Enhancing NMR Sensitivity of Natural-Abundance Low-γ Nuclei by Ultrafast Magic-Angle-Spinning Solid-State NMR Spectroscopy

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.

Atomic-Scale Resolution Insights into Structural and Dynamic Differences between Ofloxacin and Levofloxacin

This study employs advanced solid-state NMR techniques to investigate the atomic-level structure and dynamics of two enantiomers: ofloxacin and levofloxacin. The investigation focuses on critical attributes, such as the principal components of the chemical shift anisotropy (CSA) tensor, the spatial proximity of ¹H and ¹³C nuclei, and site-specific ¹³C spin-lattice relaxation time, to reveal the local electronic environment surrounding specific nuclei. Levofloxacin, the levo-isomer of ofloxacin, exhibits higher antibiotic efficacy than its counterpart, and the dissimilarities in the CSA parameters indicate significant differences in the local electronic configuration and nuclear spin dynamics between the two enantiomers. Additionally, the study employs the ¹H-¹³C frequency-switched Lee-Goldburg heteronuclear correlation (FSLGHETCOR) experiment to identify the presence of heteronuclear correlations between specific nuclei (C15 and H7 nuclei and C13 and H12 nuclei) in ofloxacin but not in levofloxacin. These observations offer insights into the link between bioavailability and nuclear spin dynamics, underscoring the significance of NMR crystallography approaches in advanced drug design.

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.

Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning

Solid-state nuclear magnetic resonance (NMR) often suffers from significant limitations due to the inherent low signal sensitivity when low-γ nuclei are involved. Herein, we report an elegant solid-state NMR approach for rapid structural analysis of minute amounts of organic solids. By encoding staggered chemical shift evolution in the indirect dimension and staggered acquisition in the ¹H dimension, a proton-detected homonuclear ¹H/¹H and heteronuclear ¹³C/¹H chemical shift correlation (HETCOR) spectrum can be obtained simultaneously in a single experiment at a fast magic-angle-spinning (MAS) condition with barely increasing the experimental time. We further show that during the conventional ¹H-detected HETCOR experimental time, multiple homonuclear ¹H/¹H correlation spectra can be recorded in addition to the HETCOR spectrum, enabling the determination of ¹H-¹H distances. We establish that abundant ¹H polarization can be efficiently manipulated and fully utilized in proton-detected solid-state NMR spectroscopy for extraction of more critical structural information and thus reduction of the total experimental time.

The role of water in the confinement of ibuprofen in SBA-15

The introduction of ibuprofen into mesopores of SBA-15 has been accomplished using the melting method. Samples exhibit from 9 to 33% of the hydrophobic drug. They are not toxic to mouse monocyte-macrophage cells and do not stimulate a pro-inflammatory response. The sample with 25% of the drug showed no crystalline ibuprofen and almost filled the mesopores, while the sample with 33% showed a total filling of the mesopores with some crystalline ibuprofen present. By means of 1D (¹H, ¹³C HPDEC, ¹³C CP MAS) and 2D (¹H-¹H NOESY) MAS NMR spectroscopy, it has been shown that water coexists with ibuprofen in mesopores and has an impact on the mobility of ibuprofen molecules and their location within the sample (outside or inside mesopores). Studies in the dehydrated state show for the first time that the high mobility of ibuprofen in mesopores is directly connected to the presence of water. Dehydrated samples show slightly slower release rates in comparison to their hydrated counterparts.

