Moisture uptake in native cellulose – the roles of different hydrogen bonds: a dynamic FT-IR study using Deuterium exchange

This paper aims at a better understanding of the interaction between cellulose and moisture. In particular, the role of different hydrogen bonds in moisture uptake is investigated. Dynamic Fourier transform infrared spectroscopy (FT-IR) has been used in combination with deuterium exchange, which permits the labelling of cellulose domains with different accessibilities. The static spectra indicate a marked exchange of deuterium for the O2–H⋯O6 bonds, but only a limited exchange for the O3–H⋯O5 bonds. In the dynamic FT-IR spectra, deuteration gives rise to the growth of a broad band at wavenumbers around 2500 cm⁻¹. The rather unstructured appearance of the band suggests that deuteration is occurring only on the surface of the cellulose crystallites, i.e. in more or less non-load-carrying parts. This is corroborated by the lack of split peaks related to OD bonds in this band. In agreement with these observations, the split peak related to O3–H⋯O5 bonds and assigned to the load carrying cellulose structure increases during both H₂O and D₂O moisture conditioning, indicating a shift of the load transfer towards the backbone of the cellulose structure.     

Reactivity of dissolving pulps modified by TEMPO-mediated oxidation

The reactivity of dissolving pulps towards derivatization or dissolution is a crucial quality parameter and is mainly determined by the accessibility of the hydroxyl groups. When dissolving pulps are produced from paper-grade pulps by cold caustic extraction (CCE), their reactivity is often inferior as compared to commercial prehydrolysis kraft dissolving pulps. It was hypothesized that pulp reactivity can be enhanced by the introduction of small amounts of substituents to facilitate interchain accessibility. In this study, CCE-treated Eucalyptus globulus kraft paper pulp was subjected to TEMPO-mediated oxidation to initiate partial oxidation of the C₆-hydroxyl groups to carboxyl groups. The effect of this pulp modification on the reactivity towards xanthation and the subsequent dissolution in diluted aqueous alkali solution (viscose process) as well as the dissolution in complexing and non-complexing solvents, respectively, was thoroughly examined. The results revealed that the oxidized pulps rich in C₆-carboxylate groups impeded the xanthation reaction obviously because of the reduced availability of hydroxyl groups. When N-methylmorpholine-N-oxide monohydrate was used as a direct solvent, a very high content of C₆-carboxylate groups was found to reduce the solubility of the pulp fibers as less hydrogen bonds can be formed with NMMO·H₂O. In the case of dissolution in the complexing solvent cupriethylenediamine, the dissolution mechanism of cellulose was not deteriorated by the high content of C₆-carboxylate groups. Instead, the oxidation procedure increased the hydrophilic character and the swelling capacity of the outer cell wall layers allowed homogeneous dissolution.     

Compression molded wood pulp biocomposites: a study of hemicellulose influence on cellulose supramolecular structure and material properties

In this study, the importance of hemicellulose content and structure in chemical pulps on the property relationships in compression molded wood pulp biocomposites is examined. Three different softwood pulps are compared; an acid sulfite dissolving grade pulp with high cellulose purity, an acid sulfite paper grade pulp and a paper grade kraft pulp, the latter two both containing higher amounts of hemicelluloses. Biocomposites based the acid sulfite pulps exhibit twice as high Young’s modulus as the composite based on paper grade kraft pulp, 11–12 and 6 GPa, respectively, and the explanation is most likely the difference in beating response of the pulps. Also the water retention value (WRV) is similarly low for the two molded sulfite pulps (0.5 g/g) as compared to the molded kraft pulp (0.9 g/g). The carbohydrate composition is determined by neutral sugar analysis and average molar masses by SEC. The cellulose supramolecular structure (cellulose fibril aggregation) is studied by solid state CP/MAS ¹³C-NMR and two forms of hemicellulose are assigned. During compression molding, cellulose fibril aggregation occurs to higher extent in the acid sulfite pulps as compared to the kraft pulp. In conclusion, the most important observation from this study is that the difference in hemicellulose content and structure seems to affect the aggregation behaviour and WRV of the investigated biocomposites.     

