High-resolution Solid-state 13C NMR Research Articles

Understanding the Solid-State Structure of Riboflavin through a Multitechnique Approach.

Crystalline riboflavin (vitamin B2) performs an important biological role as an optically functional material in the tapetum lucidum of certain animals, notably lemurs and cats. The tapetum lucidum is a reflecting layer behind the retina, which serves to enhance photon capture and vision in low-light settings. Motivated by the aim of rationalizing its biological role, and given that the structure of biogenic solid-state riboflavin remains unknown, we have used a range of experimental and computational techniques to determine the solid-state structure of synthetic riboflavin. Our multitechnique approach included microcrystal XRD, powder XRD, three-dimensional electron diffraction (3D-ED), high-resolution solid-state 13C NMR spectroscopy, and dispersion-augmented density functional theory (DFT-D) calculations. Although an independent report of the crystal structure of riboflavin was published recently, our structural investigations reported herein provide a different interpretation of the intermolecular hydrogen-bonding arrangement in this material, supported by all the experimental and computational approaches utilized in our study. We also discuss, more generally, potential pitfalls that may arise in applying DFT-D geometry optimization as a bridging step between structure solution and Rietveld refinement in the structure determination of hydrogen-bonded materials from powder XRD data. Finally, we report experimental and computational values for the refractive index of riboflavin, with implications for its optical function.

Mechanochemical synthesis of mechanical bonds in M12L8 poly-[n]-catenanes.

Using mechanochemistry by grinding TPB and ZnBr2, an amorphous poly-[n]-catenane of interlocked M12L8 nanocages is obtained in good yields (∼80%) and within 15 minutes. The mechanical bond among the icosahedral M12L8 cages in the amorphous phase has been demonstrated by single crystal XRD, powder XRD and FT-IR spectroscopy following an amorphous-to-crystalline transformation by guest uptake of the amorphous phase. High-resolution solid-state 13C NMR spectroscopy gives insights into the local structure of the amorphous catenane focusing on TPB aromatic-aromatic interactions.

Evaluation of chitosan crystallinity: A high-resolution solid-state NMR spectroscopy approach

We propose a novel approach relied on high-resolution solid-state 13C NMR spectroscopy to quantify the crystallinity index of chitosans (Ch) prepared with variable average degrees of acetylation (DA¯) from 5% to 60 % and average weight molecular weight (M¯w) ranged in 0.15 × 106 g mol−1–1.2 × 106 g mol−1. The Dipolar Chemical Shift Correlation (DIPSHIFT) curve of the C(6)OH segment revealed increased mobility dynamic, which induced different distribution from trans-to-gauche conformations in relation to C(4). Indeed, 1H-13C Heteronuclear Correlation (2D HETCOR) showed that distinguished C4 chemical shifts correlates with the same aliphatic protons. The short-range ordering can be assigned to C4/C6 signals on 13C CPMAS and, for our case, the deconvolution procedure between disordered and ordered phases revealed increasing crystallinity with DA¯, as confirmed by SVD multivariate analysis. This work extended the knowledge regarding the use of 13C CPMAS technique to predict the crystallinity of chitosans without the use of amorphous standards.

Self-healing UV-curable polymer network with reversible Diels-Alder bonds for applications in ambient conditions

The ambient-temperature self-healing potential of a poly(methacrylate) network, containing reversible furan-maleimide Diels-Alder crosslinks, is studied. A reversible bis(methacrylate) monomer, containing Diels-Alder bonds, is synthesized and characterized by means of 1H NMR spectroscopy. Subsequent polymerization via UV-cure yields a reversible polymer network, consisting of thermoplastic poly(methacrylate), crosslinked by Diels-Alder bonds. Characterization of the polymer network via high-resolution solid-state 13C NMR indicates that undesirable side reactions, such as the formation of irreversible maleimide homo- and copolymers, are prevented by protecting the maleimide functionality in the Diels-Alder adduct. The thermal reversibility of the polymer network is studied by means of Fourier transform infrared spectroscopy and differential scanning calorimetry, and it is confirmed that the reversibility of the Diels-Alder reaction remains during several thermal cycles between −40 °C and 85 °C. Dynamic mechanical analysis confirms that the reversible polymer network maintains sufficient mechanical properties even at elevated temperatures up to 110 °C, at which retro Diels-Alder reaction is favored. Self-healing of the polymer network at ambient temperature, even in the fully vitrified state at 20 °C, is demonstrated by grinding the polymer material into a powder and then healing the powder (after compression) to form a rectangular bar with recovered mechanical properties. Ambient temperature healing without human intervention is therefore feasible, making this material suitable as e.g. encapsulant in photovoltaic modules for outside applications.

