Acceleration of natural-abundance solid-state MAS NMR measurements on bone by paramagnetic relaxation from gadolinium-DTPA

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.

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.

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.

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.

Detection of gadolinium deposition in cortical bone with ultrashort echo time T1 mapping: an ex vivo study in a rabbit model

OBJECTIVES

To investigate the capacity of ultrashort echo time (UTE) T₁ mapping to non-invasively assess gadolinium deposition in cortical bone after gadolinium-based contrast agent (GBCA) administration.

METHODS

Twenty-eight New Zealand rabbits (male, 3.0-3.5 kg) were randomly allocated into control, macrocyclic, high-dose macrocyclic, and linear GBCA groups (n = 7 for each group), and respectively given daily doses of 0.9 ml/kg bodyweight saline, 0.3 mmol/kg bodyweight gadobutrol, 0.9 mmol/kg bodyweight gadobutrol, and 0.3 mmol/kg bodyweight gadopentetate dimeglumine for five consecutive days per week over a period of 4 weeks. After a subsequent 4 weeks of recovery, the rabbits were sacrificed and their tibiae harvested. T₁ value of cortical bone was measured using a combination of UTE actual flip angle imaging and variable repetition time on a 7T animal scanner. Gadolinium concentration in cortical bone was measured using inductively coupled plasma mass spectrometry (ICP-MS). Pearson's correlation between R₁ value (R₁ = 1/T₁) and gadolinium concentration in cortical bone was assessed.

RESULTS

Bone T₁ values were significantly lower in the lower-dose macrocyclic (329.2 ± 21.0 ms, p < 0.05), higher-dose macrocyclic (316.8 ± 21.7 ms, p < 0.01), and linear (296.8 ± 24.1 ms, p < 0.001) GBCA groups compared with the control group (356.3 ± 19.4 ms). Gadolinium concentrations measured by ICP-MS in the control, lower-dose macrocyclic, higher-dose macrocyclic, and linear GBCA groups were 0.04 ± 0.02 μg/g, 2.60 ± 0.48 μg/g, 4.95 ± 1.17 μg/g, and 13.62 ± 1.55 μg/g, respectively. There was a strong positive correlation between R₁ values and gadolinium concentrations in cortical bone (r = 0.73, p < 0.001).

CONCLUSIONS

These results suggest that UTE T₁ mapping has the potential to provide a non-invasive assessment of gadolinium deposition in cortical bone following GBCA administration.

KEY POINTS

• Changes in T₁ value related to gadolinium deposition were found in bone after both linear and macrocyclic GBCA administrations. • R₁ relaxometry correlates strongly with gadolinium concentration in cortical bone. • UTE T₁ mapping provides a potential tool for non-invasively monitoring gadolinium deposition in cortical bone.

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.

Surface Characterization of TiO2 Polymorphic Nanocrystals through 1H-TD-NMR

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using ¹H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times ( T₂) as the function of the concentration and the specific surface area (δ_p· C_m) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity ( R₂) and their concentration C_m. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m^-2 s^-1 of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution.

Application of paramagnetic relaxation enhancements to accelerate the acquisition of 2D and 3D solid-state NMR spectra of oriented membrane proteins

Oriented sample solid-state NMR (OS-ssNMR) spectroscopy is uniquely suited to determine membrane protein topology at the atomic resolution in liquid crystalline bilayers under physiological temperature. However, the inherent low sensitivity of this technique has hindered the throughput of multidimensional experiments necessary for resonance assignments and structure determination. In this work, we show that doping membrane protein bicelle preparations with paramagnetic ion chelated lipids and exploiting paramagnetic relaxation effects it is possible to accelerate the acquisition of both 2D and 3D multidimensional experiments with significant saving in time. We demonstrate the efficacy of this method for a small membrane protein, sarcolipin, reconstituted in DMPC/POPC/DHPC oriented bicelles. In particular, using Cu²⁺-DMPE-DTPA as a dopant, we observed a decrease of ¹H T₁ of sarcolipin by 2/3, allowing us to reduce the recycle delay up to 3 times. We anticipate that these new developments will enable the routine acquisition of multidimensional OS-ssNMR experiments.

Selective detection and complete identification of triglycerides in cortical bone by high-resolution (1)H MAS NMR spectroscopy

Using (1)H-based magic angle spinning solid-state NMR spectroscopy, we report an atomistic-level characterization of triglycerides in compact cortical bone. By suppressing contributions from immobile molecules present in bone, we show that a (1)H-based constant-time uniform-sign cross-peak (CTUC) two-dimensional COSY-type experiment that correlates the chemical shifts of protons can selectively detect a mobile triglyceride layer as the main component of small lipid droplets embedded on the surface of collagen fibrils. High sensitivity and resolution afforded by this NMR approach could be potentially utilized to investigate the origin of triglycerides and their pathological roles associated with bone fractures, diseases, and aging.

