Proton-Detected Solid-State NMR Spectroscopy of Bone with Ultrafast Magic Angle Spinning

Regulating polystyrene glass transition temperature by varying the hydration levels of aromatic ring/Li+ interaction

Polymer properties can be altered via lithium ion doping, whereby adsorbed Li+ binds with H2O within the polymer chain. However, direct spectroscopic evidence of the tightness of Li+/H2O binding in the solid state is limited, and the impact of Li+ on polymer sidechain packing is rarely reported. Here, we investigate a polystyrene/H2O/LiCl system using solid-state NMR, from which we determined a dipolar coupling of 11.4 kHz between adsorbed Li+ and H2O protons. This coupling corroborates a model whereby Li+ interacts with the oxygen atom in H2O via charge affinity, which we believe is the main driving force of Li+ binding. We demonstrated the impact of hydrated Li+ on sidechain packing and dynamics in polystyrene using proton-detected solid-state NMR. Experimental data and density functional theory (DFT) simulations revealed that the addition of Li+ and the increase in the hydration levels of Li+, coupled with aromatic ring binding, change the energy barrier of sidechain packing and dynamics and, consequently, changes the glass transition temperature of polystyrene.

1 H-13 C cross-polarization kinetics to probe hydration-dependent organic components of bone extracellular matrix

Bone is a living tissue made up of organic proteins, inorganic minerals, and water. The organic component of bone (mainly made up of Type-I collagen) provides flexibility and tensile strength. Solid-state nuclear magnetic resonance (ssNMR) is one of the few techniques that can provide atomic-level structural insights of such biomaterials in their native state. In the present article, we employed the variable contact time cross-polarization (¹ H-¹³ C CP) kinetics experiments to study the hydration-dependent atomic-level structural changes in the bone extracellular matrix (ECM). The natural abundant ¹³ C CP intensity of the bone ECM is measured by varying CP contact time and best fitted to the nonclassical kinetic model. Different relaxation parameters were measured by the best-fit equation corresponding to the different hydration conditions of the bone ECM. The associated changes in the measured parameters due to varying levels of hydration observed at different sites of collagen protein have provided its structural arrangements and interaction with water molecules in bone ECM. Overall, the present study reveals a better understanding of the kinetics of the organic part inside the bone ECM that will help in comprehending the disease-associated pathways.

Nuclear spin diffusion under fast magic-angle spinning in solid-state NMR

Solid-state nuclear spin diffusion is the coherent and reversible process through which spin order is transferred via dipolar couplings. With the recent increases in magic-angle spinning (MAS) frequencies and magnetic fields becoming routinely applied in solid-state nuclear magnetic resonance, understanding how the increased 1H resolution obtained affects spin diffusion is necessary for interpretation of several common experiments. To investigate the coherent contributions to spin diffusion with fast MAS, we have developed a low-order correlation in Liouville space model based on the work of Dumez et al. (J. Chem. Phys. 33, 224501, 2010). Specifically, we introduce a new method for basis set selection, which accounts for the resonance-offset dependence at fast MAS. Furthermore, we consider the necessity of including chemical shift, both isotropic and anisotropic, in the modeling of spin diffusion. Using this model, we explore how different experimental factors change the nature of spin diffusion. Then, we show case studies to exemplify the issues that arise in using spin diffusion techniques at fast spinning. We show that the efficiency of polarization transfer via spin diffusion occurring within a deuterated and 100% back-exchanged protein sample at 60 kHz MAS is almost entirely dependent on resonance offset. We additionally identify temperature-dependent magnetization transfer in beta-aspartyl L-alanine, which could be explained by the influence of an incoherent relaxation-based nuclear Overhauser effect.

Selective excitation with recoupling pulse schemes uncover properties of disordered mineral phases in bone-like apatite grown with bone proteins

Bone construction has been under intensive scrutiny for many years using numerous techniques. Solid-state NMR spectroscopy helped unravel key characteristics of the mineral structure in bone owing to its capability of analyzing crystalline and disordered phases at high-resolution. This has invoked new questions regarding the roles of persistent disordered phases in structural integrity and mechanical function of mature bone as well as regarding regulation of early events in formation of apatite by bone proteins which interact intimately with the different mineral phases to exert biological control. Here, spectral editing tethered to standard NMR techniques is employed to analyze bone-like apatite minerals prepared synthetically in the presence and absence of two non-collagenous bone proteins, osteocalcin and osteonectin. A ¹H spectral editing block allows excitation of species from the crystalline and disordered phases selectively, facilitating analysis of phosphate or carbon species in each phase by magnetization transfer via cross polarization. Further characterization of phosphate proximities using SEDRA dipolar recoupling, cross-phase magnetization transfer using DARR and T₁/T₂ relaxation times demonstrate that the mineral phases formed in the presence of bone proteins are more complex than bimodal. They reveal disparities in the physical properties of the mineral layers, indicate the layers in which the proteins reside and highlight the effect that each protein imparts across the mineral layers.

Investigating bone resorption in Atlantic herring fish intermuscular bones with solid-state NMR

Bones are connective tissues mainly made of collagen proteins with calcium phosphate deposits. They undergo constant remodeling, including destroying existing bones tissues (known as bone resorption) and rebuilding new ones. Bone remodeling has been well-described in mammals, but it is not the case in fish. Here, we focused on the mobile phase of the bone vascular system by carefully preserving moisture in adult Atlantic herring intermuscular bones. We detected pore water with high ionic strength and soluble degraded peptides whose ¹H-transverse relaxation times, T₂s, exceed 15 milliseconds. With favorable T₂s, we incorporated a solution state spinlock scheme into the INEPT techniques to unequivocally demonstrate collagen degradation. In addition, we detected a substantial amount of inorganic phosphate in solution with ³¹P-NMR in the considerable background of solid hydroxyapatite calcium phosphate by saturation recovery experiment. It is consistent with the idea that bone resorption degrades bone collagen and releases calcium ions and phosphate ions in the pore water with increased ionic strength. Our report is the first to probe the resorption process in the heterogenous bone microstructure with a rigorous characterization of ¹H and ¹³C relaxation behavior and direct assignments. In addition, we contribute to the fish bones literature by investigating fish bone remodeling using NMR for the first time.