Multiplexing experiments in NMR and multi-nuclear MRI

Multiplexing NMR experiments by direct detection of multiple free induction decays (FIDs) in a single experiment offers a dramatic increase in the spectral information content and often yields significant improvement in sensitivity per unit time. Experiments with multi-FID detection have been designed with both homonuclear and multinuclear acquisition, and the advent of multiple receivers on commercial spectrometers opens up new possibilities for recording spectra from different nuclear species in parallel. Here we provide an extensive overview of such techniques, designed for applications in liquid- and solid-state NMR as well as in hyperpolarized samples. A brief overview of multinuclear MRI is also provided, to stimulate cross fertilization of ideas between the two areas of research (NMR and MRI). It is shown how such techniques enable the design of experiments that allow structure elucidation of small molecules from a single measurement. Likewise, in biomolecular NMR experiments multi-FID detection allows complete resonance assignment in proteins. Probes with multiple RF microcoils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems, effectively reducing the cost of NMR analysis and increasing the information content at the same time. Solid-state NMR experiments have also benefited immensely from both parallel and sequential multi-FID detection in a variety of multi-dimensional pulse schemes. We are confident that multi-FID detection will become an essential component of future NMR methodologies, effectively increasing the sensitivity and information content of NMR measurements.

Sensitivity enhancement in 2D Double Cross Polarization experiments under fast MAS by recycling unused protons

We demonstrate sensitivity enhancement via recycling of proton magnetization in 2D Double Cross Polarization (Double CP) experiments performed on fully protonated and uniformly labeled (¹³C, ¹⁵N) samples at a magic angle spinning rate of 60 kHz. Unused proton magnetization is preserved during t₁ evolution either by locking it with CW irradiation or by employing rotor-synchronized pi pulses. A flip-back pulse together with a modified second CP block preserves unused proton magnetization resulting in enhanced sensitivity. We have achieved sensitivity enhancements of 15-20% and 25-28% in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. At shorter recycle delays (∼0.25T₁), relative sensitivity enhancements of 40-45% and 55% were obtained in ¹H-¹³C and ¹H-¹⁵N 2D Double CP experiments respectively. An analysis of the sensitivity enhancements and theoretical estimation of lineshapes in indirect dimension in the presence of proton recycling is provided.

Molecular packing of pharmaceuticals analyzed with paramagnetic relaxation enhancement and ultrafast magic angle pinning NMR

Probing molecular details of fluorinated pharmaceutical compounds at a faster acquisition utilizing paramagnetic relaxation enhancement and better resolution from ultrafast magic angle spinning (ν_rot = 110 kHz) and high magnetic field (B₀ = 18.8 T).

Exploiting heterogeneous time scale of dynamics to enhance 2D HETCOR solid-state NMR sensitivity

Multidimensional solid-state NMR spectroscopy plays a significant role in offering atomic-level insights into molecular systems. In particular, heteronuclear chemical shift correlation (HETCOR) experiments could provide local chemical and structural information in terms of spatial heteronuclear proximity and through-bond connectivity. In solid state, the transfer of magnetization between heteronuclei, a key step in HETCOR experiments, is usually achieved using cross-polarization (CP) or insensitive nuclei enhanced by polarization transfer (INEPT) depending on the sample characteristics and magic-angle-spinning (MAS) frequency. But, for a multiphase system constituting molecular components that differ in their time scales of mobilities, CP efficiency is pretty low for mobile components because of the averaging of heteronuclear dipolar couplings whereas INEPT is inefficient for immobile components due to the short T₂ and can yield through-space connectivity due to strong proton spin diffusion for immobile components especially under moderate spinning speeds. Herein, in this study we present two 2D pulse sequences that enable the sequential acquisition of ¹³C/¹H HETCOR NMR spectra for the rigid and mobile components by taking full advantage of the abundant proton magnetization in a single experiment with barely increasing the overall experimental time. In particular, the ¹³C-detected HETCOR experiment could be applied under slow MAS conditions, where a multiple-pulse sequence is typically employed to enhance ¹H spectral resolution in the indirect dimension. In contrast, the ¹H-detected HETCOR experiment should be applied under ultrafast MAS, where CP and heteronuclear nuclear Overhauser effect (NOE) polarization transfer are combined to enhance ¹³C signal intensities for mobile components. These pulse sequences are experimentally demonstrated on two model systems to obtain 2D ¹³C/¹H chemical shift correlation spectra of rigid and mobile components independently and separately. These pulse sequences can be used for dynamics based spectral editing and resonance assignments. Therefore, we believe the proposed 2D HETCOR NMR pulse sequences will be beneficial for the structural studies of heterogeneous systems containing molecular components that differ in their time scale of motions for understanding the interplay of structures and properties.

