Journal of Magnetic Resonance Open最新文献

Higher magic angle spinning (MAS) frequencies than currently available are desirable to improve spectral resolution in NMR and EPR systems. While conventional strategies employ pneumatic spinning limited by fluid dynamics, this paper demonstrates the development of an optical spinning technique in which vacuum quality dictates the maximum achievable spinning frequency. Using optical traps, we levitated a range of micron-sized samples. Under vacuum we achieved optical rotation of a single ∼10 μm diameter particle of vaterite at several mbar up to hundreds of Hz and of 20 μm diameter SiO₂ particles at ≤10⁻² mbar at several kHz. At ambient conditions, we optically levitated γ-irradiated alanine particles of 20–50 μm diameter. Additionally, using a single chip EPR detector operating at 11 GHz, we measured the EPR spectrum for a 30 μm γ-irradiated alanine particle in contact with the chip surface (i.e., without optical levitation) in a single scan lasting 92 s. These observations suggest that a γ-irradiated alanine particle having a diameter in the order of 30 μm is a promising candidate for our aim of demonstrating the first magnetic resonance experiment on optically levitated samples. Furthermore, we discuss strategies, limitations, and the potential of implementing MAS with optical traps for NMR and EPR.

Tuberculosis and non-tuberculosis mycobacterial infections are rising each year and often result in chronic incurable disease. Important antibiotics target cell-wall biosynthesis, yet some mycobacteria are alarmingly resistant or tolerant to currently available antibiotics. This resistance is often attributed to assumed differences in composition of the complex cell wall of different mycobacterial strains and species. However, due to the highly crosslinked and insoluble nature of mycobacterial cell walls, direct comparative determinations of cell-wall composition pose a challenge to analysis through conventional biochemical analyses. We introduce an approach to directly observe the chemical composition of mycobacterial cell walls using solid-state NMR spectroscopy. ¹³C CPMAS spectra are provided of individual components (peptidoglycan, arabinogalactan, and mycolic acids) and of in situ cell-wall complexes. We assigned the spectroscopic contributions of each component in the cell-wall spectrum. We uncovered a higher arabinogalactan-to-peptidoglycan ratio in the cell wall of M. abscessus, an organism noted for its antibiotic resistance, relative to M. smegmatis. Furthermore, differentiating influences of different types of cell-wall targeting antibiotics were observed in spectra of antibiotic-treated whole cells. This platform will be of value in evaluating cell-wall composition and antibiotic activity among different mycobacteria and in considering the most effective combination treatment regimens.

Commercial availability, ease of printing and cost effectiveness have rendered 3D printing an essential part of magnetic resonance (MR) experimental design. However, the magnetic properties of several materials contemporarily used for 3D printing are lacking in literature to some extent. A database of the magnetic susceptibilities of several commonly used 3D printing materials is provided, which may aid MR experiment design. Here, we exploit the capability of magnetic resonance imaging (MRI) to map the local magnetic field variations caused by these materials when placed in the scanner's B₀ field. Exact analytical solutions of the magnetic flux density distribution for a cylindrical geometry are utilized to fit experimentally obtained data with theory in order to quantify the magnetic susceptibilities. A detailed explanation of the data processing and fitting procedure is presented and validated by measuring the susceptibility of air along with high resolution MR measurements. Furthermore, an initiative is taken to address the need for a comprehensive database comprising of not only the magnetic susceptibilities of 3D printing materials, but also information on the 3D printing parameters, the printers used, and other information available for the materials that may also influence the measured magnetic properties. An open platform with the magnetic susceptibilities of materials reported in this work besides existing literature values is provided here, with the aim to invite researchers to enable further extension and development towards an open database to characterize commonly used 3D printing materials based on their magnetic properties.