Ultrafast magic angle spinning NMR characterization of pharmaceutical solid polymorphism: A posaconazole example

Protons represent the most NMR-sensitive nucleus in pharmaceutical compounds. Therefore, proton-detected solid-state NMR techniques under fast magic angle spinning are among the few solutions to overcome the challenge of low sensitivity to analyze natural abundant drug substances and products. In this study, we report the structural characterization of crystal polymorphs of a commercial drug molecule, posaconazole, with a relatively large molecular weight of 700.8 g·mol^-1 and at the natural abundance. The enhanced sensitivity and resolution at 100 kHz MAS enables the exploration of the distinct intermolecular packing in posaconazole forms I, III, and γ. These results demonstrate that proton-detected homo- and heteronuclear correlation methods can probe the structural details of pharmaceutical polymorphism.

Solid-state NMR methods for the characterization of bioconjugations and protein-material interactions

Protein solid-state NMR has evolved dramatically over the last two decades, with the development of new hardware and sample preparation methodologies. This technique is now ripe for complex applications, among which one can count bioconjugation, protein chemistry and functional biomaterials. In this review, we provide our account on this aspect of protein solid-state NMR.

Bone hydration: How we can evaluate it, what can it tell us, and is it an effective therapeutic target?

Water constitutes roughly a quarter of the cortical bone by volume yet can greatly influence mechanical properties and tissue quality. There is a growing appreciation for how water can dynamically change due to age, disease, and treatment. A key emerging area related to bone mechanical and tissue properties lies in differentiating the role of water in its four different compartments, including free/pore water, water loosely bound at the collagen/mineral interfaces, water tightly bound within collagen triple helices, and structural water within the mineral. This review summarizes our current knowledge of bone water across the four functional compartments and discusses how alterations in each compartment relate to mechanical changes. It provides an overview on the advent of- and improvements to- imaging and spectroscopic techniques able to probe nano-and molecular scales of bone water. These technical advances have led to an emerging understanding of how bone water changes in various conditions, of which aging, chronic kidney disease, diabetes, osteoporosis, and osteogenesis imperfecta are reviewed. Finally, it summarizes work focused on therapeutically targeting water to improve mechanical properties.

1H-Detected Biomolecular NMR under Fast Magic-Angle Spinning

Since the first pioneering studies on small deuterated peptides dating more than 20 years ago, 1H detection has evolved into the most efficient approach for investigation of biomolecular structure, dynamics, and interactions by solid-state NMR. The development of faster and faster magic-angle spinning (MAS) rates (up to 150 kHz today) at ultrahigh magnetic fields has triggered a real revolution in the field. This new spinning regime reduces the 1H-1H dipolar couplings, so that a direct detection of 1H signals, for long impossible without proton dilution, has become possible at high resolution. The switch from the traditional MAS NMR approaches with 13C and 15N detection to 1H boosts the signal by more than an order of magnitude, accelerating the site-specific analysis and opening the way to more complex immobilized biological systems of higher molecular weight and available in limited amounts. This paper reviews the concepts underlying this recent leap forward in sensitivity and resolution, presents a detailed description of the experimental aspects of acquisition of multidimensional correlation spectra with fast MAS, and summarizes the most successful strategies for the assignment of the resonances and for the elucidation of protein structure and conformational dynamics. It finally outlines the many examples where 1H-detected MAS NMR has contributed to the detailed characterization of a variety of crystalline and noncrystalline biomolecular targets involved in biological processes ranging from catalysis through drug binding, viral infectivity, amyloid fibril formation, to transport across lipid membranes.

Preferential adsorption sites for propane/propylene separation on ZIF-8 as revealed by solid-state NMR spectroscopy

Solid-state NMR spectroscopy in conjunction with theoretical calculation was employed to investigate the adsorbent-adsorbate host-guest interactions during propane/propylene separation on ZIF-8. ¹H NMR chemical shifts of free gaseous and adsorbed propane/propylene are unambiguously assigned with the assistance of two-dimensional (2D) ¹H-¹H correlation spectroscopy (COSY) MAS NMR spectra. Meanwhile, the adsorption selectivity for propane/propylene mixtures on ZIF-8 at a pressure in range of 1.9-9.6 bar is quantitatively determined using ¹H MAS NMR experiments, which agreed well with the ideal adsorbed solution theory (IAST) predictions. The preferential adsorption of propane compared with propylene on ZIF-8 is directly visualized from the 2D ¹H-¹H spin diffusion homo-nuclear correlation (HOMCOR) MAS NMR spectroscopy. Moreover, the preferential adsorption sites for propane and propylene are deduced from the ¹H-¹H spin diffusion buildup curves, which is further confirmed by DFT theoretical calculations. This work provides insights to understand the structure-property relationship during the propane/propylene separation on ZIF-8 as adsorbent.

Solid-State NMR: Methods for Biological Solids

In the last two decades, solid-state nuclear magnetic resonance (ssNMR) spectroscopy has transformed from a spectroscopic technique investigating small molecules and industrial polymers to a potent tool decrypting structure and underlying dynamics of complex biological systems, such as membrane proteins, fibrils, and assemblies, in near-physiological environments and temperatures. This transformation can be ascribed to improvements in hardware design, sample preparation, pulsed methods, isotope labeling strategies, resolution, and sensitivity. The fundamental engagement between nuclear spins and radio-frequency pulses in the presence of a strong static magnetic field is identical between solution and ssNMR, but the experimental procedures vastly differ because of the absence of molecular tumbling in solids. This review discusses routinely employed state-of-the-art static and MAS pulsed NMR methods relevant for biological samples with rotational correlation times exceeding 100's of nanoseconds. Recent developments in signal filtering approaches, proton methodologies, and multiple acquisition techniques to boost sensitivity and speed up data acquisition at fast MAS are also discussed. Several examples of protein structures (globular, membrane, fibrils, and assemblies) solved with ssNMR spectroscopy have been considered. We also discuss integrated approaches to structurally characterize challenging biological systems and some newly emanating subdisciplines in ssNMR spectroscopy.

From Angstroms to Nanometers: Measuring Interatomic Distances by Solid-State NMR

Internuclear distances represent one of the main structural constraints in molecular structure determination using solid-state NMR spectroscopy, complementing chemical shifts and orientational restraints. Although a large number of magic-angle-spinning (MAS) NMR techniques have been available for distance measurements, traditional ¹³C and ¹⁵N NMR experiments are inherently limited to distances of a few angstroms due to the low gyromagnetic ratios of these nuclei. Recent development of fast MAS triple-resonance ¹⁹F and ¹H NMR probes has stimulated the design of MAS NMR experiments that measure distances in the 1-2 nm range with high sensitivity. This review describes the principles and applications of these multiplexed multidimensional correlation distance NMR experiments, with an emphasis on ¹⁹F- and ¹H-based distance experiments. Representative applications of these long-distance NMR methods to biological macromolecules as well as small molecules are reviewed.

