Structural analysis of cellulose triacetate polymorphs by two-dimensional solid-state C-C and H-C correlation NMR spectroscopies

For the elucidation of the crystal structures of the two crystalline allomorphs of cellulose triacetate (CTA), namely CTA I and CTA II, two-dimensional (2D) solid-state through-bond ¹³C-¹³C and ¹H-¹³C correlations NMR techniques applied to the two crystalline allomorphs of CTA. As a result, the ¹³C and ¹H chemical shifts of the glucopyranose ring of CTA I and CTA II were completely assigned by the 2D NMR spectra of these allomorphs. On the 2D ¹³C-¹³C correlation spectrum of CTA II, two sets of the ¹³C-¹³C correlations from C1 to C6 were observed. This indicated that the CP/MAS ¹³C NMR spectrum of CTA II can be characterized by its overlapping of the ¹³C subspectra of two kinds of 2,3,6-triacetyl-anhydroglucopyranose units and that there are two magnetically non-equivalent sites in the unit cell of CTA II. In the case of CTA I, the numbers of respective ¹³C and ¹H shifts of CTA I agreed with the those of the glucopyranose residue in the allomorph, strongly suggesting that the asymmetric unit of CTA I is only one glucose residue. In addition, conformational differences in the exocyclic C5-C6 bonds between CTA I and CTA II were strongly suggested by the notable differences in the ¹H and ¹³C chemical shifts at the C6 sites of these allomorphs.     

Characterisation of the supramolecular structure of chemically and physically modified regenerated cellulosic fibres by means of high-resolution Carbon-13 solid-state NMR

Carbon-13 high-resolution solid-state NMR techniques have been invaluable in elucidating the structure of regenerated cellulosic materials. Studies of a range of fibres have shown systematic changes in chemical shifts, which can be related to the influences of physical processing or chemical modification. A constrained curve fitting method has been applied, where the C4 spectral envelope is represented as the sum of contributions from polymer in ordered, partially-ordered and disordered environments, associated with differing conformational arrangements of the cellulose hydroxymethyl and glycocidic bonds. The empirical gamma-gauche effect seems likely to provide the best rationalization for the relationship between C4 shifts and conformational order, taking into account the increased range of bond angles in disordered environments. The quantification of proportions of polymer units within different conformational groupings will provide new insights into the development of supramolecular texture. This will allow better appreciation of the relationships between fibre processing and ultimate fibre performance.     

High-resolution solid-state NMR structure of an anticancer agent

We demonstrate that solid-state NMR methods can be used to rapidly determine the high-resolution 3D structure of Epothilone B in the polycrystalline state. The solid-state NMR structures exhibit an average heavy atom RMSD to the mean structure of 0.14 A. The 3D-structural analysis leads to stereospecific assignments and provides insight into the influence of intermolecular interactions upon ssNMR chemical-shift parameters. Our results pave the way to the study of ligand-microtubule interactions in a noncrystalline and insoluble environment at atomic level.     

The study of water behaviour in regenerated cellulosic fibres by low-resolution proton NMR

Regenerated cellulosic fibres and comparative materials were studied in the hydrated state by low-resolution proton NMR. Experiments at variable pH and temperatures have shown that the shortened T₂ relaxation times of water within fully swollen cellulosic fibres are dominated by proton exchange with accessible cellulose hydroxyl groups. Proton exchange is accelerated by both acid and base catalysis, with relaxation data used to estimate rate constants for acid, base and neutral mechanisms. Complementary deuterium exchange measurements suggest that accessible cellulose regions below the immediate water interface may not contribute effectively to the proton exchange relaxation mechanism, with two-site relaxation models sensitive only to the direct pore surface area. Differences between surface-relaxing water and deuterium-exchanging water can therefore be used to determine an apparent depth of the accessible cellulose, which is greater for viscose and modal compared to lyocell. However, from relaxation data lyocell has a higher pore surface area. This work also confirmed that water interacting with accessible cellulose experiences motional restriction, allowing an intra-molecular dipolar contribution to relaxation. However, in the fully swollen state water molecules are diffusing rapidly between all internal fibre environments and there is no evidence of specific binding.     

