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  • Chemistry  (28)
  • Conducting polymers
  • 1985-1989  (27)
  • 1945-1949  (1)
  • 1
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures. (1) Such polymerizations may allow the polymer to retain functionality that would be destroyed under normal polymerization conditions. (2) The selectivity provided by enzyme catalysts may permit polymers, including optically active polymers, to be prepared that are either not accessible or accessible only with difficulty by other methods. (3) The characteristics of the enzyme and the mild polymerization conditions may permit formation of polymers having highly regular sizes and backbone structures. This report describes the first successful use of an enzyme-catalyzed polycondensation to prepare a chiral (AA-BB)x polyesters of more than a few repeat units. Polymerization of bis(2,2,2-trichloroethyl) alkanedioates (BB) with diols (AA) using the enzyme porcine pancreatic lipase (PPL) as a catalyst is detailed. The polycondensations were carried out at ambient temperature in anhydrous, low polarity organic solvents such as ether, THF, and methylene chloride. End group analysis by NMR provided Mn values of 1300-8200 daltons while GPC provided Mw values of 2800-14900 daltons for the polymers. Based on proton NMR spectra obtained during the polymerization, relatively rapid formation of an AA-BB “dimer” and an AA-BB-AA “trimer,” slower formation of a BB-AA-BB “trimer,” and subsequent condensation of these to give higher polymers are suggested to be components of the polymerization mechanism.
    Additional Material: 2 Ill.
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  • 2
    ISSN: 0887-3585
    Keywords: membrane proteins ; channels ; circular dichroism spectroscopy ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The secondary structure of alamethicin, a membrane channel-forming polypeptide, has been examined by circular dichroism spectroscopy to determine the relationship of its conformation in organic solution to its conformation in a membrane-bound state. The spectrum of alamethicin in small unilamellar dimyristoyl phosphatidylcholine vesicles is significantly different from its spectrum in 10% methanol/acetonitrile, the solvent from which it was crystallized (Fox and Richards: Nature 300:325-330, 1982), as well as its spectrum in methanol, the solvent in which NMR studies have been done (Banerjee and Chan: Biochemistry 22:3709-3713, 1983). This suggests that structural models based on studies of the molecule in organic solvents may not be entirely appropriate for the membrane-bound state. To distinguish between different models for channel formation and insertion, two different methods were used to associate the alamethicin with vesicles; in addition, the effect of oligomerization on the conformation of the membrane-bound state was investigated. These studies are consistent with a modified insertion model in which alamethicin monomers, dimers, or trimers associate with the bilayer and then spontaneously oligomerize to form a prechannel with a higher helix content. This aggregate could then “open” upon application of an appropriate gating transmembrane potential.
    Additional Material: 4 Ill.
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  • 3
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantioselective polymerization of bis(2,2,2-trichloroethyl) trans-3,4-epoxyadipate with 1,4-butanediol using the enzyme porcine pancreatic lipase as a catalyst is described. The polymerization was carried out at ambient temperature in anhydrous ethyl ether. End group analysis provided MN = 5,300 daltons, whereas GPC provided Mw = 7,900 daltons for the polymer. The unchanged (+)-enantiomer of the diester was shown to have an enantiomeric purity of 〉 95% by proton NMR in the presence of the chiral shift reagent Eu(hfc)3. The stereochemical purity of the (-)-polymer was estimated at 〉 96% by consideration of the amount of the slower reacting enantiomer that could have been incorporated and still attain the observed degree of polymerization (25) when the starting ratio of racemic diester to diol was 2:1. Direct determination of the stereochemical purity of the polymer using Eu(hfc)3 was unsuccessful. Similar studies on polymer having random stereochemical orientations of the epoxide showed that such polymers do not behave as if they are racemic in the presence of the shift reagent. The polymer required for the latter studies was prepared by epoxidation of the product from enzyme catalyzed polymerization of bis(2,2,2-trichloroethyl) trans-3-hexenedioate with 1,4-butanediol.
    Additional Material: 2 Ill.
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  • 4
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 5 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 24 (1985), S. 1705-1720 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A model has been developed for approximating the free energy of collagen fibril formation (ΔFf) and the equilibrium solubility of collagen under physiological conditions. The model utilizes an expression of Flory for rodlike polymers, with the modification that the “pure” anisotropic phase is defined as a collagen fibril containing about 0.3 g water/g collagen. The model also assumes that χ1, the polymer-solvent interaction term, is entirely due to hydrophobic effects. χ1 is estimated from hydrophobic bond energies of amino acid side chains, using the results of Némethy and Scheraga. The temperature dependence of χ1 is utilized to calculate equilibrium solubilities and ΔFf as a function of temperature.
    Additional Material: 6 Ill.
