Applied Magnetic Resonance最新文献

The immobilization of dimeric [M₂(COD)₂(μ–Cl)₂] complexes (M – Rh or Ir) by the interaction with -SH groups of 3-mercaptopropyl-functionalized silica gel leads to Rh_Cl–S–SiO₂ and Ir_Cl–S–SiO₂ catalysts active in hydrogenations of propene and propyne. Nuclear magnetic resonance enhancement in parahydrogen-induced polarization experiments was studied in a wide range of hydrogenations conditions (25–120 °C, 1.0–3.9 bar). The structural transformations were studied using ex situ X-ray photoelectron spectroscopy (XPS). It was established that Ir_Cl–S–SiO₂ demonstrated greater thermal stability in the hydrogenation of both propene and propyne in comparison with Rh_Cl–S–SiO₂. The beneficial effect of propyne was elucidated for thermal stability of studied catalysts and for the efficiency of the pairwise hydrogen addition. This can be explained by more efficient binding of the C≡C triple bond to an active center. The increase in reaction pressure typically leads to higher conversion in hydrogenations for both catalysts, but also decreases the temperatures sufficient for the reduction of anchored complexes with the formation of metal nanoparticles, which was confirmed by XPS.

Eighty years ago, the advent of electron paramagnetic resonance (EPR) revolutionized our ability to observe the physical world of unpaired electron spins. The inception of EPR spawned multiple scientific areas with a focus on discerning the roles of paramagnetic metals and organic radicals in an array of processes and materials. More recently, the emergence of site-directed spin labeling combined with distance measurement technology and molecular modeling has harnessed the power of EPR, to ‘watch proteins move’. Spin labels have enabled the measurement of distance constraints and site-specific dynamics in biomolecules to provide rich details of structure and structural changes that are tightly linked to biological function. Historically, nitroxide radicals are the most common spin labels. However, decades of method development and technological innovation have created a plethora of spin label types to extend the reach of EPR throughout the realm of biophysics. In this review we overview recent developments that improve the sensitivity of distance measurements using Cu(II) labels. These achievements over the last three years promise advancements in the ability of EPR to measure structural and dynamical constraints beyond what is possible using common spin labels. First, we briefly discuss pulsed and continuous-wave EPR techniques that discern the coordination of Cu(II) to monitor spin-labeling efficiency and binding in biological environments. Next, we outline the  bottlenecks that impact sensitivity  in pulsed dipolar spectroscopy and the strategic steps taken to remove these bottlenecks to collect distance measurements in hours. More precisely, we focus on the fast-spin phase memory relaxation time, the broad EPR spectrum due to anisotropy, and orientational selectivity effects inherent to Cu(II). Finally, we showcase the versatile application of Cu(II) spin labels in biological systems and the advantages of Cu(II) in pulsed dipolar spectroscopy to access nanomolar protein concentrations.

A method for calculating the matrix elements of the long-range Coulomb interaction is proposed. The method is based on the Slater-type orbitals (STO) of ions forming an infinite crystal lattice. One-center matrix elements of the Coulomb interaction of an electron of a selected ion with a crystal lattice taken in the ionic approximation are considered. All obtained expressions are absolutely and rapidly convergent series in the space of reciprocal lattice vectors. The STO method is compared with the Gaussian-type orditals method and is illustrated by examples.

Chlorophyll is a pivotal molecule in photosynthesis due to its ability to absorb solar light and start the photochemical process. The chlorophyll triplet state is easily populated from the light-induced singlet excited state via Intersystem Crossing (ISC) and can react with oxygen generating singlet oxygen, posing a threat to the stability of chlorophyll, but also an opportunity for photodynamic therapy. Here, we focus on the study of the photo-physical properties of chlorophyll a and WSCP (Water-Soluble Chlorophyll Protein, a protein binding the pigment), which have been adsorbed in mesoporous silica matrices. We adopt SBA-15, a silica matrix with well-ordered hexagonal structure with pores of 70 Å of diameter. The triplet formed upon illumination in these systems is studied by time-resolved EPR spectroscopy. Analysis of the EPR spectra shows that the triplet state is very sensitive to the inclusion in the pores of SBA-15 modifying its spin polarization. Chl a in SBA-15 loses its central metal ion, while its structure remains unchanged if the mesoporous silica is previously soaked into a basic solution before Chl a adsorption, as revealed by its zero-field splitting parameters of the triplet state. WSCP is readily included in the silica matrix, but its embedded pigments are no longer protected by the protein core.

Single crystals of Yb³⁺-doped Cs₂NaScF₆ are synthesized under hydrothermal conditions and dopant Yb³⁺ ions are studied using electron paramagnetic resonance spectroscopy. The paramagnetic Yb³⁺ center of cubic symmetry is detected. The parameters of the corresponding spin Hamiltonians and the ground state Г₆ are determined. A structural model of the complex under study is proposed. The experimental results are analyzed by comparison with those for the paramagnetic Yb³⁺ ion in other lattices.

