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  • 1990-1994  (1,240)
  • 1915-1919
  • 1991  (1,240)
  • Chemistry  (1,240)
  • 1
    Electronic Resource
    Electronic Resource
    Positively and negatively charged water cluster ions generated via liquid chromatography/thermospray mass spectrometry (1991)
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 5 (1991), S. 188-191 
    Details
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The formation of singly protonated water clusters in the positive-ion mode and singly deprotonated water clusters as well as singly negatively charged water clusters in the negative-ion mode with a thermospray system running on pure water and in the discharge-on mode is described. The influence of the potential at the repeller electrode opposite to the sampling cone recalls previous mechanistic discussions on the repeller influence. Application of these water cluster ions to tuning and calibration in the thermospray mode is proposed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/rcm.1290050409
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  • 2
    Electronic Resource
    Electronic Resource
    Mechanisms of repeller-induced effects in thermospray liquid chromatography/mass spectrometry (1991)
    Chichester : Wiley-Blackwell
    Biological Mass Spectrometry 26 (1991), S. 519-527 
    Details
    ISSN: 0030-493X
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The application of a high potential at the repeller electrode, positioned opposite to the sampling cone in order to increase the sampling efficiency, can induce fragmentation in thermospray mass Spectrometry. Until now, this fragmentation has been attributed to collision-induced dissociation. As a result of studies on the changes in the reagent gas composition in the thermospray buffer ionization mode as a function of the repeller potential in the positive-ion mode, it appears that three different processes are occurring. At low repeller potentials, the thermospray mass spectra of the eluent are determined by the proton affinities and the concentrations of the various solvent constituents, and the stabilities of the formed cluster ions under the ion source conditions. With an increase in the repeller potential, collision-induced dissociation of the background ions starts to occur. When the kinetic energy of the ions and cluster ions becomes high enough, endothermic proton transfer and solvent-switching reaction pathways are opened. For the relatively volatile analytes studied, e.g. aniline, acetophenone, benzaldehyde and benzoic acid, similar effects are observed.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/oms.1210260528
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  • 3
    Electronic Resource
    Electronic Resource
    Immobilization of catalase on membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors (1991)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 29 (1991), S. 269-274 
    Details
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The immobilization of catalase on grafted membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors is described. The introduction of carboxylic acid groups onto a hydrophobic support provides a good environment for subsequent enzyme immobilization. This single membrane, hydrogen peroxide sensor showed significant improvement with respect to the double membrane versions. The response is very rapid, the linear range being from 10 μM up to 6 mM, with a detection limit of 4.7 μM, and a lifetime of more than 4 months.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pola.1991.080290215
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  • 4
    Electronic Resource
    Electronic Resource
    Lipase kinetics: Hydrolysis of triacetin by lipase from Candida cylindracea in a hollow-fiber membrane reactor (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 38 (1991), S. 727-732 
    Details
    ISSN: 0006-3592
    Keywords: lipase kinetics ; Candida cylindracea ; hydrolysis of triacetin ; hollow-fiber membrane reactor ; immobilization ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The aptitude of a hollow-fiber membrane reactor to determine lipase kinetics was investigated using the hydrolysis of triacetin catalyzed by lipase from Canadida cylindracea as a model system. The binding of the lipase to the membrane appears not to be very specific (surface adsorption), and probably its conformation is hardly altered by immobilization, resulting in an activity comparable to that of the enzyme in its native form. The reaction kinetics defined on the membrane surface area were found to obey Michaelis-Menten kinetics. The specific activity of the lipase in the membrane reactor was found to be significantly higher than in an emulsion reactor. The activity and stability of the enzyme immobilized on a hydrophilic membrane surface seem not to be influenced significantly by the choice of the membrane material. The hollow-fiber membrane reactor is a suitable tool to assess lipase kinetics in a fast and convenient way.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bit.260380706