Although very effective in decreasing NMR relaxation of large proteins, homogeneous deuteration can be costly, and anyway unsuitable for recombinant production in metazoan systems. We sought to explore other deuteration schemes, which would be adapted to protein expression in mammalian cells. Here, we evaluate the benefits of the deuteration on alpha- and beta-positions of amino acids for a typical middle size protein domain, namely the model 40 kDa-large kinase p38α. We report the position-specific deuteration of free amino acids by using enzyme-assisted H/D exchange, executed by the cystathionine gamma-synthase and a newly designed high-performance mutant E325A. Then, we used cell-free expression in bacterial extracts to avoid any scrambling and back-protonation of the tested isotopically labelled amino acids (Ala, Leu, Lys, Ser, Asp, Glu, Gly). Our results show signal enhancements up to three in ¹H-¹⁵N spectra when these α/β-deuterated amino acids are integrated. Because our approach relies on single ²H_α/β-¹⁵N-amino acid labeling, an additional three-fold increase in sensitivity is obtained by the possible use of moderate resolution SOFAST-HMQC instead of the classical HSQC or TROSY experiments. This allows recording residue-resolved solution ¹H-¹⁵N NMR spectra of 100 μg of p38α in one hour with S/N∼10.

Magnetic resonance imaging (MRI) is a well-known and widespread imaging modality for neuroscience studies and the clinical diagnoses of neurological disorders, mainly due to its capability to visualize brain microstructures and quantify various metabolites. Additionally, its noninvasive nature makes possible the correlation of high-resolution MRI from ex vivo brain samples with histology, supporting the study of neurodegenerative disorders such as Alzheimer's or Parkinson's disease. However, the quality and resolution of ex vivo MRI highly depend on the availability of specialized radiofrequency coils with maximized filling factors for the different sizes and shapes of the samples to be studied. For instance, small, dedicated radiofrequency (RF) coils are not always commercially available in ultrahigh field whole-body MRI scanners. Even for ultrahigh field preclinical scanners, specific RF coils for ex vivo MRI are expensive and not always available. Here, we describe the design and construction of two RF coils based on the solenoid geometry for ex vivo MRI of human brain tissues in a 7T whole-body scanner and for ex vivo MRI of marmoset brain samples in a 9.4T preclinical scanner. We designed the 7T solenoid RF coil to maximize the filling factor of human brain samples conditioned on cassettes for histology, while the 9.4T solenoid was constructed to accommodate marmoset brain samples conditioned in 50 ml centrifuge tubes. Both solenoid designs operate in transceiver mode. The measured B₁⁺ maps show a high level of homogeneity in the imaging volume of interest, with a high signal-to-noise ratio over the imaging volume. High-resolution (80 µm in plane, 500 µm slice thickness) images of human brain samples were acquired with the 7T solenoid, while marmoset brain samples were acquired with an isotropic resolution of 60 µm using the 9.4T solenoid coil.

Plant defensins (PDs) display a CSαβ-fold that lacks a canonical hydrophobic core. They display almost all hydrophobic residues on the protein surface. The exposed hydrophobic residues form surface clusters stabilized by the vicinity of hydrophilic residues and the hydration shell. Here, we used Psd2 as a model to study the formation and stabilization of these local foldons named surface hydrophobic clusters (SHC). We characterized the temperature dependence of ¹⁵N CPMG relaxation dispersion profiles to describe the complex dynamics of Psd2 and indirectly study the thermodynamics of SHCs in PDs. We show a correlation between residues undergoing conformational exchange and the SHCs. Chemical shift changes between the native ground state and the first thermally accessible excited state enabled us to map the major conformational changes in Psd2 conformational equilibrium. The observation of a cold-driven excited state revealed that SHCs are stabilized by hydrophobic contacts, which are exposed at low temperatures, leading to a favorable decrease in enthalpy compensated by an unfavorable entropy reduction. At higher temperatures, we detected another excited conformer that may play a role in membrane-specific interaction, as previously described for other defensins.