Carboxymethyl cellulose on a fiber substrate: the interactions with cationic polyelectrolytes

High and low molecular weight (M_w) carboxymethyl celluloses (CMC) were adsorbed on a well-characterized fiber substrate (long fibers of a commercial bleached birch kraft pulp with the carboxylic acid groups in Na-form) to increase the charge of the fibers in a controlled fashion. The M_w played a role in the utilization of CMCs as a strength additive in paper sheets nearly doubling the tensile strength with the high M_w CMC. Swelling properties of the CMC treated fibers were measured with water retention value (WRV). The WRV increased more with the high M_w CMC. The swelling was further tuned by two highly cationic polyelectrolytes; high M_w poly(diallyldimethyl ammonium chloride) (PDADMAC) and low M_w polybrene (hexadimethrine bromide, [3,6]-ionene). They were chosen because of their known ability to neutralize the anionic charge either exclusively on the surface or in the whole fiber, respectively. Adsorption of PDADMAC could reduce WRV of the CMC pre-treated fibers to the level of the untreated reference, while polybrene adsorbed pulps with 3–10 times more cationic polyelectrolyte deswelled the fibers only slightly more than the surface neutralized fibers. These results indicated surface conformation differences with low and high M_w CMCs. While the conformation did play a role after physical alteration (drying and rewetting) of the fibers, the paper sheets produced from these fibers showed remarkable differences. In extreme cases, the strength of the paper could be retained after drying (low M_w CMC + PDADMAC) or paper, resistant to disintegration, could be achieved (CMC + polybrene).     

Helical concept of the fine structure of cellulose

The helical concept of the fine structure of cellulose as proposed by Manley is discussed. The deuterium exchange experiments with cotton, cotton crystallites, and regenerated cellulose I, in which accessibility was determined by comparing the ratio of the infrared absorbance of the O? D peak to the O? H peak, revealed that the accessibility of cotton linters decreased on acid hydrolysis, whereas it increased on treatment with ethylamine followed by washing with water. This is in contrast to the finding of Manley, who had evaluated the accessibility by a gravimetric D₂O exchange method and had come to the conclusion that acid hydrolysis did not change the accessibility of cellulose and hence cellulose did not contain crystalline and amorphous phases but was all crystalline. On the basis of Manley's protofibril and the accessibility data obtained in this investigation, a concept of the fine structure of cellulose is proposed, in which the role of Manley's protofibril is analogous to the role of individual molecule in the fringe micellar model. This concept explains the properties of cellulose that are otherwise explainable on the currently accepted fringe micellar theory. In addition, it explains the marked shrinkage in the length of cotton and rayon fibers when placed in 16% sodium hydroxide solution.     

Near-infrared spectroscopic analysis of aging degradation in antique washi paper using a deuterium exchange method

The diffusion process of deuterated water (D₂O) in washi (Japanese traditional paper) was investigated by means of a deuterium exchange method and Fourier-transform near-infrared (FT-NIR) transmission spectroscopy. The samples were the modern (AD 2003) hand-made washi and those from an archival collection of cultural artifacts (AD 1791 and 1615). Four absorption bands were identified in the NIR spectral range from 7200 to 6000 cm⁻¹ which are due to OH groups in the amorphous, semi-crystalline and two types of crystalline regions of cellulose. The accessibility of D₂O increased with decreasing state of order of cellulose, and the saturation accessibility increased with the age of the samples. It was suggested that during aging hemicellulose, which forms a composite with cellulose in paper, was progressively hydrolyzed, resulting in the expansion of inter-molecular distance between cellulose chains. The oldest sample showed a low diffusion rate compared with the others. SEM observation of the textile structures indicated that the oldest sample had two layers due to beating. It was estimated that the tight surface layer blocked the diffusant in the initial stage of the diffusion process.     

The effect of eucalypt pulp xylan content on its bleachability,refinability and drainability

Currently, bleached eucalypt pulps are largely used for printing and writing (P&W) and sanitary (tissue) paper grades. Among the many pulp quality requirements for P&W and tissue paper production the xylan content is one of the most significant. For P&W papers, increasing xylans improve pulp refinability and strength properties but negatively affect bulk and drainability. For tissue paper, xylans are purportedly advantageous during paper drying in the Yankee cylinder but negatively affect paper bulk and may increase dusting during paper manufacture. On the other hand, bleachability is a very important parameter for both P&W and tissue grade pulps since bleaching cost is the second most significant in eucalypt bleached kraft pulp production. The aim of this study was evaluating the influence of eucalyptus pulp xylan content on its bleachability, refinability and drainability. A sample of industrial unbleached eucalyptus kraft pulp containing 15.6?% xylans was treated with various alkali charges at room temperature in order to obtain materials with different xylan contents. The pulps were bleached to 90 % ISO brightness with the O–D_HT–(EP)–D sequence and evaluated for their refinability and drainability. By increasing the alkali concentration in the range of 10–70 g/L pulps of 14.5–5.9 % xylans were produced with no significant impact on cellulose crystallinity. The decrease of xylan content significantly decreased pulp bleaching chemical demand, water retention value and refinability and increased pulp drainability.     

