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  • 1
    Digitale Medien
    Digitale Medien
    Oligomerization of 3′-amino-3′-deoxyguanosine-5′-phosphorimidazolidate on a d(CpCpCpCpC) template (1987)
    Springer
    Journal of molecular evolution 25 (1987), S. 97-99 
    Details
    ISSN: 1432-1432
    Schlagwort(e): Template synthesis ; Nucleotide analogs ; Oligomerization
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Summary 3′-Amino-3′-deoxyguanosine-5′-phosphorimidazolidate (ImpGnh 2) oligomerizes more rapidly and regiospecifically than related nucleotide derivatives on a d(CpCpCpCpC) template. The greater nucleophilicity of the amino group leads to efficient oligomerization even when the structure of the double-helical complex formed by the template and the substrate is not optimal for reaction. The use of amine-containing analogues should permit us to develop models of potentially prebiotic polymerization reactions that cannot be studied easily using natural nucleotides.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02101750
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  • 2
    Digitale Medien
    Digitale Medien
    Physical and chemical properties of the nitrogenase proteins from Azotobacter vinelandii (1974)
    Springer
    Archives of microbiology 98 (1974), S. 93-100 
    Details
    ISSN: 1432-072X
    Schlagwort(e): Azotobacter ; Nitrogenase ; Iron Sulfur Proteins ; Properties
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie
    Notizen: Abstract The purified Mo-Fe protein and Fe protein of nitrogenase from Azotobacter vinelandii have molecular weights (MW) of about 216000 and 64000, respectively. The Mo-Fe protein is composed of subunits of about 56000 MW, and the Fe protein has 2 equivalent subunits of about 33000 MW. The isoelectric point of the Mo-Fe protein is 5.2 and that of the Fe protein is 4.7. Amino acid compositions reflect the acidic nature of the proteins. The Mo-Fe protein yielded 24 atoms of Fe, 20 atoms of acid-labile sulfide and 1.54 atoms of Mo per molecule. Analysis of the Fe protein showed 3.45 atoms of Fe per molecule.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00425272
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  • 3
    Digitale Medien
    Digitale Medien
    Further elucidation of a pertussis toxin-sensitive transmembrane signaling mechanism involved in central α2-adrenoceptor activation in the rat (1993)
    Springer
    Journal of biomedical science 1 (1993), S. 13-18 
    Details
    ISSN: 1423-0127
    Schlagwort(e): Central α2-adrenoceptor ; Guanabenz ; Nucleus reticularis gigantocellularis ; GTP-binding regulatory proteins ; Pertussis toxin
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Biologie , Medizin
    Notizen: Abstract In adult male Sprague-Dawley rats anesthetized with pentobarbital sodium, we elucidated the molecular consequence of central α2-adrenoceptor activation. The hypotensive and negative chronotropic and inotropic actions of the α2-adrenoceptor agonist guanabenz were used as our experimental index. Intracerebroventricular administration of pertussis toxin (2.5 µg) significantly attenuated the cardiovascular suppressant effects of the aminoguanidine compound (100 µg/kg i.v.). However, application of N-ethylmaleimide (0.125 or 0.250 µg), phorbol 12-myristate 13-acetate (1.25 or 2.50 µg), cholera toxin (1.25 or 2.50 µg) or forskolin (12.5 or 25.0 µg) into the lateral cerebral ventricle elicited no appreciable blunting effect on the circulatory depression produced by guanabenz. These results were essentially duplicated when pertussis toxin (0.125 or 0.250 µg), N-ethylmaleimide (0.0125 or 0.05 µg), phorbol 12-myristate 13-acetate (0.125 or 0.25 µg), cholera toxin (0.125 or 0.25 µg) or forskolin (1.25 or 2.50 µg) was microinjected bilaterally to the nucleus reticularis gigantocellularis, a medullary site believed to be intimately related to the antihypertensive action of guanabenz. These findings suggest that stimulation of the α2-adrenoceptors in the medulla oblongata may result in the activation of a pertussis toxin-sensitive GTP-binding regulatory protein. They further suggest that the biologic signals subsequent to this action may not be linked to Gs, Gi or Gp but possibly Go.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02258335
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  • 4
    Digitale Medien
    Digitale Medien
    On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects (1995)
    Springer
    Applied physics 61 (1995), S. 397-405 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 61.70 ; 66.30
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, anamphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancyV Ga 2− and the triply positively charged defect complex (ASGa+V As)3+, with AsGa being the antisite defect of an As atom occupying a Ga site andV As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect systemV Ga 2− /(AsGa+V As)3+ is supposed to be able to pin the GaAs Fermi level at approximately theE v +0.6 eV level position, which requires that the net free energy of theV Ga/(AsGa+V As) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about theE v +1.2 eV level position instead of the neededE v +0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01540114
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  • 5
