Solid-state NMR spectroscopy insights for resolving different water pools in alginate hydrogels

Heat-Induced Preparation of Myofibrillar Protein Gels Reinforced Through Ferulic Acid, α-Cyclodextrin and Fe(III)

Phenolic acids have a positive effect on the processing quality of myofibrillar protein (MP) gels. However, in this study, the addition of ferulic acid (FA) did not have a positive effect on MP gels. To address this issue, we performed the addition and observed the effects on the structure of MP gels by both surface coating and internal cross-linking: addition of FA alone, addition of α-cyclodextrin (CD) to encapsulate FA (MP-FA/CD), and addition of Fe(III) to form a metal-phenolic network structure (Fe @MP-FA) and a metal-cyclodextrin-phenolic acid structure (Fe@MP-FA /CD). It was found that both Fe @MP-FA formed by surface coating and internal cross-linking were able to improve the textural properties of MP gels, including hardness, elasticity, chewability, adhesion, etc. FA effectively promoted the conversion of some of the non-fluidizable water to the bound water morphology, and the addition of Fe(III) effectively enhanced this trend. In particular, the composite network structure formed by Fe@MP-FA/CD more significantly promoted the conversion to bound water and improved the water retention of the gel. Hydrophobic interactions and hydrogen bonding in non-covalent bonding as well as disulfide bonding in covalent bonding were always the main factors promoting the formation of gels from MP after different additions. Meanwhile, different gel treatments lead to changes in the structure of different proteins. Internal cross-linking with the addition of FA promotes protein oxidation, whereas CD reduces the occurrence of oxidation and promotes a homogeneous gel structure. Surface coating with the addition of FA/CD resulted in a reduction in pores in the MP gels and a denser gel structure. However, the addition of internal cross-linking resulted in a gel with a loose and rough network structure. In this study, we compared the common methods of gel enhancement, with the objective of providing a reference for the improvement in the gel texture of meat products.

Probing the Effect of Fluorine on Hydrogen Bonding Interactions in a Pharmaceutical Hydrate Using Advanced Solid-State NMR

Structural studies of pharmaceutical hydrates are essential to understanding stability-related issues, especially during the heating process of formulation. A thorough understanding of the hydration and dehydration behavior of active pharmaceutical ingredient (API) hydrate is also important since phase transitions can occur during the formulation process. This is because dehydration could result in a considerable rearrangement in the structure if water-API hydrogen bonding is present. We perform advanced solid-state NMR experiments on regorafenib monohydrate to investigate the role of fluorine in hydrogen bonding interaction, and the results are compared to its anhydrous form and its structural analogue, namely, sorafenib. Our results show that significant structural changes could not be observed on dehydration. Based on our study, it can be concluded that the introduction of fluorine restricts the intramolecular hydrogen bonding and the asymmetry in the structure of regorafenib monohydrate is absent, in comparison to sorafenib.

Solid-state NMR protocols for unveiling dynamics and (drug) interactions of membrane-bound proteins

Magic angle spinning solid-state NMR (MAS ssNMR) is a versatile tool for studying the structure and dynamics of membrane proteins, as well as their interactions with ligands and drugs. Its power lies in the ability to provide atomic-level information on samples under physiological-like conditions. Moreover, it can illuminate dynamics across a wide range of timescales with great relevance to membrane protein function and dysfunction. In this protocol paper, we highlight key aspects of sample preparation, data acquisition, and interpretation, based on our own experience and the broader literature. We discuss key protocol steps along with important considerations for sample preparation and parameters for ssNMR measurements, with reference to the special requirements of membrane-based samples. Such samples display physiologically relevant dynamics across different motional regimes that can be probed by NMR but also can interfere with certain NMR measurements. We guide the reader through the whole process from sample preparation to complex NMR characterization techniques. Throughout the report, we refer back to examples from our own prior work on the interactions between cytochrome c and cardiolipin-containing membranes, with a discussion of the lipid dependence and interactions with a peroxidase-activity inhibitor. We conclude with a short discussion of alternative and new methods that are further boosting the power and versatility of ssNMR as a tool to study membrane-bound proteins and their ligands or drug interactions.

