On the accessibility and structure of xylan in birch kraft pulp

The structure of -(14)-xylan, both in isolated form and as a component of bleached birch kraft pulp, was studied employing CP/MAS ¹³C NMR spectroscopy. Bleached birch kraft pulp was treated with xylanases or alkali in order to distinguish between accessible and inaccessible xylan. In xylan which was alkali-extracted from bleached birch kraft pulp, the relative contents of xylose and 4-O-methylglucuronic acid were 99.4 and 0.6 weight %, respectively, and the degree of polymerization was 70. The supermolecular structure of xylan is very sensitive to the surrounding environment. All extracted xylan chains were accessible to water and methanol and the solvent molecules easily exchanged. In bleached birch kraft pulp, cellulose fibrils interact with xylan chains, causing these to adopt a conformation similar to one of the configurations observed for dry xylan. In birch pulp, about 1/3 of the xylan was found to be accessible to digestion by xylanases or extraction with 5% w/w potassium hydroxide (aq). A signal at 81.7ppm in the C-4 region of the CP/MAS ¹³C NMR spectrum of bleached birch kraft pulp originated from xylan at the accessible fibril surfaces. A portion of a broad signal at 83.5ppm reflected inaccessible xylan, which is probably present as co-aggregates with cellulose fibril aggregates.     

Characterization of the supermolecular structure of cellulose in wood pulp fibres

The hierarchic organization of cellulose fibrils (microfibrils) was investigated in holocellulose, sulphite pulp and kraft pulp using TEM, XRD, ED and FTIR. There were remarkable differences in both the fibril structure and fibril aggregation between the samples. TEM observations revealed more intimately associated fibrils in the kraft pulp compared to the sulphite pulp and the holocellulose, results in agreement with previous CP/MAS ¹³C-NMR data [Hult E.-L. et al. (2002) Holzforschung 56: 231–234]. Furthermore, the cellulose crystallinity was higher in the kraft pulp sample. With respect to the cellulose I and I allomorphs, these samples were controversial when different analytical techniques were applied. Due to the small fibril size and the low degree of order of cellulose in these samples, the concept of crystalline triclinic and monoclinic components as determined by diffraction analysis may not be adequate. Instead the fibril can be regarded to have different degrees of lateral order (including paracrystalline ordering) that can be reoriented to I type conformation and packing upon pulping.     

Surface properties of lignocellulosic fibers bearing carboxylic groups

Fibers with various amounts of carboxylic acid functionalities as determined with FTIR and conductometric titration were prepared by chemical modification of high bleached kraft pulp (CP) and chemical thermomechanical pulp (CTMP) with succinic anhydride. The degree of the modification was dependent on reaction time and the type of fiber used. The modification levelled off after 15h of reaction, and this effect was similar for both fiber substrates. The amount of carboxylic acid attached to CTMP, determined by weight gain, was however less than half of the amount of carboxylic moieties introduced to CP fibers at any reaction time. ESCA characterization of the succinylated fibers indicates that the carboxylic acid functionalities are predominantly introduced at the fiber surface. The wettability in water, measured as contact angle, of the succinylated CTMP fibers was significantly improved by the modification, whereas the wettability of CP fibers was slightly decreased. The differences in wettability are caused by the dispersive and polar characteristics of succinic acid attached to the fiber surface and its interaction with the fiber surface. The character of the linkage group in the anhydride used for modification as well as the composition of the cellulose fiber surface are suggested to play a crucial role in the surface energy of the modified fibers and hence their wetting properties.     

SEC–MALLS analysis of cellulose using LiCl/1,3-dimethyl-2-imidazolidinone as an eluent

The lithium chloride/1,3-dimethyl-2-imidazolidinone (LiCl/DMI) solvent system for cellulose was adopted as a mobile phase of size-exclusion chromatographic (SEC) analysis of cellulose, and the applicability of this system was examined using multi-angle laser light scattering and ¹³C-NMR analysis. The results indicate that 8% (w/v) LiCl/DMI ID a true solvent for cellulose, and that cellulose molecules dissolving ID 1% (w/v) LiCl/DMI are separated orderly depending on their molecular mass (MM) or root-mean-square (RMS) radius by the SEC system. Practically, no aggregates were detected ID the dilute cellulose/LiCl/DMI solutions. Furthermore, high stability of cellulose/LiCl/DMI solutions has been demonstrated; only a few percent of decline ID average MM was observed even after storage for 6 months at room temperature. Relationships between RMS radius and MM for hardwood bleached kraft pulp ID 1% LiCl/DMI was estimated as the following equation: _g^0.59, corresponding to a Mark–Houwink–Sakurada exponent of 0.77.     