Gadolinium Complexes as Contrast Agent for Cellular NMR Spectroscopy.

Aqua Gd3+ and Gd-DOTA (gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacete) complexes were studied as a contrast agent in cellular NMR (nuclear magnetic resonance) spectroscopy for distinguishing between intracellular and extracellular spaces. The contrast agents for this purpose should provide strong paramagnetic relaxation enhancement and localize in the extracellular space without disturbing biological functions. Cell membrane permeability to Gd complexes was evaluated from the concentrations of gadolinium complexes in the inside and outside of E. coli cells measured by the 1H-NMR relaxation. The site-specific binding of the complexes to E. coli cells was also analyzed by high-resolution solid-state 13C-NMR. The aqua Gd3+ complex did not enhance T1 relaxation in proportion to the amount of added Gd3+. This Gd3+ concentration dependence and the 13C-NMR indicated that its strong cytotoxicity should be due to the binding of the paramagnetic ions to cellular components especially at the lipid membranes. In contrast, Gd-DOTA stayed in the solution states and enhanced relaxation in proportion to the added amount. This agent exhibited strong T1 contrast between the intra- and extracellular spaces by a factor of ten at high concentrations under which the cells were viable over a long experimental time of days. These properties make Gd-DOTA suitable for selectively contrasting the living cellular space in NMR spectroscopy primarily owing to its weak interaction with cellular components.

Investigation of the network structure of new bisphenol A non-intent interior food can coatings using high-resolution solid-state 13C-NMR spectroscopy

Facing recent controversial debates about commonly used epoxy resins in food contact applications, an understanding of newly developed polyester-based BPA-non-intent (BPA-NI) interior food can coatings is necessary. So far, little is known about the polymer structure of the polyester-based blends which have been brought to the European market.We report a detailed study on a polyester-phenol blend model system suitable for interior can coating in terms of its mechanical properties such as high flexibility, very good adhesion and hardness. We employ vibrational spectroscopy (FTIR), thermoanalysis (DSC) and an in-depth high-resolution solid-state 13C-NMR analysis to reveal that an overall homogeneity on molecular scale of the two main components in the blend is sustained in the solid state. This is remarkable, taking into account the overall different characteristics of polyesters and phenolic resins and explains the exceptional material properties of this coating.

Remarkably enhanced H2 evolution activity of oxidized graphitic carbon nitride by an extremely facile K2CO3-activation approach

The photocatalytic activity enhancement of graphitic carbon nitride (g-C3N4) in water splitting were commonly achieved by introducing various functional groups onto/into its planar structure. Herein, oxidized g-C3N4 (CNO) was prepared via an extremely facile K2CO3 activation approach. The high-resolution XPS spectra and solid-state 13C NMR spectra confirmed the successful introduction of O-containing groups and O atoms were inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged bonded with C atoms rather than N atoms in the tri-s-triazine structure of g-C3N4. CNO inherited the original molecular framework of g-C3N4, but exhibited effectively enlarged surface area and enhanced visible light absorption, and more attrractively, the band structure of g-C3N4 could be well-tuned with the introduction of oxygen species. Importantly, the surface O-containing moieties could strongly interact with cocatalyst Pt and thus remarkably promote interfacial electron transfer and consequent photo-generated charge carrier separation. Consequently, the as-obtained CNO exhibited the remarkably enhanced photocatalytic performance with a H2 evolution rate of 199.7 μmol h−1, which is 9 times that of g-C3N4 (22.2 μmol h−1).