Characterization of Structural Bone Properties through Portable Single-Sided NMR Devices: State of the Art and Future Perspectives

Nuclear Magnetic Resonance (NMR) is a well-suited methodology to study bone composition and structural properties. This is because the NMR parameters, such as the T2 relaxation time, are sensitive to the chemical and physical environment of the 1H nuclei. Although magnetic resonance imaging (MRI) allows bone structure assessment in vivo, its cost limits the suitability of conventional MRI for routine bone screening. With difficulty accessing clinically suitable exams, the diagnosis of bone diseases, such as osteoporosis, and the associated fracture risk estimation is based on the assessment of bone mineral density (BMD), obtained by the dual-energy X-ray absorptiometry (DXA). However, integrating the information about the structure of the bone with the bone mineral density has been shown to improve fracture risk estimation related to osteoporosis. Portable NMR, based on low-field single-sided NMR devices, is a promising and appealing approach to assess NMR properties of biological tissues with the aim of medical applications. Since these scanners detect the signal from a sensitive volume external to the magnet, they can be used to perform NMR measurement without the need to fit a sample inside a bore of a magnet, allowing, in principle, in vivo application. Techniques based on NMR single-sided devices have the potential to provide a high impact on the clinical routine because of low purchasing and running costs and low maintenance of such scanners. In this review, the development of new methodologies to investigate structural properties of trabecular bone exploiting single-sided NMR devices is reviewed, and current limitations and future perspectives are discussed.

Proton Detected Solid-State NMR of Membrane Proteins at 28 Tesla (1.2 GHz) and 100 kHz Magic-Angle Spinning

The available magnetic field strength for high resolution NMR in persistent superconducting magnets has recently improved from 23.5 to 28 Tesla, increasing the proton resonance frequency from 1 to 1.2 GHz. For magic-angle spinning (MAS) NMR, this is expected to improve resolution, provided the sample preparation results in homogeneous broadening. We compare two-dimensional (2D) proton detected MAS NMR spectra of four membrane proteins at 950 and 1200 MHz. We find a consistent improvement in resolution that scales superlinearly with the increase in magnetic field for three of the four examples. In 3D and 4D spectra, which are now routinely acquired, this improvement indicates the ability to resolve at least 2 and 2.5 times as many signals, respectively.

Mechanistic Insights into the Structural Stability of Collagen-Containing Biomaterials Such as Bones and Cartilage

Structural stability of various collagen-containing biomaterials such as bones and cartilage is still a mystery. Despite the spectroscopic development of several decades, the detailed mechanism of collagen interaction with citrate in bones and glycosaminoglycans (GAGs) in the cartilage extracellular matrix (ECM) in its native state is unobservable. We present a significant advancement to probe the collagen interactions with citrate and GAGs in the ECM of native bones and cartilage along with specific/non-specific interactions inside the collagen assembly at the nanoscopic level through natural-abundance dynamic nuclear polarization-based solid-state nuclear magnetic resonance spectroscopy. The detected molecular-level interactions between citrate-collagen and GAG-collagen inside the native bone and cartilage matrices and other backbone and side-chain interactions in the collagen assembly are responsible for the structural stability and other biomechanical properties of these important classes of biomaterials.

Long-Chain Fatty Acids in Bones and Their Link to Submicroscopic Vascularization Network: NMR Assignment and Relaxation Studies under Magic Angle Spinning Conditions in Intramuscular Bones of Atlantic Herring Fish

The long-lasting proton signals in bones are identified as long-chain fatty acids, including saturated, mono-, and di-unsaturated fatty acids, with direct nuclear magnetic resonance evidence. We used intramuscular bones from Atlantic Herring fish to avoid interference from lipid-rich marrows. The key is to recognize that these signals are from mobile phase materials and study them with J-coupled correlation spectroscopies under magic angle spinning conditions. We kept extensive ¹H-spin-echo records that allowed us to examine the effect of magic angle spinning on the transverse relaxation time of water and lipids over time. While it is impossible to distinguish based on chemical shifts, the relaxation data suggest that the signals are more consistent with the interpretation of phospholipid membranes than triglycerides in lipid droplets. In particular, the simultaneous T₂ changes in water and lipids suggest that the centrifugal impact of magic angle spinning alters the lipid's structure in very tight spaces. Additional evidence of phospholipid membranes came from the choline-γ resonance at 3.2 ppm in fresh samples, which disappears with magic angle spinning. Thus, the fatty acid signals are at least partially from membrane bilayer structures, and we propose that they are linked to the submicroscopic vascularization channels similar to the dense canaliculi network in mammalian bones. Our detection of phospholipids from bones depended critically on two factors: (1) the elimination of the overwhelming triglyceride signals from marrows and (2) the preservation of water that biomembranes require. The relaxation data reveal aspects of lipid fluidity that have not been elucidated by previous order parameter studies on model membranes. Relaxation times have long been considered difficult to interpret. A robust and renewed understanding may be beneficial.

Technical Note: Post-burial alteration of bones: Quantitative characterization with solid-state 1H MAS NMR

The identification of markers of the modifications occurring in human bones after death and of the sedimentary and post-sedimentary processes affecting their state of preservation, is of interest for several scientific disciplines. A new index, obtained from spectral deconvolution of the 1H MAS NMR spectra of bones, relating the number of organic protons to the amount of hydrogen nuclei in the OH- groups of bioapatite, is proposed as indicator of the state of preservation of the organic fraction. In the osteological material from three different archaeological sites, this index resulted positively correlated with the extent of collagen loss derived from infrared spectroscopy. Its sensitivity to changes in the physical and chemical characteristics of bone allows to identify distinct diagenetic pathways specific to each site and to distinguish different trajectories within the same site.