An electron diffraction study of the crystal structure of native cellulose

The crystal lattice of native cellulose from four sources has been investigated by electron diffraction techniques. The four sources were: cotton, ramie, a bacterial cellulose (Acetobacter xylinum), and an algal cellulose (Valonia ventricosa). Evidence for the existence of at least two different unit cells is provided. There were no systematic absences of odd-order OkO reflections in any of the cellulose patterns, therefore, it was concluded that neither cellulose cell falls into the P2₁ space group.     

Probing the degree of crosslinking of a cellulose based superabsorbing hydrogel through traditional and NMR techniques

The network structure of a cellulose-based superabsorbing material has been probed by using three different techniques: ¹³C solid state NMR, free swelling in water and uniaxial compression of water swollen samples. A good agreement between the three apporaches has been found in terms of concentration of crosslinks per unit volume.The results have been discussed taking into account that NMR technique is able to detect only chemically effective crosslinks while free swelling and compression are sensitive to elastically effective physical and chemical crosslinks.A depression of swelling capacity and an apparent increase of degree of crosslinking with time, promoted by ageing of the cellulosic material, has been experimentally evidenced and discussed in terms of development of intermolecular physical interactions.     

On the structure and oxygen transmission rate of biodegradable cellulose nanobarriers

ABSTRACT: Cellulose nanofibrils have been proposed for novel barrier concepts, based on their capability to form smooth, strong and transparent films, with high oxygen barrier properties. A series of cellulose-based films were manufactured and tested with respect to their oxygen transmission rate (OTR) capabilities. The obtained OTR levels were considerably better than the levels recommended for packaging applications. Part of the nanofibrillated material applied in this study was produced with 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) mediated oxidation as pretreatment. Films made of TEMPO-pretreated samples yielded lower OTR values. The minimum obtained OTR value was 3.0 mL m-2 day-1 atm-1 with a corresponding oxygen permeability of 0.04 mL mm m-2 day-1 atm-1, tested at 50% relative humidity. The good barrier properties are due to the compact and dense structure of the films, as revealed by field-emission scanning electron microscopy. A relationship between OTR and the structure of the corresponding nanofibril-based films was confirmed.     

Synthesis and spectroscopic analysis of cellulose sulfates with regulable total degrees of substitution and sulfation patterns via C NMR and FT Raman spectroscopy

Synthesis and spectroscopic characterisation of cellulose sulfate (CS) were reported. Various CS exhibiting diverse degrees of sulfation (DS_S) were prepared through acetosulfation or direct sulfation of cellulose. During the acetosulfation, intermediate product - cellulose acetate sulfate (CAS) - was formed after the comparative esterification and subsequent deacetylation of CAS led to CS. The direct sulfation proceeded quasi-homogeneously and heterogeneously in N,N-dimethylformamide (DMF) or homogeneously in N,N-dimethylacetamide (DMAc)/LiCl mixture. The total DS_S between 0.21-2.59 and partial DS_S6 as well as DS_S2 of up to 1 were determined via elemental analysis and ¹³C NMR spectroscopy. Besides, solid-state CP/MAS ¹³C NMR could characterise CS regarding the sulfation. Subsequently, FT Raman investigation of obtained CS was conducted with the aim to establish analysis methods quantifying the total DS_S. The intensities of Raman bands ascribed to the vibrations of OSO and C-O-S groups were used as analysis parameters, yielding calibration curves with high correlation coefficients of more than 0.96.     

NMR study of the phosphonomethylation reaction on chitosan

N-Phosphonomethylation of chitosan reaction was studied and optimized using different reaction conditions. NMR spectroscopy was an important tool in this work to study this reaction and the α-aminomethylphosphonic acid function introduced onto chitosan was unequivocally characterized by ³¹P and ¹³C NMR analyses. But surprisingly, whatever the reaction conditions used, N-phosphonomethylation reaction of chitosan cannot be dissociated from a side reaction: the N-methylation reaction of chitosan. A mechanism was proposed to explain the formation of this side product.     