The application of inside-out magnetic resonance sensors has emerged as a focal point in soil moisture detection, offering a non-invasive approach crucial for evaluating soil physical structure. While existing inside-out magnetic resonance sensors with a unilateral structure can effectively measure the shallow soil layer, they fall short in assessing deeper soil layers (1–10 cm). However, the lack of optimal design and horizontal comparison among relevant sensors has impeded the widespread adoption of magnetic resonance sensors in soil moisture measurement. This paper addresses this gap by optimizing a magnetic resonance sensor with a dumbbell structure. The magnet structure is fine-tuned using a non-derivative optimization method, specifically the coordinate search, under practical engineering constraints to establish a uniform field in the region of interest. Employing the coordinate search method enhances the sensor’s relative signal-to-noise ratio (SNR) to its maximum. Artificial soil samples are then tested, and the T2 spectrum distribution reveals insights into water distribution characteristics across different pores.

To formulate the relationship between the dextrin hydrate concentration and T1 relaxation rate and create a magnetic resonance imaging (MRI) phantom with an arbitrary T1 value. Dextrin solution with nine different concentrations was prepared by dissolving 0–20 g (2.5 g increments) of dextrin in 25 g of purified water. The T1 values of the phantoms were measured using a 1.5 T MR scanner, and the relationship between the R1 value and dextrin concentration was regressed using linear and quadratic equations. Phantoms with concentrations adjusted to T1 values of 500, 1000, and 1500 ms were created from each regression equation, and the errors between the measured T1 and target values were evaluated. In addition, the temporal changes in the T1 and T2 values of the phantoms were also evaluated. The T1 and T2 values ranged from 367.4 ± 14.1 to 2577.6 ± 76.5 ms and 20.0 ± 0.9 to 1805.3 ± 8.3 ms, respectively. The linear and quadratic regression equations were (y=2.9631x+0.2043) and (y=1.8295{x}^{2}+1.4995x+0.37), with coefficients of determination of 0.9763 and 0.9954, respectively. The maximum errors were 12.3% and 2.1% for the linear and quadratic equations, respectively. The T1 value was maintained at a fluctuation rate of approximately 10% during the first 4 weeks. The T2 value decreased by approximately 20% after 4 weeks. MRI phantoms with arbitrary T1 values in the range of 500–1500 ms with an error within 2.1% can be created using dextrin, which can be used as human tissue-equivalent MRI phantoms for T1 of the grey or white matter of the brain, liver, pancreas, spleen, and prostate.

The dynamics of the photogenerated triplet electron spin polarization (ESP) of two perylenebisimide (PBI) derivatives, PBI-2Br and PBI-Br-NH, is studied by the time-resolved electron paramagnetic resonance (TREPR). The ESP inversion in the triplet TREPR spectra of two compounds is detected at delays after the laser pulse (DAF) of ~ 500–900 ns. A fairly efficient algorithm for the exact numerical solution of the equation of motion of the spin density matrix of an ensemble of photoexcited triplets taking into account the anisotropy of the decay rates of sublevels of the triplet state, spin–lattice and spin–spin relaxations is elaborated in order to analyze the ESP inversion of TREPR spectra. The calculated time evolution of the TREPR spectra and its fitting experimental ones shows that even a slight difference in decay rates for the triplet sublevels of PBI-2Br results in the ESP inversion in the TREPR spectra already at DAF of ~ 500 ns. At the same time, decay rates ( τ_X⁻¹ = 4 µs⁻¹; τ_Y⁻¹ = 4.4 µs⁻¹; τ_Z⁻¹ = 3.3 µs⁻¹) are higher than those for the available examples of the ESP inversion of the triplet TREPR spectra. The replacement of a Br atom by NH leads to noticeable changes in the photoinduced properties of the triplet including decay rates (τ_X⁻¹ = 1.25 µs⁻¹; τ_Y⁻¹ = 0.125 µs⁻¹; τ_Z⁻¹ = 0.125 µs⁻¹). PBI-Br-NH is characterized by an axial decay rate anisotropy with τ_x⁻¹, which is ten times higher than τ_Y⁻¹ and τ_Z⁻¹. The trend in the triplet lifetime change obtained from the TREPR spectra is consistent with the data of the nanosecond transient absorption spectra.

In recent years, the theory of the effect of saturation of EPR spectra of free radicals undergoing spin exchange has been extended to spin exchange frequencies where “peculiar” behavior occurs. In a paper by Salikhov (Appl. Magn. Reson. (2021) 52:1063–1091), analytic expressions were developed that predict the dependence of measurable EPR parameters on the exchange frequencies and the microwave field strength. This work is an experimental test of that theory where, in principle, there are no adjustable parameters.