Solid-State Nuclear Magnetic Resonance Identifies Abnormal Calcium Phosphate Formation in Diseased Bones

The crystallites of calcium phosphate (CaP) in bones consist of hydroxyl apatite (HA) and amorphous calcium phosphate (ACP). These nanoscale structures of CaP are sculptured by biological bone formation and resorption processes and are one of the crucial factors that determine the overall strength of the constructs. We used one- and two-dimensional ¹H-³¹P solid-state nuclear magnetic resonance (SSNMR) to investigate the nanoscopic structural changes of CaP. Two quantitative measurables are deduced based on the heterogeneous linewidth of ³¹P signal and the ratio of ACP to HA, which characterize the mineral crystallinity and the relative proportion of ACP, respectively. We analyzed bones from different murine models of osteopetrosis and osteoporosis and from human samples with osteoporosis and osteoarthritis. It shows that the ACP content increases notably in osteopetrotic bones that are characterized by defective osteoclastic resorption, whereas the overall crystallinity increases in osteoporotic bones that are marked by overactive osteoclastic resorption. Similar pathological characteristics are observed for the sclerotic bones of late-stage osteoarthritis, as compared to those of the osteopetrotic bones. These findings suggest that osteoclast-related bone diseases not only alter the bone density macroscopically but also lead to abnormal formation of CaP crystallites. The quantitative measurement by SSNMR provides a unique perspective on the pathology of bone diseases at the nanoscopic level.

Probing short and long-range interactions in native collagen inside the bone matrix by BioSolids CryoProbe

Solid-state nuclear magnetic resonance is a promising technique to probe bone mineralization and interaction of collagen protein in the native state. However, many of the developments are hampered due to the low sensitivity of the technique. In this article, we report solid-state nuclear magnetic resonance (NMR) experiments using the newly developed BioSolids CryoProbe™ to access its applicability for elucidating the atomic-level structural details of collagen protein in native state inside the bone. We report here approximately a fourfold sensitivity enhancement in the natural abundance ¹³ C spectrum compared with the room temperature conventional solid-state NMR probe. With the advantage of sensitivity enhancement, we have been able to perform natural abundance ¹⁵ N cross-polarization magic angle spinning (CPMAS) and two-dimensional (2D) ¹ H-¹³ C heteronuclear correlation (HETCOR) experiments of native collagen within a reasonable timeframe. Due to high sensitivity, 2D ¹ H/¹³ C HETCOR experiments have helped in detecting several short and long-range interactions of native collagen assembly, thus significantly expanding the scope of the method to such challenging biomaterials.

Characterization of hydrogenated dentin components by advanced 1H solid-state NMR experiments

Collecting information about molecular organisation on biological materials such as bone and dentin represents a major challenge in attaining a better understanding of their mechanical properties. To that end, solid state Nuclear Magnetic Resonance (ssNMR) spectroscopic study is an appropriate strategy to provide atomic structural details on these amorphous composite materials. However, species like water molecules and hydroxyl groups are usually observed through ¹H magic angle spinning (MAS) ssNMR that suffers from poor resolution due to strong signal overlapping, making their identification difficult. This paper proposes a set of ssNMR experiments for ¹H characterization of the main components of human dentin, based on homo- and hetero-nuclear dipolar couplings and composed mostly of fast 1D experiments. The ¹H assignment is assisted by straightforward sample modifications: vacuum drying, deuterium exchange and demineralization. These experiments allow the hydrogen signal edition of dentin species like water molecules, HPO₄^2- and OH^- groups, depending on their localization (bound to the organic phase, linked to apatite or at the interface) and their dynamic behaviour. This ssNMR toolbox has the potential to provide important structural and dynamic information on chemical and physical modifications of biomaterials. STATEMENT OF SIGNIFICANCE: Molecular characterisation of apatitic biomaterials by biophysical techniques is extremely difficult due to their complex and amorphous nature. It is, however, crucial to obtain such information if we want to understand their mechanical properties in relation to their physical state, for example their hydration levels. In this article we used a set of solid state NMR experiments and sample modifications to distinguish ¹H signal of human dentin components with a particular attention to water molecules, known for their major role in biomaterial structuring.

Single-sided NMR to estimate morphological parameters of the trabecular bone structure

PURPOSE

Single-sided ¹ H-NMR is proposed for the estimation of morphological parameters of trabecular bone, and potentially the detection of pathophysiological alterations of bone structure. In this study, a new methodology was used to estimate such parameters without using an external reference signal, and to study intratrabecular and intertrabecular porosities, with a view to eventually scanning patients.

METHODS

Animal trabecular bone samples were analyzed by a single-sided device. The Carr-Purcell-Meiboom-Gill sequence of ¹ H nuclei of fluids, including marrow, confined inside the bone, was analyzed by quasi-continuous T₂ distributions and separated into two ¹ H pools: short and long T₂ components. The NMR parameters were estimated using models of trabecular bone structure, and compared with the corresponding micro-CT.

RESULTS

Without any further assumptions, the internal reference parameter (short T₂ signal intensity fraction) enabled prediction of the micro-CT parameters BV/TV (volume of the trabeculae/total sample volume) and BS/TV (external surface of the trabeculae/total sample volume) with linear correlation coefficient >0.80. The assignment of the two pools to intratrabecular and intertrabecular components yielded an estimate of average intratrabecular porosity (33 ± 5)%. Using the proposed models, the NMR-estimated BV/TV and BS/TV were found to be linearly related to the corresponding micro-CT values with high correlation (>0.90 for BV/TV; >0.80 for BS/TV) and agreement coefficients.

CONCLUSION

Low-field, low-cost portable devices that rely on intrinsic magnetic field gradients and do not use ionizing radiation are viable tools for in vitro preclinical studies of pathophysiological structural alterations of trabecular bone.

Selectively Enhanced 1H-1H Correlations in Proton-Detected Solid-State NMR under Ultrafast MAS Conditions

Proton-detected solid-state NMR has emerged as a powerful analytical technique in structural elucidation via ¹H-¹H correlations, which are mostly established by broadband methods. We propose a new class of frequency-selective homonuclear recoupling methods to selectively enhance ¹H-¹H correlations of interest under ultrafast magic-angle spinning (MAS). These methods, dubbed as selective phase-optimized recoupling (SPR), can provide a sensitivity enhancement by a factor of ∼3 over the widely used radio-frequency-driven recoupling (RFDR) to observe ¹H_N-¹H_N contacts in a protonated tripeptide N-formyl-Met-Leu-Phe (fMLF) under 150 kHz MAS and are successfully utilized to probe a long-range ¹H-¹H contact in a pharmaceutical molecule, the hydrochloride form of pioglitazone (PIO-HCl). SPR is not only highly efficient in frequency-selective recoupling but also easy to implement, imparting to it great potential to probe ¹H-¹H contacts for the structural elucidation of organic solids such as proteins and pharmaceuticals under ultrafast MAS conditions.

