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  • 1
    Electronic Resource
    Electronic Resource
    Molecular modeling of closed circular DNA thermodynamic ensembles (1996)
    New York : Wiley-Blackwell
    Biopolymers 39 (1996), S. 243-258 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many modeling studies of supercoiled DNA are based on equilibrium structures from theoretical calculations or energy minimization. Since closed circular DNAs are flexible, it is possible that errors are introduced by calculating properties from a single minimum energy structure, rather than from a complete thermodynamic ensemble. We have investigated this question using molecular dynamics simulations on a low resolution molecular mechanics model in which each base pair is represented by three points (a plane). This allows the inclusion of sequence-dependent variations of tip, inclination, and twist. Three kinds of sequences were tested: (1) homogeneous DNA, in which all base pairs have the helicoidal parameters of an ideal, average B-DNA; (2) random sequence DNA; and (3) curved DNA. We examined the rate of convergence of various structural parameters. Convergence for most of these is slowest for homogeneous sequences, more rapid for random sequences, and most rapid for curved sequences. The most slowly converging parameter is the antipodes profile. In a plasmid with N base pairs (bp), the antipodes distance is the distance dij from base pair i to base pair j halfway around the plasmid, j = i + N/2. The antipodes profile at time t is a plot of dij over the range i = 1, N/2. In a homogeneous plasmid, convergence requires that the antipodes profile averaged over time must be flat. Even in the small plasmids examined here, the average properties of the ensembles were found to differ from those of static equilibrium structures. These effects will be even more dramatic for larger plasmids. Further, average and dynamic properties are affected by both plasmid size and sequence. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Detection of microgels in polyacrylamide solutions using microcapillary flow analysis (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 33 (1987), S. 693-702 
    Details
    ISSN: 0021-8995
    Keywords: 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 , Physics
    Notes: A new technique for quantifying the amount of multimolecular microgel material in water-soluble polymers is described. The enhanced velocity of the large microgels during flow through a 25 μm capillary is the basis for a separation between the microgels and dissolved polymer. With the use of laser-excited fluorescence detection, the arrival of fluorescently-tagged polyacrylamide samples at a downstream location is recorded. The presence of small amounts of microgel has a deleterious effect on the ability to filter polymer solutions. Analysis of samples before and after filtration suggests that a significant amount of microgel material above 8 μm in diameter is present in samples that exhibit poor filterability characteristics. Treatment of a sample with base is found to improve filterability and to decrease the amount of microgel in the sample.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070330301
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  • 3
    Electronic Resource
    Electronic Resource
    Grafting reactions of vinyl acetate onto poly[(vinyl alcohol)-co-(vinyl acetate)] (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 2581-2594 
    Details
    ISSN: 0887-624X
    Keywords: grafting ; poly(vinyl alcohol) ; poly(vinyl acetate) ; vinyl acetate ; methine carbon ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The grafting preference of vinyl acetate onto the methine carbon of poly(vinyl alcohol) (PVOH) versus the acetate group of poly(vinyl acetate) (PVAc) was determined as part of an attempt to prepare novel branched PVOH from partially hydrolyzed PVAc. The results showed long chain grafting on the acetate groups of the PVAc units rather than the methine carbons of the PVOH or PVAc units. Decreasing the monomer or initiator concentration decreased the molecular weight of the graft copolymer formed. Of the initiators studied, ammonium persulfate gave the largest increase in copolymer molecular weight. Both hydrolysis and reacetylation combined with gel permeation chromatography (GPC) and 13C-NMR of the fully hydrolyzed material were used to estimate the number and location of grafts. © 1996 John Wiley & Sons, Inc.
    Additional Material: 11 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Characterization of cobalt-modified polyimides (1987)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 25 (1987), S. 685-702 
    Details
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Modified polyimide films containing cobalt have been prepared by the addition of cobalt(II) chloride to a solution containing one of the diamines 4,4′-oxydianiline (ODA) or 4,4′-diaminodiphenylsulfide (DDS) and one of the dianhydrides 3.3′, 4,4′-benzophenonetetracarboxylic dianhydride (BTDA) or 4,4′-bis(3,4-dicarboxyphenoxy)diphenylsulfide dianhydride (BDSDA) and characterized by thermal methods, UV-visible spectra, room-temperature direct-current electrical resistivity measurements, and X-ray photoelectron and Auger electron spectroscopy. A principal goal of this work was to establish if there was coordination of the potential donor atoms of the polymide to cobalt. UV-visible spectra of the modified polyamic acid solutions and polyimide films and a titration study of a model system do not show any appreciable coordination with either the polyamic acid or the polyimide; rather, the cobalt(II) appears to be coordinated to the solvent, N,N-dimethylacetamide (DMAc), as [Co(DMAc)4]2+, until the temperature is raised above 200°C. X-ray photoelectron spectra of films cured only to 200°C also do not show significant shifts in the binding energies of the potential donor atoms from those binding energies of the undoped polymers, confirming little direct coordination of the cobalt to atoms of the polyimide. Heating the films to 300°C in a forced-air oven causes the formation of a cobalt oxide layer on the air side of the polymer. Direct-current electrical resistivity measurements on this surface show a 104-106 reduction in resistivity due to this layer.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pola.1987.080250220
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulations of small DNA plasmids: Effects of sequence and supercoiling on intramolecular motions (1996)
    New York : Wiley-Blackwell
    Biopolymers 39 (1996), S. 259-278 
    Details
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Small (600 base pair) DNA plasmids were modeled with a simplified representation (3DNA) and the intramolecular motions were studied using molecular mechanics and molecular dynamics techniques. The model is detailed enough to incorporate sequence effects. At the same time, it is simple enough to allow long molecular dynamics simulations. The simulations revealed that large-scale slithering occurs in a homogeneous sequence. In a heterogeneous sequence, containing numerous small intrinsic curves, the centers of the curves are preferentially positioned at the tips of loops. With more curves than loop tips (two in unbranched supercoiled DNA), the heterogeneous sequence plasmid slithers short distances to reposition other curves into the loop tips. However, the DNA is immobilized most of the time, with the loop tips positioned over a few favored curve centers. Branching or looping also appears in the heterogeneous sequence as a new method of repositioning the loop tips. Instead of a smooth progression of increasing writhing with increasing linking difference, theoretical studies have predicted that there is a threshold between unwrithed and writhed DNA at a linking difference between one and two. This has previously been observed in simulations of static structures and is demonstrated here for dynamic homogeneous closed DNA. Such an abrupt transition is not found in the heterogeneous sequence in both the static and dynamic cases. © 1996 John Wiley & Sons, Inc.