Characterisation of cellulose treated by the steam explosion method. Part 3: Effect of crystal forms (cellulose I,II and III) of original cellulose on changes in morphology,degree of polymerisaion,solubility and supermolecular structure by steam explosion

An attempt was made to clarify the effect of the crystal form of untreated cellulose on the morphological and structural changes of cellulose during steam explosion treatment (steam pressure P = 2.9MPa (T = 508K), treatment time t = 15-300 s). For this purpose, the crystal form of soft wood pulp (cellulose I) was converted by solid-to-solid transition, with minimal unavoidable change in other structural characteristics including morphology and average degree of polymerisation, into cellulose II or cellulose III. It was proved by both X-ray and solid-state cross-polarisation/magic-angle sample-spinning (CP/MAS) ¹³C NMR analyses that even a simple addition of water at room temperature brought about a significant structural change in the steam-untreated cellulose samples. The solubility towards 9.1 wt% aqueous sodium hydroxide, S_a, of the cellulose samples of crystal forms I and III could be improved from 31-33% up to almost 100% by selecting appropriate steam explosion conditions (for example, P = 2.9MPa, t = 30 s). Such a magnificent increase in S_a by the steam explosion treatment was not observed for the cellulose II sample, even under the rather severe conditions of the steam explosion treatment at which the cellulose III crystal was converted to a large extent to cellulose I, as confirmed by X-ray diffraction. X-ray diffraction analysis showed that crystallisation of samples with cellulose I or II crystal occurred to some extent during the steam explosion treatment. Contrary to this, the degree of breakdown of the intramolecular hydrogen bond O₃…O'₅, as estimated by CP/MAS ¹³C NMR analysis, significantly increased for cellulose I and I11 during the treatment. The decrease in the viscosity-average degree of polymerisation, P, observed for all treated samples can be roughly categorised into two or three steps of the first-order decomposition reaction with different reaction rates.     

Control of the crystal structure of microbial cellulose during nascent stage

The structure of the product, from an Acetobacter culture in the presence of Fluorescent Brightener, Direct Red 28, and Direct Blue 1, 14, 15 and 53, characterized by an X‐ray diffractormeter is a crystalline complex. On the other hand, solid‐state ¹³C‐NMR spectroscopy reveals that the product is noncrystalline. However, the X‐ray result of the product sample suggests that the dye molecule is included in the form of a monolayer between the cellulose sheets in the complex corresponding to the (11¯0) plane of microbial cellulose. But the celluloses regenerated from the Fluorescent Brightener product, the Direct Red 28 product, and the rest of the dye products are celluloses I, IV, and II, respectively. More specifically, the ¹³C‐NMR spectra revealed that the crystal types of cellulose from the Fluorescent Brightener and Direct Red 28 products are I_β and IV_I, respectively. Thus, the crystal structure of the product and the regenerated cellulose depends mainly on the position and number of the sulfonate groups in a direct dye and the interactions of the dye with the noncrystalline microbial cellulose in the nascent stage. The conformation and arrangement of the nascent cellulose chain changes when a direct dye adheres to it. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1726–1734, 2001     

A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars

The study presented in this paper is devoted to improve the knowledge on the influence of cellulose ethers (CE) on the freshly-mixed mortars water retention. Indeed, this crucial property is the most important imparted by these polysaccharides. One of the assumptions proposed to explain this phenomenon is that CE acts as diffusion barrier to the water. To test this hypothesis, the CE effect on the self-diffusion coefficient of water in solution and on the water mobility between two fresh cement pastes was studied by Nuclear Magnetic Resonance. CE does not significantly modify the water self-diffusion coefficient in CE solution or in admixed cement pastes. Moreover the interdiffusion imaging experiments demonstrated that the water diffusion at the paste/paste interface is not affected by the presence of cellulosic admixture.     

