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  • 1
    ISSN: 1573-4951
    Keywords: G-protein-coupled receptor ; Hartree-Fock calculations ; Histamine H2 receptor ; Molecular mechanics ; Receptor models
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary In the first part (pp. 461–478 in this issue) of this study regarding the histamine H2 receptor agonistic binding site, the best possible interactions of histamine with an α-helical oligopeptide, mimicking a part of the fifth transmembrane α-helical domain (TM5) of the histamine H2 receptor, were considered. It was established that histamine can only bind via two H-bonds with a pure α-helical TM5, when the binding site consists of Tyr182/Asp186 and not of the Asp186/Thr190 couple. In this second part, two particular three-dimensional models of G-protein-coupled receptors previously reported in the literature are compared in relation to agonist binding at the histamine H2 receptor. The differences between these two receptor models are discussed in relation to the general benefits and limitations of such receptor models. Also the pros and cons of simplifying receptor models to a relatively easy-to-deal-with oligopeptide for mimicking agonistic binding to an agonistic binding site are addressed. Within complete receptor models, the simultaneous interaction of histamine with both TM3 and TM5 can be analysed. The earlier suggested three-point interaction of histamine with the histamine H2 receptor can be explored. Our results demonstrate that a three-point interaction cannot be established for the Asp98/Asp186/Thr190 binding site in either of the investigated receptor models, whereas histamine can form three H-bonds in case the agonistic binding site is constituted by the Asp98/Tyr182/Asp186 triplet. Furthermore this latter triplet is seen to be able to accommodate a series of substituted histamine analogues with known histamine H2 agonistic activity as well.
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  • 2
    ISSN: 1573-4951
    Keywords: α-helical model system ; Conformational analysis ; Counterpoise method ; Hartree-Fock calculations ; Histamine H2 receptor ; Molecular mechanics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary Mutation studies on the histamine H2 receptor were reported by Gantz et al. [J. Biol. Chem., 267 (1992) 20840], which indicate that both the mutation of the fifth transmembrane Asp186 (to Ala186) alone or in combination with Thr190 (to Ala190) maintained, albeit partially, the cAMP response to histamine. Recently, we have shown that histamine binds to the histamine H2 receptor as a monocation in its proximal tautomeric form, and, moreover, we suggested that a proton is donated from the receptor towards the tele-position of the agonist, thereby triggering the biological effect [Nederkoorn et al., J. Mol. Graph., 12 (1994) 242; Eriks et al., Mol. Pharmacol., 44 (1993) 886]. These findings result in a close resemblance with the catalytic triad (consisting of Ser, His and Asp) found in serine proteases. Thr190 resembles a triad's serine residue closely, and could also act as a proton donor. However, the mutation of Thr190 to Ala190 — the latter is unable to function as a proton donor — does not completely abolish the agonistic cAMP response. At the fifth transmembrane α-helix of the histamine H2 receptor near the extracellular surface, another amino acid is present, i.e. Tyr182, so an alternative couple of amino acids, Tyr182 and Asp186, could constitute the histamine binding site at the fifth α-helix instead of the (mutated) couple Asp186 and Thr190. In the first part of our present study, this hypothesis is investigated with the aid of an oligopeptide with an α-helical backbone, which represents a part of the fifth transmembrane helix. Both molecular mechanics and ab initio data lead to the conclusion that the Tyr182/Asp186 couple is most likely to act as the binding site for the imidazole ring present in histamine.