The Degree of Disorder in Hardwood Kraft Pulps Studied by Means of LODP

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.     

Hydroxyl accessibility in wood cell walls as affected by drying and re-wetting procedures

The first drying of wood cell walls from the native state has sometimes been described as producing irreversible structural changes which reduce the accessibility to water, a phenomenon often referred to as hornification. This study demonstrates that while changes do seem to take place, these are more complex than what has hitherto been described. The accessibility of wood cell wall hydroxyls to deuteration in the form of liquid water was not found to be affected by drying, since vacuum impregnation with liquid water restores the native cell wall accessibility. Contrary to this, hydroxyl accessibility to deuteration by water vapour was found to decrease to different levels depending on the drying conditions. Vacuum drying at 60 °C for 3 days reduced the accessibility more than drying for 1 day at 103 °C without vacuum. Drying for 3 days at 103 °C increased the hydroxyl accessibility compared to 1 day. Moreover, the decrease in hydroxyl accessibility to deuteration by water vapour induced by the first drying could be at least partially erased by subsequent vacuum impregnation with liquid water, indicating reversibility. For the drying of solid, non-degraded wood cell walls the results challenge the often supposed process of hornification, understood as a permanent decrease in hydroxyl accessibility to water.     

Aromatic ring favorable and efficient H-D exchange reaction catalyzed by Pt/C

An effective and applicable Pt/C-catalyzed deuteration method of aromatic rings using D₂O as a deuterium source under hydrogen atmosphere was developed. Five percent Pt/C would lead to quite effective H-D exchange results on the aromatic ring systems. The reaction is general for a variety of aromatic compounds including biologically active compounds.     

Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods

Solid-state ¹³C NMR spectroscopy was used to determine the degree of cellulose crystallinity (CrI) in kraft, flow-through kraft and polysulphide–anthraquinone (PS–AQ) pulps of pine and birch containing various amounts of hemicelluloses. The applicability of acid hydrolysis and the purely spectroscopic proton spin-relaxation based spectral edition (PSRE) method to remove the interfering hemicellulose signals prior to the determination of CrI were also compared. For softwood pulps, the spectroscopic removal of hemicelluloses by PSRE was found to be more efficient than the removal of hemicelluloses by acid hydrolysis. In addition to that, the PSRE method also provides information on the associations between cellulose and hemicelluloses. On the basis of the incomplete removal of xylan from the cellulose subspectra by PSRE, the deposition of xylan on cellulose fibrils and therefore an ordered ultrastructure of xylan in birch pulps was suggested. The ordered structure of xylan in birch pulps was also supported by the observed change of xylan conformation after regeneration. Similarly, glucomannan in pine pulps may have an ordered structure. According to the ¹³C CPMAS measurements conducted after acid hydrolysis, the degree of cellulose crystallinity was found to be slightly lower in birch pulps than in the pine pulps. Any significant differences in cellulose crystallinity were not found between the pulps obtained by the various pulping methods. Only in pine PS–AQ pulp, the degree of cellulose crystallinity may be slightly lower than in the kraft pulps containing less hemicelluloses.     

Dissolving-grade birch pulps produced under various prehydrolysis intensities: quality,structure and applications

Aqueous-phase prehydrolysis followed by alkaline pulping is a viable process to produce wood-based dissolving pulps. However, detailed characterisation of the achievable pulp quality, performance and cellulose structure is yet lacking. In this study, the production of hemicellulose-lean birch soda-anthraquinone pulps after prehydrolysis under various intensities was investigated. Increasing prehydrolysis intensity resulted in pulps of higher purity but lower cellulose yield and degree of polymerisation. Higher cellulose yield by using sodium borohydride during pulping was achieved at the expense of reducing pulp purity. Cellulose crystallinity was similar in all pulps indicating simultaneous degradation of both crystalline and amorphous cellulose regions. Reinforced prehydrolysis seemingly increased the cellulose crystal size and the interfibrillar distances. Moderate intensity prehydrolysis (170 °C) resulted in a pulp well suited for viscose application, whereas reinforced prehydrolysis favoured the production of acceptable cellulose triacetate dope. The performance of the pulps in viscose and acetate applications was strongly related to the chemical and structural properties.     