Orientation of immobilized antigens on common surfaces by a simple computational model: Exposition of SARS-CoV-2 Spike protein RBD epitopes

The possibility of immobilizing a protein with antigenic properties on a solid support offers significant possibilities in the development of immunosensors and vaccine formulations. For both applications, the orientation of the antigen should ensure ready accessibility of the antibodies to the epitope. However, an experimental assessment of the orientational preferences necessarily proceeds through the preparation/isolation of the antigen, the immobilization on different surfaces and one or more biophysical characterization steps. To predict a priori whether favorable orientations can be achieved or not would allow one to select the most promising experimental routes, partly mitigating the time cost towards the final product. In this manuscript, we apply a simple computational model, based on united-residue modelling, to the prediction of the orientation of the receptor binding domain of the SARS-CoV-2 spike protein on surfaces commonly used in lateral-flow devices. These calculations can account for the experimental observation that direct immobilization on gold gives sufficient exposure of the epitope to obtain a response in immunochemical assays.

Osteopontin regulates biomimetic calcium phosphate crystallization from disordered mineral layers covering apatite crystallites

Details of apatite formation and development in bone below the nanometer scale remain enigmatic. Regulation of mineralization was shown to be governed by the activity of non-collagenous proteins with many bone diseases stemming from improper activity of these proteins. Apatite crystal growth inhibition or enhancement is thought to involve direct interaction of these proteins with exposed faces of apatite crystals. However, experimental evidence of the molecular binding events that occur and that allow these proteins to exert their functions are lacking. Moreover, recent high-resolution measurements of apatite crystallites in bone have shown that individual crystallites are covered by a persistent layer of amorphous calcium phosphate. It is therefore unclear whether non-collagenous proteins can interact with the faces of the mineral crystallites directly and what are the consequences of the presence of a disordered mineral layer to their functionality. In this work, the regulatory effect of recombinant osteopontin on biomimetic apatite is shown to produce platelet-shaped apatite crystallites with disordered layers coating them. The protein is also shown to regulate the content and properties of the disordered mineral phase (and sublayers within it). Through solid-state NMR atomic carbon-phosphorous distance measurements, the protein is shown to be located in the disordered phases, reaching out to interact with the surfaces of the crystals only through very few sidechains. These observations suggest that non-phosphorylated osteopontin acts as regulator of the coating mineral layers and exerts its effect on apatite crystal growth processes mostly from afar with a limited number of contact points with the crystal.

Spatial molecular-dynamically ordered NMR spectroscopy of intact bodies and heterogeneous systems

Noninvasive evaluation of the spatial distribution of chemical composition and diffusion behavior of materials is becoming possible by advanced nuclear magnetic resonance (NMR) pulse sequence editing. However, there is room for improvement in the spectral resolution and analytical method for application to heterogeneous samples. Here, we develop applications for comprehensively evaluating compounds and their dynamics in intact bodies and heterogeneous systems from NMR data, including spatial z-position, chemical shift, and diffusion or relaxation. This experiment is collectively named spatial molecular-dynamically ordered spectroscopy (SMOOSY). Pseudo-three-dimensional (3D) SMOOSY spectra of an intact shrimp and two heterogeneous systems are recorded to evaluate this methodology. Information about dynamics is mapped onto two-dimensional (2D) chemical shift imaging spectra using a pseudo-spectral imaging method with a processing tool named SMOOSY processor. Pseudo-2D SMOOSY spectral images can non-invasively assess the different dynamics of the compounds at each spatial z-position of the shrimp's body and two heterogeneous systems.

Water-lipid interactions in native bone by high-resolution solid-state NMR spectroscopy

The study of structural and dynamical properties of lipid and its associated interaction with different components of bone is essential to understand its role at a different level of bone homeostasis such as bone mineralization and bone metabolism. In this article, we present water-dependent dynamical changes observed in lipids (triglycerides) in its absolute native environment inside bone by high-resolution ¹H solid-state nuclear magnetic resonance spectroscopy (ssNMR). Relaxation measurement (T₂ measurement) ssNMR experiments were performed at different levels of water network induced by dehydration and H/D exchange in native bone. Our measurements reflect the changes in the local environment and dynamical properties of triglyceride due to different hydration levels. The present study explains the role of water in stabilizing the structural properties of triglycerides in bone hence will help understand its pathological role associated with bone physiology and bone disorders.

Ultrafast 1H MAS NMR Crystallography for Natural Abundance Pharmaceutical Compounds

Magic angle spinning (MAS) NMR is a powerful method for the study of pharmaceutical compounds, and probes with spinning frequencies above 100 kHz enable an atomic-resolution analysis of sub-micromole quantities of fully protonated solids. Here, we present an ultrafast NMR crystallography approach for structural characterization of organic solids at MAS frequencies of 100-111 kHz. We assess the efficiency of ¹H-detected experiments in the solid state and demonstrate the utility of 2D and 3D homo- and heteronuclear correlation spectra for resonance assignments. These experiments are demonstrated for an amino acid, U-¹³C,¹⁵N-histidine, and also for the significantly larger, natural product Posaconazole, an antifungal compound investigated at natural abundance. Our results illustrate the power for characterizing organic molecules, enabled by exploiting the increased ¹H resolution and sensitivity at MAS frequencies above 100 kHz.

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).

Design Parameters of Tissue-Engineering Scaffolds at the Atomic Scale

Stem-cell behavior is regulated by the material properties of the surrounding extracellular matrix, which has important implications for the design of tissue-engineering scaffolds. However, our understanding of the material properties of stem-cell scaffolds is limited to nanoscopic-to-macroscopic length scales. Herein, a solid-state NMR approach is presented that provides atomic-scale information on complex stem-cell substrates at near physiological conditions and at natural isotope abundance. Using self-assembled peptidic scaffolds designed for nervous-tissue regeneration, we show at atomic scale how scaffold-assembly degree, mechanics, and homogeneity correlate with favorable stem cell behavior. Integration of solid-state NMR data with molecular dynamics simulations reveals a highly ordered fibrillar structure as the most favorable stem-cell scaffold. This could improve the design of tissue-engineering scaffolds and other self-assembled biomaterials.