Nmr investigation of the spatial structure of quinolizidine alkaloids IV. Conformation of sophoridine in solution

Summary On the basis of PMR spectra at 100 and 300 MHz, the conformation of the molecule of the alkaloid sophoridine in solution has been determined as A/B-, A/C-, and C/D-trans, B/C-cis, with the chair conformation of rings A and D and the boat conformation of rings B and C.For Communications I–III, see [1].V. I. Lenin Tashkent University. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 744–751, November–December, 1974.     

Some Properties of Polyelectrolyte-G rafted Cellulose

Anionic and cationic polyelectrolytes were grafted on a bleached kraft pulp. Grafting an anionic polyelectrolyte (sodium poly-acrylate-polyacrylamide copolymer) resulted in modified fibers possessing outstanding affinity for water and saline solutions in the pH range where the polymer is ionized. Swelling is the result of both the grafting operation itself and of the presence of the ionized polyelectrolyte.

The swollen grafted fibers could be disintegrated under intense shear to give a colloidal solution exhibiting pseudoplastic thixo-tropic behavior. Electron microscopic examination revealed that during the shearing process the fiber had been disintegrated into its constitutive elements, long rodlike protofibrils, which are believed to be mainly responsible for the high viscosities observed.

Grafting a cationic polyelectrolyte (polydimethylamino ethyl methacrylate hydrochloride) produced fibers with lower but significant water swelling. The influence of pH on swelling was similar, although reversed, to that observed with the anionic grafted fibers. The presence of a large number of cationic groups in the porous cellulose fiber gel points to applications in ion-exchange and adsorption processes.     

What crystals of small analogs are trying to tell us about cellulose structure

The molecular geometries from crystal structures of 23 small molecules such as cellobiose were reviewed and extrapolated to give model cellulose chains. Within a given model, all monosaccharide units and their linkages are identical so the models are regular helices. Despite fairly large ranges for the glycosidic linkage torsion angles and , 29° and 57°, respectively, there is little variation in the n and h parameters of the model helices. They are extended, with h values (the advance per residue along the helix axis) of 5.04–5.27 Å. Some models were slightly right-handed, with n values up to 2.12 residues per helix turn. Left-handed models were in the majority, and their n values were as large as –2.91. These results are consistent with known structures of cellulose and its derivatives. An exception comes from a heavily derivatized cellobiose molecule. It yields right-handed helices with n 4.5 and h 3 Å. Because one half turn of this helix reverses the direction of the chain in a compact region, the linkage geometry is a model for chain-folding. Other derivatives that are unable to form the O3O5 hydrogen bond gave left-handed helices. The puckering of the glucose rings was also surveyed. A number of rings in small molecule structures are puckered to a degree that is similar to the puckering determined for methyl cellotrioside, cellotetraose, cellulose I and cellulose II.     

Changes of cellulose fiber wall structure during drying investigated using NMR self-diffusion and relaxation experiments

The self-diffusion of water sorbed in cellulose fibers was investigatedby means of NMR during slow drying in order to follow changes in the celluloseporous structure. In pulp fibers pores with at least one dimension on them scale were observed at high amounts of sorbed water and nm-scale pores atlow amounts. Beating affected the m-scale pores. Regenerated cellulosefibers had nm-scale pores up to high amounts of water. Bulk water was observedas a second component with long T ₂ in a CPMGexperiment. The sequence of water removal for kraft pulp fibers is: (1) bulkwater, (2) water in m-scale pores and (3) water in nm-scale pores.     

Conformational analysis of a farnesyltransferase peptide inhibitor, CVIM

The conformational states of the peptide Cys-Val-Ile-Met (or CVIM) were computed and characterized. CVIM inhibits farnesylation of the Ras oncogene product, p21^ras, at the cysteine residue of the C-terminal segment. CVIM is active in an extended conformation. A similar peptide (KTKCVFM) appears to bind the enzyme in the Type I bend conformation. In the present study, the conformations of CVIM were computed in an aqueous environment with the peptide in the zwitterionic state. Solvation free energy based on solvent accessible surface area and a distance dependent dielectric were used in the calculations. Final conformations of multiple independent Monte Carlo simulated annealing (MCSA) conformational searches were used as starting points for Metropolis Monte Carlo (MMC) runs. Conformations saved at intervals during MMC runs were analyzed. Conformers were separated by interactive clustering in dihedral angle coordinates. The four lowest energy conformers corresponding to a Type I bend, extended, AB-bend, and BA-bend were within 0.3 kcal/mol of each other, and dominant in terms of population. The Type I bend and extended conformers were supported by the binding studies. The extended conformer was the most populated. In the AB-bend conformer, `A' indicates the -helix conformation of Val, and `B' indicates the -strand conformation of Ile. The AB- and BA-bend conformations differed from the extended conformation in the value of Val and Ile , respectively, and from the Type I bend conformation in the value of Ile and Val , respectively. The four lowest energy conformers were characterized in terms of energy, density of low energy conformations (or entropy), structure, side chain rotamer fraction population, and interatomic distances.     