    Digitale Medien
    Digitale Medien
    On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects (1995)
    Springer
    Applied physics 61 (1995), S. 397-405 
    Details
    ISSN: 1432-0630
    Schlagwort(e): PACS: 61.70; 61.70; 66.30
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract.  Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, an amphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancy V 2- Ga and the triply positively charged defect complex (ASGa+V As)3+, with AsGa being the antisite defect of an As atom occupying a Ga site and V As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect system V 2- Ga/(AsGa+V As)3+ is supposed to be able to pin the GaAs Fermi level at approximately the E v +0.6 eV level position, which requires that the net free energy of the V Ga/(AsGa+V As) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about the E v +1.2 eV level position instead of the needed E v +0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01540114
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  • 6
    Digitale Medien
    Digitale Medien
    Mixed convection cooling of a heated, continuously stretching surface (2000)
    Springer
    Heat and mass transfer 36 (2000), S. 79-86 
    Details
    ISSN: 1432-1181
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract  An numerical study of the flow and heat transfer characteristics associated with a heated, continuously stretching surface being cooled by a mixed convection flow has been carried out. The relevant heat transfer mechanisms are of interest in a wide variety of practical applications, such as hot rolling, continuous casting, extrusion, and drawing. The surface velocity of the continuously stretching sheet was assumed to vary according to a power-law form, that is, u w (x)=Cx p . Two conditions of surface heating were considered, a variable wall temperature (VWT) in the form T w (x)−T ∞=Ax n and a variable surface heat flux (VHF) in the form q w (x)=Bx m . The governing differential equations are transformed by introducing proper nonsimilarity variables and solved numerically using a procedure based on finite difference approximations. Results for the local Nusselt number and the local friction coefficient are obtained for a wide range of governing parameters, such as the surface velocity parameter p, the wall temperature exponent n, the surface heat flux exponent m, the buoyancy force parameters (ξ for the VWT case and χ for the VHF case), and Prandtl number of the fluid. It is found that the local Nusselt number is increased with increasing the velocity exponent parameter p for the VWT case, while the opposite trend is observed for the VHF case. The local friction coefficient is increased for a decelerated stretching surface, while it is decreased for an accelerated stretching surface. Also, appreciable effects of the buoyancy force on the local Nusselt number and the local friction coefficient are observed for both VWT and VHF cases, as expected.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s002310050367
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  • 7
    Digitale Medien
    Digitale Medien
    Mixed convection heat transfer from a horizontal plate with variable surface heat flux in a porous medium (1998)
    Springer
    Heat and mass transfer 34 (1998), S. 1-7 
    Details
    ISSN: 1432-1181
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract In this article nonsimilarity solution for mixed convection from a horizontal surface in a saturated porous medium was obtained for the case of variable surface heat flux. The entire mixed convection regime, ranging from pure forced convection to pure free convection, is considered by introducing a single nonsimilarity parameter. Heat transfer results are predicted by employing four different flow models, namely, Darcy's law, the Ergun model, and the Brinkman-Forchheimer-extended Darcy model with constant and variable porosity. The variable porosity effect is approximated by an exponential function. Effects of transverse thermal dispersion are taken into consideration in the energy equation, along with variable stagnant thermal conductivities. The formulation of the present problem shows that the flow and heat transfer characteristics depend on five parameters, that is, the power in the variation of surface heat flux, the nonsimilarity mixed-convection parameter, the inertia effect parameter, the boundary effect parameter, and the ratio of thermal conductivity of the fluid phase to that of the solid phase. Numerical results for the local Nusselt number variations, based on the various flow models, are presented for the entire mixed convection regime. The impacts␣of different governing parameters on the heat transfer results are thoroughly investigated.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s002310050224
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  • 8
    Digitale Medien
    Digitale Medien
    Laminar mixed convection adjacent to vertical, continuously stretching sheets (1998)
    Springer
    Heat and mass transfer 33 (1998), S. 471-476 
    Details
    ISSN: 1432-1181