Simulation Study of the Water Ordering Effect of the β-(1,3)-Glucan Callose Biopolymer

Callose, a polysaccharide closely related to cellulose, plays a crucial role in plant development and resistance to environmental stress. These functions are often attributed to the enhancement by callose of the mechanical properties of semiordered assemblies of cellulose nanofibers. A recent study, however, suggested that the enhancement of mechanical properties by callose might be due to its ability to order neighboring water molecules, resulting in the formation, up to room temperature, of solid-like water-callose domains. This hypothesis is tested by atomistic molecular dynamics simulations using ad hoc models consisting of callose and cellulose hydrogels. The simulation results, however, do not show significant crystallinity in the callose/water samples. Moreover, the computation of the Young's modulus gives nearly the same result in callose/water and in cellulose/water samples, leaving callose's ability to link cellulose nanofibers into networks as the most likely mechanism underlying the strengthening of the plant cell wall.

Involvement of Anion-Specific Effects in Changes in the Gelation and Thermodynamic Properties of Calcium Alginate Hydrogel

The gelation process and hydrogel properties of calcium salt-induced alginate hydrogels are influenced by anion-specific effects. In this study, we investigated the effects of CaSO4, CaI2, and Ca(C5H9O3)2 [calcium β-hydroxy-β-methylbutyrate, CaHMB] on the gelation behavior of alginate hydrogels, using various mannuronic/guluronic acid (M/G) ratios to elucidate the underlying mechanisms of anion-specific effects. Here, at a high M/G ratio (2:1), the gelation time of CaSO4, as a low-solubility calcium source, delayed the formation of the calcium alginate hydrogel. The gelation time was 1.8 times that of the high-solubility calcium source CaHMB. Strongly hydrated ions (such as SO42- and C5H9O3-) caused the removal of water molecules from polysaccharide chains, resulting in the formation of small pores on the pore wall. Moreover, weakly hydrated chaotropic anions (I-) promoted the binding of alginate polysaccharide chains and water molecules, resulting in the slower thermal decomposition of water inside the gel. However, when the M/G ratio was reduced to 1:1 or 1:2, the influence of the three calcium salts on the water and thermodynamic properties of the hydrogels decreased, indicating that the anion-specific effect weakened. This study highlights the importance of anion-specific effects on the properties of alginate hydrogels and provides insights into the utilization of these effects to fabricate functional hydrogels with variable properties.

High-Resolution Atomic Force Microscopy Investigation of Alginate Hydrogel Materials in Aqueous Media

Alginate hydrogels are frequently used in 3D bioprinting and tissue repair and regeneration. Establishing the structure-property-performance correlation of these materials would benefit significantly from high-resolution structural characterization in aqueous environments from the molecular level to continuum. This study overcomes technical challenges and enables high-resolution atomic force microscopy (AFM) imaging of hydrated alginate hydrogels in aqueous media. By combining a new sample preparation protocol with extremely gentle tapping mode AFM imaging, we characterized the morphology and regional mechanical properties of the hydrated alginate. Upon cross-linking, basic units of these hydrogel materials consist of egg-box dimers, which assemble into long fibrils. These fibrils congregate and pile up, forming a sponge-like structure, whose pore size and distribution depend on the cross-linking conditions. At the exterior, surface tension impacts the piling of fibrils, leading to stripe-like features. These structural features contribute to local, regional, and macroscopic mechanics. The outcome provides new insights into its structural characteristics from nanometers to tens of micrometers, i.e., at the dimensions pertaining to biomaterial and hydrogel-cell interactions. Collectively, the results advance our knowledge of the structure and mechanics from the nanometer to continuum, facilitating advanced applications in hydrogel biomaterials.

Tuning the Interfacial Deformation of Gliadin-Flaxseed Gum Complex Particles for Improving the Foam Stability

Gliadin nanoparticle (GNP) is a promising foaming agent, but its application is hindered by the limited foam stability under low acidic conditions. Herein, we attempted to tune the foam stability of GNP by coating it with flaxseed gum (FG) and investigated the structure, interfacial behaviors, and foam functionality of gliadin-FG (GFG) particles at pH 4.5. Results showed that the formation of GFG complex particles was driven by an electrostatic interaction between positive charge patches on the surface of GNP (~17 mV) and negative charges in FG molecule (~-13 mV) at all tested ratios. The addition of appropriate amounts of FG (1:0.05) effectively improved the foam stability of GNP. This was because GFG with larger sizes and lower surface charge possessed higher rigidity after coating with FG. When they adsorbed at the air/water interface, their deformation process was slower than that of GNP, as indicated by interfacial dilatational rheology and cryo-SEM, and the covered particles seemed to be more closely distributed to form solid-like and dense interfacial films. Notably, the addition of FG at a higher ratio (1:0.3) promoted the foam stability of GNP by about five folds because the larger GFG with suitable flexibility and wettability could form a stiff interface layer with more significant elastic response, and the unabsorbed particles and FG could form a gel-like network structure in the continuous phase. These characteristics effectively prevented foam disproportionation and coalescence, as well as retard the drainage. Our findings demonstrate that coating GNPs with FG is an effective approach to improve their application in foamed foods.