Structure and morphology of cellulose in wheat straw

The structure and morphology of cellulose extracted from wheat were studied. It was found that the extraction process is effective and hemicelluloses and lignin can be extracted completely. Cellulose in wheat straw was identified as cellulose I allomorph with low crystallinity and the crystallinity of cellulose from different parts of the wheat straw has little difference. There was no metastable cellulose I crystalline modification found in wheat straw; only the more stable cellulose I crystalline modification existed. Cellulose chains in the epidermis of wheat straw were observed with their orientation along the growth direction of wheat straw, while those in parenchyma were observed with almost no preferred orientation. There are two kinds of morphologies on the surface of wheat straw. One is the fiber structure with fibrils of about 5 m diameter, and the other is the fiber structure with serration morphology at the edge of the fiber, with which the fibers are connected together. The diameter of the latter one is about 10 m. The vascular bundles consist of circular rings while spiral structure cellulose backbones covered with thin cellulose film were also observed.     

Complexation of the Non-steroidal Anti-inflammatory Drug Nabumetone with Modified and Unmodified Cyclodextrins

The inclusion of the anti-inflammatory drug, Nabumetone, in -, - and hydroxypropyl--cyclodextrin (CDs) is studied using UV-VIS absorption and steady-state fluorescence emission. Binding constants and thermodynamic parameters of complex formation are determined by spectrofluorimetry. The inclusion phenomena of Nabumetone with the three cyclodextrins is compared with that of the well known similar anti-inflammatory drug Naproxen. In the case of Nabumetone pronounced differences are observed in the complexation process with each cyclodextrin whereas the respective Naproxen complexes are nearly identical. ¹H-NMR experiments show that the inclusion process in Nabumetone can occur either through the substituents in the -2 (butanone) or -6 (methoxy) positions in the naphthalene ring.     

Inclusion Complexes of Atrazine with α-, β- and γ-Cyclodextrins. Evidence by Polarographic Kinetic Currents

The reduction of atrazine in acidic aqueous media on mercury electrodes proceeds only after the protonation reaction. In fact, the efficiency of the reduction process is very low at pHs greater than 4. However, the addition of cyclodextrins (CDs) to neutral aqueous solutions of atrazine yields a kinetically controlled polarographic reduction wave, whose limiting current depends on the size of the CD cavity, and increases with the concentration of the CD itself. In particular, the size of the increase follows the order -CD < -CD < -CD for the same CD concentration. The half-wave potential shifts toward more negative values when the concentration of CDs increases. These findings lead to the conclusion that atrazine and CDs form an inclusion complex, whose stability constant we have estimated, and also that atrazine undergoes protonation facilitated by complexation with CDs. The stability constants of 1:1 complexes evaluated from polarographic data in 0.1M-KCl and at neutral pH for -CD, -CD and -CD are 4900, 1970 and 19000, respectively. The formation of the inclusion complex was indirectly confirmed by UV-Vis measurements in the presence of methylred and phenolphthalein, which both compete with atrazine in the formation of the corresponding inclusion compounds with CDs.     

Effect of reduction temperature on the dehydrocyclizing activity of pure chromia and chromia containing K2O and CaO

The dehydrocyclization of n-heptane at 520–550°C over pure chromia and chromia with added CaO and K₂O (5 wt.% each) has been investigated. Introduction of an alkaline additive into Cr₂O₃ decreases markedly the yield of toluene. Raising of the reduction temperature of alkali-containing samples from 550 to 700°C results, in contrast to pure Cr₂O₃ and chromia-alumina systems, not in diminished, but in increased yields of toluene, the activity enhancement being particularly pronounced for the CaO-containing catalyst. These increased activities cannot be correlated either with variations in specific surface area or changes in the content of Cr⁶⁺ during subsequent oxidation, and presumably should be ascribed to more extensive catalyst reduction at higher temperatures.

