NMR for polymorphism

While calcium carbonate is well known to exist in a range of different crystalline forms, including anhydrous polymorphs and hydrated phases, a new crystalline formcalcium carbonate hemihydrate (CCHH)was reported in 2019 and has recently been observed in a biogenic material. The crystal structure of CCHH reported from diffraction studies is monoclinic, although a subsequent computational investigation suggested that an orthorhombic description of the structure may be more appropriate. Herein, we report experimental solid-state NMR characterization of CCHH, focused on solid-state 1H NMR and 13C NMR measurements, including analysis of 1H–13C heteronuclear correlation spectroscopy (HETCOR) and 13C chemical shift anisotropy (CSA) data, which reveals further insights into the structural and symmetry properties of this material. We demonstrate by means of DFT-GIPAW calculations that the monoclinic and orthorhombic descriptions of the crystal structure of CCHH are readily distinguishable on the basis of solid-state 1H and 13C NMR data. Our experimental solid-state NMR measurements are shown to support the orthorhombic description of the crystal structure rather than the monoclinic description.

Calcium orthophosphates, such as hydroxyapatite (HAP), have been shown to follow nonclassical nucleation and crystallization mechanisms involving intermediate species, including solute prenucleation species (PNS) that precede solid transient phases. Characterizing these transient soluble and solid phases in situ remains a challenge due to their short-lived nature. In this study, we combine 43Ca isotopic enrichment with dynamic nuclear polarization (MAS-DNP) under cryogenic conditions to enhance 43Ca sensitivity and capture transient calcium species in their native state in the course of HAP formation. Fine calibration of the Hartmann–Hahn cross-polarization conditions (low vs high radio frequency powers) enables us to distinguish different calcium-based phases based on the strength of their 43Ca quadrupolar couplings. Hence, we can record the 43Ca NMR fingerprint of submillimolar concentrations of PNS and two-dimensional 1H–43Ca correlation NMR spectra of calcium phosphate solid phases can be recorded within hours. Thanks to this 2D analysis, the first nucleating solid phase is identified as an amorphous calcium phosphate (ACP) rich in acidic orthophosphate ions and water molecules. Moreover, the core-layer organization of the final HAP phase is also evidenced allowing to distinguish the 43Ca spectral fingerprints of the crystalline hydroxyapatite core from the amorphous hydrated layer. This work demonstrates the potential of 43Ca MAS-DNP for characterizing the complex nucleation and growth mechanisms of calcium-containing materials, thus, offering new opportunities for studying similar systems in environmental and biological contexts.

In this chapter, we discuss recent applications of NMR crystallography in bulk organic molecular solids enabled by the sensitivity enhancement offered by MAS DNP. We draw upon examples from recent literature to illustrate how MAS DNP has significantly expanded the scope of NMR crystallography of organic molecular solids at natural isotopic abundance by clearing several longstanding bottlenecks caused by the low sensitivity of NMR. Specifically, for samples at natural isotopic abundance, we discuss how MAS DNP enabled: the detection of low-receptivity nuclei within the bulk of organic solids; the observation of spectral correlations between sparsely distributed spins; the investigation of the morphology of multi-domain solid particles on the nm- to µm-scale; the detection and characterization of structural changes in samples that evolve over time. For each of these advancements, we present and discuss a curated selection of applications where the contribution of DNP proved to be essential.

Biomimetic silica-peptide nanocomposites are promising materials for applications in drug delivery and enzyme encapsulation due to their biocompatibility, tunable morphologies, and unique structural characteristics. However, the structural dynamics of the peptide scaffold remain largely elusive, impeding rational biomimetic materials design. This shortcoming is not the least due to a lack of methods that can access such heterogeneous systems with dynamics on a wide range of time scales. Among the most studied candidates are silica particles templated by the diatom-derived peptide R5, known for its ability to guide silica precipitation under mild, toxicologically friendly conditions, leading to silica capsules filled with a peptide scaffold. Here, we describe the structural dynamics of R5 within its self-assemblies and the silica particles it templates with a combination of advanced magnetic resonance methods, including 13C-direct detected NMR, site-directive spin-labeling EPR, and sensitivity-enhanced solid-state NMR. We provide evidence that R5 self-assemblies form condensed phases with liquid-like dynamics both before and after silica encapsulation. Our suite of methods allowed us to access R5/silica composites over a comprehensive range of time scales. These results demonstrate that R5 retains a remarkable degree of internal dynamics, with distinct regions of solid-like and liquid-like behavior even within the silica particles. Specifically, the peptide scaffold comprises three dynamic species: (i) solid-like at the peptide-silica interface, (ii) liquid-like mobility within the scaffold core, and (iii) intermediate dynamics at the boundary regions between core and interface species. Our findings rationalize the high mobility of guest molecules, such as drugs or enzyme substrates, within R5-silica nanoparticles, which is crucial for their functionality in controlled release and catalytic applications. This understanding paves the way for improved rational design considerations for advanced nanomaterials and expands our knowledge of biomimetic mineralization mechanisms. At the same time, the methodological approach can be useful for many types of peptide-guided biominerals, bridging fundamental biochemistry with biotechnological innovation.