Charting the solid-state NMR signals of polysaccharides: A database-driven roadmap

Solid-state nuclear magnetic resonance (ssNMR) measurements of intact cell walls and cellular samples often generate spectra that are difficult to interpret due to the presence of many coexisting glycans and the structural polymorphism observed in native conditions. To overcome this analytical challenge, we present a statistical approach for analyzing carbohydrate signals using high-resolution ssNMR data indexed in a carbohydrate database. We generate simulated spectra to demonstrate the chemical shift dispersion and compare this with experimental data to facilitate the identification of important fungal and plant polysaccharides, such as chitin and glucans in fungi and cellulose, hemicellulose, and pectic polymers in plants. We also demonstrate that chemically distinct carbohydrates from different organisms may produce almost identical signals, highlighting the need for high-resolution spectra and validation of resonance assignments. Our study provides a means to differentiate the characteristic signals of major carbohydrates and allows us to summarize currently undetected polysaccharides in plants and fungi, which may inspire future investigations.

Biodegradable Nanocomposites Based on Blends of Poly(Butylene Adipate-Co-Terephthalate) (PBAT) and Thermoplastic Starch Filled with Montmorillonite (MMT): Physico-Mechanical Properties

Poly(butylene adipate-co-terephthalate) (PBAT) is widely used for production of biodegradable films due to its high elongation, excellent flexibility, and good processability properties. An effective way to develop more accessible PBAT-based bioplastics for wide application in packaging is blending of PBAT with thermoplastic starch (TPS) since PBAT is costly with prices approximately double or even triple the prices of traditional plastics like polyethylene. This study is focused on investigating the influence of TPS/PBAT blend ratio and montmorillonite (MMT) content on the physical and mechanical properties and molecular mobility of TPS-MMT/PBAT nanocomposites. Obtained TPS-MMT/PBAT nanocomposites through the melt blending process were characterized using tensile testing, dynamic mechanical thermal analysis (DMTA), and X-ray diffraction (XRD), as well as solid-state 1H and 13C NMR spectroscopy. Mechanical properties demonstrated that the addition of TPS to PBAT leads to a substantial decrease in the tensile strength as well as in the elongation at break, while Young's modulus is rising substantially, while the effect of the MMT addition is almost negligible on the tensile stress of the blends. DMTA results confirmed the formation of TPS domains in the PBAT matrix. With increasing TPS content, mobility of starch-rich regions of TPS domains slightly increases. However, molecular mobility in glycerol-rich regions of TPS domains in the blends was slightly restricted. Moreover, the data obtained from 13C CP/MAS NMR spectra indicated that the presence of TPS in the sample decreases the mobility of the PBAT chains, mainly those located at the TPS/PBAT interfaces.

Effects of glycerol content on structure and molecular motion in thermoplastic starch-based nanocomposites during long storage

Thermoplastic starch-based nanocomposites with varying glycerol content and montmorillonite as a nanofiller were studied using dynamic-mechanical analysis (DMA), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) during one-year storage. DMA results showed that starch-rich and glycerol-rich domains were present in the samples and during storage for up to one year the content of the amorphous phase decreased and molecular mobility changed. 13C NMR and XRD measurements confirmed that ordered structures were formed during storage and its content was larger for samples with higher glycerol content and increased with the storage time. The data obtained from deconvolutions of 1H broad line NMR spectra indicate increased overall molecular mobility in the samples up to four months of storage, while after nine months the trends were opposite. Lower free water content compared to the total water content in the samples determined according to deconvoluted 1H MAS (magic-angle spinning) NMR spectra indicated that a part of water molecules was immobilized in the ordered structures.