Structure determination of supra-molecular assemblies by solid-state NMR: Practical considerations

In the cellular environment, biomolecules assemble in large complexes which can act as molecular machines. Determining the structure of intact assemblies can reveal conformations and inter-molecular interactions that are only present in the context of the full assembly. Solid-state NMR (ssNMR) spectroscopy is a technique suitable for the study of samples with high molecular weight that allows the atomic structure determination of such large protein assemblies under nearly physiological conditions. This review provides a practical guide for the first steps of studying biological supra-molecular assemblies using ssNMR. The production of isotope-labeled samples is achievable via several means, which include recombinant expression, cell-free protein synthesis, extraction of assemblies directly from cells, or even the study of assemblies in whole cells in situ. Specialized isotope labeling schemes greatly facilitate the assignment of chemical shifts and the collection of structural data. Advanced strategies such as mixed, diluted, or segmental subunit labeling offer the possibility to study inter-molecular interfaces. Detailed and practical considerations are presented with respect to first setting up magic-angle spinning (MAS) ssNMR experiments, including the selection of the ssNMR rotor, different methods to best transfer the sample and prepare the rotor, as well as common and robust procedures for the calibration of the instrument. Diagnostic spectra to evaluate the resolution and sensitivity of the sample are presented. Possible improvements that can reduce sample heterogeneity and improve the quality of ssNMR spectra are reviewed.

Forcing the 'lazy' protons to work

The combination of cross-polarization (CP) with flip-back (FB) pulse has enabled in NMR the enhancement of 13C sensitivity and the decrease of the recycling delay at both moderate and fast magic-angle spinning (MAS) frequencies. However, only continuous-wave (CW) decoupling is presently compatible with FB-pulse (FB-CW), and depending on the CW radio-frequency (rf) field, either an insignificant sensitivity gain or an acquisition time-dependent gain and a low 13C resolution are obtained. In this study, we propose a new FB-pulse method in which radio frequency-driven recoupling (RFDR) is used as the 1H-13C decoupling scheme to overcome these drawbacks. The performances of FB-RFDR in terms of decoupling efficiency and sensitivity gain are tested on both natural abundance (NA) and uniformly 13C-15N labeled l-histidine·HCl·H2O (Hist) samples at a MAS frequency of νR = 70 kHz. The results show the superiority of RFDR over the CW decoupling with respect to these criteria. Importantly, they reveal that the sensitivity gain offered by FB-RFDR is nearly independent of the decoupling/acquisition duration. The application of FB-RFDR on NA-Hist and sucrose yields a sensitivity gain between 60 and 100% compared to conventional FB-CW and CPMAS-SPINAL experiments. Moreover, we compare the 13C sensitivities of NA-Hist obtained by our 1D FB-RFDR method and 2D 1H-{13C} double-CP acquisition. Both methods provide similar 13C sensitivity and are complementary. Indeed, the 2D method has the advantage of also providing the 1H-13C spatial proximities, but its sensitivity for quaternary carbons is limited; whereas our 1D FB-RFDR method is more independent of the type of carbon, and can provide a 13C 1D spectrum in a shorter experimental time. We also test the feasibility of FB-RFDR at a moderate frequency of νR = 20 kHz, but the experimental results demonstrate a poor resolution as well as a negligible sensitivity gain.

Maximizing the sensitivity in 13C cross-polarization magic-angle-spinning solid-state NMR measurements with flip-back pulses

The flip-back pulse combined with cross-polarization magic-angle spinning (CPMAS-FB) has been widely used to reduce the optimal repetition delay, resulting in enhancing sensitivity per unit time. Despite its common use in samples with long ¹H T₁ relaxation time, the experimental setup of the repetition delay in CPMAS-FB is not obvious. In this article, a simple model is used to derive the optimal repetition delay and expected sensitivity gain through the ¹H T₁ relaxation time and signal decay during ¹H spinlock.

