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  • 1
    Electronic Resource
    Electronic Resource
    N 5,N 10-Methenyltetrahydromethanopterin cyclohydrolase from the extremely thermophilic sulfate reducing Archaeoglobus fulgidus: comparison of its properties with those of the cyclohydrolase from the extremely thermophilic Methanopyrus kandleri (1993)
    Springer
    Archives of microbiology 159 (1993), S. 213-219 
    Details
    ISSN: 1432-072X
    Keywords: Archaea ; Methanogenic bacteria ; Hyperthermophiles ; Sulfate reducers ; Methanobacterium thermoautotrophicum ; Methanosarcina barkeri ; Tetrahydromethanopterin
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Archaeoglobus fulgidus and Methanopyrus kandleri are both extremely thermophilic Archaea with a growth temperature optimum at 83°C and 98°C, respectively. Both Archaea contain an active N 5,N 10-methenyltetrahydromethanopterin cyclohydrolase. The enzyme from M. kandleri has recently been characterized. We describe here the purification and properties of the enzyme from A. fulgidus. The cyclohydrolase from A. fulgidus was purified 180-fold to apparent homogeneity and its properties were compared with those recently published for the cyclohydrolase from M. kandleri. The two cytoplasmic enzymes were found to have very similar molecular and catalytic properties. They differed, however, significantly with respect of the effect of K2HPO4 and of other salts on the activity and the stability. The cyclohydrolase from A. fulgidus required relatively high concentrations of K2HPO4 (1 M) for optimal thermostability at 90°C but did not require salts for activity. Vice versa, the enzyme from M. kandleri was dependent on high K2HPO4 concentrations (1.5 M) for optimal activity but not for thermostability. Thus the activity and structural stability of the two thermophilic enzymes depend in a completely different way on the concentration of inorganic salts. The molecular basis for these differences are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00248474
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  • 2
    Electronic Resource
    Electronic Resource
    Formylmethanofuran: tetrahydromethanopterin formyltransferase and N 5,N 10-methylenetetrahydromethanopterin dehydrogenase from the sulfate-reducing Archaeoglobus fulgidus: similarities with the enzymes from methanogenic Archaea (1993)
    Springer
    Archives of microbiology 159 (1993), S. 225-232 
    Details
    ISSN: 1432-072X
    Keywords: Archaea ; Methanogens ; Sulfate reducers ; Tetrahydromethanopterin ; Methanofuran ; Coenzyme F420 ; C1-Enzymes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The sulfate-reducing Archaeoglobus fulgidus contains a number of enzymes previously thought to be unique for methanogenic Archaea. The purification and properties of two of these enzymes, of formylmethanofuran: tetrahydromethanopterin formyltransferase and of N 5,N 10-methylenetetrahydromethanopterin dehydrogenase (coenzyme F420 dependent) are described here. A comparison of the N-terminal amino acid sequences and of other molecular properties with those of the respective enzymes from three methanogenic Archaea revealed a high degree of similarity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00248476
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  • 3
    Electronic Resource
    Electronic Resource
    The [NiFe] hydrogenases of Methanococcus voltae: genes, enzymes and regulation (1997)
    Springer
    Archives of microbiology 167 (1997), S. 189-195 
    Details
    ISSN: 1432-072X
    Keywords: Key words [NiFe] Hydrogenases ; Methanococcus ; voltae ; Archaea ; Selenocysteine ; Selenium-dependent ; gene regulation ; Operator ; Silencer ; EPR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Methanococcus voltae carries genetic information for four [NiFe] hydrogenases. Two of the hydrogenases are predicted to contain selenocysteine on the basis of in-frame TGA codons, while the genes encoding the two other enzymes contain cysteine codons at homologous positions. Their predicted subunit compositions and their electron acceptor specificities are similar to those of the respective selenium-containing enzymes. The selenium-containing hydrogenases have been purified and characterized. Only one of them reduces the deazaflavin F420. The activity of the F420-nonreducing enzyme is exceptionally high. The selenium atom has been shown by EPR spectroscopy to be a ligand to the Ni atom in the primary reaction centers in both enzymes. The spectroscopic analyses also yielded a description of the electronic configuration around the NiFe center at different oxidation states and in the presence of the competitive inhibitor, CO. The genes encoding the selenium-free hydrogenases are expressed only in the absence of selenium. They are linked by an intergenic region in which regulatory cis elements were defined by employing reporter gene constructs and site-directed mutagenesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002030050434
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  • 4
    Electronic Resource
    Electronic Resource
    Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 849-862 
    Details
    ISSN: 0887-624X
    Keywords: vinyl acetate ; emulsion polymerization of ; poly(vinyl alcohol) emulsifier ; poly(vinyl alcohol) ; grafting onto ; acetonitrile/water extraction ; water-soluble PVA ; water-insoluble PVA ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and 1H and 13C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.