Anionic Polysaccharide Cryogels: Interaction and In Vitro Behavior of Alginate-Gum Arabic Composites

In the present study, polysaccharide-based cryogels demonstrate their potential to mimic a synthetic extracellular matrix. Alginate-based cryogel composites with different gum arabic ratios were synthesized by an external ionic cross-linking protocol, and the interaction between the anionic polysaccharides was investigated. The structural features provided by FT-IR, Raman, and MAS NMR spectra analysis indicated that a chelation mechanism is the main process linking the two biopolymers. In addition, SEM investigations revealed a porous, interconnected, and well-defined structure suitable as a scaffold in tissue engineering. The in vitro tests confirmed the bioactive character of the cryogels through the development of the apatite layer on the surface of the samples after immersion in simulated body fluid, identifying the formation of a stable phase of calcium phosphate and a small amount of calcium oxalate. Cytotoxicity tests performed on fibroblast cells demonstrated the non-toxic effect of alginate-gum arabic cryogel composites. In addition, an increase in flexibility was noted for samples with a high gum arabic content, which determines an appropriate environment to promote tissue regeneration. The newly obtained biomaterials that exhibit all these properties can be successfully involved in the regeneration of soft tissues, wound management, or controlled drug release systems.

van der Wel P. Selective observation of semi-rigid non-core residues in dynamically complex mutant huntingtin protein fibrils

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Mutant huntingtin exon 1 fibrils feature a broad range of molecular dynamics.

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Molecular motion is coupled to water dynamics outside the fiber core.

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Dynamics-based spectral editing ssNMR reveals mobile non-core residues.

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Intermediate-motion selection via dipolar dephasing of rigid sites.

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Semi-mobile glutamines outside the fiber core observed and identified.

Many amyloid-forming proteins, which are normally intrinsically disordered, undergo a disorder-to-order transition to form fibrils with a rigid β-sheet core flanked by disordered domains. Solid-state NMR (ssNMR) and cryogenic electron microscopy (cryoEM) excel at resolving the rigid structures within amyloid cores but studying the dynamically disordered domains remains challenging. This challenge is exemplified by mutant huntingtin exon 1 (HttEx1), which self-assembles into pathogenic neuronal inclusions in Huntington disease (HD). The mutant protein’s expanded polyglutamine (polyQ) segment forms a fibril core that is rigid and sequestered from the solvent. Beyond the core, solvent-exposed surface residues mediate biological interactions and other properties of fibril polymorphs. Here we deploy magic angle spinning ssNMR experiments to probe for semi-rigid residues proximal to the fibril core and examine how solvent dynamics impact the fibrils’ segmental dynamics. Dynamic spectral editing (DYSE) 2D ssNMR based on a combination of cross-polarization (CP) ssNMR with selective dipolar dephasing reveals the weak signals of solvent-mobilized glutamine residues, while suppressing the normally strong background of rigid core signals. This type of ‘intermediate motion selection’ (IMS) experiment based on cross-polarization (CP) ssNMR, is complementary to INEPT- and CP-based measurements that highlight highly flexible or highly rigid protein segments, respectively. Integration of the IMS-DYSE element in standard CP-based ssNMR experiments permits the observation of semi-rigid residues in a variety of contexts, including in membrane proteins and protein complexes. We discuss the relevance of semi-rigid solvent-facing residues outside the fibril core to the latter’s detection with specific dyes and positron emission tomography tracers.

Solid State NMR a Powerful Technique for Investigating Sustainable/Renewable Cellulose-Based Materials

Solid state nuclear magnetic resonance (ssNMR) is a powerful and attractive characterization method for obtaining insights into the chemical structure and dynamics of a wide range of materials. Current interest in cellulose-based materials, as sustainable and renewable natural polymer products, requires deep investigation and analysis of the chemical structure, molecular packing, end chain motion, functional modification, and solvent–matrix interactions, which strongly dictate the final product properties and tailor their end applications. In comparison to other spectroscopic techniques, on an atomic level, ssNMR is considered more advanced, especially in the structural analysis of cellulose-based materials; however, due to a dearth in the availability of a broad range of pulse sequences, and time consuming experiments, its capabilities are underestimated. This critical review article presents the comprehensive and up-to-date work done using ssNMR, including the most advanced NMR strategies used to overcome and resolve the structural difficulties present in different types of cellulose-based materials.