β-Cyclodextrin-epichlorohydrin polymer/graphene oxide nanocomposite: preparation and characterization

In this study, we report a novel and simple technique to synthesize water-insoluble nanocomposite macromolecule composed of graphene oxide (GO) nanosheets and β-cyclodextrin (β-CD) polymer. These nanocomposites were synthesized by the one-pot precipitation polymerization process via polycondensation of β-CD with epichlorohydrin (ECH) in an aqueous suspension of GO under alkaline condition. The obtained β-CD polymer/GO (CDP-ECH/GO) were characterized by thermal gravimetry, differential scanning calorimetry, infrared and Raman spectroscopies, high-resolution solid state 13C NMR, X-ray diffraction, electron microscopy (both SEM and TEM), and energy dispersive spectroscopy. Furthermore, the mechanism of CDP-ECH/GO nanocomposites formation is also discussed depending on the type of mixing device that has been used during polymerization. The swelling ratio of the nanocomposites as well as their absorption properties toward methylene blue (MB), phenol (PN), 1-naphtol (1-NPH) and 2-naphtol (2-NPH) have been determined. Increasing the GO content in the nanocomposites results in an increase of the adsorption capacity of the materials.

Chain packing in the noncrystalline region of deuterated UHMWPE: A solid-state 2H and 13C NMR study

Chain packing in the non-crystalline region of a semi-crystalline polymer is strongly influenced by the crystallization conditions. Using a single-site catalytic system, deuterated Ultra High Molecular Weight Polyethylenes (UHMW-PEs) are synthesized and the chain topology of the noncrystalline region is investigated by solid-state NMR. The variability in chain packing within the noncrystalline region is introduced on crystallizing the same polymer either by controlled synthesis or solution or melt. 2H NMR combined with line shape simulations yield detailed information on the geometry and frequency of the segmental motion in the noncrystalline regions of deuterated UHMWPE crystallized at three different conditions. High resolution solid-state 13C NMR reveals the temperature dependence of the conformation statistics of chain segments in the noncrystalline regions of the samples. Combining the observations from the 2H and 13C NMR, the influence of segmental motions on the conformation statistics in the noncrystalline regions of the samples are discussed.

Packing Arrangements and Intersheet Interaction of Alanine Oligopeptides As Revealed by Relaxation Parameters Obtained from High-Resolution 13C Solid-State NMR.

Alanine oligopeptides provide a key structure of the crystalline domains of the silks from spiders and wild silkworm and also the sequences included in proteins such as antifreeze proteins and amyloids. In this paper, the local dynamics of alanine oligopeptides, (Ala)3, (Ala)4, and (Ala)6 were examined by high-resolution 13C solid-state NMR. The 13C spin-lattice relaxation times (T1's) for the Cβ4 carbons of antiparallel (AP)-β-sheet (Ala)4 significantly prolonged and the correlation time was estimated as 3.6 × 10-11 s which was shorter than those of other carbons in the AP-β-sheet (Ala)4 (2.8 × 10-10 s). The T1 values for the Cβ carbons of (Ala)6 showed significantly longer correlation time (8.8 × 10-9 s) than those of AP-β-sheet (Ala)4. It is thus revealed that AP-β-sheet (Ala)6 exhibited stronger intersheet interaction than those of AP-β-sheet (Ala)4. The 13C spin-spin relaxation times (T2's) for the Cβ4 carbons showed longer than those of the other Cβ1-3 carbons of AP-β-sheet (Ala)4. T2 values of Cβ carbons reflect the slow time-scale (∼70 kHz) backbone motions. The C-terminal forms strong hydrogen bonds with water molecules and thus the backbone motion is slower than ∼70 kHz, while the central backbone motions are faster than ∼70 kHz in the AP-β-sheet (Ala)4.

