovine and donkey gelatin nanofibers treated with a 20 kGy gamma radiation dose. Microbial contamination with aerobic microorganisms, yeasts, filamentous fungi, Staphylococcus aureus, Escherichia coli, and Candida albicans of gelatin nanofibers treated with 10 kGy gamma radiation was under the limits required for pharmaceutical and topic formulations. Minor shifts of FTIR bands were observed at irradiation, indicating the preservation of secondary structure and stable properties of different-origin gelatin nanofibers. Full article

The mononuclear and dinuclear ruthenium(III) complexes trans-Ph₄P[Ru^III(acac)₂Cl₂] (1), Ph₄P[{Ru^III(acac)Cl}₂(μ-Cl)₃] (2) and trans-Ph₄P[Ru^III(acac)₂(NCS)₂]·0.5C₆H₁₄ (3·0.5C₆H₁₄) were synthesized. Single crystals of 1, 2·H₂O and 3·CH₃CN suitable for X-ray crystal structure analyses were obtained through recrystallization from DMF for 1 and 2·H₂O and from acetonitrile for 3·CH₃CN. An octahedral Ru with bis-chelate-acac ligands and axial chlorido or κ-N-thiocyanido ligands (for 1 and 3·CH₃CN) and triply µ-chlorido-bridged dinuclear Ru₂ for 2·H₂O were confirmed through the structure analyses. The Ru–Ru distance of 2.6661(2) of 2·H₂O is indicative of the existence of the direct metal–metal interaction. The room temperature magnetic moments (μ_eff) are 2.00 and 1.93 μ_B for 1 and 3·0.5C₆H₁₄, respectively, and 0.66 μ_B for 2. The temperature-dependent (2–300 K) magnetic susceptibility showed that the strong antiferromagnetic interaction (J ≤ −800 cm⁻¹) is operative between the ruthenium(III) ions within the dinuclear core. In the ¹H NMR spectra measured in CDCl₃ at 298 K, the dinuclear complex 2 showed signals for the acac ligand protons at 2.50 and 2.39 ppm (for CH₃) and 5.93 ppm (for CH), respectively, while 1 and 3·0.5C₆H₁₄ showed signals with large paramagnetic shifts; −17.59 ppm (for CH₃) and −57.01 ppm (for CH) for 1 and −16.89 and −17.36 ppm (for CH₃) and −53.67 and −55.53 ppm (for CH) for 3·0.5C₆H₁₄. Cyclic voltammograms in CH₂Cl₂ with an electrolyte of ⁿBu₄N(ClO₄) showed the Ru^III → Ru^IV redox wave at 0.23 V (vs. Fc/Fc⁺) for 1 and the Ru^III → Ru^II waves at −1.39 V for 1 and −1.25 V for 3·0.5C₆H₁₄ and the Ru^III–Ru^III → Ru^III–Ru^IV and Ru^III–Ru^III → Ru^III–Ru^IV waves at 0.91 V and −0.79 V for 2. Full article

Debye temperatures of α-Sn_xFe_1−xOOH nanoparticles (x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100x NPs) prepared by hydrothermal reaction were estimated with ⁵⁷Fe- and ¹¹⁹Sn-Mössbauer spectra measured by varying the temperature from 20 to 300 K. Electrical properties were studied by solid-state impedance spectroscopy (SS-IS). Together, the charge–discharge capacity of Li- and Na-ion batteries containing Sn100x NPs as a cathode were evaluated. ⁵⁷Fe-Mössbauer spectra of Sn10, Sn15, and Sn20 measured at 300 K showed only one doublet due to the superparamagnetic doublet, while the doublet decomposed into a sextet due to goethite at the temperature below 50 K for Sn 10, 200 K for Sn15, and 100 K for Sn20. These results suggest that Sn10, Sn15 and Sn20 had smaller particles than Sn0. On the other hand, 20 K ¹¹⁹Sn-Mössbauer spectra of Sn15 were composed of a paramagnetic doublet with an isomer shift (δ) of 0.24 mm s⁻¹ and quadrupole splitting (∆) of 3.52 mm s⁻¹. These values were larger than those of Sn10 (δ: 0.08 mm s⁻¹, ∆: 0.00 mm s⁻¹) and Sn20 (δ: 0.10 mm s⁻¹, ∆: 0.00 mm s⁻¹), suggesting that the Sn^IV-O chemical bond is shorter and the distortion of octahedral SnO₆ is larger in Sn15 than in Sn10 and Sn20 due to the increase in the covalency and polarization of the Sn^IV-O chemical bond. Debye temperatures determined from ⁵⁷Fe-Mössbauer spectra measured at the low temperature were 210 K, 228 K, and 250 K for Sn10, Sn15, and Sn20, while that of α-Fe₂O₃ was 324 K. Similarly, the Debye temperature of 199, 251, and 269 K for Sn10, Sn15, and Sn20 were estimated from the temperature-dependent ¹¹⁹Sn-Mössbauer spectra, which were significantly smaller than that of BaSnO₃ (=658 K) and SnO₂ (=382 K). These results suggest that Fe and Sn are a weakly bound lattice in goethite NPs with low crystallinity. Modification of NPs and addition of Sn has a positive effect, resulting in an increase in DC conductivity of almost 5 orders of magnitude, from a σ_DC value of 9.37 × 10⁻⁷ (Ω cm)⁻¹ for pure goethite Sn (Sn0) up to DC plateau for samples containing 0.15 and 0.20 Sn (Sn15 and Sn20) with a DC value of ~4 × 10⁻⁷ (Ω cm)⁻¹ @423 K. This non-linear conductivity pattern and levelling at a higher Sn content suggests that structural modifications have a notable impact on electron transport, which is primarily governed by the thermally activated via three-dimensional hopping of small polarons (SPH). Measurements of SIB performance, including the Sn100x cathode under a current density of 50 mA g⁻¹, showed initial capacities of 81 and 85 mAh g⁻¹ for Sn0 and Sn15, which were larger than the others. The large initial capacities were measured at a current density of 5 mA g⁻¹ found at 170 and 182 mAh g⁻¹ for Sn15 and Sn20, respectively. It is concluded that tin-goethite NPs are an excellent material for a secondary battery cathode and that Sn15 is the best cathode among the studied Sn100x NPs. Full article

Nuclear Magnetic Resonance (NMR) spectroscopy is the ideal tool to address the structure, reactivity and dynamics of both inorganic and biological substances. The knowledge of nuclear spin interaction and spin dynamics is increasingly consolidated, and this allows for tailoring pulse sequences. When dealing with paramagnetic systems, several decades of research have led to the development of rule-of-the-thumb criteria for optimizing the experiments, allowing for the detection of nuclei that are in very close proximity to the metal center. In turn, the observation of these systems, coupled with the development of robust and accessible quantum chemical methods, is promising to provide a link between the spectra and the structural features through the interpretation of the electronic structure. In this review, we list the challenges encountered and propose solutions for dealing with paramagnetic systems with the greatest satisfaction. In our intentions, this is a practical toolkit for optimizing acquisition and processing parameters for routine experiments aimed at detecting signals influenced by the hyperfine interaction. The implications of paramagnetic shift and line broadening are examined. With this endeavor, we wish to encourage non-expert users to consider the application of paramagnetic NMR to their systems. Full article