Two-dimensional correlation relaxometry studies of cement pastes performed using a new one-sided NMR magnet

We present preliminary results of the first NMR T₁-T₂ two-dimensional relaxation correlation experiments performed using a one-sided NMR system in cement based materials. Two-dimensional correlation relaxometry has itself only recently been demonstrated in cement paste where it proved to be a particularly sensitive probe of pore-water dynamics providing direct information on exchange of water between the gel and capillary pore networks. Further to this we have observed differences in the structural development of a selection of cement pastes throughout the early stages of hydration and verified the theoretical frequency dependence of the ratio T₁ / T₂. When coupled with instrumentation developments in one-sided NMR magnets the way is opened to detailed, spatially resolved studies of the development of hydration and porosity in the surface layers (top 50 mm) of cementitious materials. A new magnet, suitable for such applications, is discussed.     

A MAS NMR Study of the Bacterial ABC Transporter ArtMP

ATP‐binding cassette (ABC) transport systems facilitate the translocation of substances, like amino acids, across cell membranes energised by ATP hydrolysis. This work describes first structural studies on the ABC transporter ArtMP from Geobacillus stearothermophilus in native lipid environment by magic‐angle spinning NMR spectroscopy. The 2D crystals of ArtMP and 3D crystals of isolated ArtP were prepared in different nucleotide‐bound or ‐unbound states. From selectively ¹³C,¹⁵N‐labelled ArtP, several sequence‐specific assignments were obtained, most of which could be transferred to spectra of ArtMP. Residues Tyr133 and Pro134 protrude directly into the ATP‐binding pocket at the interface of the ArtP subunits, and hence, are sensitive monitors for structural changes during nucleotide binding and hydrolysis. Distinct sets of NMR shifts were obtained for ArtP with different phosphorylation states of the ligand. Indications were found for an asymmetric or inhomogeneous state of the ArtP dimer bound with triphosphorylated nucleotides. With this investigation, a model system was established for screening all functional states occurring in one ABC transporter in native lipid environment.     

Structure and dynamics in the amorphous region of natural rubber observed under uniaxial deformation monitored with solid-state C NMR

Solid-state ¹³C NMR experiments were performed under uniaxial deformation of natural rubber to investigate the changes in the structural and dynamical behavior of the amorphous region induced by elongation. The structural change was detected as the change in the ¹³C cross-polarization (CP) chemical shifts and line shapes. In addition, change in the CP ¹³C peak intensity as a function of time was also observed and was used to monitor the change of the dynamical behavior with time in high resolution. The relationship between ¹³C CP intensity and molecular dynamics under uniaxial deformation of natural rubber was examined by the ¹H magnetization decay behavior in the rotating frame and ¹H-¹³C CP build-up curves. Although the strain-induced crystallization occurred at around 200% strain, the ¹³C lineshapes showed no significant change in the orientation of the amorphous chains. On the other hand, the molecular dynamics of the amorphous chains was greatly affected even under lower extension, that is, the enhanced mobility of the chains was observed at 30% strain. This enhanced mobility induced by deformation decreased after the deformation was stopped.     

The domain structure and mobility of semi-crystalline poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate): A solid-state NMR study

Solid-state NMR techniques have been employed to investigate the domain structure and mobility of the bacterial biopolymeric metabolites such as poly(3-hydroxybutyrate) (PHB) and its copolymers poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) containing 2.7 mol% (PHBV2.7) and 6.5 mol% (PHBV6.5) 3-hydroxyvalerate. Both single-pulse excitation with magic-angle spinning (SPEMAS) and cross-polarization magic-angle spinning (CPMAS) ¹³C NMR results showed that these biopolymers were composed of amorphous and crystalline regions having distinct molecular dynamics. Under magic-angle spinning, ¹H T_1ρ and ¹³C T₁ showed two processes for each carbon. Proton relaxation-induced spectral editing (PRISE) techniques allowed the neat separation of the ¹³C resonances in the crystalline regions from those in the amorphous ones. The proton spin-lattice relaxation time in the tilted rotating frame, , measured using the Lee-Goldburg sequence with frequency modulation (LGFM) as the spin-locking scheme, was also double exponential and significantly longer than ¹H T_1ρ. The difference between for the amorphous and crystalline domains was greater than that of ¹H T_1ρ. Our results showed that the differences could be exploited in LGFM-CPMAS experiments to separate the signals from two distinct regions. ¹H spin-diffusion results showed that the domain size of the mobile components in PHB, PHBV2.7 and PHBV6.5 were about 13, 24 and 36 nm whereas the ordered domain sizes were smaller than 76, 65 and 55 nm, respectively. The results indicated that the introduction of 3-hydroxyvalerate into PHB led to marked molecular mobility enhancement in the biopolymers.     