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- 520–550° CaO K₂O ( 5% .). Cr₂O₃ . 550° 700° , Cr₂O₃ , , , CaO. , , .

     

A Heterogeneous Process for the Synthesis of Silicon Carbide Fibers

Study of the heterogeneous reaction of SiO₂ with activated carbon fibers has shown that the heterophase reaction (SiO₂ (gas), SiO (gas) + C) gave rise to polycrystalline fibers of - and -SiC, whereas the homophase reaction of SiO with CO on a Si cupel led to the growth of filament crystals of -SiC. In agreement with the proposed mechanism for the origin and growth of SiC crystals, the appearance of Si (l) on the surface of the product is connected to the exo-effect of the formation of silicon carbide.     

Investigation of the chemisorption of propylene on zinc oxide by temperature-programmed desorption

The interactions of propylene with zinc oxide have been studied by temperature-programmed desorption (TPD) under experimental conditions free of inter- or intraparticle limitations or readsorption phenomena.The results indicate that propylene is adsorbed on two types of active sites, and the Arrhenius parameters for the corresponding desorption processes have been calculated; moreover, on the basis of these results, important hypotheses can be made regarding the reaction mechanisms of the total oxidation and the dehydroaromatization of propylene on zinc oxide.The experiments confirm that thermal methods can be successfully used in the interesting and important field of heterogeneous catalysis.

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Zusammenfassung Die Wechselwirkungen von Propylen mit Zinkoxid wurden durch temperaturprogrammierte Desorption (TPD) unter Versuchsbedingungen frei von inter- oder intrapartikulären Begrenzungen oder Readsorptionserscheinungen untersucht.Die Ergebnisse zeigen, daß Propylen an zwei Typen von aktiven Stellen adsorbiert wird. Die Arrhenius-Parameter für die entsprechenden Desorptionsprozesse wurden berechnet. Aufgrund dieser Ergebnisse können wichtige Hypothesen bezüglich des Reaktionsmechanismus der totalen Oxidation und der Dehydroaromatisierung von Propylen an Zinkoxid aufgestellt werden.Die Versuche bestätigen, daß thermische Methoden auf dem interessanten und wichtigen Gebiet der heterogenen Katalyse mit Erfolg eingesetzt werden können.

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- , . , . . , . , .

     

Performance of a cobalt oxide catalyst in the oxidation of NH3 to NO

The cause for a fall in the conversion of NH₃ to NO within 73 hours from 97% to 78% with a cobalt oxide catalyst was investigated. Results show that sintering, change in valence and cation distribution in Co₃O₄ contribute to the decrease in activity. Incorporation of ThO₂ and K₂O improves the performance of the catalyst.

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NH₃ NO 73 97% 78%. , , Co₃O₄. ThO₂ K₂O NH₃ NO.

     

Thermal decomposition of 5-Nitro-2-Furaldehyde Semicarbazone

Nitrofurazone, 5-nitro-2-furaldehydesemicarbazone, pharmaceutical compound, has been synthesized and its thermal behaviour studied by DTA, DSC and TG. The resulting thermoanalytical curves, showing that the maximum change in mass occurred at 516.5 K, with a 67% weight loss, are in good correlation with the enthalpy of the decomposition reaction, 326.93 kJ mol^–1. On the basis of the results, it is possible to establish the mechanism of the thermal decomposition and to acquire information on the stability of the analyzed organic compound, 5-nitro-2-furaldehydesemicarbazone.

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Zusammenfassung Das Arzneimittel Nitrofurazon (5-Nitro-2-furaldehyd-Semicarbazon) wurde synthetisiert und dessen thermisches Verhalten mittels DTA, DSC und TG untersucht. Die erhaltenen thermoanalytischen Kurven, die einen Gewichtsverlust von 67 Gew.-% mit maximaler Geschwindigkeit der Gewichtsabnahme bei 516,5 K zeigen, sind in guter Übereinstimmung mit der Enthalpie der Zersetzungsreaktion von 326,93 kJ·mol^–1. Aus den Versuchsergebnissen können der Mechanismus der thermischen Zersetzung abgeleitet und Informationen über die Stabilität von 5-Nitro-2-furaldehyd-Semicarbazon erhalten werden.

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, — 5-. , (67%) 516,5 , , 326,93 ·^–1. .

     

The influence of CSH phase nuclei on Ca3SiO4Cl2 hydration

The influence of CSH phase nuclei on calcium chlorosilicate hydration has been studied by means of calorimetry, QXRD, DTA and TG methods.

