Transient effects in π-pulse sequences in MAS solid-state NMR

Measuring Dipolar Order Parameters in Nondeuterated Proteins Using Solid-State NMR at the Magic-Angle-Spinning Frequency of 100 kHz

Proteins are dynamic molecules, relying on conformational changes to carry out function. Measurement of these conformational changes can provide insight into how function is achieved. For proteins in the solid state, this can be done by measuring the decrease in the strength of anisotropic interactions due to motion-induced fluctuations. The measurement of one-bond heteronuclear dipole-dipole coupling at magic-angle-spinning (MAS) frequencies >60 kHz is ideal for this purpose. However, rotational-echo double resonance (REDOR), an otherwise gold-standard technique for the quantitative measurement of these couplings, is difficult to implement under these conditions, especially in nondeuterated samples. We present here a combination of strategies based on REDOR variants ϵ-REDOR and DEDOR (deferred REDOR) and simultaneously measure residue-specific ¹⁵N-¹H and ¹³C_α-¹H_α dipole-dipole couplings in nondeuterated systems at the MAS frequency of 100 kHz. These strategies open up avenues to access dipolar order parameters in a variety of systems at the increasingly fast MAS frequencies that are now available.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Heteronuclear and homonuclear radio-frequency-driven recoupling

The radio-frequency-driven recoupling (RFDR) pulse sequence is used in magic-angle spinning (MAS) NMR to recouple homonuclear dipolar interactions. Here we show simultaneous recoupling of both the heteronuclear and homonuclear dipolar interactions by applying RFDR pulses on two channels. We demonstrate the method, called HETeronuclear RFDR (HET-RFDR), on microcrystalline SH3 samples at 10 and 55.555 kHz MAS. Numerical simulations of both HET-RFDR and standard RFDR sequences allow for better understanding of the influence of offsets and paths of magnetization transfers for both HET-RFDR and RFDR experiments, as well as the crucial role of XY phase cycling.

Recent progress in dipolar recoupling techniques under fast MAS in solid-state NMR spectroscopy

With the recent advances in NMR hardware and probe design technology, magic-angle spinning (MAS) rates over 100 ​kHz are accessible now, even on commercial solid NMR probes. Under such fast MAS conditions, excellent spectral resolution has been achieved by efficient suppression of anisotropic interactions, which also opens an avenue to the proton-detected NMR experiments in solids. Numerous methods have been developed to take full advantage of fast MAS during the last decades. Among them, dipolar recoupling techniques under fast MAS play vital roles in the determination of the molecular structure and dynamics, and are also key elements in multi-dimensional correlation NMR experiments. Herein, we review the dipolar recoupling techniques, especially those developed in the past two decades for fast-to-ultrafast MAS conditions. A major focus for our discussion is the ratio of RF field strength (in frequency) to MAS frequency, ν₁/ν_r, in different pulse sequences, which determines whether these dipolar recoupling techniques are suitable for NMR experiments under fast MAS conditions. Systematic comparisons are made among both heteronuclear and homonuclear dipolar recoupling schemes. In addition, the schemes developed specially for proton-detection NMR experiments under ultrafast MAS conditions are highlighted as well.

Origin of the residual line width under frequency-switched Lee-Goldburg decoupling in MAS solid-state NMR

Homonuclear decoupling sequences in solid-state nuclear magnetic resonance (NMR) under magic-angle spinning (MAS) show experimentally significantly larger residual line width than expected from Floquet theory to second order. We present an in-depth theoretical and experimental analysis of the origin of the residual line width under decoupling based on frequency-switched Lee-Goldburg (FSLG) sequences. We analyze the effect of experimental pulse-shape errors (e.g., pulse transients and B 1 -field inhomogeneities) and use a Floquet-theory-based description of higher-order error terms that arise from the interference between the MAS rotation and the pulse sequence. It is shown that the magnitude of the third-order auto term of a single homo- or heteronuclear coupled spin pair is important and leads to significant line broadening under FSLG decoupling. Furthermore, we show the dependence of these third-order error terms on the angle of the effective field with the B 0 field. An analysis of second-order cross terms is presented that shows that the influence of three-spin terms is small since they are averaged by the pulse sequence. The importance of the inhomogeneity of the radio-frequency (rf) field is discussed and shown to be the main source of residual line broadening while pulse transients do not seem to play an important role. Experimentally, the influence of the combination of these error terms is shown by using restricted samples and pulse-transient compensation. The results show that all terms are additive but the major contribution to the residual line width comes from the rf-field inhomogeneity for the standard implementation of FSLG sequences, which is significant even for samples with a restricted volume.