Use of paramagnetic systems to speed-up NMR data acquisition and for structural and dynamic studies

NMR spectroscopy is a powerful experimental technique to study biological systems at the atomic resolution. However, its intrinsic low sensitivity results in long acquisition times that in extreme cases lasts for days (or even weeks) often exceeding the lifetime of the sample under investigation. Different paramagnetic agents have been used in an effort to decrease the spin-lattice (T1) relaxation times of the studied nuclei, which are the main cause for long acquisition times necessary for signal averaging to enhance the signal-to-noise ratio of NMR spectra. Consequently, most of the experimental time is "wasted" in waiting for the magnetization to recover between successive scans. In this review, we discuss how to set up an optimal paramagnetic relaxation enhancement (PRE) system to effectively reduce the T1 relaxation times avoiding significant broadening of NMR signals. Additionally, we describe how PRE-agents can be used to provide structural and dynamic information and can even be used to follow the intermediates of chemical reactions and to speed-up data acquisition. We also describe the unique challenges and benefits associated with the application of PRE to solid-state NMR spectroscopy, explaining how the use of PREs is more complex for membrane mimetic systems as PREs can also be exploited to change the alignment of oriented membrane systems. Functionalization of membrane mimetics, such as bicelles, can provide a controlled region of paramagnetic effect that has the potential, together with the desired alignment, to provide crucial biologically relevant structural information. And finally, we discuss how paramagnetic metals can be utilized to further increase the dynamic nuclear polarization (DNP) effects and how to preserve the enhancements when dissolution DNP is implemented.

Resolution enhancement and proton proximity probed by 3D TQ/DQ/SQ proton NMR spectroscopy under ultrafast magic-angle-spinning beyond 70 kHz

Proton nuclear magnetic resonance (NMR) in solid state has gained significant attention in recent years due to the remarkable resolution and sensitivity enhancement afforded by ultrafast magic-angle-spinning (MAS). In spite of the substantial suppression of ¹H-¹H dipolar couplings, the proton spectral resolution is still poor compared to that of ¹³C or ¹⁵N NMR, rendering it challenging for the structural and conformational analysis of complex chemicals or biological solids. Herein, by utilizing the benefits of double-quantum (DQ) and triple-quantum (TQ) coherences, we propose a 3D single-channel pulse sequence that correlates proton triple-quantum/double-quantum/single-quantum (TQ/DQ/SQ) chemical shifts. In addition to the two-spin proximity information, this 3D TQ/DQ/SQ pulse sequence enables more reliable extraction of three-spin proximity information compared to the regular 2D TQ/SQ correlation experiment, which could aid in revealing the proton network in solids. Furthermore, the TQ/DQ slice taken at a specific SQ chemical shift only reveals the local correlations to the corresponding SQ chemical shift, and thus it enables accurate assignments of the proton peaks along the TQ and DQ dimensions and simplifies the interpretation of proton spectra especially for dense proton networks. The high performance of this 3D pulse sequence is well demonstrated on small compounds, L-alanine and a tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (MLF). We expect that this new methodology can inspire the development of multidimensional solid-state NMR pulse sequences using the merits of TQ and DQ coherences and enable high-throughput investigations of complex solids using abundant protons.

Characterization of H2 -Splitting Products of Frustrated Lewis Pairs: Benefit of Fast Magic-Angle Spinning

Proton spectroscopy in solid-state NMR on catalytic materials offers new opportunities in structural characterization, in particular of reaction products of catalytic reactions such as hydrogenation reactions. Unfortunately, the ¹ H NMR line widths in magic-angle spinning solid-state spectra are often broadened by an incomplete averaging of ¹ H-¹ H dipolar couplings. We herein discuss two model compounds, namely the H₂ -splitting products of two phosphane-borane Frustrated Lewis Pairs (FLPs), to study potentials and limitations of proton solid-state NMR experiments employing magic-angle spinning frequencies larger than 100 kHz at a static magnetic field strength of 20.0 T. The ¹ H lines are homogeneously broadened as illustrated by spin-echo decay experiments. We study two structurally similar materials which however show significant differences in ¹ H line widths which we explain by differences in their ¹ H-¹ H dipolar networks. We discuss the benefit of fast MAS experiments up to 110 kHz to detect the resonances of the H⁺ /H^- pair in the hydrogenation products of FLPs.

Proton-detected 3D 1H anisotropic/14N/1H isotropic chemical shifts correlation NMR under fast magic angle spinning on solid samples without isotopic enrichment

The chemical shift anisotropy (CSA) interaction of a nucleus is an important indicator of the local electronic environment particularly for the contributions arising from hydrogen (H)-bonding, electrostatic and π-π interactions. CSAs of protons bonded to nitrogen atoms are of significant interest due to their common role as H-bonding partners in many chemical, pharmaceutical and biological systems. Although very fast (∼100 kHz) magic angle sample spinning (MAS) experiments have enabled the measurement of proton CSAs directly from solids, due to a narrow chemical shift (CS) distribution, overlapping NH proton resonances are common and necessitate the introduction of an additional frequency dimension to the regular 2D ¹H CSA/¹H CS correlation method to achieve sufficient resolution. While this can be accomplished by using the isotropic shift frequency of ¹⁴N or ¹⁵N nuclei, the use of the naturally-abundant ¹⁴N nucleus avoids ¹⁵N isotopic labeling and therefore would be useful for a variety of solids. To this end, we propose a proton-detected 3D ¹H CSA/¹⁴N/¹H CS correlation method under fast MAS (90 kHz) to determine the CSA tensors of NH protons in samples without isotopic enrichment. Our experimental results demonstrate that the proposed 3D NMR experiment is capable of resolving the overlapping ¹H resonances of amide (NH) groups through the ¹⁴N isotropic shift frequency dimension and enables the accurate measurement of site-specific ¹H CSAs directly from powder samples under fast MAS conditions. In addition to the 3D ¹H CSA/¹⁴N/¹H CS experiment, an approach employing ¹⁴N-edited 2D ¹H CSA/¹H CS experiment is also demonstrated as an additional means to address spectral overlap of NH resonances with aliphatic and other proton resonances in solids.

The solid-state proton NMR study of bone using a dipolar filter: apatite hydroxyl contentversusanimal age

The hydroxyl content of bone apatite mineral has been measured using proton solid-state NMR performed with a multiple-pulse dipolar filter under slow magic angle spinning (MAS). This new method succeeded in resolving and relatively enhancing the main hydroxyl peak at ca. 0 ppm from whole bone, making it amenable to rigorous quantitative analysis. The proposed methodology, involving line fitting, the measurement of the apatite concentration in the studied material and adequate calibration, was proved to be convenient and suitable for monitoring bone mineral hydroxylation in different species and over the lifetime of the animal. It was found that the hydroxyl content in the cranial bone mineral of pig and rats remained in the 5–10% range, with reference to stoichiometric hydroxyapatite. In rats, the hydroxyl content showed a non-monotonic increase with age, which was governed by biological processes rather than by chemical, thermodynamically driven apatite maturation.