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Zusammenfassung Mittels kalorimetrischen, QXRD-, DTA- und TG-Verfahren wurde der Einfluß von CSH Phasenkörpern auf die Hydratierung von Kalziumchlorosilikat untersucht.

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, , .

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C=CaO, S=SiO₂, H=H₂O     

SEC–MALLS analysis of ethylenediamine-pretreated native celluloses in LiCl/<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylacetamide: softwood kraft pulp and highly crystalline bacterial,tunicate, and algal celluloses

Softwood and hardwood bleached kraft pulps (SBKP and HBKP, respectively) and highly crystalline native celluloses such as algal, tunicate, bacterial and cotton lint celluloses were dissolved in 8 % (w/v) LiCl/N,N-dimethylacetamide (DMAc) after ethylenediamine (EDA) pretreatment. Complete dissolution of SBKP and other highly crystalline native celluloses in 8 % LiCl/DMAc was achieved after solvent exchange from EDA to DMAc through methanol. Neutral sugar composition analysis showed no significant differences between the original and EDA-treated pulps. A combination of size-exclusion chromatography and multi-angle laser light scattering (SEC–MALLS) was used to analyze the cellulose solutions after dilution to 1 % (w/v) LiCl/DMAc. The 0.05 % (w/v) solutions of highly crystalline cellulose in 1 % (w/v) LiCl/DMAc contained entangled molecules, and therefore 0.025 % (w/v) cellulose solutions in 1 % (w/v) LiCl/DMAc were used in the SEC–MALLS analysis to obtain reliable conformation plots (or double-logarithmic plots of molecular mass vs. root-mean-square radius). All the cellulose samples except SBKP gave conformation plots with slope values of 0.56–0.57, showing that these cellulose molecules had random-coil conformations. In contrast, SBKP gave a slope value of 0.35, indicating that some branched structures were present in the high-molecular-mass fraction. Double-logarithmic plots of the reduced viscosities of the cellulose solutions in 1 % (w/v) LiCl/DMAc versus the molecular mass were linear, except for SBKP, also suggesting the presence of anomalous cellulose structures in SBKP.     

Kinetics of the thermal dehydration of variscite and specific surface area of the solid decomposition products

The surface properties of active solids resulting from decomposition of AlPO₄ · 2 H₂O by thermal treatment in vacuum and in air are studied in connection with the parameter governing the kinetics of the dehydration process. The values obtained for the activation energies of decomposition under isothermal conditions agree acceptably well with those computed from TG curves by the Freeman and Carroll method. Kinetic results, which are analysed in terms of the Mampel theory, strongly suggest that the activation energy of the nucleation process determines the rate of surface formation, and justify the observed fact that the specific surface areas of samples treated in vacuum are higher than those of samples treated in air.

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Zusammenfassung Die Oberflächenbeschaffenheiten der aktiven Festkörper, welche bei der thermischen Behandlung im Vakuum oder in Luft als Zersetzungsprodukte von AlPO₄ · 2 H₂O entstehen, werden im Zusammenhang mit dem die Kinetik des Dehydratisierungsvorganges steuernden Parameter studiert. Die bei isothermen Bedingungen für die Aktivierungsenergien der Zersetzung erhaltenen Werte sind in befriedigender übereinstimmung mit denjenigen, welche mittels der Methode von Freeman und Carroll aus den TG-Kurven errechnet wurden. Die gemäß der Mampel-Theorie analysierten kinetischen Ergebnisse lassen darauf schließen, daß die Aktivierungsenergie der Nukleationsvorgänge die Geschwindigkeit der Oberflächen-Entstehung bedingt und rechtfertigen die Beobachtung, nach welcher die spezifische Oberfläche vakuumbehandelter Proben größer ist als die der luftbehandelten.

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Résumé On étudie les propriétés de surface des solides actifs provenant de la décomposition de AlPO₄ · 2 H₂O par traitement sous vide et dans l'air et on les rapporte aux paramètres gouvernant la cinétique du processus de déshydratation. Les valeurs obtenues en conditions isothermes pour les énergies d'activation de la décomposition sont en accord acceptable avec celles calculées à partir des courbes TG par la méthode de Freeman et Carroll. Les résultats cinétiques qui sont analysés dans le sens de la théorie de Mampel font conclure que c'est l'énergie d'activation du processus de nucléation qui détermine la vitesse de la formation de la surface; ils justifient le fait observé que la surface spécifique des échantillons traités sous vide est plus grande que celle des échantillons traités dans l'air.

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, Al ₄ · 2₂ , , . , , , . , , , , , , , , .