Solid-State NMR and the Crystallization of Aspartic and Glutamic Acids

We used high-resolution solid-state 13C NMR, with cross-polarization and magic-angle spinning, to study chirality in the crystallization of aspartic acid. We show that, contrary to a recent report, dl-aspartic acid crystallizes over most of its temperature range as racemic crystals rather than a conglomerate of enantiomeric crystals, regardless of whether the solution was prepared by solution of the racemate or by mixing solutions of the pure enantiomers. Over virtually the entire solution temperature range at 1 bar pressure, racemic crystals of aspartic acid are thermodynamically stable, whereas the conglomerate is metastable. In contrast, in agreement with the literature, glutamic acid crystallizes as a conglomerate under thermodynamic conditions. Solid-state NMR results are confirmed by powder X-ray diffraction.

Exploiting Powder X-ray Diffraction to Establish the Solvent-Assisted Solid-State Supramolecular Assembly of Pillar[5]quinone

We report the solvent-mediated supramolecular assembly of pillar[5]quinone (P[5]Q), a symmetric cyclamer containing five benzoquinone moieties bridged by five alternating methylene units. The supramolecular assembly of P[5]Q is shown to be facilitated by 1,1,2,2-tetrachloroethane (TCE) as solvent, producing a microcrystalline solvate material P[5]Q·2TCE with a fluffy texture. Optical and electron microscopy reveal that this material has a rod-shaped morphology, extending to several micrometers in length. Due to the microcrystalline nature of the material, structure determination was carried out directly from powder X-ray diffraction data, augmented by high-resolution solid-state 13C NMR. The two crystallographically distinct TCE molecules occupy different types of void in the structure and have different dynamic properties. Crystallization of P[5]Q was attempted from a large number of different solvents, but only TCE was found to facilitate the formation of a crystalline phase. Indeed, features of the crystal structure suggest that the TCE component plays an important role in promoting the columnar assembly of P[5]Q molecules.

Preparation and characterization of sol–gel-modified pineapple leaf fiber/polylactic acid composites

In this study, the fibers generated from agricultural waste-pineapple leaf are obtained through exposure, drying, crushing and sifting. A novel sol–gel method is utilized to modify the pineapple leaf fiber (PALF), which is subsequently treated by a coupling agent. The aim is to improve the compatibility between PALF and the polymer matrix and to enhance the heat resistance and mechanical properties of the composite material. Furthermore, a series of modified PALF/polylactic acid (PLA) composites are prepared. FTIR, high resolution solid-state 13C and 29Si NMR experiments show that the PALF was successfully modified by the silane coupling agent and sol–gel method. Polarizing optical microscopy analysis reveals PLA crystal growth of a sufficiently high density along the polymer-fiber interface. Moreover, the storage and loss moduli of PLA are increased by adding the modified PALF. Apart from the enhancement of the mechanical properties, the incorporation of modified PALF reduces the amount of agricultural waste and extends the application of PLA.

Comparative study of samples of powdered and microcrystalline celluloses of different natural origins: Supermolecular structure and the chemical composition of powdered samples

Methods of wide-angle X-ray scattering (WAXS), high resolution solid-state 13C NMR, and Fourier transform IR-spectroscopy are applied to study supermolecular structures and functional compositions of lignocellulose samples of wood and grass origins and powdered celluloses (PC) obtained from them under identical hydrolysis conditions. It was shown by WAXS that the structure of cellulose I is preserved in samples of powdered celluloses, however, an increased degree of crystallinity and cross-section sizes of crystallites are observed in PC samples. Specific features of changes in the supermolecular structure of cellulose occurred after the hydrolysis, i.e., an increase in the content of cellulose Iβ in PC compared to the initial samples, are established by 13C NMR method. It was shown by means of 13C NMR and Fourier transform IR-spectroscopy that the functional chemical composition of lignocelluloses is weakly affected by the hydrolysis. The presence of residual lignin functional groups in the samples is confirmed.

