Journal of magnetic resonance最新文献

Motionally averaged dipolar couplings are an important tool for understanding the complex dynamics of catalysts, polymers, and biomolecules. While there is a plethora of solid-state NMR pulse sequences available for their measurement, in can be difficult to gauge the methods’ strengths and weaknesses. In particular, there has not been a comprehensive comparison of their performance in natural abundance samples, where ¹H homonuclear dipolar couplings are important and the use of large MAS rotors may be required for sensitivity reasons. In this work, we directly compared some of the more common methods for measuring C–H dipolar couplings in natural abundance samples using L-alanine (L-Ala) and the N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLF) tripeptide as model systems. We evaluated their performance in terms of accuracy, resolution, sensitivity, and ease of implementation. We found that, despite the presence of ¹H homonuclear dipolar interactions, all methods, with the exception of REDOR, were able to yield the reasonable dipolar coupling strengths for both mobile and static moieties. Of these methods, PDLF provides the most convenient workflow and precision at the expense of low sensitivity. In low-sensitivity cases, MAS-PISEMA and DIPSHIFT appear to be the better options.

Bacterial cell walls are gigadalton-large cross-linked polymers with a wide range of motional amplitudes, including rather rigid as well as highly flexible parts. Magic-angle spinning NMR is a powerful method to obtain atomic-level information about intact cell walls. Here we investigate sensitivity and information content of different homonuclear ¹³C¹³C and heteronuclear ¹H¹⁵N, ¹H¹³C and ¹⁵N¹³C correlation experiments. We demonstrate that a CPMAS CryoProbe yields ca. 8-fold increased signal-to-noise over a room-temperature probe, or a ca. 3–4-fold larger per-mass sensitivity. The increased sensitivity allowed to obtain high-resolution spectra even on intact bacteria. Moreover, we compare resolution and sensitivity of ¹H MAS experiments obtained at 100 kHz vs. 55 kHz. Our study provides useful hints for choosing experiments to extract atomic-level details on cell-wall samples.

While pulsed field gradient stimulated echo nuclear magnetic resonance (PFGSTE NMR) spectroscopy has found widespread use in the quantification of self-diffusivity for many NMR-active nuclei, extending this technique to uncommon nuclei with unfavorable NMR properties remains an active area of research. Potassium-39 (³⁹K) is an archetypical NMR nucleus exhibiting an unfavorable gyromagnetic ratio combined with a very low Larmor frequency. Despite these unfavorable properties, this work demonstrates that ³⁹K PFGSTE NMR experiments are possible in aqueous solutions of concentrated potassium nitrite. Analysis of the results indicates that ³⁹K NMR diffusometry is feasible when the nuclei exhibit spin–lattice and spin–spin relaxation coefficients on the order of 60–100 ms and 50–100 ms, respectively. The diffusivity of ³⁹K followed Arrhenius behavior, and comparative ²³Na, ⁷Li, and ¹H PFGSTE NMR studies of equimolal sodium nitrite and lithium nitrite solutions led to correlations between the enthalpy of hydration with the activation energy governing self-diffusion of the cations and also of water. Realizing the feasibility of ³⁹K PFGSTE NMR spectroscopy has a widespread impact across energy sciences because potassium is a common alkali element in energy storage materials and other applications.

Over the last decade chemical exchange saturation transfer (CEST) NMR methods have emerged as powerful tools to characterize biomolecular conformational dynamics occurring between a visible major state and ‘invisible’ minor states. The ability of the CEST experiment to detect these minor states, and provide precise exchange parameters, hinges on using appropriate B₁ field strengths during the saturation period. Typically, a pair of B₁ fields with ${\omega }_1$ (=${2\pi B}_1$) values around the exchange rate k_ex are chosen. Here we show that the transverse relaxation rate of the minor state resonance ($R_{2,B}$) also plays a crucial role in determining the B₁ fields that lead to the most informative datasets. Using $K={\left(k_{\mathrm{ex}}\left(k_{\mathrm{ex}}+R_{2,B}\right)\right)}^{\frac{1}{2}}$ ≥ k_ex, to guide the choice of B₁, instead of k_ex, leads to data wherefrom substantially more accurate exchange parameters can be derived. The need for higher B₁ fields, guided by $K$, is demonstrated by studying the conformational exchange in two mutants of the 71 residue FF domain with $k_{ex}$ ∼ 11 s⁻¹ and ∼ 72 s⁻¹, respectively. In both cases analysis of CEST datasets recorded using B₁ field values guided by $k_{ex}$ lead to imprecise exchange parameters, whereas using B₁ values guided by $K$ resulted in precise site-specific exchange parameters. The conclusions presented here will be valuable while using CEST to study slow processes at sites with large intrinsic relaxation rates, including carbonyl sites in small to medium sized proteins, amide ¹⁵N sites in large proteins and when the minor state dips are broadened due to exchange among the minor states.

Multiple-quantum (MQ) NMR experiments were performed at a special orientation of a hambergite (Be₂BO₃OH) single crystal, which consists of alternating zigzag proton chains. At the chosen orientation, one of the dipolar coupling constants in the chain becomes zero and the system becomes a set of well-isolated dipolar coupled spin pairs. The relaxation of the spin pairs in the MQ NMR experiment was studied on the basis of the Lindblad equation. Fermi’s golden rule was used to investigate the relaxation mechanism. The agreement of the calculated relaxation time with the experimental value (125 μs) suggests that the dipole–dipole interactions with protons surrounding the pair are responsible for the relaxation of MQ coherences.