Mineral hydroxylation in whole bone can be accurately studied using proton MAS NMR with a multiple-pulse dipolar filter.

Structural Analyses of Alanine Trimer and Tetramer Crystals with Antiparallel and Parallel β-Sheet Structures Using Solid-State 1H Spin-Diffusion 2D Correlation NMR Spectroscopy

Poly-l-alanine (PLA) sequences are key elements of the crystalline domains of spider dragline and wild silkworm silks. In the present work, ¹H spin-diffusion two-dimensional (2D) correlation NMR spectra were observed for selectively deuterated (Ala)₃ and (Ala)₄ crystals to develop the analytical method for the structure of PLA sequences. The build-up curves of the cross peaks for three kinds of ¹H pairs in selectively deuterated (Ala)₃ and (Ala)₄ crystals were observed to obtain spin-diffusion rate constant k _{j, k} from relaxation master equations P _{i, j}(τ_m). The k _{j, k} values subsequently lead to effective interproton distance r _{j, k}^eff (obs) values for individual proton-proton pairs, which include intra- and intermolecular contributions. The r _{j, k}^eff (obs) values were compared to r _{j, k}^eff (calc) values obtained from the experimentally determined atomic coordinates of antiparallel (AP) β-sheet (Ala)₃ and (Ala)₄ and parallel (P) β-sheet of (Ala)₃ and (Ala)₄ crystals. The agreement between the r _{j, k}^eff (obs) and r _{j, k}^eff (calc) values was good for AP β-sheet (Ala)₃ and (Ala)₄ crystals but poor for P β-sheet (Ala)₃ and (Ala)₄ crystals. These deviations were obtained from the interproton distances of the interchain contributions due to different packing arrangements. The packing arrangements of the PLA region are important when considering the relevant structure and the mechanical properties of silks.

Diatoms response to salinity changes: investigations using single pulse and cross polarisation magic angle spinning 29Si NMR spectra

Diatoms Thalassiosira pseudonana and Chaetoceros muelleri (Bacillariophyceae) were cultured at three different salinities (26, 36 and 46 practical salinity units (PSU)) and their silica content examined using natural abundance 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The samples were investigated using both single pulse (SP) and cross-polarisation (CP) MAS experiments. In addition, samples of T. pseudonana were examined using variable contact time CP MAS experiments allowing the dynamics (TSiH and T1rρH) of CP to be determined. Comparison of SP and CP results revealed a change to a less condensed silica state when diatoms were cultured at salinities away from optimal (36 PSU). Further, an increased amount of organic material (but not its composition) was determined to be present for such samples affecting CP experiments. The location of the organic material, on the diatom surface or within the frustule, was unable to be determined.

Measurement of proton chemical shift anisotropy in solid-state NMR spectroscopy

Proton chemical shift anisotropy (CSA) is significantly important as it provides the information of the dynamics and local environmental structure of the proton. The measurement of proton CSA keeps drawing the attention of NMR researchers, and great efforts have been expended. In the early years, measuring proton CSA in solid-state NMR, especially with the strong ¹H-¹H dipolar network, was hampered by ineffective decoupling or selectively recoupling techniques, and the applications were only limited to those with sparse proton sites or single crystals. Till the latest decades, the dramatic progress on NMR methodology and magic-angle spinning (MAS) technology enable accurate detection of proton CSA in complicated powder samples even proteins. In this review, following a brief description of the measurement of proton CSA in solution and LCs NMR, a retrospect of the experimental development of proton CSA measurement in solid state NMR is presented, from the continuous wave (CW) and multiple pulse sequences for static solid samples, to combined rotation and multiple pulse spectroscopy (CRAMPS), then to the latest methods including rotary resonance, CSA amplification and R-symmetry pulse sequences under MAS conditions.

Two-dimensional 1H and 1H-detected NMR study of a heterogeneous biocatalyst using fast MAS at high magnetic fields

Nuclear magnetic resonance (NMR) is a powerful tool for investigating atomic-scale structure in heterogeneous or composite materials where long-range order is absent. In this work solid-state ¹H and ¹H-detected NMR experiments were performed with fast magic angle spinning (ν_R = 75 kHz) and at high magnetic fields (B₀ = 20 T) and used to gain structural insight into a heterogeneous biocatalyst consisting of an enzyme, human carbonic anhydrase II (hCA II), covalently immobilized on epoxy-functionalized silica. Two-dimensional ¹H-¹H NOESY-type correlation experiments were able to provide information on ¹H environments in silica, epoxy-silica and the immobilized enzyme. Two distinct signals originating from water protons were observed: water associated with the surface of the silica and the water associated with the immobilized enzyme. Additional two-dimensional ¹H-¹H double-single quantum (DQ-SQ) correlation experiments suggested that the immobilized enzyme is not in close contact with the silica surface. Most significantly, comparison of two-dimensional ¹H-¹⁵N spectra of the immobilized enzyme and the solution-state enzyme confirmed that the structural integrity of the protein is well preserved upon covalent immobilization.

Structural characterization of fluoride species in shark teeth

In shark teeth we have identified the species fluorapatite, hydroxyfluorapatite and its defect site, calcium fluoride, and potassium fluoride. Their relative amounts in teeth at different development stages have been quantified. Calcium fluoride and potassium fluoride may be associated with the fluoridation mechanism in shark teeth.

1H magic-angle spinning NMR evolves as a powerful new tool for membrane proteins

Building on a decade of continuous advances of the community, the recent development of very fast (60 kHz and above) magic-angle spinning (MAS) probes has revolutionised the field of solid-state NMR. This new spinning regime reduces the ¹H-¹H dipolar couplings, so that direct detection of the larger magnetic moment available from ¹H is now possible at high resolution, not only in deuterated molecules but also in fully-protonated substrates. Such capabilities allow rapid "fingerprinting" of samples with a ten-fold reduction of the required sample amounts with respect to conventional approaches, and permit extensive, robust and expeditious assignment of small-to-medium sized proteins (up to ca. 300 residues), and the determination of inter-nuclear proximities, relative orientations of secondary structural elements, protein-cofactor interactions, local and global dynamics. Fast MAS and ¹H detection techniques have nowadays been shown to be applicable to membrane-bound systems. This paper reviews the strategies underlying this recent leap forward in sensitivity and resolution, describing its potential for the detailed characterization of membrane proteins.