聚酯混和物组分及其结构的核磁共振鉴定

利用AV500核磁共振(NMR)波谱仪测定了聚酯多元醇原料的组成及结构。并且利用2DNMR方法测定了其中主要成分聚酯的序列结构及其分布。实验证明混和物不经分离可以直接测定其组成。     

Heterogeneous structure of poly(glycolic acid) fiber studied with differential scanning calorimeter, X-ray diffraction, solid-state NMR and molecular dynamic simulation

The heterogeneous structures of poly(glycolic acid) (PGA) fibers which have been used as bio-degradable suture were studied by differential scanning calorimeter (DSC),X-ray diffraction and ¹³C solid state NMR. The ¹³C cross polarization NMR spectra without magic angle spinning of the stretched fibers observed by changing the angle between the fiber axis and the magnetic field clearly showed the heterogeneous structures which consist of three components; well-oriented, poorly-oriented and isotropic amorphous components. The local structure, distribution of the fiber axis and fraction of each component were determined quantitatively. Change in the heterogeneous structure by changing the stretching method in the sample preparation and by changing the stretching ratio was also monitored. The X-ray diffraction data of the fibers are in good agreement with the ¹³C CP NMR data. Change in the heterogeneous structures correlate with change in the thermal properties observed by DSC method. The molecular dynamic simulation showed the generation of trans conformation of PGA chain and also change in the fraction of other conformations by stretching, which supports the experimental results obtained above and gives additional structural information.     

Light scattering study on the dynamic behaviour of cellulose inclusion complex in LiOH/urea aqueous solution

In our previous study, the rapid dissolution of cellulose in alkali/urea aqueous solution at low temperature induced by a dynamic self-assembly process has been demonstrated [1]. The cellulose solution was meta-stable, and its stability could be influenced by system fluctuations (temperature, concentration or time). In the present work, cellulose dissolved in 4.6wt% LiOH/15.0wt% urea aqueous solution pre-cooled to −12 °C was studied by means of dynamic light scattering (DLS). The results revealed that cellulose existed as single inclusion complexes (ICs) associated with LiOH and urea hydrate which could surround the ICs at their surface. And the ICs were stiff, as revealed by results from transmission electron microscopy (TEM) and light scattering (LS). When there was a system fluctuation, the self-association of cellulose with each other took place, resulted from the destruction of the urea shell, and leading to the aggregation of the ICs. For that reason, the ICs stability could be evaluated by the aggregation behaviour. In our findings, the hydrodynamic radius (R_h,app) for the cellulose dispersion in dilute solution shifted to higher values with an increase of the temperature, the concentration or the storage time, indicating an IC aggregation phenomenon.     

ATR-FTIR and NMR spectroscopic studies on the structure of polymeric gel electrolytes for biomedical applications

The structure of polymeric gel membranes to be used as electrolytes in the recording of bioelectrical signals has been investigated by means of ATR-FTIR and ⁷Li and ¹³C MAS NMR spectroscopies. The membranes, based on PMMA, 1,2-diethoxyethane and lithium perchlorate, showed different ionic conductivity as a function of their composition. Such differences have been analyzed on the basis of spectroscopic data and the existence of interactions with the ester function in the polymeric matrix was determined. Spectroscopic data allowed to establish the optimal lithium concentration needed to achieve best ionic conductivity.