The diethyl dithiophosphate complex of tetraphenylantimony(V) and its solvated form, [Sb(C6H5)4{S2P(OC2H5)2}] · 1/2 C6H6: Synthesis, crystal structure, and 13C, 31P CP/MAS NMR study. an example of monodentate coordination of dithio ligands

The O,O′-diethyl dithiophosphate complex of tetraphenylantimony(V) [Sb(C6H5)4{S2P(OC2H5)2}] (I) and its benzene-solvated form I · 1/2C6H6 (II) were synthesized and studied by high-resolution solid-state 13C and 31P NMR (MAS NMR). The diethyl dithiophosphate (Dtph) groups in I and II were quantitatively characterized by the 31P chemical shift anisotropy (δaniso), the asymmetry parameter (η), and the principal values of chemical shift tensors (δxx, δyy, δzz). The calculation of the anisotropy parameters included construction of χ2 statistic diagrams from full 31P MAS NMR spectra. In both complexes, the Dtph groups were found to have mainly axially symmetric 31P chemical shift tensors (for δzz < δxx ≈ δyy) with similar anisotropy parameters (δaniso and η), which is due to their identical S-monodentate function. According to X-ray diffraction data, II has a trigonal bipyramidal (TBP) molecular structure with Smonodentate coordination of Dtph in the TBP axial position and outer-sphere position of the benzene molecule. The desorption of the outer-sphere benzene solvent molecules from structure II, which was noted in MAS NMR experiment, passes through the formation of three intermediate solvated forms with benzene content n < 1/2.

Effects of post-reactor functionalization on the phase behaviour of an ethylene-1-octene copolymer studied using solid-state high resolution 13C NMR spectroscopy

The effects of post-reactor functionalization with naphthoate-TEMPO on the structure and morphology of an ethylene-1-octene copolymer were investigated by means of solid-state NMR techniques and DSC measurements. Selective (13)C MAS experiments allowed the orthorhombic and the monoclinic crystalline phases and two amorphous phases with different degree of mobility to be detected and quantified. (13)C and (1)H relaxation time measurements and spin diffusion experiments gave insight into the polymer dynamics within the different phases, the crystalline domain dimensions, and the rate of chain diffusion between amorphous and crystalline phases. Comparison of the results obtained for the pristine copolymer and the functionalized samples clearly indicated that the functionalization procedure causes redistribution within the crystalline and the amorphous phases with no relevant change in the degree of crystallinity or in the crystalline domain average size, and slows down chain diffusion.

Structures of crystalline and amorphous phases of the poly(ethylene oxide)/lithium trifluoromethanesulfonate complexes as studied by solid-state high-resolution 13C nuclear magnetic resonance

Solid-state high-resolution 13C NMR studies are carried out on a series of PEO/LiCF3SO3 complexes with different EO/Li molar ratios at room temperature. The obtained spectra clearly show that there are four phases, namely the crystalline PEO, the amorphous PEO, the PEO3:LiCF3SO3 crystalline complex and the PEO3:LiCF3SO3 amorphous complex, coexisting in the samples with EO/Li molar ratios larger than 3. The solid-state 13C NMR spectrum of the PEO3:LiCF3SO3 crystalline complex exhibits six peaks with high resolution at room temperature, while the previously reported 13C NMR spectra showed only one peak. These six peaks are assigned with the combination of the 13C dipolar-INADEQUATE and the two-dimensional exchange spectra. Moreover, remarkable helical jumps of PEO segments are demonstrated to exist in the crystalline complex. Our study paves the way towards a better understanding of the phase structures and subsequently the conduction mechanism of PEO-based solid polymer electrolytes.

Dynamics property recovery of archaeological-wood fibers treated with polyethylene glycol demonstrated by high-resolution solid-state NMR

We used high-resolution Solid-State 13C NMR to better understand and optimize the conservation process of archaeological waterlogged woods by polyethylene glycol (PEG) impregnation via the study of the molecular interactions between PEG and residual celluloses. By both deconvoluting NMR spectra and analyzing the behavior of 13C magnetization build-up under proton to carbon cross-polarization conditions, we were able to quantify PEG penetration and extract parameters sensitive to molecular dynamics such as proton spin lattice-relaxation-time constants in the rotating frame T1ρH and the cross-relaxation time constant TCH. By exploring a large range of PEG concentrations for the impregnating solutions we show that the PEG penetrates inside the fibers and interacts at a molecular level with the cellulose fibrils thus restoring the dynamics properties of the damaged molecular cell wall network. At high PEG concentration, the polymer accumulates in the remaining free volume with more and looser molecular interactions with the residual wood components. This feature explains the facility for these hydroscopic materials to exude from the wood and led to deleterious consequences for the restored artefacts.