Mitral annular plane systolic excursion by cardiac MR is an easy tool for optimized prognosis assessment in ST-elevation myocardial infarction

OBJECTIVES

The purpose of this study was to assess the comparative prognostic value of mitral annular plane systolic excursion (MAPSE) versus left ventricular ejection fraction (LVEF), measured by cardiac magnetic resonance (CMR) imaging in patients with ST-elevation myocardial infarction (STEMI) treated with primary percutaneous coronary intervention (pPCI).

METHODS

CMR was performed in 255 STEMI patients within 2 days (interquartile range (IQR) 2-4 days) after infarction. CMR included MAPSE measurement on CINE 4-chamber view. Patients were followed for major adverse cardiovascular events (MACE)-death, non-fatal myocardial re-infarction, stroke, and new congestive heart failure.

RESULTS

Patients with MACE (n = 35, 14%, median follow-up 3 years [IQR 1-4 years]) showed significantly lower MAPSE (8 mm [7-8.8] vs. 9.6 mm [8.1-11.5], p < 0.001). The association between decreased MAPSE (< 9 mm, optimal cut-off value by c-statistics) remained significant after adjustment for independent clinical and CMR predictors of MACE. The AUC of MAPSE for the prediction of MACE was 0.74 (CI 95% 0.65-0.82), significantly higher than that of LVEF (0.61 [CI 95% 0.50-0.71]; p < 0.001).

CONCLUSIONS

Reduced long-axis function assessed with MAPSE measurement using CINE CMR independently predicts long-term prognosis following STEMI. Moreover, MAPSE provided significantly higher prognostic implication in comparison with conventional LVEF measurement.

KEY POINTS

• MAPSE determined by CMR independently predicts long-term prognosis following STEMI. • MACE-free survival is significantly higher in patients with MAPSE ≥ 9 mm than < 9 mm. • MAPSE provides significantly higher prognostic implication than conventional LVEF.

Optimization of band-selective homonuclear dipolar recoupling in solid-state NMR by a numerical phase search

Spin polarization transfers among aliphatic ¹³C nuclei, especially ¹³Cα-¹³Cβ transfers, permit correlations of their nuclear magnetic resonance (NMR) frequencies that are essential for signal assignments in multidimensional solid-state NMR of proteins. We derive and demonstrate a new radio-frequency (RF) excitation sequence for homonuclear dipolar recoupling that enhances spin polarization transfers among aliphatic ¹³C nuclei at moderate magic-angle spinning (MAS) frequencies. The phase-optimized recoupling sequence with five π pulses per MAS rotation period (denoted as PR5) is derived initially from systematic numerical simulations in which only the RF phases are varied. Subsequent theoretical analysis by average Hamiltonian theory explains the favorable properties of numerically optimized phase schemes. The high efficiency of spin polarization transfers in simulations is preserved in experiments, in part because the RF field amplitude in PR5 is only 2.5 times the MAS frequency so that relatively low ¹H decoupling powers are required. Experiments on a microcrystalline sample of the β1 immunoglobulin binding domain of protein G demonstrate an average enhancement factor of 1.6 for ¹³Cα → ¹³Cβ polarization transfers, compared to the standard ¹³C-¹³C spin-diffusion method, implying a two-fold time saving in relevant 2D and 3D experiments.

Direct amide 15N to 13C transfers for solid-state assignment experiments in deuterated proteins

The assignment of protein backbone and side-chain NMR chemical shifts is the first step towards the characterization of protein structure. The recent introduction of proton detection in combination with fast MAS has opened up novel opportunities for assignment experiments. However, typical 3D sequential-assignment experiments using proton detection under fast MAS lead to signal intensities much smaller than the theoretically expected ones due to the low transfer efficiency of some of the steps. Here, we present a selective 3D experiment for deuterated and (amide) proton back-exchanged proteins where polarization is directly transferred from backbone nitrogen to selected backbone or sidechain carbons. The proposed pulse sequence uses only ¹H-¹⁵N cross-polarization (CP) transfers, which are, for deuterated proteins, about 30% more efficient than ¹H-¹³C CP transfers, and employs a dipolar version of the INEPT experiment for N-C transfer. By avoiding H^N-C (H^N stands for amide protons) and C-C CP transfers, we could achieve higher selectivity and increased signal intensities compared to other pulse sequences containing long-range CP transfers. The REDOR transfer is designed with an additional selective π pulse, which enables the selective transfer of the polarization to the desired ¹³C spins.