Stereo‐ and Regioselective Direct Multi‐Deuterium‐Labeling Methods for Sugars

Deuterium‐labeled sugars can be utilized as powerful tools for the architectural analyses of high‐sugar‐containing molecules represented by the nucleic acids and glycoproteins, and chiral building blocks for the syntheses of new drug candidates (heavy drugs) due to their potential characteristics, such as simplifying the ¹H NMR spectra and the stability of C? D bonds compared with C? H bonds. We have established a direct and efficient synthetic method of deuterated sugars from non‐labeled sugars by using the heterogeneous Ru/C‐catalyzed H–D exchange reaction in D₂O under a hydrogen atmosphere with perfect chemo‐ and stereoselectivities. The direct H–D exchange reaction can selectively proceed on carbons adjacent to the free hydroxyl groups, and the deuterium labeling of various pyranosides (such as glucose and disaccharides), as well as furanosides, represented by ribose and deoxyribose was realized. Furthermore, the desired number of deuterium atoms can be freely incorporated into selected positions by the site‐selective protection of the hydroxyl groups using acetal‐type protective groups because the deuterium exchange reaction never proceeds on positions adjacent to the protected hydroxyl groups.     

Effect of cellulase-assisted refining on the properties of dried and never-dried eucalyptus pulp

The effect of two different cellulases on the hornification phenomenon,in which drainability (Schopper–Riegler method) and mechanical propertiesdiminish when pulps are dried, was studied. The enzyme applications testedincluded a commercial enzyme named ComC (Pergalase A40 from CIBA) and alaboratory enzyme from Paenibacillus sp. strain BP-23namedCelB. Industrial never-dried Eucalyptus globulus bleachedkraft pulp was split in two halves and one of them was dried at ambientcontrolled conditions. We compared enzyme effects on both pulps (wet pulp anddried pulp) before and after PFI mill refining. Enzyme applications increaseddrainability (Schopper–Riegler method) and water retention value (WRV) ofnever-dried bleached pulp, although this did not imply an enhancement of themechanical properties of paper. Cellulase treatment of dried pulps, bycontrast,gave rise to increased drainability and WRV and also to improved mechanicalproperties. The changes caused by drying became less significant after enzymeapplication. Handsheets from CelB-treated dried pulps showed an improvement oftensile and burst indexes while tear decreased. The effect produced by CelB canbe considered a biorefining step. In fact, by means of enzyme treatment withCelB the properties of paper manufactured from dried pulp equalled theproperties attained from wet fibres, with the exception of tear index. Changeswere also found in surface fibre morphology, such as flakes and peeling due tocellulase treatment. The surface modification of fibres with cellulases givesrise to better bonding properties and a closer structure of paper. The finalconclusion is that treatment with cellulases could compensate the hornificationeffect and lead to an important saving of refining energy. The novel enzyme,CelB, was the most effective in improving paper properties and counterbalancingthe hornification effect caused by drying.     

Electrocatalytic Deuteration of Halides with D2O as the Deuterium Source over a Copper Nanowire Arrays Cathode

Precise deuterium incorporation with controllable deuterated sites is extremely desirable. Here, a facile and efficient electrocatalytic deuterodehalogenation of halides using D₂O as the deuteration reagent and copper nanowire arrays (Cu NWAs) electrochemically formed in situ as the cathode was demonstrated. A cross-coupling of carbon and deuterium free radicals might be involved for this ipso-selective deuteration. This method exhibited excellent chemoselectivity and high compatibility with the easily reducible functional groups (C=C, C≡C, C=O, C=N, C≡N). The C−H to C−D transformations were achieved with high yields and deuterium ratios through a one-pot halogenation–deuterodehalogenation process. Efficient deuteration of less-active bromide substrates, specific deuterium incorporation into top-selling pharmaceuticals, and oxidant-free paired anodic synthesis of high-value chemicals with low energy input highlighted the potential practicality.     