Calcium ion complexation in aqueous solutions is of paramount importance in biology as it is related to cell signaling, muscle contraction, or biomineralization. However, Ca2+-complexes are dynamic soluble entities challenging to describe at the molecular level. Nuclear magnetic resonance appears as a method of choice to probe Ca2+-complexes. However, 43Ca NMR exhibits severe limitations arising from the low natural abundance coupled to the low gyromagnetic ratio and the quadrupolar nature of 43Ca, which overall make it a very unreceptive nucleus. Here, we show that 43Ca dynamic nuclear polarization (DNP) NMR of 43Ca-labeled frozen solutions is an efficient approach to enhance the NMR receptivity of 43Ca and to obtain structural insights about calcium ions complexed with representative ligands including water molecules, ethylenediaminetetraacetic acid (EDTA), and l-aspartic acid (l-Asp). In these conditions and in combination with numerical simulations and calculations, we show that 43Ca nuclei belonging to Ca2+ complexed to the investigated ligands exhibit rather low quadrupolar couplings (with CQ typically ranging from 0.6 to 1 MHz) due to high symmetrical environments and potential residual dynamics in vitrified solutions at a temperature of 100 K. As a consequence, when 1H→43Ca cross-polarization (CP) is used to observe 43Ca central transition, “high-power” νRF(43Ca) conditions, typically used to detect spin 1/2 nuclei, provide ∼120 times larger sensitivity than “low-power” conditions usually employed for detection of quadrupolar nuclei. These “high-power” CPMAS conditions allow two-dimensional (2D) 1H–43Ca HetCor spectra to be readily recorded, highlighting various Ca2+–ligand interactions in solution. This significant increase in 43Ca NMR sensitivity results from the combination of distinct advantages: (i) an efficient 1H-mediated polarization transfer from DNP, resembling the case of low-natural-abundance spin 1/2 nuclei, (ii) a reduced dynamics, allowing the use of CP as a sensitivity enhancement technique, and (iii) the presence of a relatively highly symmetrical Ca environment, which, combined to residual dynamics, leads to the averaging of the quadrupolar interaction and hence to efficient high-power CP conditions. Interestingly, these results indicate that the use of high-power CP conditions is an effective way of selecting symmetrical and/or dynamic 43Ca environments of calcium-containing frozen solution, capable of filtering out more rigid and/or anisotropic 43Ca sites characterized by larger quadrupolar constants. This approach could open the way to the atomic-level investigation of calcium environments in more complex, heterogeneous frozen solutions, such as those encountered at the early stages of calcium phosphate or calcium carbonate biomineralization events.

Radio-frequency (RF) field calibration is essential in NMR spectroscopy. A common practice is to collect a nutation curve by varying the pulse length in a direct single-pulse excitation experiment or in a cross-polarization
magic-angle spinning with a flip-back pulse experiment. From the null points on this curve, one can calculate the
RF field strength. Nevertheless, the practical implementation is not always straightforward or can even be unrealizable,
especially for low-receptivity nuclei owing to their associated low sensitivity. Several researchers used
an approach that involves utilizing other nuclei with more sensitivity but nearly identical Larmor frequencies to
that of the nucleus of interest. However, such an approach has not been a common practice so far. In this work,
we have systematically revisited this approach using 3.2 mm rotors on different sets of nuclei covering a Larmor
frequency range up to 80 MHz. The effect of solid- and solution-states on RF field strength measurements has
been investigated. The detection of each set of nuclei is then carried out with a resonant circuit in the NMR probe
consisting of identical coils and capacitors. Our methodology is illustrated by recording 135/137Ba NMR spectra of
BaTiO3 without prior 135/137Ba RF field calibration.

In this study, nuclear magnetic resonance (NMR) is used to investigate the crystallisation behaviour of aspirin within a mesoporous SBA-15 silica material. The potential of dynamic nuclear polarisation (DNP) experiments is also investigated using specifically designed porous materials that incorporate polarising agents within their walls. The formation of the metastable crystalline form II is observed when crystallisation occurs within the pores of the mesoporous structure. Conversely, bulk crystallisation yields the most stable form, namely form I, of aspirin. Remarkably, the metastable form II remains trapped within the pores of mesoporous SBA-15 silica material even 30 days after impregnation, underscoring its persistent stability within this confined environment.