DNP enhanced NMR with flip-back recovery

DNP methods can provide significant sensitivity enhancements in magic angle spinning solid-state NMR, but in systems with long polarization build up times long recycling periods are required to optimize sensitivity. We show how the sensitivity of such experiments can be improved by the classic flip-back method to recover bulk proton magnetization following continuous wave proton heteronuclear decoupling. Experiments were performed on formulations with characteristic build-up times spanning two orders of magnitude: a bulk BDPA radical doped o-terphenyl glass and microcrystalline samples of theophylline, l-histidine monohydrochloride monohydrate, and salicylic acid impregnated by incipient wetness. For these systems, addition of flip-back is simple, improves the sensitivity beyond that provided by modern heteronuclear decoupling methods such as SPINAL-64, and provides optimal sensitivity at shorter recycle delays. We show how to acquire DNP enhanced 2D refocused CP-INADEQUATE spectra with flip-back recovery, and demonstrate that the flip-back recovery method is particularly useful in rapid recycling regimes. We also report Overhauser effect DNP enhancements of over 70 at 592.6 GHz/900 MHz.

Proton-Based Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy

Protons are vastly abundant in a wide range of exciting macromolecules and thus can be a powerful probe to investigate the structure and dynamics at atomic resolution using solid-state NMR (ssNMR) spectroscopy. Unfortunately, the high signal sensitivity, afforded by the high natural-abundance and high gyromagnetic ratio of protons, is greatly compromised by severe line broadening due to the very strong ¹H-¹H dipolar couplings. As a result, protons are rarely used, in spite of the desperate need for enhancing the sensitivity of ssNMR to study a variety of systems that are not amenable for high resolution investigation using other techniques including X-ray crystallography, cryo-electron microscopy, and solution NMR spectroscopy. Thanks to the remarkable improvement in proton spectral resolution afforded by the significant advances in magic-angle-spinning (MAS) probe technology, ¹H ssNMR spectroscopy has recently attracted considerable attention in the structural and dynamics studies of various molecular systems. However, it still remains a challenge to obtain narrow ¹H spectral lines, especially from proteins, without resorting to deuteration. In this Account, we review recent proton-based ssNMR strategies that have been developed in our laboratory to further improve proton spectral resolution without resorting to chemical deuteration for the purposes of gaining atomistic-level insights into molecular structures of various crystalline solid systems, using small molecules and peptides as illustrative examples. The proton spectral resolution enhancement afforded by the ultrafast MAS frequencies up to 120 kHz is initially discussed, followed by a description of an ensemble of multidimensional NMR pulse sequences, all based on proton detection, that have been developed to obtain in-depth information from dipolar couplings and chemical shift anisotropy (CSA). Simple single channel multidimensional proton NMR experiments could be performed to probe the proximity of protons for structure determination using ¹H-¹H dipolar couplings and to evaluate the changes in chemical environments as well as the relative orientation to the external magnetic field using proton CSA. Due to the boost in signal sensitivity enabled by proton detection under ultrafast MAS, by virtue of high proton natural abundance and gyromagnetic ratio, proton-detected multidimensional experiments involving low-γ nuclei can now be accomplished within a reasonable time, while the higher dimension also offers additional resolution enhancement. In addition, the application of proton-based ssNMR spectroscopy under ultrafast MAS in various challenging and crystalline systems is also presented. Finally, we briefly discuss the limitations and challenges pertaining to proton-based ssNMR spectroscopy under ultrafast MAS conditions, such as the presence of high-order dipolar couplings, friction-induced sample heating, and limited sample volume. Although there are still a number of challenges that must be circumvented by further developments in radio frequency pulse sequences, MAS probe technology and approaches to prepare NMR-friendly samples, proton-based ssNMR has already gained much popularity in various research domains, especially in proteins where uniform or site-selective deuteration can be relatively easily achieved. In addition, implementation of the recently developed fast data acquisition approaches would also enable further developments in the design and applications of proton-based ultrafast MAS multidimensional ssNMR techniques.