General method of obtaining deuterium-labeled heterocyclic compounds using neutral D2O with heterogeneous Pd/C

A protocol of a versatile H-D exchange reaction of heterocyclic compounds catalyzed by heterogeneous Pd/C in D₂O is described. The reaction of various nitrogen-containing heterocycles with 10% Pd/C (10 wt % of the substrate) under hydrogen atmosphere in D₂O as a deuterium source at 110-180 °C for 24 h afforded the corresponding deuterated compounds with satisfactory efficiency of deuteration in moderate to excellent isolated yields. Furthermore, the Pd/C-H₂-D₂O system can be extended to the direct deuteration of biologically active compounds such as sulfamethazine, which is used as a synthetic antibacterial drug for fat stocks and would be applied as a general method for the preparation of the standard materials for the analysis of residual chemicals in foods and so on.     

Multiple Deuteration of Water-Soluble Olefinic Acids with a [Pd(Alizarin Monosulfonate)2] Catalyst

Hydrogenations in aqueous systems with the soluble [Pd(alizarin monosulfonate)₂] catalyst resulted in extensive deuteration of crotonic, trans-2-pentenoic and itaconic acids regardless of whether the deuterium source was D₂ or D₂O. Itaconic acid was deuterated up to 3.6 D/methylsuccinic acid. Detailed ¹H- and ¹³C-NMR studies identified six isotopomers of the deuterated methylsuccinic acid product and revealed an important role of the H/D exchange on the catalytically active Pd-intermediate.     

Monitoring allostery in D2O: a necessary control in studies using hydrogen/deuterium exchange to characterize allosteric regulation

There is currently a renewed focus aimed at understanding allosteric mechanisms at atomic resolution. This current interest seeks to understand how both changes in protein conformations and changes in protein dynamics contribute to relaying an allosteric signal between two ligand binding sites on a protein (e.g., active and allosteric sites). Both nuclear magnetic resonance (NMR), by monitoring protein dynamics directly, and hydrogen/deuterium exchange, by monitoring solvent accessibility of backbone amides, offer insights into protein dynamics. Unfortunately, many allosteric proteins exceed the size limitations of standard NMR techniques. Although hydrogen/deuterium exchange as detected by mass spectrometry (H/DX-MS) offers an alternative evaluation method, any application of hydrogen/deuterium exchange requires that the property being measured functions in both H₂O and D₂O. Due to the promising future H/DX-MS has in the evaluation of allosteric mechanisms in large proteins, we demonstrate an evaluation of allosteric regulation in D₂O. Exemplified using phenylalanine inhibition of rabbit muscle pyruvate kinase, we find that binding of the inhibitor is greatly reduced in D₂O, but the effector continues to elicit an allosteric response.     

Purification of cellulosic pulp by hot water extraction

Hot water extraction (HWE) of pulp in a flow-through reactor was evaluated as a method to purify paper-grade pulps. About 50–80 % of the xylan and up to 50 % of the lignin in unbleached birch Kraft pulp was extracted by the HWE without losses in cellulose yield. The residual xylan content in the extracted pulps was predominantly too high for dissolving-grade applications, but some of the pulps with a xylan content of 5–7 % might still be suitable as rayon-grade pulps. Increasing extraction temperature lowered the xylan content at which cellulose yield started to decrease. Furthermore, at any given xylan content, increasing extraction temperature resulted in cellulosic pulp with higher degree of polymerization. The extracted xylan was recovered almost quantitatively as xylo-oligosaccharides. The results suggest that HWEs at elevated temperatures may be applied to purify cellulosic pulps, preferably containing a low xylan content, and to recover the extracted sugars.     

1H-NMR-untersuchungen an sialinsäuren markierung von N-acetyl-D-neuraminsäure mit deuterium

An efficient method for the deuteration of N-acetyl-D-neuraminic-acid (NeuAc) at C-3 is described. In alkaline D₂O solution both protons of NeuAc (H_3a and H_3e) will be substituted by deuterium, whereby the substitution effect was found to be faster for H_3a than for H_3e. The rate of this exchange depends strongly on the pD value: the rate increases with increasing pD value. In acidic solution no effect could be observed. The process of H-D-exchange is reversible.