Simple physical models for restricted diffusion in a potential, which provide important insights for NMR spin relaxation, usually are based on free diffusion within rigid boundaries or diffusion in relatively simple continuous potential energy surfaces. The diffusion-in-a-cone model is an example of the former and diffusion in an $N$-fold cosine potential is an example of the latter. The present work models restricted diffusion for arbitrary potential energy functions on the surface of a cone or a sphere, by expanding the potentials in Fourier or spherical harmonic series, respectively. The results exhibit simple relationships between generalized order parameters and effective correlation times, critical for analysis of experimental spin relaxation data, and illustrate the transition from diffusive-like to jump-like behavior in multi-well potentials.

It is advantageous to investigate milli-to-micro-second conformational exchange data contained in the solution NMR protein relaxation data other than ¹⁵N nuclei. Not only does one search under another lamp post, one also looks at dynamics at other time scales. The HSQC-ROESY ¹HN relaxation dispersion experiment for amide protons as introduced by Ishima, et al (1998). J. Am. Soc. 120, 10534–10542, is such an experiment, but has by the authors been advised to only be used for perdeuterated proteins to avoid complication with the ¹H–¹H multiple-spin effects. This is regretful, since not all proteins can be perdeuterated.

Here we analyze in detail the ¹HN relaxation terms for this experiment for a fully proteated protein. Indeed, the ¹HN relaxation theory is in this case complex and includes dipolar-dipolar relaxation interference and TOCSY transfers. With simulate both of these effects and show that the interference can be exploited for detecting exchange broadening. The TOCSY effect is shown to minor, and when it is not, a solution is provided. We apply the HSQC-ROESY experiment, with a small modification to suppress ROESY crosspeaks, to a 7 kDa GB1 protein that is just ¹⁵N and ¹³C labeled. At 10 °C we cannot detect any conformational exchange broadening: the ¹HN R₂ relaxation rates with 1.357 kHz spinlock field not larger than those recorded with a 12.136 kHz spinlock field. This means that there is no exchange broadening that can be differentially suppressed with the applied fields. Either there is no broadening, or the broadening is effectively suppressed by all fields, or the broadening cannot be suppressed by either of the fields. While initially this seems to be a disappointing result, we feel that this work establishes that the HSQC-ROESY experiment is very robust. It can indeed be utilized for proteated proteins upto about 30 kDa. This could be opening the study the milli-microsecond conformational dynamics as reported by ¹HN exchange broadening for many more proteins.

Magic-angle spinning (MAS) solid-state NMR methods are crucial in many areas of biology and materials science. Conventional probe designs have often been specified with 0.1 part per million (ppm) or 100 part per billion (ppb) magnetic field resolution, which is a limitation for many modern scientific applications. Here we describe a novel 5-mm MAS module design that significantly improves the linewidth and line shape for solid samples by an improved understanding of the magnetic susceptibility of probe materials and geometrical symmetry considerations, optimized to minimize the overall perturbation to the applied magnetic field (B₀). The improved spinning module requires only first and second order shimming adjustments to achieve a sub-Hz resolution of ¹³C resonances of adamantane at 150 MHz Larmor frequency (14.1 Tesla magnetic field). Minimal use of third and higher order shims improves experimental reproducibility upon sample changes and the exact placement within the magnet. Furthermore, the shimming procedure is faster, and the required gradients smaller, thus minimizing thermal drift of the room temperature (RT) shims. We demonstrate these results with direct polarization (Bloch decay) and cross polarization experiments on adamantane over a range of sample geometries and with multiple superconducting magnet systems. For a direct polarization experiment utilizing the entire active sample volume of a 5-mm rotor (90 µl), we achieved full width at half maximum (FWHM) of 0.76 Hz (5 ppb) and baseline resolved the ¹³C satellite peaks for adamantane as a consequent of the 7.31 Hz (59 ppb) width at 2% intensity. We expect these approaches to be increasingly pivotal for high-resolution solid-state NMR spectroscopy at and above 1 GHz ¹H frequencies.

Magnetic Resonance Imaging (MRI) often encounters image quality degradation due to magnetic field inhomogeneities. Conventional passive shimming techniques involve the manual placement of discrete magnetic materials, imposing limitations on correcting complex inhomogeneities. To overcome this, we propose a novel 3D printing method utilizing binder jetting technology to enable precise deposition of a continuous range of concentrations of ferromagnetic ink. This approach grants complete control of the magnitude of the magnetic moment within the passive shim enabling tailored corrections of B₀ field inhomogeneities. By optimizing the magnetic field distribution using linear programming and an in-house written Computer-Aided Design (CAD) generation software, we printed shims with promising results in generating low spherical harmonic corrections. Experimental evaluations demonstrate feasibility of these 3D printed passive shims to induce target magnetic fields corresponding to second-order spherical harmonic, as evidenced by acquired B₀ maps. The electrically insulating properties of the printed shims eliminate the risk of eddy currents and heating, thus ensuring safety. The dimensional fabrication accuracy of the printed shims surpasses previous methods, enabling more precise and localized correction of subject-specific inhomogeneities. The findings highlight the potential of binder-jetted 3D printed passive shims in MRI shimming as a versatile and efficient solution for fabricating passive shims, with the potential to enhance the quality of MRI imaging while also being applicable to other types of Magnetic Resonance systems.

Experimental confirmation of the manifestations of new spin exchange paradigm in EPR spectra of ¹⁴N nitroxide radical solutions is presented. It was shown that in the region of relatively low concentrations of radicals, the two side components of the spectrum have a mixed shape (the sum of the absorptive line and dispersive line). The dispersion contributions in these two lines have opposite signs. As the concentration of radicals increases, the contribution of dispersion passes through an extremum and in the region of maximum contribution of dispersion, the contribution of absorption to these two lines changes sign. In the region of high concentrations of radicals, when one homogeneously broadened line is practically observed, it turns out that these side components have resonant frequencies that do not coincide with the frequency of the center of gravity of the spectrum.