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  • 5
    Electronic Resource
    Electronic Resource
    Inhibition of microbial growth and metabolism by excess turbulence (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Biotechnology and Bioengineering 38 (1991), S. 552-556 
    Details
    ISSN: 0006-3592
    Keywords: stirring ; turbulence ; shear effects ; lysine fermentation ; Brevibacterium flavum ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Excess turbulence caused by high-intensity stirring inhibited microbial growth and metabolism. In stirred tank bioreactors, the growth rate and lysine biosynthesis decreased in Brevibacterium flavum beyond 900 rpm, the growth rate of Trichoderma reesei on wheat straw beyond 150 rpm, and the growth rate of Saccharomyces cerevisae beyond 800 rpm. The term turbohypobiosis was introduced to describe this inhibition. Turbohypobiosis was characterized by a stress factor Fstr expressing the interaction of medium flow with microbial cells in local turbulent zones, dependent on the energy distribution of the stirring regime. Lysine synthesis was inhibited at significantly lower Fstr values than the growth of B. flavum. The main reason for the inhibition was shear effects causing decreased adenosine triphosphate (ATP) generation, lower O2 uptake, and lower specific growth rate of bacteria.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bit.260380514
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  • 6
    Electronic Resource
    Electronic Resource
    Structural versatility of peptides from Cα,α-disubstituted glycines: Preferred conformation of the chiral isovaline residue (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 1135-1148 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecular structures of four protected isovaline- (Iva-) containing peptides to the pentamer level have been determined by x-ray diffraction. The peptides are t-Boc-Ala-(S)-Iva-Ala-OMe (t-Boc : tert-butyloxycarbonyl; OMe : methoxy) and its (R)-Iva diastereomer, and t-Boc-[Ala-(R)-Iva]2-Ala-OH and its (S)-Iva diastereomeric methyl ester analogue. The two tripeptides are folded in an open type II β-bend conformation. The fully developed right-handed 310-helix formed by the (R)-Iva pentapeptide, which includes an unusual intramolecular (acid) O—H⃛O=C(peptide) H bond, is partially unfolded (near the C-terminus) in the (S) -Iva pentapeptide. 1H-nmr and Fourier transform ir absorption studies suggest that in CDCl3 solution (a) the two tripeptides maintain a type II β-bend conformation of comparable stability and (b) both diastereomeric pentapeptide sequences adopt a fully developed 310-helix. A comparison with the preferred conformation of other extensively investigated Cα,α-disubstituted glycines is made and the implications for the use of the Iva residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360311002
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  • 7
    Electronic Resource
    Electronic Resource
    Precipitation temperature of poly(ethylene glycol) in some electrolyte solutions (1991)
    New York, NY : Wiley-Blackwell
    Polymers for Advanced Technologies 2 (1991), S. 269-273 
    Details
    ISSN: 1042-7147
    Keywords: Poly(ethylene glycol) ; Precipitation temperature ; Cloud points ; θ-temperature ; Salt effect ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: The effect of sodium fluoride, sodium bromide, tetramethyl ammonium chloride, tetramethyl ammonium bromide and sodium formate on both precipitation and θ-temperatures of poly(ethylene glycol) (Mw = 4000) in aqueous solution have been determined. Curves have been drawn to represent the effect of such ions on both the θ and the precipitation temperatures of the polymer. The effectiveness increases in the order (CH3)4 NBr, NaBr, (CH3)4NCl, NaF and HCOONa. The Mw-temperatures obtained at infinite dilution are 445 K for NaBr, NaF, (CH3)4 NBr and (CH3)4NCl and 415 K for HCOONa.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pat.1991.220020602
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Inositol lipids in friend erythroleukemia cells: Evidence for changes in nuclear metabolism after differntiation (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Cell Biochemistry and Function 9 (1991), S. 135-145 
    Details
    ISSN: 0263-6484