    Additional Material: 19 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Compatibilization of the PBA/PMMA core/shell latex interphase. I. Effect of PMMA macromonomer (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 3173-3181 
    Details
    ISSN: 0887-624X
    Keywords: macromonomer ; core/shell latexes ; interphase ; compatibilizing agent ; NMR relaxation studies ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work poly(methyl methacrylate) (PMMA) macromonomer is used as a compatibilizing agent in a poly(butyl acrylate) (PBA)/PMMA core/shell latex system. The incorporation of the PMMA macromonomer was achieved by copolymerizing it with BA monomer using miniemulsion polymerization. PBA seed latex was also synthesized without the macromonomer present to compare the compatibilizing effects with the PMMA macromonomer. The second stage methyl methacrylate monomer was added semi-continuously to the PBA seed latexes under monomer-starved conditions. Solid-state 13C-NMR [H]T1ρ relaxation studies were used to determine the effect of PMMA compatibilizer on these PBA/PMMA core/shell latex interphase regions. The thickness of the interphase of the core/shell particles prepared with and without the PMMA macromonomer compatibilizing agent are calculated to be in the range of 15-16 nm and 10-12 nm, respectively. Electron microscopy revealed that the seed latex prepared with the PMMA macromonomer achieved a more uniform coverage with the second stage PMMA polymer as compared to the latex synthesized without the compatibilizing agent present. It is concluded that the PMMA macromonomer is effective in increasing the thickness of the interphase region and also the amount of interfacial PMMA. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Role of the nonionic surfactant Triton X-405 in emulsion polymerization. I. Homopolymerization of styrene (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 3813-3825 
    Details
    ISSN: 0887-624X
    Keywords: reaction calorimetry ; emulsion polymerization ; kinetics ; styrene ; nonionic surfactant ; Triton X-405 ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The emulsion polymerization of styrene was studied using the nonionic surfactant Triton X-405 (octylphenoxy polyethoxy ethanol). Two separate nucleation periods were noted in these polymerizations resulting in bimodal final latex particle size distributions. The partitioning of the surfactant between the phases was found to play the major role in determining the nucleation mechanism(s) in these polymerizations. Although the total concentration of the emulsifier was always added at a level above its critical micelle concentration (CMC) based on the water phase in the recipe, it was found that the portion of the surfactant initially present in the aqueous phase was below its CMC due to the partitioning. This CMC was also found to increase with increasing total surfactant because the distribution of the surfactant (varying ethylene oxide chain length) depended on the partitioning between the phases. Under these conditions, the first of the two nucleation periods was attributed to homogeneous nucleation, while the second was attributed to micellar nucleation. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3813-3825, 1997
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Details of the emulsion polymerization of styrene using a reaction calorimeter (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 461-473 
    Details
    ISSN: 0887-624X
    Keywords: calorimetry ; styrene ; kinetics ; emulsion polymerization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An automated reaction calorimeter was used to directly monitor the rate of emulsion polymerization of styrene using different emulsifier (sodium lauryl sulfate) and initiator (potassium persulfate) concentrations. By using this technique in conjunction with off-line measurements of the evolution of the particle size distributions, important details of the process were observed. The classical constant rate period (Interval II) often reported for the batch emulsion polymerization of styrene was not seen in this work. Instead, the experimental results suggest that the end of nucleation and the disappearance of monomer droplets take place at approximately the same conversion (36-40%). From the polymerization rate data, important parameters such as the monomer concentration in the polymer particles and the average number of radicals per particle were calculated. © 1996 John Wiley & Sons, Inc.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Role of the nonionic surfactant Triton X-405 in emulsion polymerization. II. Homopolymerization of n-butyl acrylate (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 3827-3835 
    Details
    ISSN: 0887-624X