    Additional Material: 15 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    The viscosity of concentrated polymer solutions containing low molecular weight solvents (1986)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 31 (1986), S. 763-776 
    Details
    ISSN: 0021-8995
    Keywords: 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 , Physics
    Notes: The zero shear rate viscosities of polystyrene/ethylbenzene solutions having polymer weight fractions ranging from 0.5 to 1.0 have been measured using a novel sealed rheometer cell over a temperature range of 50 to 200°C. The concentration and temperature dependence of the solution viscosity has been found to be well described by the relation η0 = K caMw3.4ζ(c, T) where the monomeric friction coefficient ζ is determined by the free volume of the solution. Following the procedure of Berry, the free volume parameters, αf(c)/γ and T∞ (c), and the fractional free volume, f(c,T)/γ, have been determined. After using these parameters to account for the concentration dependence of the friction coefficient, the concentration exponent a has been evaluated and found to be in reasonable agreement with the value of 3.4 obtained by Berry and Fox for other polymer/solvent systems. A comparison of the relative conributions made by the friction coefficient and the term c3.4 to the overall concentration dependence of the viscosity of these highly concentrated solutions shows the friction coefficient to be the dominant factor
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1986.070310302
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  • 7
    Electronic Resource
    Electronic Resource
    Electric birefringence measurements of native, denatured, and renatured xanthan (1987)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 33 (1987), S. 825-834 
    Details
    ISSN: 0021-8995
    Keywords: 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 , Physics
    Notes: Electric birefringence measurements were performed on three xanthan solutions to determine conformational differences between native xanthan, denatured xanthan (heated in 4 M urea to 90°C and cooled), and renatured xanthan (heated and cooled). The birefringence of samples exposed to pulses of 1000 V for 1 ms were analyzed to determine longest relaxation times and steady-state birefringence. The results showed the following order for steady state birefringence, native 〉 renatured 〉 denatured, and longest relaxation time, renatured 〉 native 〉 denatured. The results are interpreted in terms of helix reformation and aggregation upon renaturing.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1987.070330312
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  • 8
    Electronic Resource
    Electronic Resource
    Synergistic induction of tumor necrosis factor α by bacterial lipopolysaccharide and lipoteichoic acid in combination with polytetrafluoroethylene particles in a murine macrophage cell line RAW 264.7 (1996)
    Hoboken, NJ : Wiley-Blackwell
    Journal of Biomedical Materials Research 31 (1996), S. 251-256 
    Details
    ISSN: 0021-9304
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine , Technology
    Notes: The induction of tumor necrosis factor α (TNF-α) by polytetrafluoroethylene (PTFE) particles (5-50 μm) and by bacterial lipopolysaccharide (LPS) and lipoteichoic acid (LTA) was examined in RAW cell cultures. Twenty-four-hour culture supernatants from the treated and control cells were assayed for TNF-α using a mouse L929 cell cytotoxicity assay. Untreated RAW cells produced low levels of endogenous TNF-α in the culture supernatants. Addition of 0.5 ng to 1 μg/ml LPS or 1 ng to 1 μg/ml LTA increased the TNF-α production by 7-3570-fold and 2-815-fold, respectively. Addition of 1-5 mg PTFE increased the TNF-α production by 6-17-fold over the untreated control cell levels. The cells exposed to PTFE and 0.5 ng/ml LPS or 5 ng/ml LTA produced TNF-α levels that were significantly higher than those produced by any inducer alone. Thus, both LTA, a Gram-positive bacterial cell wall component and LPS, a Gram-negative bacterial cell wall component, can induce TNF-α production, which is further enhanced by PTFE particles in RAW cells. © 1996 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
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