Sensitivity enhanced (14)N/(14)N correlations to probe inter-beta-sheet interactions using fast magic angle spinning solid-state NMR in biological solids

(14)N/(14)N correlations are vital for structural studies of solid samples, especially those in which (15)N isotopic enrichment is challenging, time-consuming and expensive. Although (14)N nuclei have high isotopic abundance (99.6%), there are inherent difficulties in observing (14)N/(14)N correlations due to limited resolution and sensitivity related to: (i) low (14)N gyromagnetic ratio (γ), (ii) large (14)N quadrupolar couplings, (iii) integer (14)N spin quantum number (I = 1), and (iv) very weak (14)N-(14)N dipolar couplings. Previously, we demonstrated a proton-detected 3D (14)N/(14)N/(1)H correlation experiment at fast magic angle spinning (MAS) on l-histidine·HCl·H2O utilizing a through-bond (J) and residual dipolar-splitting (RDS) based heteronuclear multiple quantum correlation (J-HMQC) sequence mediated through (1)H/(1)H radio-frequency driven recoupling (RFDR). As an extension of our previous work, in this study we show the utility of dipolar-based HMQC (D-HMQC) in combination with (1)H/(1)H RFDR mixing to obtain sensitivity enhanced (14)N/(14)N correlations in more complex biological solids such as a glycyl-l-alanine (Gly-l-Ala) dipeptide, and parallel (P) and antiparallel (AP) β-strand alanine tripeptides (P-(Ala)3 and AP-(Ala)3, respectively). These systems highlight the mandatory necessity of 3D (14)N/(14)N/(1)H measurements to get (14)N/(14)N correlations when the amide proton resonances are overlapped. Moreover, the application of long selective (14)N pulses, instead of short hard ones, is shown to improve the sensitivity. Globally, we demonstrate that replacing J-scalar with dipolar interaction and hard- with selective-(14)N pulses allows gaining a factor of ca. 360 in experimental time. On the basis of intermolecular NH/NH distances and (14)N quadrupolar tensor orientations, (14)N/(14)N correlations are effectively utilized to make a clear distinction between the parallel and antiparallel arrangements of the β-strands in (Ala)3 through the observation of inter-β-sheet correlations.

NMR of Macromolecular Assemblies and Machines at 1 GHz and Beyond: New Transformative Opportunities for Molecular Structural Biology

As a result of profound gains in sensitivity and resolution afforded by ultrahigh magnetic fields, transformative applications in the fields of structural biology and materials science are being realized. The development of dual low temperature superconducting (LTS)/high-temperature superconducting (HTS) magnets has enabled the achievement of magnetic fields above 1 GHz (23.5 T), which will open doors to an unprecedented new range of applications. In this contribution, we discuss the promise of ultrahigh field magnetic resonance. We highlight several methodological developments pertinent at high-magnetic fields including measurement of 1H-1H distances and 1H chemical shift anisotropy in the solid state as well as studies of quadrupolar nuclei such as 17O. Higher magnetic fields have advanced heteronuclear detection in solution NMR, valuable for applications including metabolomics and disordered proteins, as well as expanded use of proton detection in the solid state in conjunction with ultrafast magic angle spinning. We also present several recent applications to structural studies of the AP205 bacteriophage, the M2 channel from Influenza A, and biomaterials such as human bone. Gains in sensitivity and resolution from increased field strengths will enable advanced applications of NMR spectroscopy including in vivo studies of whole cells and intact virions.

In situ characterization of advanced glycation end products (AGEs) in collagen and model extracellular matrix by solid state NMR

Non-enzymatic glycation of extracellular matrix with (U-13C5)-d-ribose-5-phosphate (R5P), enables in situ 2D ssNMR identification of many deleterious protein modifications and crosslinks, including previously unreported oxalamido and hemiaminal (CH3-CH(OH)NHR) substructures. Changes in charged residue proportions and distribution may be as important as crosslinking in provoking and understanding harmful tissue changes.

Is protein deuteration beneficial for proton detected solid-state NMR at and above 100 kHz magic-angle spinning?

¹H-detection in solid-state NMR of proteins has been traditionally combined with deuteration for both resolution and sensitivity reasons, with the optimal level of proton dilution being dependent on MAS rate. Here we present ¹H-detected ¹⁵N and ¹³C CP-HSQC spectra on two microcrystalline samples acquired at 60 and 111 kHz MAS and at ultra-high field. We critically compare the benefits of three labeling schemes yielding different levels of proton content in terms of resolution, coherence lifetimes and feasibility of scalar-based 2D correlations under these experimental conditions. We observe unexpectedly high resolution and sensitivity of aromatic resonances in 2D ¹³C-¹H correlation spectra of protonated samples. Ultrafast MAS reduces or even removes the necessity of ¹H dilution for high-resolution ¹H-detection in biomolecular solid-state NMR. It yields ¹⁵N,¹H and ¹³C,¹H fingerprint spectra of exceptional resolution for fully protonated samples, with notably superior ¹H and ¹³C lineshapes for side-chain resonances.

High-Resolution Solid-State NMR Characterization of Ligand Binding to a Protein Immobilized in a Silica Matrix

Solid-state NMR is becoming a powerful tool to detect atomic-level structural features of biomolecules even when they are bound to (or trapped in) solid systems that lack long-range three-dimensional order. We here demonstrate that it is possible to probe protein-ligand interactions from a protein-based perspective also when the protein is entrapped in silica, thus translating into biomolecular solid-state NMR all of the considerations that are usually made to understand the chemical nature of the interaction of a protein with its ligands. This work provides a proof of concept that also immobilized enzymes can be used for protein-based NMR protein-ligand interactions for drug discovery.

Proton-Based Structural Analysis of a Heptahelical Transmembrane Protein in Lipid Bilayers

The structures and properties of membrane proteins in lipid bilayers are expected to closely resemble those in native cell-membrane environments, although they have been difficult to elucidate. By performing solid-state NMR measurements at very fast (100 kHz) magic-angle spinning rates and at high (23.5 T) magnetic field, severe sensitivity and resolution challenges are overcome, enabling the atomic-level characterization of membrane proteins in lipid environments. This is demonstrated by extensive ¹H-based resonance assignments of the fully protonated heptahelical membrane protein proteorhodopsin, and the efficient identification of numerous ¹H–¹H dipolar interactions, which provide distance constraints, inter-residue proximities, relative orientations of secondary structural elements, and protein–cofactor interactions in the hydrophobic transmembrane regions. These results establish a general approach for high-resolution structural studies of membrane proteins in lipid environments via solid-state NMR.