In this study, supercritical CO2 (scCO2) was used to impregnate polymers with paramagnetic polarizing agents to prepare samples for dynamic nuclear polarization (DNP) solid-state NMR (ssNMR) experiments. As a proof of concept, we impregnated polystyrene samples with bTbK, which stands for bis-TEMPO-bisketal where TEMPO is 2,2,6,6-tetra-methylpiperindin-1-oxyl. Substantial DNP signal enhancements could be measured on DNP-enhanced 1H→13C cross-polarization (CP) magic-angle spinning (MAS) spectra recorded at 9.4 T and ~100 K, reaching a maximum value of 8 in the most favorable case, which appeared comparable or even higher than what is typically obtained on similar systems for former sample preparation methods. These results highlight the potential of scCO2 impregnation as an efficient and possibly versatile methodology to prepare polymer samples for DNP ssNMR investigations.

Abstract We show here that the Electronic Mixing-Mediated Annihilation (EMMA) method, previously reported for the suppression of background signals in solid-state nuclear magnetic resonance spectra, can be successfully applied to remove the solvent signals observed in the case of nuclear magnetic resonance spectra obtained with dynamic nuclear polarization. The methodology presented here is applied to two standard sample preparation methods for dynamic nuclear polarization, namely, glass forming and incipient wetness impregnation. It is demonstrated that the Electronic Mixing-Mediated Annihilation method is complementary to the different methods for solvent suppression based on relaxation filters and that it can be used to preserve the quantitative information that might be present in the pristine spectra.

A method based on highly concentrated radical solutions is investigated for the suppression of the NMR signals arising from solvents that are usually used for dynamic nuclear polarization experiments. The presented method is suitable in the case of powders, which are impregnated with a radical-containing solution. It is also demonstrated that the intensity and the resolution of the signals due to the sample of interest is not affected by the high concentration of radicals. The method proposed here is therefore valuable when sensitivity is of the utmost importance, namely samples at natural isotopic abundance.

Solid-state nuclear magnetic resonance (SSNMR) has established itself as an essential structural elucidation technique in polymer science because it can non-destructively provide unique molecular-level information with atomic resolution on complex macromolecular materials{,} especially when combined with complementary techniques such as scattering and computer simulation. The Achilles' heel of NMR{,} however{,} is its low sensitivity due to the weak nuclear spin polarisation at thermal equilibrium{,} even at very high magnetic fields. One intriguing way to enhance NMR sensitivity is dynamic nuclear polarisation (DNP){,} which relies on the microwave-driven transfer of thermal electron spin polarisation to nuclei. DNP is attracting scientific attention owing to the availability of both high-power/high-frequency microwave sources and efficient DNP polarising agents (i.e.{,} paramagnetic species used for doping diamagnetic samples to provide the source of electron polarisation for DNP). At moderately high magnetic fields (∼10 T){,} large DNP signal enhancements can now be readily obtained{,} which has led to unprecedented SSNMR applications. In this review{,} we describe the requirements for high-field DNP SSNMR and provide examples of its use for the structural analysis of organic polymer materials{,} so as to highlight its advantages and (current) limitations for the field.

The issue of incomplete release of poorly soluble drugs from sustained-release oral formulations is addressed using prednisolone (PDS) as the model drug and a novel highly swelling hydrogel as the rate-controlling material. The hydrogel was formed by heating N-carboxymethylchitosan (CMC) to 80 degrees C for 24 h. Swelling, alkalimetry, FTIR, DSC, and solid-state NMR studies showed that the treatment produced physical crosslinking, i.e., polymer chain entanglement. A controlled-release system was prepared by coating an inert compacted support of ethylcellulose (50 mg; diameter, 6 mm) with a CMC layer containing dispersed PDS powder (10-50 microm). The system was heated to crosslink the CMC coating, then drug release to simulated GI fluids was studied in vitro. The drug release pattern and term were modulated via the layer mass (LM) (10 or 14 mg cm(-2)) and/or the drug-polymer wt ratio (D/P) (1:5 or 2:5). The rate parameter, K, and the time exponent, n, of the Peppas equation were: k=26.6+/-0.3 h(-n), n = 0.78+/-0.02 (LM, 10 mg cm(-2); D/P, 1:5); k = 24.7+/-0.7 h(-n), n = 0.56+/-0.02 (LM, 14 mg cm(-2); D/P, 1:5); k = 20.7+/-0.3 h(-n), n = 0.76+/-0.01 (LM, 10 mg cm(-2); D/P, 2:5). Hydrogel swelling was faster than drug release. This was controlled, in a first stage, by drug dissolution-diffusion in the swollen gel, and subsequently, by diffusion. The drug release rate was unaffected by the GI pH variations, and slightly affected by the environmental hydrodynamics. The system promises an extended and complete release of poorly soluble drugs in the GI tract.

The aim of this study was to investigate, at a molecular level, the structural and dynamic properties of the acidic and sodium salt forms of ibuprofen and their solid dispersions with Eudragit RL100, obtained by two different preparation methods (physical mixtures and co-evaporates), which may affect the release properties of these drugs in their dispersed forms.