Proton chemical shift tensors determined by 3D ultrafast MAS double-quantum NMR spectroscopy

Proton NMR spectroscopy in the solid state has recently attracted much attention owing to the significant enhancement in spectral resolution afforded by the remarkable advances in ultrafast magic angle spinning (MAS) capabilities. In particular, proton chemical shift anisotropy (CSA) has become an important tool for obtaining specific insights into inter/intra-molecular hydrogen bonding. However, even at the highest currently feasible spinning frequencies (110-120 kHz), (1)H MAS NMR spectra of rigid solids still suffer from poor resolution and severe peak overlap caused by the strong (1)H-(1)H homonuclear dipolar couplings and narrow (1)H chemical shift (CS) ranges, which render it difficult to determine the CSA of specific proton sites in the standard CSA/single-quantum (SQ) chemical shift correlation experiment. Herein, we propose a three-dimensional (3D) (1)H double-quantum (DQ) chemical shift/CSA/SQ chemical shift correlation experiment to extract the CS tensors of proton sites whose signals are not well resolved along the single-quantum chemical shift dimension. As extracted from the 3D spectrum, the F1/F3 (DQ/SQ) projection provides valuable information about (1)H-(1)H proximities, which might also reveal the hydrogen-bonding connectivities. In addition, the F2/F3 (CSA/SQ) correlation spectrum, which is similar to the regular 2D CSA/SQ correlation experiment, yields chemical shift anisotropic line shapes at different isotropic chemical shifts. More importantly, since the F2/F1 (CSA/DQ) spectrum correlates the CSA with the DQ signal induced by two neighboring proton sites, the CSA spectrum sliced at a specific DQ chemical shift position contains the CSA information of two neighboring spins indicated by the DQ chemical shift. If these two spins have different CS tensors, both tensors can be extracted by numerical fitting. We believe that this robust and elegant single-channel proton-based 3D experiment provides useful atomistic-level structural and dynamical information for a variety of solid systems that possess high proton density.

Selective excitation enables assignment of proton resonances and (1)H-(1)H distance measurement in ultrafast magic angle spinning solid state NMR spectroscopy

Remarkable developments in ultrafast magic angle spinning (MAS) solid-state NMR spectroscopy enabled proton-based high-resolution multidimensional experiments on solids. To fully utilize the benefits rendered by proton-based ultrafast MAS experiments, assignment of (1)H resonances becomes absolutely necessary. Herein, we propose an approach to identify different proton peaks by using dipolar-coupled heteronuclei such as (13)C or (15)N. In this method, after the initial preparation of proton magnetization and cross-polarization to (13)C nuclei, transverse magnetization of desired (13)C nuclei is selectively prepared by using DANTE (Delays Alternating with Nutations for Tailored Excitation) sequence and then, it is transferred to bonded protons with a short-contact-time cross polarization. Our experimental results demonstrate that protons bonded to specific (13)C atoms can be identified and overlapping proton peaks can also be assigned. In contrast to the regular 2D HETCOR experiment, only a few 1D experiments are required for the complete assignment of peaks in the proton spectrum. Furthermore, the finite-pulse radio frequency driven recoupling sequence could be incorporated right after the selection of specific proton signals to monitor the intensity buildup for other proton signals. This enables the extraction of (1)H-(1)H distances between different pairs of protons. Therefore, we believe that the proposed method will greatly aid in fast assignment of peaks in proton spectra and will be useful in the development of proton-based multi-dimensional solid-state NMR experiments to study atomic-level resolution structure and dynamics of solids.

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

While obtaining high-resolution structural details from bone is highly important to better understand its mechanical strength and the effects of aging and disease on bone ultrastructure, it has been a major challenge to do so with existing biophysical techniques. Though solid-state NMR spectroscopy has the potential to reveal the structural details of bone, it suffers from poor spectral resolution and sensitivity. Nonetheless, recent developments in magic angle spinning (MAS) NMR technology have made it possible to spin solid samples up to 110 kHz frequency. With such remarkable capabilities, (1)H-detected NMR experiments that have traditionally been challenging on rigid solids can now be implemented. Here, we report the first application of multidimensional (1)H-detected NMR measurements on bone under ultrafast MAS conditions to provide atomistic-level elucidation of the complex heterogeneous structure of bone. Our investigations demonstrate that two-dimensional (1)H/(1)H chemical shift correlation spectra for bone are obtainable using fp-RFDR (finite-pulse radio-frequency-driven dipolar recoupling) pulse sequence under ultrafast MAS. Our results infer that water exhibits distinct (1)H-(1)H dipolar coupling networks with the backbone and side-chain regions in collagen. These results show the promising potential of proton-detected ultrafast MAS NMR for monitoring structural and dynamic changes caused by mechanical loading and disease in bone.

Direct Evidence of Imino Acid-Aromatic Interactions in Native Collagen Protein by DNP-Enhanced Solid-State NMR Spectroscopy

Aromatic amino acids (AAAs) have rare presence (∼1.4% abundance of Phe) inside of collagen protein, which is the most abundant animal protein playing a functional role in skin, bone, and connective tissues. The role of AAAs is very crucial and has been debated. We present here experimental results depicting interaction of AAAs with imino acids in a native collagen protein sample. The interaction is probed by solid-state NMR (ssNMR) spectroscopy experiments such as (1)H-(13)C heteronuclear correlation (HETCOR) performed on a native collagen sample. The natural abundance (13)C spectrum was obtained by dynamic nuclear polarization (DNP) sensitivity enhancement coupled with ssNMR, providing ∼30-fold signal enhancement. Our results also open up new avenues of probing collagen structure/dynamics closest to the native state by ssNMR experiments coupled with DNP.