    Keywords: reaction calorimetry ; emulsion polymerization ; n-butyl acrylate ; nonionic surfactant ; Triton X-405 ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the emulsion polymerization of n-butyl acrylate, the nonionic surfactant Triton X-405 (octylphenoxy polyethoxy ethanol) partitions primarily into the aqueous phase leading to nucleation in the presence of micelles and unimodal final latex particle size distributions. Nucleation is accompanied by limited aggregation in the early stages of the reactions with particles being formed in some cases past 50% conversion depending on the experimental conditions. High values of the average number of radicals per particle (n̄ 〉 10) are observed during these reactions which explains a secondary increase in the heat of reaction. This increase was not observed when the initiator (K2S2O8) concentration was reduced from 6.4 to 0.6 mM or the temperature was decreased from 70°C to 60°C; contrary to what was expected, an inverse relationship was observed between the rate of polymerization and the final number of particles for these experiments. This was attributed to more extensive limited aggregation at the higher temperature and initiator concentration. Limited aggregation is thus shown to be an important mechanism in these polymerizations. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3827-3835, 1997
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Compatibilization of the PBA/PMMA Core/Shell Latex Interphase. II. Effect of PMMA Macromonomer (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 34 (1996), S. 3183-3190 
    Details
    ISSN: 0887-624X
    Keywords: macromonomer ; core/shell latexes ; interphase ; compatibilizing agent ; NMR relaxation studies ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work poly(methyl methacrylate) (PMMA) macromonomer is used as a compatibilizing agent in a poly(butyl acrylate) (PBA)/PMMA core/shell latex system. The incorporation of the PMMA macromonomer was achieved by copolymerizing it with BA monomer using miniemulsion polymerization. PBA seed latex was also synthesized without the macromonomer present to compare the compatibilizing effects with the PMMA macromonomer. The second stage methyl methacrylate monomer was added semi-continuously to the PBA seed latexes under monomer-starved conditions. Solid-state 13C-NMR [H]T1ρ relaxation studies were used to determine the effect of PMMA compatibilizer on these PBA/PMMA core/shell latex interphase regions. The thickness of the interphase of the core/shell particles prepared with and without the PMMA macromonomer compatibilizing agent are calculated to be in the range of 15-16 nm and 10-12 nm, respectively. Electron microscopy revealed that the seed latex prepared with the PMMA macromonomer achieved a more uniform coverage with the second stage PMMA polymer as compared to the latex synthesized without the compatibilizing agent present. It is concluded that the PMMA macromonomer is effective in increasing the thickness of the interphase region and also the amount of interfacial PMMA. © 1996 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pola.1996.869
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  • 10
    Electronic Resource
    Electronic Resource
    Role of the nonionic surfactant Triton X-405 in emulsion polymerization. III. Copolymerization of styrene and n-butyl acrylate (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 3837-3846 
    Details
    ISSN: 0887-624X
    Keywords: reaction calorimetry ; emulsion copolymerization ; styrene ; n-butyl acrylate ; nonionic surfactant ; Triton X-405 ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Emulsion copolymerizations of styrene and n-butyl acrylate were conducted at 70°C using varying amounts of Triton X-405 (octyl phenoxy polyethoxyethanol) as emulsifier. The kinetic behavior was found to vary widely depending on the emulsifier concentration. Unimodal particle size distributions were produced at the lowest (4.2 mM) and the highest levels (12.5 mM, 16.2 mM) of emulsifier while at intermediate levels (6.3 mM and 8.4 mM) bimodal distributions were produced; these were reflected in the reaction kinetics. These results were attributed to the surfactant partitioning behavior in the system which led to homogeneous/coagulative nucleation at the lowest level to homogeneous/coagulative nucleation followed by micellar nucleation at the intermediate levels to micellar nucleation at the highest levels. Although the added surfactant levels were all well above that required to exceed its CMC in the aqueous phase, the substantial partitioning into the oil phase lead to conditions well below the CMC of the portion of the surfactant present in the aqueous phase for the lowest and the intermediate levels of the Triton X-405. Consequences of this partitioning were seen in a semibatch reaction where all the surfactant was initially present in the aqueous phase; in this case, too many particle were nucleated leading to massive coagulation due to insufficient surfactant being available to stabilize the particles. © John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3837-3846, 1997
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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