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  • 3
    ISSN: 1573-5001
    Keywords: MORASS ; NOESY-NOESY ; relaxation matrix analysis ; three-way junction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Homonuclear 3D NOESY-NOESY has shown great promise for the structural refinement of large biomolecules. A computationally efficient hybrid-hybrid relaxation matrix refinement methodology, using 3D NOESY-NOESY data, was used to refine the structure of a DNA three-way junction having two unpaired bases at the branch point of the junction. The NMR data and the relaxation matrix refinement confirm that the DNA three-way junction exists in a folded conformation with two of the helical stems stacked upon each other. The third unstacked stem extends away from the junction, forming an acute angle (∼60° ) with the stacked stems. The two unpaired bases are stacked upon each other and are exposed to the solvent. Helical parameters for the bases in all three strands show slight deviations from typical values expected for right-handed B-form DNA. Inter-nucleotide imino-imino NOEs between the bases at the branch point of the junction show that the junction region is well defined. The helical stems show mobility (± 20° ) indicating dynamic processes around the junction region. The unstacked helical stem adjacent to the unpaired bases shows greater mobility compared to the other two stems. The results from this study indicate that the 3D hybrid-hybrid matrix MORASS refinement methodology, by combining the spectral dispersion of 3D NOESY-NOESY and the computational efficiency of 2D refinement programs, provides an accurate and robust means for structure determination of large biomolecules. Our results also indicate that the 3D MORASS method gives higher quality structures compared to the 2D complete relaxation matrix refinement method.
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  • 4
    ISSN: 1573-4951
    Keywords: Histamine ; H1-receptor ; H1-agonists ; H1-antagonists ; G-protein coupled receptor
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A modelling study has been carried out, investigating the binding of histamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2-PhHist) at two postulated agonistic binding sites on transmembrane domain 5 (TM5) of the histamine H1-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys200, Thr203 and Asn207, for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys200/Asn207 pair. Therefore, Lys200 was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H1-receptor stimulation by histamine. Altogether, the data suggest that the Lys200/Asn207 pair is important for activation of the H1-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histamine agonists and an alternative mode of binding is proposed in which the 2-phenyl ring binds to the same receptor location as one of the aromatic rings of classical histamine H1-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H1-antagonist cyproheptadine were evaluated in three different seven-α-helical models of the H1-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp116 (TM3) within the receptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon agonist binding is expected to play an essential role in receptor stimulation.
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  • 5
    ISSN: 0933-5137
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Microstructural, mechanical and corrosive properties of friction stir welded aluminium jointsFriction stir welding (FSW) is a novel solid state welding process. It allows joining of high strength aluminum alloys, generally considered as difficult-to-weld with conventional technologies, without loss in joint strength. Results of investigations on selfmade FSW butt joints of the aluminum alloys 2024-T3 and 6013-T4 are presented. First, the microstructure of the weld seam and heat affected zone is characterised metallographically and by hardness measurements. By tensile, fatigue endurance (SN) and fatigue crack propagation tests it is demonstrated, that especially the FSW-joints of 2024-T3 sustain high mechanical loadings. Investigations on the corrosion properties reveal a certain sensitivity of the 2024-T3 joints to intergranular and exfoliation corrosion.
    Notes: Das Reibrührschweißen (friction stir welding FSW) ist ein neues Fügeverfahren, mit dem auch die schwer schmelzschweißbaren, höherfesten Aluminiumlegierungen ohne große Festigkeitsverluste verschweißt werden können. Erste Ergebnisse von Untersuchungen an selbst hergestellten FSW-Stumpfnähten der Aluminiumlegierungen 2024-T3 und 6013-T4 werden vorgestellt. Zunächst wird das Gefüge der Naht und der Wärmeeinflußzone mikrostrukturell und durch Härteverläufe charakterisiert. Weiterhin wird mit Zug-, Schwingfestigkeits- und Rißfortschrittsversuchen nachgewiesen, daß insbesondere die FSW-Nähte der Legierung 2024-T3 außerordentlich hohe mechanische Belastungen ertragen. Abschließende Korrosionsuntersuchungen der FSW-Nähte zeigen eine gewisse Anfälligkeit der abschreckempfindlichen Legierung 2024-T3 für interkristalline Korrosion und Schichtkorrosion.
    Additional Material: 15 Ill.