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The analysis of laminar mixed convection in boundary layers adjacent to a vertical, continuously stretching sheet has been presented. The velocity and temperature of the sheet were assumed to vary in a power-law form, that is, u w (x)=Bx m and T w (x)−T ∞=Ax n . In the presence of buoyancy force effects, similarity solutions were reported for the following two cases: (a) n=0 and m=0.5, which corresponds to an isothermal sheet moving with a velocity of the form u w =Bx 0.5 and (b) n=1 and m=1, which corresponds to a continuous, linearly stretching sheet with a linear surface temperature distribution, i.e. T w −T ∞=Ax. Formulation of the present problem shows that the heat transfer characteristics depends on four governing parameters, namely, the velocity exponent parameter m, the temperature exponent parameter n, the buoyancy force parameter G *, and Prandtl number of the fluid. Numerical solutions were generated from a finite difference method. Results for the local Nusselt number, the local friction coefficient, and temperature profiles are presented for different governing parameters. Effects of buoyancy force and Prandtl number on the flow and heat transfer characteristics are thoroughly examined.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s002310050217
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  • 9
    Digitale Medien
    Digitale Medien
    Transient combustion of a heating droplet in a gravitational environment (1996)
    Springer
    Heat and mass transfer 31 (1996), S. 339-346 
    Details
    ISSN: 1432-1181
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung Der instationäre Verbrennungsprozeß eines plötzlich einem Flammenfeld in atmosphärischer Umgebung ausgesetzten Tröpfchens wird numerisch untersucht. Die Verbrennung läßt sich in die zwei Phasen unterteilen: (1) Aufheizphase des Tröpfchens, (2) konstante Tröpfchentemperatur. Die natürliche Konvektionsströmung ist nicht von vorneherein bekannt, sie wird als Teil der Lösung bereitgestellt. Während der Aufheizphase steigen Temperatur und Verdampfungstrate des Tröpfchens stark an, während das Quadrat des Durchmessers langsam abnimmt. In der folgenden Phase verringern sich Verdampfungstrate und Tröpfchendurchmesser mit der Zeit. Das Strömungsfeld imfolge natürlicher Konvektion wird ebenfalls ermittelt, um seince Wechsel-wirkung mit der Flamme und dem Verbrennungsprozeß zu veranschaulichen. Schließlich läßt sich zeigen, daß in der Umgebung des Tröpfchens eine Anhäufung unverbrannten Brenngases erfolgt, was zu einer Abnahme der Verdampfungs-rate führt.
    Notizen: Abstract The transient combustion characteristics of a droplet suddenly exposed to the envelope flames in an atmospheric environment are studied numerically. Combustion can be divided into a droplet heating-up and a constant-droplet-temperature burning. The naturally-convective flow is not knowna priori, but provided as part of the solution. During the heating-up stage, the temperature and evaporation rate of droplet increase sharply, and the square of diameter decreases slightly as time proceeds. In the following stage, the droplet temperature remains constant, the evaporation rate and droplet diameter decrease with time. The flowfield of natural convection is also presented to demonstrate its interaction with the flame and the transient process. Finally, the fuel accumulation phenomenon is identified and it results in an reduction of evaporation constant.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02184048
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  • 10
    Digitale Medien
    Digitale Medien
    Natural convection from a vertical cylinder in a thermally stratified porous medium (1999)
    Springer
    Heat and mass transfer 34 (1999), S. 423-428 
    Details
    ISSN: 1432-1181
    Schlagwort(e): Keywords Non-Darcy natural convection ; Thermal stratification ; and Vertical cylinder.
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The problem of non-Darcy natural convection adjacent to a vertical cylinder embedded in a thermally stratified porous medium has been analyzed. Nonsimilarity solutions are obtained for the case that the ambient temperature increases linearly with height of the cylinder. A generalized flow model was used in the present study to include the effects of the macroscopic viscous term and the microscopic inertial force. Also, the thermal dispersion effect is considered in the energy equation. Thus, the main aim of this work is to examine the effects of thermal stratification and non-Darcy flow phenomena on the free convection flow and heat transfer characteristics. It was found that the present problem depends on six parameters, namely, the local thermal stratification parameter ξ, the boundary effect parameter Bp, the modified Grashof number Gr*, wall temperature exponent m, the curvature parameter ω, and the modified Rayleigh number based on pore diameter Ra d . The impacts of these governing parameters on the local heat transfer parameter are discussed in great detail. Also, representative velocity and temperature profiles are presented at selected values of the thermal stratification parameter. In general, the local heat transfer parameter is increased with increasing the values of m, ω, and Ra d ; while it is decreased with increasing the values of ξ, Bp, and Gr*.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s002310050278
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