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  • 1
    Electronic Resource
    Electronic Resource
    Physicochemical properties of (1 → 6)-branched (1 → 3)-β-D-glucans. 1. Physical dimensions estimated from hydrodynamic and electron microscopic data (1993)
    New York : Wiley-Blackwell
    Biopolymers 33 (1993), S. 561-573 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The physical dimensions of several (1 → 6) branched (1 → 3) -β-D-glucan samples obtained from different organisms and their derivatives have been studied by electron microscopy, light scattering measurements, viscometry, and gel permeation chromatography. The electron micrographs indicate that in most samples these biopolymers are adequately described as linear worm-like coils. A sample reconstituted from alkaline media appeared as a blend of the linear, circular, and aggregated polymer morphologies. The average mass per unit length, ML = Mw/Lw for the macroscopically linear samples, was estimated to be 2100 ± 200 g mol-1 nm-1. The parameter mL was determined from the contour lengths obtained by electron microscopy and the molecular weight by light scattering measurements. The observed ML was consistent with the triple-helical structure reported from x-ray diffraction studies and observed degree of side-chain substitution. From the molecular snapshots shown in the electron micrographs, the persistence lengths of these β-D-glucans were determined to be 140 ± 30 nm. The experimentally determined intrinsic viscosities were consistent with these estimates of ML and persistence length. Comparison of the molecular weight distributions obtained from gel permeation chromatography and those deduced from the electron micrographs indicates that number and weight average contour lengths are more reliable than z and z + 1 averages. © 1993 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360330406
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  • 2
    Electronic Resource
    Electronic Resource
    Temperature-Induced conformational transition in xanthans with partially hydrolyzed side chains (1993)
    New York : Wiley-Blackwell
    Biopolymers 33 (1993), S. 151-161 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational properties of xanthans with partially hydrolyzed side chains were in vestigated by optical rotation, CD, and differential scanning calorimetry (DSC). All variants displayed the well-known temperature-driven, cooperative order-disorder transition, and both optical rotation and DSC showed that the transition temperature was essentially independent of the content of terminal β-mannose. It was found that up to 80% of the changes in the specific optical rotation accompanying the transition reflects conformational changes linked to the terminal β-mannose in the side chains. Modification of the sidechains also affected the CD when xanthan was in the ordered state, but in this case the data suggest that the glucuronic acid is the major component determining the magnitude of the CD signal. DSC measurements showed that the transition enthalpy (ΔHcal) increased linearly with the fraction of β-mannose, again indicating that a significant part (up to 80%) of ΔHcal reflects conformational changes in the side chains. The conformational transition of the xanthan variants generally showed a higher degree of cooperativity (sharper transition) than unmodified, pyruvated xanthan. Calculation of the cooperativity parameter σ by means of the Zimm-Bragg theory (OR data) or from the ratio between ΔHcal and the van't Hoff enthalpy (ΔHvH) using DSC data showed a correlation between σ and the content of β-mannose, but the two methods gave different results when the content of β-mannose approached 100%. The ionic strength dependence of the transition temperature, expressed as d (log I)/d(T-1m), was nearly identical for intact xanthan and a sample containing only 6% of the terminal β-mannose. Application of the Manning polyelectrolyte theory does not readily account for the observed ΔHcal values, neither does it provide new information on the nature of the ordered and disordered conformations in xanthan. © 1993 John Wiley & Sons, Inc.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360330115
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  • 3
    Electronic Resource
    Electronic Resource
    Conformational transitions of schizophyllan in aqueous alkaline solution (1996)
    New York : Wiley-Blackwell
    Biopolymers 39 (1996), S. 407-416 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational transitions of schizophyllan were studied in aqueous alkaline solutions by high-sensitivity differential scanning calorimetry (DSC) and optical rotation measurements. The temperature of half completion for reversible intramolecular conformational transition determined by DSC, centered at 7.4°C in water, increases to 37.2°C at 0.01M KOH with increasing alkaline concentration. The transition enthalpy per mole of the polysaccharide repeating unit is 2.62 ± 0.23 kJ mol-1 independent of the alkaline concentration. The cooperative unit size for the transition decreases with increasing alkaline concentration. Optical rotation was measured as a function of pH at 25 and 60°C. A sharp decrease in optical rotation was observed at pH = 13, which is ascribed to the triple helix-coil transition. From data obtained by DSC and optical rotation measurements, in combination with results reported previously, a phase diagram for the conformation of schizophyllan as a function of temperature and pH is proposed. The irreversibility of the triple helix to single coil transition, induced by strong alkali, was investigated as a function of polymer concentration by gel permeation chromatography and electron microscopy. The renatured samples at polymer concentrations 〈 1.0 mg/mL, which are prepared by dissolution in 0.25M KOH followed by neutralization with HCl, are observed as a mixture of globular, linear, and circular structures, and larger aggregates with less-defined morphology by electron microscopy. Higher concentrations lead to increased proportions of multichain clusters (aggregates). Subsequent annealing of the renatured samples at 115-120°C increases the proportion of circular species. The change in molecular weight distribution of samples that accompanies the renaturation and annealing mentioned above can be well interpreted in terms of the proportion of species having different morphology as observed by electron microscopy. © 1996 John Wiley & Sons, Inc.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Thermally induced conformational transition of double-stranded xanthan in aqueous salt solutions (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1243-1255 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermally induced conformational transition of double-stranded xanthans (degree of pyruvate substitution, DSP = 0.45) having Mw = 3.1, 5.7, and 20.3 × 105 has been studied in aqueous salt solutions by high-sensitivity differential scanning calorimetry (DSC). The double strandedness of these samples in the ordered conformation was ascertained by the value of mass per unit length, ML = 2090 ± 270 g mol-1 nm-1, which was determined from the contour length obtained by electron microscopic observations and the molecular weight by light scattering measurements. The temperature at half completion of the transition T½ for these samples increased linearly with the logarithm of the cation (Na+, K +) concentration. The plot of 1/T½ vs the natural logarithm of cation (Na + ) concentration in mM for the sample with Mw = 5.7 × 105 (15-SX) yielded the equation 103/T½ = 3.45-0.159 in[Na +]. The specific enthalpy Δhcal, for 15-SX, essentially independent of salt concentration above 20 mM, was 8.31 ± 0.39 J/g (SD, n = 6). No systematic dependence of molecular weight on the transition temperature and the enthalpy was observed. Application of the Manning polyelectrolyte theory to the system using the DSC data suggested that the separation of the double strand of xanthan into two single chains was not completed at the temperature where the endothermic peak was finished. This suggestion is consistent with recent findings by light scattering measurements as a function of temperature. Our DSC study was extended to include four other samples from various sources. It was found that T½ and Δhcal depend on the pyruvate contents of the samples. For example, the t½(t½/° C = T½/K - 237.15) values for samples with high pyruvate content (DSp = 0.9) and depyruvated (DSp = 0.14) in 20 m Maqueous NaCl were 48.8 and 85.3°C, respectively. Two other samples showed relatively broad DSC curves having shoulders, which were resolved into two independent components. Thermodynamic parameters for each component were examined as a function of salt concentration, and the results obtained were interpreted in terms of the heterogeneity of the pyruvate content of the samples.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360311102
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