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  • 1990-1994  (16)
  • 1950-1954
  • 1925-1929
  • Polymer and Materials Science  (12)
  • Analytical Chemistry and Spectroscopy  (3)
  • Basal cells  (1)
Material
Years
Year
Keywords
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Protoplasma 159 (1990), S. 77-84 
    ISSN: 1615-6102
    Keywords: Stem cells ; Phenotypic markers ; Proliferation potential ; Keratins ; Basal cells ; Preneoplasias ; Development
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The evidence for mammary epithelial stem cells and their phenotypic characteristics in normal and neoplastic development is reviewed. The presence of stem cells in all parts of the mammary parenchyma at all stages of differentiation has been demonstrated by transplantation experiments. The phenotypic characterization of stem cells has been defined by a battery of monospecific antibodies. These studies suggest that a mammary epithelium stem cell compartment exists in the basal layer of the gland as well as in the end bud. Whether these same stem cells are expressed in mammary preneoplasias and neoplasias has not been answered conclusively. Phenotypic markers specific for stem cells and stably expressed in transformed cell populations are needed to follow the fate of stem cells.
    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 crystal structure of a synthetic analogue of valinomycin, cyclo [-(D-Val-L-Hyi-L-Val-D-Hyi)2-] (octa-meso-valinomycin) (I) (C40H68N4O12 · 1.5 · C4H8O2, Mr = 937.01 + 88.10), has been determined. Crystals grown from dioxane are monoclinic, space group P21/a, with cell parameters a = 21.487(8), b = 16.836(5), c = 16.089(4) Å, β = 111.70(4), and Z = 4. The atomic coordinates for nonhydrogen atoms were refined in the anisotropic thermal motion approximation. H atom positions were included in the structure factor calculations at their geometrically expected positions. Values of the standard and weighted R factors after refinement are 0.11 and 0.13, respectively. The conformation of the depsipeptide crystallized from dioxane is different from that crystallized from chloroform (II). The molecule adopts a rectangular shape with two type IV β-turns containing a hydrogen bond and possesses pseudorotational symmetry. The side chains are located on the molecular periphery. The orientation of the carbonyl groups of the molecule is not conducive for efficient metal-ion coordination and in the observed conformation cannot behave as an ionophore.In the crystal the molecules form infinite chains parallel to the c axis, and are stabilized by two intermolecular hydrogen bonds that are shorter and have better geometry than the intramolecular hydrogen bonds.A φ/Ψ plot for dodecadepsipeptides with a (DLLD)3 sequence has well-defined areas for Val and Hyi residues only in cases when the crystals have been grown from nonpolar or medium-polar solvents. The Φ/Ψ plot for octadepsipeptides crystallized from chloroform (II) shows this behavior also. There also is a correlation between the polarity of the solvent from which crystals of octa-meso-valinomycin or valinomycin analogues with a (DLLD) sequence of configuration have been grown and the number of the intramolecular hydrogen bonds that are formed. The more polar the solvent the fewer the number of intramolecular hydrogen bonds.Empirical energy calculations on octa-meso-valinomycin indicate that for isolated molecules, the energy of the bracelet form (II) is 4.7 kcal/mole lower than that of the rectangular form (I).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Biopolymers 32 (1992), S. 441-445 
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Different hexameric forms of insulin have been crystallized from a variety of conditions. In the presence of 1% phenol, 1.0 M sodium chloride, and at a pH of 8.5, a rhombohedral form is produced with two monomers in the asymmetric unit, space group R3, a = 79.92 Å and c = 40.39 Å. The structure has been solved and refined, using data between 8.0 and 2.5 Å resolution, to a residual of 0.157. Each of the monomers adopts an R conformation, that is residues B1-B8 are α-helical. As a result of the T to R transition, an elliptical cavity is created between symmetry-related monomers and is occupied by a phenol molecule. A region of density within bonding distance to one of the zinc ions has been interpreted as an additional phenol molecule.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The use of a laser-desorption mass spectrometer to examine a coal liquefaction extract known to contain high-molecular-mass material (from size-exclusion chromatography) has shown the presence of components of molecular mass in excess of 104Da for the first time. The molecules detected differ from each other in molecular mass by a few hundred or a few thousand Da. These preliminary results indicate that the basic coal-structure molecules probably consist of aromatic clusters on a chin, in reasonable agreement with current theories of coal structure.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Molecular masses up to 270 000 u have been observed during matrix-assisted laser desorption mass spectroscopy on a sample of bituminous coal; the major fraction of desorbed ions appeared between m/z values of 800 and 5000. Similar mass distributions have been found in a coal-liquefaction extract and a pyrolysis tar. The highest molecular masses identified in this work far exceed previously reported high molecular masses found in coal-derived products (12 000 u), which were also detected by laser-desorption mass spectrometry.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 7 (1993), S. 360-362 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: The production of carbon clusters (but not fullerenes) from coal-derived material by the use of laser desorption mass spectrometry is described. Benzo(e)pyrene, a standard polynuclear hydrocarbon found in coal tars, also gave carbon clusters despite using low laser power to avoid pyrolysis of target molecules. Positive- and negative-ion spectra of the standard included carbon clusters. These clusters are formed by the laser energy and can be confused with aromatic hydrocarbons of the same nominal masses. They have no relevance for studies of coal structure.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Chichester [u.a.] : Wiley-Blackwell