Oxide-based materials are of key technological importance in different areas including advanced functional materials, solid state chemistry and catalysis. Many of the key questions concerning these areas involve understanding the chemical bond between the metal and the oxygen ions in the first or subsequent coordinating shells. The spectroscopic study of oxygen is therefore of fundamental importance to elucidate the complex interfacial coordination chemistry that underlies the development of metal-oxide supported catalysts and other advanced materials. Oxygen atoms at solid surfaces or lining the pores of zeolite frameworks play a vital role in stabilizing and defining the electronic and geometric structure of single metal atoms or clusters that act as catalytically active sites. In the case of paramagnetic species, EPR and its related hyperfine techniques offer a unique opportunity to explore and understand the nature of the chemical bonding in metal-oxide systems through the detection of the ¹⁷O hyperfine interaction. In this perspective we offer an overview of experimental considerations and relevant examples specific to ¹⁷O hyperfine spectroscopy of transition metal ions in zeolites relevant to catalysis. ¹⁷O hyperfine coupling values are obtained, which allow discriminating σ- and π-bonding channels in metal-oxygen bonds involving first-row transition metal ions. An exhaustive collection of ¹⁷O hyperfine and nuclear quadrupole couplings in different systems including molecular and biomolecular chemistry is provided, emphasizing the connection between interfacial and molecular inorganic coordination chemistry.

The theoretical foundations of NMR and EPR are very similar. Nevertheless, the fields differ significantly in practical aspects. This primer is an attempt to bridge some of the gaps between the fields, and at the same time it highlights some applications of pulse EPR. Basic EPR pulse sequences to detect nuclei in the vicinity of unpaired electrons are introduced. An emphasis is placed on explaining the concept to researchers familiar with NMR. The theoretical background as well as examples are given for electron-nuclear double resonance (ENDOR), electron-spin echo envelope modulation (ESEEM), including hyperfine sublevel correlation spectroscopy (HYSCORE), and electron double resonance (ELDOR)-detected NMR.

At low magnetic fields (<1 T), the low loss of radio frequency (RF) coils can cause RF pulses, in magnetic resonance (MR) experiments, to suffer from long rise and fall times. When severe, long rise and fall times can result in RF pulses with undesirable and/or unexpected shapes, lengths, and amplitudes leading to inadequate spin control during an MR experiment. Experimentally, the lack of spin control becomes evident in the shape of the nutation curve and the inversion efficiency, the degree to which full inversion is accomplished following a 180° pulse. Lowering the quality factor (Q) of tuned coil is shown to reduce the duration of the rise and fall times. The effects of long rise and fall times on the spin behavior during an MR experiment is explored. It is shown that the inversion efficiency can be used to provide a threshold that ensures high fidelity pulses with adequate spin control.

An electron spin echo in a nitroxide-containing polymer cathode film for organic radical batteries is observed for various states of charge at cryogenic temperatures. The EPR-detected state of charge (ESOC), as inferred from the number of paramagnetic centers in the film, is compared to the results of Coulomb counting based on galvanostatic charging. Spin concentration, longitudinal relaxation times $T_1$ and phase memory times $T_m$ strongly correlate with the ESOC. In the discharged film, the spin concentration reaches $\left(5\pm 3\right)\times 10^{20}$ cm⁻³, causing a phase memory time $T_m\ll$ 100 ns (shorter than the resonator ring-down time) that hinders the detection of the spin echo. In the charged film, the decreased spin concentration results in a longer $T_m$ between 100 ns and 300 ns that enables spin-echo detection, yet limits the length of the microwave pulse sequence. The short, broad-band pulses cause instantaneous diffusion in the unoxidized domains across the oxidized film, affecting the relative peak intensities in the pulsed EPR spectrum. By simulating the spectral distortion caused by instantaneous diffusion, we obtain information on the local spin concentration, which complements the information on the ‘bulk’ spin concentration determined by electrochemistry and continuous-wave EPR spectroscopy.