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  • 6
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The heat denaturation of pepsinized bovine nonfibrillar and fibrillar collagen was studied by differential scanning calorimetry. For fibrillar preparations that had been rapidly precipitated with stirring at low ionic strength, then resuspended at physiological ionic strength, multiple denaturational transitions were observed. At heating rates of 10°C/min, melting endotherms occurred at about 44, 50, 53, and 57°C. Fibrillar collagen that was slowly gelled without stirring at physiological ionic strength exhibited a similar series of endotherms, but the lower melting transitions were less conspicuous. In contrast, nonfibrillar bovine collagen in acidic solution showed only a single denaturational transition at 40°C. Nonfibrillar solutions at pH 7, to which inhibitors of fibrillogenesis were added, showed a major endotherm as high as 46°C. These results suggest that reconstituted fibrillar collagen contains a heterogeneous fibril population, possibly including molecules in a nonfibrillar state. It was proposed that the multiple melting endotherms of such preparations were due to sequential melting of molecular and fibril classes, each with a distinct melting temperature. The fibrillar classes may represent three or more types of banded and nonbanded species that differ from each other in packing order, collagen concentration, and possibly also in fibril width and level of cross-linking.
    Additional Material: 8 Ill.
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  • 7
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Analogs of natural proteins have application in basic research, in medicine, and increasingly, in industry. Of the many methods developed for the fabrication of modified proteins, semisynthesis has so far been the most productive [R. E. Offord (1983) Proc. Eur. Peptide Symp., 17 31-42]. The technique of fragment-condensation semisynthesis takes as its basic raw material the native protein; fragments derived from it are manipulated by sequential degradation and resynthesis before recombination. While gene manipulation may overtake protein manipulation in general usefulness, certain types of analogs will remain accessible only by semisynthesis. Fragment condensation semisynthesis, as described above, has been used in preparing cytochrome c analogs [C. J. A. Wallace (1979) Proc. Am. Peptide Symp. 6, 609-612]. To overcome the difficulty of replacing amino acids far from convenient cleavage sites, a number of variants of the method have also been used. These include fragment-specific chemical modification [C. J. A. Wallace & K. Rose (1983) Biochem. J. 215, 651-658] and solid-phase synthesis of small [P. J. Boon, G. I. Tesser, H. H. K. Brinkhof & R. J. F. Nivard (1981) Proc. Eur. Peptide Symp. 16, 127-130] or large fragments [L. E. Barstow, R. S. Young, E. Yakali, J. J. Sharp, J. C. O'Brien, P. W. Berman & H. A. Harbury (1977) Proc. Natl. Acad. Sci. USA 74, 4148-4250]. The latter study gave low final recoveries, so we have prepared large peptides (38-42 residues) by classical solution methods. We have examined both their use in the reformation of the complete protein, and the physical and biochemical properties of the product analogs compared with those of the native proteins.
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  • 8
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report details of the chemical synthesis of two fragments reproducing the C-terminal sequences 71-108 and 70-108 of Saccharomices cerevisiae cytochrome c. Preparation of the fragments employed classical solution methods and a fragment-condensation strategy; they have been used, together with a third fragment (sequence 67-108) [L. Moroder, B. Filippi, G. Borin & F. Marchiori (1975) Biopolymers 14, 2061-2074], in the semisynthesis of chimeric cytochromes [C. J. A. Wallace, G. Corradin, F. Marchiori & G. Borin (1986) Biopolymers 25, 2121-2132].
    Additional Material: 2 Ill.
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  • 9
    ISSN: 0006-3592
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Aspen (Populus tremuloides) and black cottonwood (Populus trichocarpa) organosolv pulps produced in a wide range of solvent composition (between 30 and 70% by volume of methanol) and catalysts (H2SO4 and H3PO4) such that the cooking liquor pH ≤ 3 are easily digested by enzymes. The total yields of hydrolysis residues (pulps) are in the 40-60% range; the acid-catalyzed delignification followed by enzyme hydrolysis can generate 70-88% of the original six-carbon sugars contained in the wood. Glucomannan and arablnogalactan are dissolved into the pulping liquor in the pH range of 2-4.5. Lower pH (≤3) leads to additional solubilization of six-carbon sugars. These sugars may be fermented directly. From the insoluble hydrolysis residues, 36-41% conversions of wood into fermentable sugars were obtained after enzyme hydrolysis; the starting feedstocks contain 50.8 and 46.6% hexosans, respectively, for aspen and black cotton-wood. The kinetics of enzymatic hydrolysis of cellulose can be formally treated as two simultaneous pseudo-first-order reactions in which fast and slow hydrolyses of cellulose occur. Correlations between the glucan digestibility and the effect of the pretreatment have been made. The higher residual xylan content reduces the amount of the rapidly hydrolyzable glucan fraction and lowers the glucan digestibility. The proposed simple kinetic treatment is very helpful in assessing the effect of the pretreatment on pulp enzyme hydrolyzability.
    Additional Material: 3 Ill.
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  • 10
    ISSN: 1040-0397
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Various methods have been investigated to stabilize a ruthenium polymer-modified electrode for use in flow-injection analysis. These include treatment with ultraviolet irradiation as well as coating the modified electrode with electrochemically generated conductive and nonconductive polymer layers. The stabilization procedures tested proved effective in improving the operational half-life of a nonstabilized electrode from 8 hours to more than 48 hours for the determination of test analyte species such as nitrite and nickel bis-2-hydroxyethyl dithiocarbamate.
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