    Keywords: Inositol lipids ; differentiation ; nucleus ; Friend erythroleukemia cells ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: The incorporation of 32Pi into phospholipids was studied in Friend erythroleukemia cells either induced or not to erythroid differentiation with 4 mM hexamethylenebisacetamide (HMBA). The effect of the differentiating agent on the recovery of radiolabelled phospholipids was compared in whole cells, isolated nuclei and nuclear matrix after in vivo labelling for 1 hr. The procedure employed for the isolation of nuclei was demonstrated to allow only negligible lipid redistribution caused by cell manipulations. Among the lipids extractable from nuclei, acidic phospholipids, and particularly polyphosphoinositides, were more represented than in whole cells, while small differences were found in the other phospholipid classes examined. The comparison between the uninduced and induced condition showed that the relative amounts of nuclear inositol lipids were modified by HMBA treatment of the cells, with a decreased recovery of phosphatidylinositol 4,5 bisphosphate.These results indicate that phosphatidylinositol and its phosphorylation products synthesized in vivo show a different metabolism in nuclei and whole cells. They appear to be tightly bound nuclear components, also present in membrane-deprived nuclei and nuclear matrix, and are probably related to the nuclear events involved in erythroid differentiation.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cbf.290090211
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  • 9
    Electronic Resource
    Electronic Resource
    Study of the conduction mechanisms in polycarbonate (1991)
    Weinheim : Wiley-Blackwell
    Angewandte Makromolekulare Chemie 193 (1991), S. 13-20 
    Details
    ISSN: 0003-3146
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Description / Table of Contents: Die Strom-Spannungs-Charakteristik eines Bisphenol-A-Polycarbonats im Span-nungsbereich 50-1000 V und der Einfluß von UV-Bestrahlung (λ = 225 und 300 nm) und Tempern auf die elektrische Leitfähigkeit (σ) des Polycarbonats wurden untersucht. Die Abhängigkeit von σ gegen 1/T deutet auf zwei verschiedene Leitfähigkeits-mechanismen hin. Aus den Ergebnissen wurden die Aktivierungsenergien der Leitfähigkeit und die Sprungdistanzen berechnet.
    Notes: Current-voltage characteristics of polycarbonate samples (PC) have been studied in the voltage range 50-1000 V. The effect of UV-irradiation at λ = 225 and 300 nm, and thermal annealing on the electrical conductivity (σ) of PC was studied. The σ vs. 1/T relationship shows two slopes, i.e. two possible conduction mechanisms. The obtained results made it possible to determine a complete set of conduction parameters including activation energy and jump distance.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/apmc.1991.051930102
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  • 10
    Electronic Resource
    Electronic Resource
    Time-Resolved study of network self-organization from a biopolymeric solution (1991)
    New York : Wiley-Blackwell
    Biopolymers 31 (1991), S. 859-868 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Time-resolved studies of network self-organization from homogeneous solutions of the representative biostructural polymer agarose are presented. Solutions are temperature quenched and observed by several techniques. Consistent with previous suggestions by the authors, experiments at concentrations up to about 1.75% w/v provide direct kinetic evidence for the occurrence of at least two distinct processes, leading, in sequence, to self-assembly. These are as follows: (a) a liquid-liquid phase separation of the solution occurring via spinodal demixing and resulting in two sets of regions that have, respectively, higher and lower than average concentrations of random-coiled polymers; and (b) the subsequent 2 coils → double helix transition and accompanying cross-linking and gelation (due to branching of double helices), occurring in the high-concentration regions. The size of the high-concentration regions depends upon agarose concentration and quenching temperature, and is in the range from a fraction of micrometers to a few micrometers, in agreement with earlier experiments. Bundling of the double-helical segments is known to follow self-assembly and can be considered as a third step (gel curing). This follows from the thermo-dynamic instability of the helical segments in the solvent, behaving as a system of rod-like particles connected by more or less flexible joints.The two processes leading in succession to self-assembly are discussed in terms of a phase diagram consistent with available data. Their time scales differ remarkably. At the end of the first process, all polymers remain random coiled and freely drifting. Much later coil-helix transition is observed, always in coincidence with polymer cross-linking and gelation. The enhancement of concentration of random-coiled polymers in specific regions of the sol caused by spinodal demixing is thus a prerequisite for self-assembly of these biostructural gels in the concentration interval studied. Conceptually, concentration enhancements of this type can provide a new pathway for promotion of functional biomolecular interactions even at very low average concentrations. The mechanism will work identically if the region of instability is reached by varying the polymer concentration (e.g., by biosynthesis), rather than by temperature quenching.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/bip.360310706
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