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The polarimeter system for ITER features 10 viewing chords covering almost the full poloidal cross section of the plasma in a fan-like arrangement. The 10 laser beams all enter the plasma via 150 mm diam penetrations through the diagnostic plug at the low-field side. Small retroreflectors at the high-field side mirror the laser beams back towards the penetrations at the low-field side. Extensive feasibility calculations have led to the conclusion that the optimum wavelength for the polarimeter is around 100 μm. At this wavelength counterproductive effects like refraction and electron cyclotron absorption are negligibly small. However, the anticipated Faraday rotation angles are still very large, such that several important plasma parameters can be measured within the requirements set by the ITER team. In this article the status of the conceptual design of the polarimeter system for ITER will be presented. © 1999 American Institute of Physics.
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  • 7
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In this article the effect of mode scrambling on the operation of pulsed radar reflectometers working in the ordinary polarization mode on devices with a high magnetic shear is studied. Mode scrambling occurs when the magnetic field changes considerably on length and/or time scales which are similar to or smaller than those of the probing wave. In this case the polarization of the mode is not conserved during passage through the plasma, and power is transferred from ordinary to extraordinary mode and vice versa. Calculations with a full-wave code are performed for two different high-shear devices: the RFX reversed field pinch and the LHD torsatron. The conclusion of the work is that the density profiles in these devices can excellently be measured by a pulsed radar reflectometer operating in the ordinary mode. © 1997 American Institute of Physics.
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  • 8
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A four-channel pulsed radar reflectometry system has been developed for the Rijnhuizen Tokamak Project. The system combines a high spatial and temporal resolution with limited access demands to the tokamak plasma. Steady-state density profiles as measured with the pulsed radar system match very well to the profiles as measured by Thomson scattering and by interferometry. As will be illustrated in this paper, the diagnostic is able to follow fast phenomena as pellet injection and plasma disruptions as well as MHD oscillations. © 1995 American Institute of Physics.
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  • 9
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The 20-channel heterodyne radiometer of the Rijnhuizen Tokamak Project (RTP) measures microwaves in the range between 86 and 146 GHz. The spatial resolution is ∼7–16 mm parallel and 25–1300 mm2 perpendicular to the viewing direction, depending on the position in the torus. A waveguide network gives the opportunity to do alternate measurements of X- and O-mode electron cyclotron emission (ECE) from the low-field side (LFS) and X-mode ECE from the high-field side (HFS). A backward wave oscillator is installed at the HFS, for the purpose of measuring the electron cyclotron absorption (ECA) of the plasma. This set-up also provides a possibility for fast relative calibration of the radiometer. To reduce the effects of multiple-pass reflections and standing waves, Macor viewing dumps are mounted at both sides. A comparison of HFS and LFS ECE measurements during electron cyclotron heating will be given as a first result. First results of ECA measurements were obtained but will be presented in the future. © 1995 American Institute of Physics.
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  • 10
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A novel method is presented to analyze internal disruptions (sawteeth), observed with the five-camera soft-x-ray diagnostic at the Rijnhuizen Tokamak Project (RTP). Using four or five cameras, tomographic images with seven poloidal harmonics have been obtained throughout sawtooth crashes and precursor oscillations. The detection of cross-field transport and magnetic reconnection from the reconstructed tomographic images is complicated by the fact that (except during the final phase of the collapse) the time dependence is dominated by the m=1 ideal MHD displacement of the hot core in combination with plasma rotation. It is possible to define quantities, e.g., the plasma volume where the emissivity is within a certain range, whose change is only determined by cross-field transport or reconnection, and are not affected by displacement and rotation. The good spatial resolution of the 80-channel system enables us to compute these quantities with such an accuracy that their sensitivity to plasma rotation and MHD convection is small. Using this approach we find in the sawteeth studied so far that m=1 magnetic reconnection may take place, but affects less than half of the magnetic flux within the q=1 surface, thus ruling out complete reconnection as a mechanism for the final collapse of the hot core in these cases. © 1995 American Institute of Physics.
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