Quantifying the fraction of alanine residues in an α-helical conformation in hornet silk using solid-state NMR

Here, we demonstrate the first successful isotope labeling of Ala carbons in hornet silk produced by the larvae of Vespa (Vespinae, Vespidae) mandarinia. This labeled hornet silk was examined by high-resolution 13C solid-state NMR, and it was found that the fraction of Ala residues in α-helical conformations compared with Ala residues in the overall conformation of hornet silk can be quantitatively determined from Ala Cα NMR peaks. The value for this α-helical Ala fraction is close to that of the fraction of Ala residues in coiled-coil structures estimated in the four major hornet silk proteins by coiled-coil prediction analysis. This result indicates that most of the Ala residues in α-helices occur in those α-helices with a coiled-coil structure, and that the number of Ala residues in α-helices without a coiled-coil structure is small. Moreover, coiled-coil prediction analysis indicated that the potential coiled-coil domains are located only in the central portion of the protein chains of the major hornet silk proteins. From these results, we confirmed that the α-helical conformation mostly forms in the central portion of the hornet silk chains, whereas the ends of the protein chains are nearly devoid of α-helical structure. We deduce that the ends of the protein chains would preferentially adopt a β-sheet conformation. An isotope-labeled hornet (Vespa) cocoon in which the Ala carbons of silk proteins were substituted with 13C was successfully obtained by feeding a mixture of larval saliva with [13C3]Ala to mature larvae. The Ala fraction in the α-helix conformation of hornet silk was estimated by obtaining high-resolution 13C solid-state NMR spectra of the labeled cocoon, which also helped in understanding the molecular structure of hornet silk.

Structure and crystallization of sub-elementary fibrils of bacterial cellulose isolated by using a fluorescent brightening agent

The structure and crystallization of carefully isolated sub-elementary fibrils (SEFs) of bacterial cellulose have been investigated using TEM, WAXD, and high-resolution solid-state 13C NMR. The addition of a suitable amount of fluorescent brightener (FB) to the incubation medium of Acetobacter xylinum effectively suppressed the aggregation of the SEFs into the microfibrils, as previously reported. However, this study confirmed for the first time that serious structural change in the SEFs occurs during the removal of excess FB by washing with buffer solutions having pH values higher than 6 or with the alkaline aqueous solution that was frequently used in previous studies. In contrast, the isolation of unmodified SEFs was successfully performed by utilizing a washing protocol employing pH 7 citrate–phosphate buffer solution containing 1% sodium dodecyl sulfate. High-resolution solid-state 13C NMR and WAXD measurements revealed that the SEFs thus isolated are in the noncrystalline state in which the pyranose rings of the almost parallel cellulose chains appear to be stacked on each other. The respective CH2OH groups of the SEFs adopt the gt conformation instead of the tg conformation found in cellulose Iα and Iβ crystals, and undergo significantly enhanced molecular motion in the absence of intermolecular hydrogen bonding associated with these groups. The main chains are also subject to rapid motional fluctuations while maintaining the parallel orientation of the respective chains, indicating that the SEFs have a liquid crystal-like structure with high molecular mobility. Moreover, the SEFs crystallize into cellulose Iβ when the FB molecules that may adhere to the surface of the SEFs are removed by extraction with boiling 70 v/v% ethanol and 0.1N NaOH aqueous solution. On the basis of these results, the crystallization of the SEFs into the Iα and Iβ forms is discussed, including the possible formation of the crystalline-noncrystalline periodic structure in native cellulose.