    Surface and Interface Analysis 15 (1990), S. 701-704 
    ISSN: 0142-2421
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: AES spectra, when measured in the constant ΔE/E mode, may be distorted by the varying efficiency of the electron detector, usually a channel electron multiplier (CEM). In the analogue mode, this is a problem that increases as the emitted electron energy falls below 200 eV. Instruments may be modified by adding transparent meshes and by biasing the front of the CEM by 200 eV. In this way, the spectral distortion may be reduced by a factor of 10 at 50 eV, so that the resultant spectral shape is within ±10% of the true shape over the energy range 25-2000 eV. This small distortion may be calibrated accurately so that true spectra may be derived from all modified instruments with greater reliability and confidence than in unmodified instruments.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 0142-2421
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: For the use of published general or theoretical sensitivity factors in quantitative AES and XPS, the energy dependence of both the spectrometer transmission function and the detector sensitivity must be known. Here, we develop simple procedures that allow these dependencies to be determined experimentally. Detailed measurements for a modified VG Scientific ESCALAB II, the metrology spectrometer, operated in both the constant ΔE/E and constant ΔE modes, are presented and compared with theoretical estimates. It is shown that an exceptionally detailed electron-optical calculation, involving proprietary information, would be required to match the accuracy of the experimental procedures developed. Removal of the spectrometer transmission function and the detector sensitivity terms allow the measured spectrum to be converted to the true electron emission spectrum, irrespective of the mode of operation. This provides the first step to the provision of reference samples to calibrate the transmission functions and detector sensitivities of all instruments so that they, in turn, may produce true electron emission spectra. This is vital if: (1) all instruments are to give consistent results; (2) theoretical terms are to be used in quantifying either AES or XPS; (3) reference data banks are to be established for AES or XPS.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Chichester [u.a.] : Wiley-Blackwell
    Surface and Interface Analysis 17 (1991), S. 855-874 
    ISSN: 0142-2421
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: An interlaboratory study of the calibration of the intensity scales of Auger electron spectrometers has been completed. Thirty-eight respondees have completed intensity measurements using pre-calibrated reference materials of Cu, Ag and Au in either the direct or differential modes. The reference material calibrations, made at NPL, involve the recording of true spectra under defined conditions using the metrology spectrometer, in which all instrumental contributions to intensity have been accurately clibrated. The ratio of the spectral intensity form the respondee to that derived at NPL gives the intensity-energy response function of the respondee's instrument. In the direct mode this function may be calibrated at 1 eV intervals from 10 to 2500 eV using any one of the reference materials. By using all three materials it is shown that the average repeatability of the calibrations for the 38 respondees is better than 3% over the energy range 10-2500 eV, improving to 〈 2% over the range 100-1000eV. The mot serious unknown parameter that has given rise to uncertainty in the intensity function in the past has been the channel electron multiplier ditector efficiency. It is shown here that this term behaves as predicted theoretically and, if ignored, can lead to a factor of two error in intensity. With the present calibration the errors should be reduced to 2% for spectrometers in which there is little internal scattering. In the differential mode twelve peaks are used to define the intensity-energy calibration. This calibration is less accurate than that in the direct mode and, at the present time, a one-standard deviation uncertainty of ∼11% is more appropriate. In both modes a loss of intensity owing to multiplier overloading may cause severe errors in the high-energy peaks.
    Additional Material: 20 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0142-2421
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: Experiments have been made to provide calibrated kinetic energy values for AES peaks in order to calibrate Auger electron spectrometers of various resolving powers. The kinetic energies are measured using a VG Scientific ESCALAB II, which has power supplies appropriate for AES measurements in both the constant ΔE and constant ΔE/E modes. The absolute calibration of the energy scale is obtained by the development of a new measurement chain, which, in turn, is calibrated in terms of the post-1990 representation of electron-volts using XPS peaks with a traceable kinetic energy accuracy of 0.02 eV. The effects of instrumental and operating parameters, including the spectrometer dispersion and stray magnetic fields, are all assessed and contribute errors for three peaks not exceeding 0.06 eV and for two peaks not exceeding 0.03 eV. Calibrated positions in the direct spectrum are given for the Cu M2,3VV, Au N6,7VV, Ag M4NN, Cu L3VV and Au M5N6,7N6,7 transitions at 0.2 eV resolution, referred to both the Standard Vacuum Level and the Fermi level. For the differential spectrum, the positions of the negative excursions are derived numerically by computer from these data and are established with the same accuracy. Data are tabulated for the above peaks in both the direct and differentiated modes for the popular resolutions of 0.15%, 0.3% and 0.6% produced by Gaussian broadening of the high-resolution spectra. Differentiations are effected by both sinusoidal modulation and Savitzky-Golay functions of 2 eV and 5 eV peak-to-peak.
    Additional Material: 19 Ill.
    Type of Medium: Electronic Resource
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