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  • 1
    Electronic Resource
    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-6266
    Keywords: latex films ; particle coalescence ; cross-linking ; small-angle neutron scattering (SANS) ; dynamic micromechanical analysis (DMA) ; chain segment mobility ; diffusion coefficient ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Interdiffusion of polymer chains across particle-particle interfaces in latex films is one of the key issues, because this further coalescence process usually controls the cohesive strength of the films. Here we examined local dynamics of polymer chains within latex films and showed that it is related to the occurrence of particle coalescence. For this purpose, structural and dynamic micromechanical properties of partly cross-linked polymer films from styrenebutadiene (SB) copolymer latexes were investigated. The gel fraction of the emulsion copolymers was varied within a large range (15-90%) by adding different chain transfer agents during the polymerization process. The structure of the films was determined using small-angle neutron scattering (SANS) and transmission electron microscopy (TEM). A qualitative description of the mechanism that likely triggers further coalescence in latex films is given, based on critical disjoining pressure exerted by particle cores on interfacial membranes. In addition, from dynamic micromechanical analysis (DMA) spectra, a key molecular parameter reflecting local segmental mobility was derived. Average diffusion coefficients of polymer segments were calculated and shown to be consistently related to the occurrence of particle coalescence as evidenced from SANS and TEM structural investigations. Finally, the noncovalent physical intermolecular interaction loci, which act as cross-link knots, were shown to play a significant part in the restriction of the coalescence process, increasing the state of constraints encountered by free chain segments embedded in the covalent network. © 1995 John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The high-resolution nmr spectrum of the anticodon hairpin from E. coli tRNAfMet has been obtained at a number of different temperatures. The positions of the resonances from interior Watson-Crick base pairs are well accounted for (within 0.1 ppm) by a semi-empirical ring current shift theory, but the terminal base pairs are susceptible to the exact orientation of adjacent bases in single-stranded regions. From a careful examination of the exact way in which resonances disappear at elevated temperatures, we conclude that melting in the nmr experiments occurs when the lifetime of a base pair is reduced to several milliseconds. On the basis of these experiments we are able to assign an nmr Tm to each individual base pair and these should be useful in interpreting the melting behavior of the intact molecule. An “extra” resonance is observed at ∼11.3 ppm and, on the basis of its position and temperature sensitivity, it is tentatively assigned to the ring nitrogen proton of a “protected” U residue in the anticodon loop. A strong preference for stacking of a nonbase-paired A residue on an adjacent GC base pair is observed even at temperatures in excess of 52°C.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0006-291X
    Keywords: [abr] FMLP; formyl-methionyl-leucyl-phenylalanine ; [abr] G protein; guanine nucleotide binding protein ; [abr] GDP-β-S; guanosine-5'-O-(2-thiodiphosphate) ; [abr] GTP-γ-S; guanosine-5'-O-(3-thiotriphosphate) ; [abr] Gi; the inhibitory G protein associated with adenylate cyclase ; [abr] GppNHp; 5'-guanylylimidodiphosphate ; [abr] Gs; the stimulatory G protein associated with adenylate cyclsae ; [abr] PAF; platelet-activating factor ; [abr] cAMP; adenosine-3'-5'-cyclic monophosphate ; [abr] cGMP; guanosine-3'-5'-cyclic monophosphate
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0006-291X
    Keywords: [abr] FMLP; formyl-methionyl-leucyl-phenylalanine ; [abr] G protein; guanine nucleotide binding protein ; [abr] GDP-β-S; guanosine-5'-O-(2-thiodiphosphate) ; [abr] GTP-γ-S; guanosine-5'-O-(3-thiotriphosphate) ; [abr] Gi; the inhibitory G protein associated with adenylate cyclase ; [abr] GppNHp; 5'-guanylylimidodiphosphate ; [abr] Gs; the stimulatory G protein associated with adenylate cyclsae ; [abr] PAF; platelet-activating factor ; [abr] cAMP; adenosine-3'-5'-cyclic monophosphate ; [abr] cGMP; guanosine-3'-5'-cyclic monophosphate
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The high-resolution (300 MHz) proton nmr spectrum of E. coli tRNAfMet has been examined in 0.17M NaCl, with and without Mg2+, and at various temperatures. In light of recent studies of other E. coli tRNA and fragments of tRNAfMet, some low field (11-15 ppm) resonances previously assigned to secondary structure base pairs are reassigned to a tertiary structure A14-S4U8 base pair and a protected uridine residue in the anticodon loop. These two resonances and other low field resonances which are assigned to secondary structure base pairs are used to monitor the thermal unfolding of the molecule. In the absence of Mg2+ the tertiary structure base pair is present only to ∼45°C, but in the presence of Mg2+ it remains until at least 70°C. Analysis of the temperature dependence of other low field resonances indicates that the melting of the dihydrouridine stem occurs more or less simultaneously with the loss of tertiary structure. The observation of the resonance from the A14-S4U8 base pair proves that tertiary structure is present in this molecule below 40°C, even in the absence of Mg2+.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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